CA2069944C - Method and apparatus for transmitting broadband amplitude modulated radio frequency signals over optical links - Google Patents
Method and apparatus for transmitting broadband amplitude modulated radio frequency signals over optical linksInfo
- Publication number
- CA2069944C CA2069944C CA002069944A CA2069944A CA2069944C CA 2069944 C CA2069944 C CA 2069944C CA 002069944 A CA002069944 A CA 002069944A CA 2069944 A CA2069944 A CA 2069944A CA 2069944 C CA2069944 C CA 2069944C
- Authority
- CA
- Canada
- Prior art keywords
- signals
- radio frequency
- optical
- modulated
- carrier signals
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N7/00—Television systems
- H04N7/22—Adaptations for optical transmission
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/504—Laser transmitters using direct modulation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/506—Multiwavelength transmitters
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/02—Wavelength-division multiplex systems
- H04J14/0298—Wavelength-division multiplex systems with sub-carrier multiplexing [SCM]
Abstract
A plurality of radio frequency channels are separated into two or more contiguous groups of channels, each of such groups occupying less than one octave. A first group of channels occupies a 54 MHz to 106 MHz band, a second band of chan-nels occupies a 108 MHz to 214 MHz band, and optionally, additional channel groups may occupy the bands of 214 MHz through 426 MHz and 426 MHz through 850 MHz.
Each sub-band is used to modulate the intensity of the output of a laser (1, 2, 3). Each resulting amplitude modulated light signal is transmitted over an optical link (F1, F2, F3, F4) to a receiving location. Alternatively, the lasers (1, 2, 3) may be selected to generate light at different wavelengths with all outputs transmitted over a single optical link. At the receiving location, each of the optical signals is received and demodulated by a re-spective individual receiver (51, 52, 53). The demodulated signal is passed through a bandpass filter which reduces harmonics, in-termodulation distortions, and noise produced in transmission. The resulting signals are then matched in amplitude before being applied to combiner (69, 70) which converts them back into a single broadband signal. The broadband signal is then processed to reduce discontinuities at the transitions between the sub-bands. All of the baseband television signals can be modulated onto car-rier frequencies in the highest sub-band of interest (e.g., in the range of 330 to 550 MHz). Because of the separation of signals provided by multiple optical links (F1, F2, F3, F4), the same carrier frequency may be utilized on each of the links to carry a channel of baseband video information.
Each sub-band is used to modulate the intensity of the output of a laser (1, 2, 3). Each resulting amplitude modulated light signal is transmitted over an optical link (F1, F2, F3, F4) to a receiving location. Alternatively, the lasers (1, 2, 3) may be selected to generate light at different wavelengths with all outputs transmitted over a single optical link. At the receiving location, each of the optical signals is received and demodulated by a re-spective individual receiver (51, 52, 53). The demodulated signal is passed through a bandpass filter which reduces harmonics, in-termodulation distortions, and noise produced in transmission. The resulting signals are then matched in amplitude before being applied to combiner (69, 70) which converts them back into a single broadband signal. The broadband signal is then processed to reduce discontinuities at the transitions between the sub-bands. All of the baseband television signals can be modulated onto car-rier frequencies in the highest sub-band of interest (e.g., in the range of 330 to 550 MHz). Because of the separation of signals provided by multiple optical links (F1, F2, F3, F4), the same carrier frequency may be utilized on each of the links to carry a channel of baseband video information.
Description
20~9944 M~T~OD AND APPA~AT~8 FOR TRAN8NITTING
BRO~nR~ND AMP~ITUDE MOD~ATED RADIO FR~QUENCY
8IGNAL8 OV~R OPTICAL LINR8 T~C~NICA~ FIE~D
The present invention relates to optical signal transmission systems and more particularly to a system for 10 transmitting a broadband of radio frequency channels covering more than an octave of bandwidth over an optical link wherein the radio frequencies are amplitude modulated onto the optical signals transmitted over the link.
R~ r~u~v ART
In recent years there has been a great deal of interest in the transmission of various types of information television signals via optical fiber. Presently, most CATV systems for 20 distributing television signals operate by modulating the video, audio, and other information for each television channel onto a respective radio frequency carrier signal.
Each of these carrier signals typically has a bandwidth of 6MHz (4.5 MHz of information and a 1.5 MHz guard band). A
25 plurality of these signals covering a broadband of radio frequencies (e.a., in the range of 54-550 MHz) are distributed via networks comprising 75 ohm coaxial cables and appropriate signal amplifiers and taps.
Optical fibers intrinsically have more information 30 carrying capacity than do the coaxial cables which are used in present cATV systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals.
Consequently, optical fibers are capable of sp~nn;ng longer 35 distances between signals regenerators or amplifiers than are coaxial cables. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by 20~999~
electrical shorting or radio frequency pickup. Finally, optical fiber is immune to ambient electromagnetic interference ("EMI") and generates no EMI of its own.
A number of means are available for transmitting 5 television signals and or other types of information over optical fibers or other optical transmission media. For example, the 6 MHz baseband television signal may be converted to digital form. This digital information may be used to modulate a light signal and transmitted via an optical link.
10 Transmission of such a digitized 6 MHz video signal requires a digital data transmission rate of at least 45 megabits per second. High definition video ("HDTV") may require a digital data transmission rate of up to 145 megabits per second.
Moreover, encoders and decoders for~ converting analog 15 television signals to digital form and for reconverting these digital signals to analog form for viewing on a conventional television set are quite expensive. Consequently, analog transmission of television signals by optical means is, potentially, much more economical than digital transmission 20 of such signals.
One such means of analog transmission is to use the baseband television signal to frequency modulate a radio frequency carrier. This modulated radio frequency carrier is in turn used to modulate an optical signal. Such frequency 25 modulation is less susceptible to noise than is amplitude modulation but it requires more bandwidth for each television channel transmitted than are required by amplitude modulation methods. Thus, the number of television ch~nnels which can be carried by each optical transmission link (e.a., each 30 optical fiber) in an FM-based system may be somewhat limited.
Moreover, since the standard NTSC format for video calls for amplitude modulation of the video carrier, means for converting FM signals to NTSC AM format are required either at the television set or at the point at which the fiber 35 transmission trunk is connected to a coaxial cable distribution network. The need for such FM to NTSC AM
conversion increases the cost of the system.
206994~
,..
In view of the above, a system in which the video baseband signal amplitude modulates a radio frequency carrier signal which in turn amplitude modulates an optical signal is preferable to other systems from the standpoint of cost and 5 simplicity. However, several phenomena limit the number of radio frequency channels which can be carried by present day optical links where the intensity of light signals is amplitude modulated. A first of these phenomena is a limitation of the amount of radio frequency energy which may 10 be supplied as a modulating signal to a laser or other light generating device before various types of distortions are generated by the light generating device. This power limitation relates to the sum of the radio frequency power contributions of each radio frequency channel. Thus, if it 15 is desired to transmit 80 radio frequency channels over a single optical link, each of these channels can be powered with only half of the power which would be available to it if only 40 channels were transmitted by the link. Such a limitation on the power of each radio frequency carrier brings 20 each of these carriers closer to the white noise level of the system, thus, adversely affecting the signal to noise ratio of the system. Decreasing the number of channels carried by each optical link in order to improve the signal to noise ratio increases the number of lasers which must be used and 25 the overall complexity and cost of the system. On the other hand trying to increase the amount of radio frequency power supplied to the laser beyond certain limits causes the laser to produce several types of distortion which are discussed below.
When the modulating signal supplied to a laser causes the laser to be driven into a nonlinear portion of its input-signal-to-light-outputcharacteristic,harmonicdistortionmay be produced. The products of this type of distortion are signals which are integer multiples of the "primary"
35 frequency. The second harmonic of 54 MHz is, for example, 108 MHz. Thus, if the bandwidth accommodated by a system is such that there are channels at both 54 MHz and 108 MHz, second harmonics of the 54 NHz channel will interfere with the signals on the 108 MHz channel.
Intermodulation distortion is of particular concern in amplitude modulated systems. Such distortion results in distortion products having frequencies which are the sum products or difference between two other frequencies.
The sums and difference of two primary frequencies are called second order distortion products and are par-ticularly troublesome. For example, a video channel at150 MHz and another video channel at 204 MHz may produce a second order distortion product at 54 MHz (the difference frequency) and at 354 MHz (the sum frequency).
Third order distortion products are produced by the mixing of a primary frequency and a second order distortion product producing third order distortion products equal to the sum and difference between the primary frequency and the second order distortion product. Third order products may also be generated by the using of three frequencies or by third harmonic generation.
Several methods have been proposed to alleviate the problems caused by harmonic distortion and inter-modulation distortion in amplitude modulated optical links. One such method is described in U. S. Patent No.
5,379,141 entitled "CATV Distribution Networks Using Lightwave Transmission Lines" and assigned to the Assignee of the present application. In accordance with the method described in the above patent, distortion of a broadband CATV signal is reduced by translating of the signal to a higher frequency range at which the bandwidth signal constitutes less than an octave. This translated broadband signal is applied as a radio frequency modulating signal to an optical signal generating device (e.a., a laser or laser diode) and transmitted over an optical link (e.g., a fiber optical path) to an optical ~ ' _ 4a 2069q44 receiver and demodulator at a remote location. After demodulation radio frequency signals outside the band of interest (e.q., harmonics and intermodulation distortion products) are filtered out by a bandpass filter having a passband covering only the frequencies of interest. The
BRO~nR~ND AMP~ITUDE MOD~ATED RADIO FR~QUENCY
8IGNAL8 OV~R OPTICAL LINR8 T~C~NICA~ FIE~D
The present invention relates to optical signal transmission systems and more particularly to a system for 10 transmitting a broadband of radio frequency channels covering more than an octave of bandwidth over an optical link wherein the radio frequencies are amplitude modulated onto the optical signals transmitted over the link.
R~ r~u~v ART
In recent years there has been a great deal of interest in the transmission of various types of information television signals via optical fiber. Presently, most CATV systems for 20 distributing television signals operate by modulating the video, audio, and other information for each television channel onto a respective radio frequency carrier signal.
Each of these carrier signals typically has a bandwidth of 6MHz (4.5 MHz of information and a 1.5 MHz guard band). A
25 plurality of these signals covering a broadband of radio frequencies (e.a., in the range of 54-550 MHz) are distributed via networks comprising 75 ohm coaxial cables and appropriate signal amplifiers and taps.
Optical fibers intrinsically have more information 30 carrying capacity than do the coaxial cables which are used in present cATV systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals.
Consequently, optical fibers are capable of sp~nn;ng longer 35 distances between signals regenerators or amplifiers than are coaxial cables. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by 20~999~
electrical shorting or radio frequency pickup. Finally, optical fiber is immune to ambient electromagnetic interference ("EMI") and generates no EMI of its own.
A number of means are available for transmitting 5 television signals and or other types of information over optical fibers or other optical transmission media. For example, the 6 MHz baseband television signal may be converted to digital form. This digital information may be used to modulate a light signal and transmitted via an optical link.
10 Transmission of such a digitized 6 MHz video signal requires a digital data transmission rate of at least 45 megabits per second. High definition video ("HDTV") may require a digital data transmission rate of up to 145 megabits per second.
Moreover, encoders and decoders for~ converting analog 15 television signals to digital form and for reconverting these digital signals to analog form for viewing on a conventional television set are quite expensive. Consequently, analog transmission of television signals by optical means is, potentially, much more economical than digital transmission 20 of such signals.
One such means of analog transmission is to use the baseband television signal to frequency modulate a radio frequency carrier. This modulated radio frequency carrier is in turn used to modulate an optical signal. Such frequency 25 modulation is less susceptible to noise than is amplitude modulation but it requires more bandwidth for each television channel transmitted than are required by amplitude modulation methods. Thus, the number of television ch~nnels which can be carried by each optical transmission link (e.a., each 30 optical fiber) in an FM-based system may be somewhat limited.
Moreover, since the standard NTSC format for video calls for amplitude modulation of the video carrier, means for converting FM signals to NTSC AM format are required either at the television set or at the point at which the fiber 35 transmission trunk is connected to a coaxial cable distribution network. The need for such FM to NTSC AM
conversion increases the cost of the system.
206994~
,..
In view of the above, a system in which the video baseband signal amplitude modulates a radio frequency carrier signal which in turn amplitude modulates an optical signal is preferable to other systems from the standpoint of cost and 5 simplicity. However, several phenomena limit the number of radio frequency channels which can be carried by present day optical links where the intensity of light signals is amplitude modulated. A first of these phenomena is a limitation of the amount of radio frequency energy which may 10 be supplied as a modulating signal to a laser or other light generating device before various types of distortions are generated by the light generating device. This power limitation relates to the sum of the radio frequency power contributions of each radio frequency channel. Thus, if it 15 is desired to transmit 80 radio frequency channels over a single optical link, each of these channels can be powered with only half of the power which would be available to it if only 40 channels were transmitted by the link. Such a limitation on the power of each radio frequency carrier brings 20 each of these carriers closer to the white noise level of the system, thus, adversely affecting the signal to noise ratio of the system. Decreasing the number of channels carried by each optical link in order to improve the signal to noise ratio increases the number of lasers which must be used and 25 the overall complexity and cost of the system. On the other hand trying to increase the amount of radio frequency power supplied to the laser beyond certain limits causes the laser to produce several types of distortion which are discussed below.
When the modulating signal supplied to a laser causes the laser to be driven into a nonlinear portion of its input-signal-to-light-outputcharacteristic,harmonicdistortionmay be produced. The products of this type of distortion are signals which are integer multiples of the "primary"
35 frequency. The second harmonic of 54 MHz is, for example, 108 MHz. Thus, if the bandwidth accommodated by a system is such that there are channels at both 54 MHz and 108 MHz, second harmonics of the 54 NHz channel will interfere with the signals on the 108 MHz channel.
Intermodulation distortion is of particular concern in amplitude modulated systems. Such distortion results in distortion products having frequencies which are the sum products or difference between two other frequencies.
The sums and difference of two primary frequencies are called second order distortion products and are par-ticularly troublesome. For example, a video channel at150 MHz and another video channel at 204 MHz may produce a second order distortion product at 54 MHz (the difference frequency) and at 354 MHz (the sum frequency).
Third order distortion products are produced by the mixing of a primary frequency and a second order distortion product producing third order distortion products equal to the sum and difference between the primary frequency and the second order distortion product. Third order products may also be generated by the using of three frequencies or by third harmonic generation.
Several methods have been proposed to alleviate the problems caused by harmonic distortion and inter-modulation distortion in amplitude modulated optical links. One such method is described in U. S. Patent No.
5,379,141 entitled "CATV Distribution Networks Using Lightwave Transmission Lines" and assigned to the Assignee of the present application. In accordance with the method described in the above patent, distortion of a broadband CATV signal is reduced by translating of the signal to a higher frequency range at which the bandwidth signal constitutes less than an octave. This translated broadband signal is applied as a radio frequency modulating signal to an optical signal generating device (e.a., a laser or laser diode) and transmitted over an optical link (e.g., a fiber optical path) to an optical ~ ' _ 4a 2069q44 receiver and demodulator at a remote location. After demodulation radio frequency signals outside the band of interest (e.q., harmonics and intermodulation distortion products) are filtered out by a bandpass filter having a passband covering only the frequencies of interest. The
2 06~q 44 passband is then reconverted to the original frequency range (e.g., 54-550 MHz) and is available for distribution to subscribers of conventional CATV components.
One deficiency of the above method is that the processes 5 of "up-conversion" of the frequency band of interest and subsequent "down-conversion" may themselves introduce noise and/or distortion into the system;
DI8CLO8~RE OF INVENTION
In accordance with the present invention therefor, a method has been found to limit the bandwidth of radio frequency signals used to modulate light emitting sources used in a broadband CATV distribution network to less than 15 an octave. The method is implemented using a minimum number of optical links and does not require block conversion of the entire frequency band of interest to a higher band of frequencies and reconversion to the original band, although such block conversion may be utilized for some of the lower 20 frequency channels in the frequency band of interest as set forth below.
To implement the invention, a plurality of radio frequency channels covering more than one octave of bandwidth are separated into two or more contiguous groups of channels, 25 each of such qroups occupying less than one octave of frequency. For example, a first group of channels may occupy the 54 MHz to 106 MHz band (just under one octave).
Similarly, a second band of ~h~nels may occupy the 108 MHz to 214 MHz band. Optionally, additional groups of frequencies 30 may occupy the bands of 214 MHz through 426 MHz and 426 MHz through 850 MHz. Each of these sub-bands, constituting a group of radio frequency channels, is used to modulate the intensity of the output of a light emitting device such as a laser. Each resulting amplitude modulated light signal is 35 transmitted over an optical link to the receiving location.
Each of these optical links may be, for example, a single strand of optical fiber. Alternatively, each of the lasers may be selected to generate light of different wavelengths than the other lasers and all of the laser outputs may be transmitted over a single optical fiber or other physical light transmitting means. In this latter case, at the 5 receiving mode, each wavelength is separated by a wavelength division multiplier and sent to individual receivers. At the receiving location, each of the optical signals is received and demodulated by a respective individual receiver. The demodulated radio frequency signal produced by each of these 10 receivers may be passed through a frequency selective or bandpass filter which serves to alleviate harmonic and/or intermodulation distortions and/or noise produced in the optical transmission system. Each of the resulting radio frequency signals may then be suitably~attenuated so as to 15 match the amplitude of the other radio frequency signals before being applied to combiner and trim circuitry to combine the sub-bands of frequencies back into a single broadband radio frequency signal. This resulting broadband radio frequency signal may be operated on by additional circuitry 20 which serves to alleviate any discontinuities of the signal at the transitions between the sub-bands.
It will be noted that in the above system, far fewer 6 MHz channels can be accommodated by the optical links serving the lower frequency sub-bands than by the optical links 25 accommodating the higher frequency sub-bands since there is room for fewer 6 MHz channels in a lower frequency octave.
In accordance with one aspect of the invention, the number of optical links required to implement the above system, may be reduced by initially modulating all of the baseband television 30 signals onto radio frequency carrier frequencies in the highest sub-band of interest (e.q., in the range of 330-550 MHz). Because of the separation of signals provided by multiple~ optical links, the same carrier frequency may be utilized on each of the links to carry a channel of baseband 35 video information. At the receive side one of the demodulated signals must be down-converted to a lower frequency range so as not to interfere with the signals from the other optical çh~nnPlc when the two signals are mixed. However, since this solution involves only one down~..vcl~ion for only a portion or portions of the frequency band of interest. Moreover, since the method does not require up~-~ ion for any 5 of the ch~nnP-lc, it is believed to introduce far less distortion and noise to the ~ncmiccion system then the previous metho~c of block COI vclling all of the ch~nnehc, to a higher frequency and then lo~on~ling all ch~nnP-hc to their origin~l frequenriP~s.
Other aspects of this invention are as follows:
A method of Ll~ns.. ;~l;ng a plurality of radio rl~u~ ch~nmPlc covering more than one octave of bandwidth from a first lt-c~tion to a second location, said method compricing the steps of:
sPkP~ting two or more groups of contiguous radio frequency ch~nn~Plc, each of said groups co~,e~ing a bandwidth of less than one octave;
~cm.dting a pluraliq of optical signals at said first location, each said optical signal col~ ding to one of said groups;
m~ul~hng the intensity of each optical signal with the radio frequency çh~nnPlc of its ~ ive group;
tr~ncmitting each resultinp m~~ tP~ optical signal to the second 20 location via a S~)aldtC optical link; and demod~ tin~ each ~ ns~..;ll~ optical signal at the second location into its lcspec~ivc group of radio frequency ch~nnPlc.
A method of tran~ l;~ a plurality of ch~nnplc of inform~tion from a first location to a second loc~tinn said method comprising t,he st,eps of:
~efining two or more groups of çh~nmPlc wl~elcin each rh~nnel is ~ccign~d to at least one of said groups;
mo~ul~ting a radio frequency carrier signal with each ch~nnel in each group, wherein each radio frequency carrier signal uniquely i~iPntifiPs oneçh~nnPl in a group;
generating a pluraliq of opt,ical signals, each coll~,~nding to one of said groups of ~h~nnP.lc,;
7a 2069944 ~mplitude mod~ ting each of said optical signals with the mod~ ted radio frequency carriers coll.,a~nding to its respective group of ch~nn~l c;
t~ncmitting each of said optical signals to the second location;
dem~~ tinP each of said optical signals at the second location into its ~spacLve m~lll~ted radio frequency carrier signals;
cG"~Glling at least one of said mod~ ted radio rl~uency carrier signals to a different rrG~ue~ band of carrier signals which does not overlap with any other t~n.$mitted radio frequency carrier cig~lc; and combining the resulting mod~ ted radio frequency carrier signals in a single signal.
In a system for co------un;c~tin~ ;nro~ ;on signals with a plurality of optical ci~r~lc, each of which being modlll~tc^;d with a plurality of carrier signals, a method of receiving said optical signals comrcing the steps of:
ting each of said optical signals into its ~ ive carrier signals, wh~m a frequency of one of said carrier signals is s~st;~nti~lly equal to a r~uenc~ of another of said carrier signals; and con~,~ling the frequency of the one carrier signal to a frequency different than that of the anolll~r carrier signal to lLe,~b~ produce carrier signals having dirÇ~nt frequen(ies.
A system for tr~ncmittinp a plurality of cl~nnelc of info...,~l;on from a first l~tion to a second loc~tion, said system comrricing an optical ~ nsll~ includin~ means for l~iving signals col~ ~n~ling to each c~nnPl of info~ n, means for genP~ting a plurality of radio frequency carrier signals, means for geu~-.,.l;ng a plurality of optical ~ig~,~lc, means for modul~tinE each of said info.... ~I;on signals onto at least one of said radio carrier frequencies to produce ,~..~;tivc m~~ t~ radio 30 fic~lu~ ~ carrier ~ig~ls, means for ~lecti~ a first group of said mod~ t~l radio frequency carrier signals, none of which have the same radio carrier frequency, means for ~l~ting a second group of said mod~ t~ radio - 7b 2069944 frequency carrier signals, none of which have the same sdio frequency carrier, means for modnl~ting a first optical signal with said first group of mod sdio rlc~luency carrier signals, and means for m xlul~ting a second optical signal with said second group of mod~ tp~ sdio frequency carrier signals;
an optical rccci~r in~ ling means for demodl~1~ting each said optical signal into its l~ iVC modul~t~ sdio frequency carrier sign~l~, and means for combining said modlll~tpd sdio frequency carrier signals into a single signal; and a pluslity of optical links c4nnF~ g each of said optical signal modlll~tinf! means with a l~s~ive one of said optical signal demod~ ting means.
A method for lCCCiVillg a plurality of optical sign~l~, each of which is m~lll~t~ with at least one rnodlll~t~d radio frequency carrier signal, said method compri~ing the steps of:
~emod~ ting each optical signal into its Lci.~æ~ive m~lll~t~pd sdio rrc~lueacy carrier signals, wLc~ at least one of said rn~lll~ted sdio frequency carrier signals has a frequency band that u~c.laps with another of said modul~tPd sdio frequency carrier signals;
con~.ling the at least one of said modul~tP~ sdio frequency carrier signals to a dirr"~nl frequency band of carrier signals which does not overlap with any other modlll~t~ sdio frequency carrier signal mod~ ting said optical si~ ; and colllbi~ g the res lting mo~ tP~ radio frequency carrier signals in a single signal path.
A t~n~mittP~ for t~n~mittin~ a plurality of cl-~nn~l~ of inform~tion compri~ing:
means for lcceiving signals coll~onding to each c~nnel of info....~l;on;
means for gencldting a plurality of sdio frequency carrier .~ign~
means for g,~ne.,-t;ng a plurality of optical signals;
7c 2069944 means for modul~ting each said inform~tion signals onto at least one of said radio frequency carrier signals to produce a l~L,e~;live modu radio frequency carrier signal;
S means for sP1~tinp a first group of said modul~tP~ radio frequency carrier signals wherein none of the signals of the first group have the same carrier frequency as any other;
means for sel~ting a second group of mod~ tP~d radio r,~u~nc~
carrier signals whe~ein none of the signals of the second group have the same carrier frequency as any other;
means for m~dnl~ting one of said optical signals with the m~lll~ted radio frequency carrier signals of said first group; and means for modlll~ting al~lh~. of said optical signals with the m~lll~tP~ radio frequency carrier signals of said second group.
A method for ~ ;~ a plurality of radio frequency ~h~nmPlc covering more than one octave of bandwidth from a first location to a second location, said method compricinp the steps of:
sPl~p~ting two or more groups of contiguous radio frequency ch~nnels, each of said groups covering a bandwidth of less than one octave;
gen~dlii g a plurality of optical signals of dirr~,ent wavelPngths at said first location wherein each of said optical signals co~ ds to at least one group of radio frequency ch~nnPlc;
mod~ ting the intensily of each optical signal with the radio frequency ch~nnpl~ of its l~xclive group;
I.~n~.,;ll;,~g each res~ltin~ m~l~l~t~d optical signal to the second l~tion via a s.~ ~e optical link; and demodlll~ting each l.~n~."ill~ optical signal at the second location into its l~;live group of radio frequency ch~nnPls A method of l~eiving inlcnsil~-m xlul~t~P~d optical signals over optical links to produce a plurality of radior~u~cy ch~nnPlc covering more than one octave of bandwidth, compri~ing:
. ;~, 7d 2069944 receiving at least three intensity-modl-lqtPd optical sign_ls over optical links;
dçm xl-llqting each received int~,~s,l~-m~dl-lqtP,d optical signal to produce a co"~onding group of mr~lllqt~Pd radiof~u~ncy carrier signals;
filtP.ring each said group of modulqtçd radiorl_luel ey carrier signals to roll off noise and distortion products, ll,e,~)~ produc.ing l~,~CIiVegroups of modlllqtçd radiofrequency carrier sig~qls, each group of modlllqtPd radior~equellcy cqrrier signals having a bandwidth of contiguous radiofrequency10 ch~nnPl~ covering less than one octave; and combining the groups of modlllat~d radiofrequency carrier signals to produce a single signal having a plurality of radioL~u~ r cll~nnel~ covering more than one octave of bandwidth.
An optical l~C~ . for ~iving inte,~sily-modulqt~d optical signals over optical links to produce a plurality of radioL~u~l~cy cll~nn~
covering more than one octave of bandwidth, complicing:
at least three opt~~ unic recei~,s for lc~iving at least three h~ sil~-modulqt~d optical signals over optical links and for dçmod~ q-ting each lcceived hllensil~-m~l-lqt~d optical signal to produce a coll~ ng group of0 mo~ul~qted radior~uenc~ carrier signals;
at least three filters, each said filter c~ ,~n~l;ng to one of said optoelectronic lCC~iV~ i, for filtP.ring the group of modulqtP.d radiofrequency carrier signals produced by its coll~onding optoPl~onic recei~ to roll off noise and distortion products lh~ producing ~ eclive groups of mod 25 radiofrequency carrier signals, each group of modlllqtP~l radiofrequency carrier signals having a bandwidth of contiguous radiofrequency c~l~nn~l~ covering less than one octave; and a signal cu,l,biner, couFlcd to said filters, for co",bh~ing the groups of modulqt*~ radiorl~uency carrier signals to produce a single signal 30 having a plurality of radiofrequency c~qnnpl~ covering more than one octave of bandwidth.
7e 2069944 It is an object of an aspect of the present invention, th~ forc, to provide a method for gen~r~ting and receiving light signals mo~ul~tP~ with bro~Ab~nA
radio frequency signals in a manner which m~simi7P the signal to noise ratio of 5 the rcceived radio frequency signals.
It is an object of an aspect of the invention to minimi7~ h~rmnnic and il-lf~ ion distortion products introduced by the optical t~ncmiccion system into the radio frequency signals.
It is an object of an aspect of the invention to accomplish reducti-~n of 10 the in~ tinn and h~..-onic distortion products in a --am ~r which minimi7~S the nu~l,bel of optical signal paths n~fc*s~ for t~nQmiQQion of a given number of radio frequency signals.
It is an object of an aspect of the invention to achieve a re~uctin~ in the number of optical signal paths or links nPc~s~aly to ~n~mit a given nl~.llbf r of 15 b~QPb~nd television ch~nnel~ without collvclling the carrier frequencies of the television ch~nn~Plc to ~_s~ætivc higher ~u'nc;~Ps~
BRIEF DESCRIPIION OF THE DRAVVINGS
The above and further objets and advantages of the invention will baco,,,c a~ t with reference to the det~il~ d;Q~10S-~ of the invention set 20 forth below and the accG",panying illllstrative figures in which:
Fig. 1 is a block tlia~m of a h~dend optical tr~nsmit-tpr for a system o~ldting in accor~ce with the 8 206~944 present invention;
Fig. 2 is a block diagram illustrating how optoelectronic nodes may be connected in an existing CATV system to provide an interface between optical signal paths and a previously 5 existing coaxial cable network;
Fig 3 is a block diagram of an optoelectronic receiver node for a system operating in accor~A~ with the present nventlon;
Fig. 4 is a block diagram of an alternative embodiment 10 of an optoelectronic receiver node operating in accordance with the present invention;
Figs. 5a through 5h are schematic diagrams of presently preferred filters which are incorporated in the optoelectronic receiver nodes;
Fig. 6 is a schematic diagram of an illustrative directi,onal coupler which may be incorporated in the receiver nodes of the invention;
Fig. 7 is a schematic diagram of a combiner which may be incorporated in the receiver node of the invention;
Fig. 8 is a schematic diagram of a diplex filter which may be incorporated in the receiver node of the present invention; and Figs. 9a and 9b are schematic diagrams of presently preferred receiver trimmers (frequency signature correction 25 networks) for use in the present invention for equalizing radio frequency carrier amplitudes at the outputs of the receiver ro~C.
MODE(S) FOR CA~K~N~ OUT T~E I~v NTION
Referring now to Fig. 1, there is shown an illustrative embodiment of a CATV headend configured in accordance with the invention. At this headend there exists a number of channels of information (channel 2 through ch~nnel n) of information 35 to transmit through the CATV network fed by the headend. In this illustrative example the channels are television channels which occupy 6 MHz of bandwidth at h~5~hA~ (4.5 MHz of video, ~ 9 206~944 audio and other information and a 1.5 MHz guard band). It will be appreciated, however, that the present invention would be equally applicable for the transmission of any type of information through a network. Each of the baseband channels 5 of information is supplied to a respective modulator of the type normally found at CATV headend installations (e.a., a Scientific Atlanta Model 6350). Each modulator serves to impose the baseband channel on a radio freguency carrier supplied by the modulator. Standard plug-in modulators are 10 in common use at CATV headends which can produce modulated radio frequency signals in the range of, for example, 54-550 MHz and higher in 6 MHz increments. In the conventional CATV
systems in use today, each baseband signal must be modulated onto a carrier frequency which is distinct from carrier 15 frequencies associated with any other baseband signal since all of the carrier frequencies must be mixed together to form a single broadband signal for transmission over a coaxial cable network wherein the trunk and distribution links of the network comprise a single 75 ohm coaxial cable path. In 20 accordance with present invention, however, several optical links may be used to replace or supplement portions of a trunk of a CATV system. In view of the plurality of optical links employed, a hA~^hAn~ signal transmitted over one link may, if desired, be modulated onto a radio 25 frequency carrier having the same frequency as the carrier of a baseband signal transmitted over a different link.
Thus, as shown in Fig. 1, a first group of basehAn~
signals designated as chAnnels 2 through k may be fed to 30 a first group of modulators having carrier frequencies in, for example, the 300-550 MHz range. Similarly, a second group of baseband signals designated as channels k+1 through m may be fed respectively to a second group of modulators which may, optionally, have the same or 35 some of the same carrier frequencies as the first group of modulators. Finally a third group of baseband signals designated as channels m+l through n may be fed to their respective group of modulators which may also have carrier frequencies in the same range as the 5 modulators of the first two groups (e.a., 300-500 MHz). It will be noted that if each of the group of bas~hAnA signals is modulated with the highest possible set of carrier frequencies within the radio frequency band of interest a ~ um number of channels may be accommodated in a single 10 signal path while maintaining the total bandwidth of the group of channels within less than an octave range. As discussed above, this objective of keeping the bandwidth below an octave is important in order to minimize problems associated with intermodulation distortion-and harmonic~distortion produced 1~ when the radio frequency signal is used to amplitude modulate the light output of a light emitting device such as a laser.
It is particularly noteworthy, that the present invention achieves this objective without resorting to up-conversion of one or more groups of modulated radio frequency signals since 20 such up-conversion can cause degradation and distortion of the signals upon which it operates.
In the illustrative example set forth herein three groups of baseband signals are designated. It will be appreciated that the invention is equally applicable to systems in which 25 two groups of baseband signals or greater than three groups are defined. In the illustrative embodiment of Fig. 1, each of the three groups of baseband signals are combined into respective broa~h~n~ signals in the range of 300-500 MHz by respective signal combiners 1, 2, and 3. Each of these 30 broadband radio frequency signals may be passed through respective buffer amplifiers and equalizers in order to provide equivalent signals to amplitude the light output of respective lasers 1, 2, and 3. In the illustrative embodiment of Fig. 1, each of the lasers 1, 2, and 3 may emit light of 35 the same wavelength since their respective optical outputs are transmitted through the network over individual fiber optic links Fl, F2, and F3. Separate physical links for 20699~4 transmitting the respective optical signals are not nec~cs~ry to practice the present invention however. Referring to Fig.
la the lasers 1, 2, and 3 may each be selected to emit light of a unique wavelength. The three amplitude modulated optical S signals may then be combined in an optical combiner device and transmitted over a single fiber strand or other type of optical link.
Fig. 2 shows the manner in which an optical signal transmission network may be interfaced with an existing 10 coaxial CATV network so as to complement the existing network.
Fig. 2a shows a portion of an existing network consisting of a cascade of trunk amplifiers. Fig. 2b shows an interfacing scheme called a reverse cascade in which two nodes or interfaces between the optical electronic systems are utilized 15 to feed signals carried by the trunk to forward and reverse cascades of amplifiers. It will be noted in this configuration that it is required to reverse the direction of some of the existing amplifiers in the CATV system. Fig 2c shows a system in which the optical signal transmission system 20 bypasses portions of the existing CATV trunk. In this system each node receives both an optical signal and a radio frequency signal from the cascade of trunk amplifiers between the node and the headend of the system.
Fig. 2d shows a system similar to the system set forth 25 in Fig. 2c which requires one-half the number of nodes. The question of whether the system of Fig. 2c is more or less reliable than the system of Fig. 2d depends in great part on the reliability of the node circuitry. Thus, if the node circuitry is relatively reliable compared to trunk amplifier 30 circuitry, the circuitry of Fig. 2c may be more reliable on a system from a system standpoint. On the other hand, if the node circuitry proves to be relatively unreliable when compared with the trunk amplifier circuitry or the likelihood that the existing coaxial trunk system will suffer outages for 35 other reasons, the network illustrated by Fig. 2d will tend to have a better system reliability. Fig. 3 illustrates an embodiment of a node which is adapted to serve as an interface between an existing coaxial signal distribution system and an optical signal distribution system such as is set forth in Fig. 1. The node circuitry set forth in Fig. 3 is particularly adapted for nodes operating in accordance with 5 the requirements of the systems illustrated in Figs. 2c and 2d.
Referring to Fig. 1, it will ,be noted that the light signals amplitude are modulated with radio frequency carriers at the radio frequency carriers are themselves be modulated 10 with video or other types of information. The modulated light signals are delivered to the node via optical fiber strands F1, F2, and F3. In this connection it should be noted that the node of Fig. 3 is equally applicable to a system in which only one physical fiber strand delivers multiple optical 15 signals having different wave lengths to the node. In this case, the optical signals delivered by the single fiber may be physically separated by optical means to enable each optical signal to be conveyed to a corresponding optical signal receiver/demodulator.
In either of the above cases, each amplitude modulated optical signal is provided as an input to a corresponding optical receiver/demodulator. In the illustrative embodiment of Fig. 3, optical receivers 51, 52, and 53 serve this purpose. These receivers may be any type of receiver which 25 has characteristics which are suitably linear to adapt it to serve as a device for demodulating an analog radio frequency signal from an amplitude modulated light beam. One such type of receiver is called a high imrD~nCD receiver and another type of receiver suitable for this purpose is a transimpedance 30 receiver. A type of receiver which is particularly adapted to implement receivers 51, 52, and 53 is the receiver described in U.S. Patent Application Serial No. 481,436 entitled "Push-Pull Optical Receiver" which is also assigned to the assignee of the present application. In addition to 35 the above receivers a number of other types of optoelectronic receivers are currently available.
One or more of the receivers 51, 52, or 53 may feed down-converters such as down-converters S4 and 55 if the band of radio frequency carriers assigned to these receivers overlaps with the bands of radio frequency carriers assigned to another receiver such as receiver 53. It will be recalled that in the 5 illustrative signal transmitter illustrated by Fig. 1, the radio frequency carriers carried by optical links Fl, F2, and F3 were all in the range of 300-550 MHz so as to ~x;~ize the number of channels which could be carried by the link and still contain the channels of each link within one octave.
10 Assuming that the transmitter of Fig. 1 feeds the receivers of Fig. 3, it will be necessary for suitable oscillators 56 and 57 to feed converters 54 and 55 so as to down-convert or up-convert the radio frequency signals at the outputs of receivers 51 and 52 so that they do not overlap the frequency 15 band at the output of receiver 53 or with each other. The outputs of receivers 51 and 52 and their respective optional down-converters 54 and 55 are connected respectively to filter circuits 60 and 61. The ouL~uL of receiver 53 is connected to filter 62. Each of these filters is designed to attenuate 20 frequencies outside of the frequency band of interest on a particular signal path. Circuits for presently preferred embodiments of these filters are discussed in more detail on connection with the discussion of 5a through 5h below. Each of the filters 60, 61, and 62 is connected to a respective 25 radio frequency test point 63, 64 or 65. These radio frequency test points are directional couplers which are described in greater detail in connection with the discussion of Fig. 5 below. The radio frequency test points 63, 64, and 65 are in turn connected to Pads 66, 67, and 68 which are of 30 conventional design. Pads 66 and 67 are connected to a combiner 69 and the output of the combiner 69 and of the Pad 68 are connected to a combiner 70. The output of this combiner 70 is connected to an equalizer trim circuit 71 which in turn feeds a hybrid amplifier 72. The u~u~ of the hybrid 35 amplifier feeds a diplexer or diplex filter 73, illustrative circuitry for which is illustrated in Fig. 8. The diplex filter in turn feeds a return laser or LED 74 via a test point 75. The common output of the diplex filter 73 goes to a relay 76. The common point 77 of the relay is connected to a 75 ohm radio frequency output port of the node via an AC bypass circuit 78 and to another port of the node via another AC
bypass circuit 79. When the common arm 77 of the relay is in its other position 80 the optical signal path is bypassed and the radio frequency output port is connected to a radio frequency input port via a Pad 81, an equalizer 82, and an AC
bypass circuit 83 and 84.
The operation of the circuitry described above is as follows. Filters 60, 61, and 62 are set up to roll off noise and distortion products outside the band of interest of their respective receivers 51, 52, and 53. In a typical arrangement receiver 51 and down converter 54 will produce about 6 MHz radio frequency video carrier channels beginning at channel 2 beginning at 54 MHz. The receiver 52 and down-converter 55 may accommodate the next 30 radio frequency channels in the frequency band and the remainder of the channels would be handled by receiver 53. In such a configuration, filter 60 would be a low pass filter allowing only the frequencies up to and including the highest frequency channel in the frequency band of interest to pass. Filter 61 would be of the band-pass variety, attenuating noise and distortion at frequencies above and below the band of interest. Filter 62 would be a high pass filter allowing all frequencies above the lowest frequency channel of the band of interest through.
Alternatively a two-path system is possible utilizing only optical links F1 and F2. In such a system, approximately one-half of the channels are accommodated by each of the receivers 51 and 52. Here filter 60 would be a low pass filter with a break point at about 330 MHz and filter 61 would be a high pass filter with a similar break point.
The respective filters 60, 61, and 62 feed combiners 69 and 70 through test points 63, 64, 65 and Pads 66, 67, and 68. The pads are pluggable units which are selected to equalize the amplitude of the signals fed to the inputs of combiners 69 and 70. The attenuation required of each pads may be determined by monitoring the outputs of filters 60, 61, and 62 via radio frequency test points 63, 64, and 65.
The purpose of the equalizer trim circuit 74 is to flatten out the output signal from combiner 70. In this 5 regard two types of adjustments may be required to the signal.
Firstly, the signal from the combiner may be "tilted." That is, there may be differences between the amplitudes of the higher frequency signals and lower frequency signals which vary across the frequency band in a linear manner. Secondly, 10 at certain points in the frequency and, especially at frequencies at the interfaces between the sub-bands carried by the various optical links, there may be notches in the response wherein some channels or group of channels have a higher or lower amplitude than the channels surrounding them.
15 It is desireable to smooth out these notches or peaks in the frequency response. Fig. 9a illustrates a presently preferred embodiment of a equalizer trim circuit which is particularly adapted for systems utilizing two optical links. Fig. 9b illustrates a presently preferred embodiment of a circuit 20 particularly adapted to provide equalizer trim in a system with three optical links.
In the presently preferred emho~ nt of the invention, the equalizer trim circuit 71 feeds a hybrid amplifier 72 such as a Model SHW 571 manufa~L~ed by Motorola.
At this point it should be noted that in many of today's coaxial CATV systems television signals are transmitted from the headend to subscriber locations in a frequency band of 54-550 MHz. Some systems also have the capability of transmitting signals from subscriber locations back to the 30 headend. In most such systems, these return signals are carried in a frequency band ranging from about 5-30 MHz.
Referring to Fig. 3, the output of the hybrid amplifier 72 feeds a 54-550 MHz broadband signal into port A of the diplexer 73. This signal exits the diplexer 73 at port B
35 thereby passing through the relay 76 and the AC bypass 78 to the radio frequency output. It will be noted that a return signal in the band of 5-30 MHz enters the node from the radio ,.~
2069~44 frequency output, passes through the AC bypass and the relay 76 and enters the diplexer 73 at its port B. This signal exits the diplexer 73 at port C and is supplied to the return laser or LED 74 where it modulates the optical signal 5 generated by this device. This modulated optical signal may be transmitted back to the headend of the system via the optical link F4 which may, for example, be an additional fiber optical strand.
It should also be noted that the relay 77 is conditioned 10 to go to its other position when a problem occurs in signal transmission along the optical path. Under these conditions the broadband 54-550 MHz signal enters the node through radio frequency input port 90 via AC bypass 83, equalizer 82, and a Pad 81. The signal is routed through relay 76 and AC bypass 15 78 to reach the radio frequency output port 91. Similarly, the 5-30 MHz return signal entering at radio frequency output port 91 is routed through AC bypass 78, the relay 76, and the AC bypass 84 so as to exit the node in the return direction at radio frequency input port 90. A relay which is suitable 20 for use as the relay 76 of Fig. 3 is the OMRON~M Model G5Y-154P
(catalog No. K37-E3-1).
Fig. 4 illustrates another embodiment of an optoelectronic bridger node which is similar in most respects to the optoelectronic bridger node of Fig. 3 except for the 25 differences which are set forth below. It will be noted that in the node of Fig. 4 the output of the combiner is fed directly to the hybrid amplifier 72 rather than feeding the hybrid amplifier through an equalizer/trim circuit 71. In Fig. 4, the hybrid amplifier has a separate trim adjustment 30 101 and equalizer 103 which are connected to each other through a selectable Pad 102. This separation of the trim and equalizer adjustments provide more flexibility for trimming the frequency response of the system to eliminate minor deviations caused by the split band technique of the present 35 invention. The Pad 102 provides means for adjusting the level of the entire 54-550 MHz band prior to its being fed to the diplexer. The equalizer 103 adjusts for tilt in the signal .~:",.
, . . :~, ~
response. Another feature of the optoelectronic bridger node of Fig. 4 is that it includes a transponder 106 which has a capability of monitoring all of the important operating modes of the node by monitoring key analog parameters, including 5 power supply voltages. Each of the test points illustrated in Fig. 4 may provide signals to the transponder 106 indicative of the status of the radio frequency signals at the corresponding test point. This transponder also allows remote control of a relay 108 which can bypass the optical path 85 10 and return the node to the original radio frequency path 80.
In this regard, the transponder may receiver messages from the headend to, for example, acti~ate the back up mode by switching to the radio frequency path 80 or to perform tests on the node and to return data back to the headend reporting 15 on these tests.
Referring to Figs. 5a through 5h there are illustrated a number of radio frequency filters which can be used as the radio frequency filters 60, 61, and 62 of Figs. 3 and 4. As noted previously, these filters may be selected by the user 20 as appropriate depending upon the manner in which the frequency band of interest is split among the various optical signal links. In preferred embodiments of these filters the components of the filter may be plugged into a standard circuit board. Thus, in certain of the filters below a 25 particular leg of the filter may either be jumpered to ground or may not be used (i.e., no components plugged in).
With reference to the figures, Figs. 5a and 5b represent 330 MHz low pass and high pass filters respectively. Figs.
5c and 5d represent 270 MHz low pass and high pass 30 respectively. Figs. 5e and 5f represent 234 MHz low pass and high pass filters respectively. Finally, Figs. 5g and 5h represent 174 MHz low pass and high pass filters respectively.
It will be noted, of course, that various combinations of these high pass and low pass filters may also be used to 35 obtain bandpass characteristics for middle h~nAC when the total frequency band of interest is split into more than two bands.
,, , Fig. 6 is a schematic diagram of a directional coupler which is the presently preferred embodiment of the test points (e.a., test points 63, 64, and 65 of Fig. 3) which are utilized in the optoelectronic receiver nodes of the present 5 invention. This directional coupler comprises a radio frequency transformer 203 which is adapted for sampling a signal 17db down from the signal which is applied to the transformer. A radio frequency input signal is applied to terminal 3 of the transformer and exits at terminal 5 where 10 this radio frequency signal is passed on to the next component in the circuitry. A secondary winding of the transformer is connected between terminals 1 and 4. Externally connected to terminal 4 is a resistor 202 which preferably is a 75 ohm 5%
1/8 watt resistor. This resistor is connected to ground.
15 Also connected to ground from terminal 4 and parallel with resistor 202 is a capacitor 201 which, preferably, has a value of 2pf and is rated at 100 volts, 5%. Terminal 1 of the transformer is the output which supplies a sample of the radio frequency input signal to a conventional Pad 204 and thence 20 to the physical test point 205.
Fig. 7 is a circuit diagram of a radio frequency combiner circuit which may be used, for example, to implement the radio frequency combiners 69 or 70 of Fig. 3. Referring to Fig. 7, the circuit includes a pair of input terminals 320 and 321 25 which are adapted to be connected to respective radio frequency input signals. A resistor R301 in series with a capacitor 302 are connected across the input terminals 321 and 322. The resistor R301 is preferably 130 ohm and the capacitor 302 is preferably 510pf. Resistor R301 is connected 30 through a three-turn radio frequency choke to a point 323.
Capacitor 302 is also connected through a three-turn radio frequency choke to a point 323. It will be noted that there is ferrite coupling between the chokes 303 and 304. Point 323 is connected through a capacitor 305 to ground. This 3S capacitor is preferably .3pf. Point 323 is connected through a radio frequency choke 307 to the output terminal 322.
Output terminal 322 is also connected through a capacitor 306 to ground. ~his capacitor 306 is pr~ferably a .lpf capaeitor.
~oint 323 is further connected through an R~ chokQ (preferably 2 turns) to ground.
~hQ above circuit opQrates in a ~anner t~ combine two 5 radio frQquQncy input signals of diffe~ent frequency bands appl~Qd at ~nput t~r~inAl~ 320 and 321 into a single broadband at output terminal 322. This bro~bA~ signal constitute~ all of the frequency channels contained in ~oth of the input signals.
0 RefQrrin~ now to Fig. 8 ther~ is shown a schematic diagram o~ a presQntly preferred but ne~ertheless illustrative embodiment o~ a circuit which i~ suitable for use as the dipl~xer 73 of Fig. 3.
Referring to Figs. 9a and 9b there are ~hown illustrat~ve 15 schematic diagrams of trim circuitQ which may ~Q used for smoothing out the o~u~s o~ the combinsr~ of Figs. 3 and 4.
Fig. 9a illustrates a trim circuit which is applicable to systsm~ in which the freguency b~nd of interest ~8 split into two sub-bands ~or tran~ ion ovQr two optical linXs. ThQ
20 circuit diagram of Fig. 9b i~ ustrativ~ of a trim circuit which may be particularly useful for systQms ~n wh~ch the band o~ intere~t has b~en split into threQ sub-bands ~or transmis~ion ovQr threQ optical links ~nd then recombined.
It will bQ apparent that thQre has b~en described above 25 sevQral preferred but ne~Qrth~less, illu~trativQ embodiments o~ apparatus for transmitting br~h~ amplitude modulated radio frequency ~gnals o~er opt~cal l~nks and recei~ing such signals. MQthod~ hav~ al80 b~en ~escr~Qd for utilizing the illustrati~e app~r~tus ~or tran~tt~ng the signals in a
One deficiency of the above method is that the processes 5 of "up-conversion" of the frequency band of interest and subsequent "down-conversion" may themselves introduce noise and/or distortion into the system;
DI8CLO8~RE OF INVENTION
In accordance with the present invention therefor, a method has been found to limit the bandwidth of radio frequency signals used to modulate light emitting sources used in a broadband CATV distribution network to less than 15 an octave. The method is implemented using a minimum number of optical links and does not require block conversion of the entire frequency band of interest to a higher band of frequencies and reconversion to the original band, although such block conversion may be utilized for some of the lower 20 frequency channels in the frequency band of interest as set forth below.
To implement the invention, a plurality of radio frequency channels covering more than one octave of bandwidth are separated into two or more contiguous groups of channels, 25 each of such qroups occupying less than one octave of frequency. For example, a first group of channels may occupy the 54 MHz to 106 MHz band (just under one octave).
Similarly, a second band of ~h~nels may occupy the 108 MHz to 214 MHz band. Optionally, additional groups of frequencies 30 may occupy the bands of 214 MHz through 426 MHz and 426 MHz through 850 MHz. Each of these sub-bands, constituting a group of radio frequency channels, is used to modulate the intensity of the output of a light emitting device such as a laser. Each resulting amplitude modulated light signal is 35 transmitted over an optical link to the receiving location.
Each of these optical links may be, for example, a single strand of optical fiber. Alternatively, each of the lasers may be selected to generate light of different wavelengths than the other lasers and all of the laser outputs may be transmitted over a single optical fiber or other physical light transmitting means. In this latter case, at the 5 receiving mode, each wavelength is separated by a wavelength division multiplier and sent to individual receivers. At the receiving location, each of the optical signals is received and demodulated by a respective individual receiver. The demodulated radio frequency signal produced by each of these 10 receivers may be passed through a frequency selective or bandpass filter which serves to alleviate harmonic and/or intermodulation distortions and/or noise produced in the optical transmission system. Each of the resulting radio frequency signals may then be suitably~attenuated so as to 15 match the amplitude of the other radio frequency signals before being applied to combiner and trim circuitry to combine the sub-bands of frequencies back into a single broadband radio frequency signal. This resulting broadband radio frequency signal may be operated on by additional circuitry 20 which serves to alleviate any discontinuities of the signal at the transitions between the sub-bands.
It will be noted that in the above system, far fewer 6 MHz channels can be accommodated by the optical links serving the lower frequency sub-bands than by the optical links 25 accommodating the higher frequency sub-bands since there is room for fewer 6 MHz channels in a lower frequency octave.
In accordance with one aspect of the invention, the number of optical links required to implement the above system, may be reduced by initially modulating all of the baseband television 30 signals onto radio frequency carrier frequencies in the highest sub-band of interest (e.q., in the range of 330-550 MHz). Because of the separation of signals provided by multiple~ optical links, the same carrier frequency may be utilized on each of the links to carry a channel of baseband 35 video information. At the receive side one of the demodulated signals must be down-converted to a lower frequency range so as not to interfere with the signals from the other optical çh~nnPlc when the two signals are mixed. However, since this solution involves only one down~..vcl~ion for only a portion or portions of the frequency band of interest. Moreover, since the method does not require up~-~ ion for any 5 of the ch~nnP-lc, it is believed to introduce far less distortion and noise to the ~ncmiccion system then the previous metho~c of block COI vclling all of the ch~nnehc, to a higher frequency and then lo~on~ling all ch~nnP-hc to their origin~l frequenriP~s.
Other aspects of this invention are as follows:
A method of Ll~ns.. ;~l;ng a plurality of radio rl~u~ ch~nmPlc covering more than one octave of bandwidth from a first lt-c~tion to a second location, said method compricing the steps of:
sPkP~ting two or more groups of contiguous radio frequency ch~nn~Plc, each of said groups co~,e~ing a bandwidth of less than one octave;
~cm.dting a pluraliq of optical signals at said first location, each said optical signal col~ ding to one of said groups;
m~ul~hng the intensity of each optical signal with the radio frequency çh~nnPlc of its ~ ive group;
tr~ncmitting each resultinp m~~ tP~ optical signal to the second 20 location via a S~)aldtC optical link; and demod~ tin~ each ~ ns~..;ll~ optical signal at the second location into its lcspec~ivc group of radio frequency ch~nnPlc.
A method of tran~ l;~ a plurality of ch~nnplc of inform~tion from a first location to a second loc~tinn said method comprising t,he st,eps of:
~efining two or more groups of çh~nmPlc wl~elcin each rh~nnel is ~ccign~d to at least one of said groups;
mo~ul~ting a radio frequency carrier signal with each ch~nnel in each group, wherein each radio frequency carrier signal uniquely i~iPntifiPs oneçh~nnPl in a group;
generating a pluraliq of opt,ical signals, each coll~,~nding to one of said groups of ~h~nnP.lc,;
7a 2069944 ~mplitude mod~ ting each of said optical signals with the mod~ ted radio frequency carriers coll.,a~nding to its respective group of ch~nn~l c;
t~ncmitting each of said optical signals to the second location;
dem~~ tinP each of said optical signals at the second location into its ~spacLve m~lll~ted radio frequency carrier signals;
cG"~Glling at least one of said mod~ ted radio rl~uency carrier signals to a different rrG~ue~ band of carrier signals which does not overlap with any other t~n.$mitted radio frequency carrier cig~lc; and combining the resulting mod~ ted radio frequency carrier signals in a single signal.
In a system for co------un;c~tin~ ;nro~ ;on signals with a plurality of optical ci~r~lc, each of which being modlll~tc^;d with a plurality of carrier signals, a method of receiving said optical signals comrcing the steps of:
ting each of said optical signals into its ~ ive carrier signals, wh~m a frequency of one of said carrier signals is s~st;~nti~lly equal to a r~uenc~ of another of said carrier signals; and con~,~ling the frequency of the one carrier signal to a frequency different than that of the anolll~r carrier signal to lLe,~b~ produce carrier signals having dirÇ~nt frequen(ies.
A system for tr~ncmittinp a plurality of cl~nnelc of info...,~l;on from a first l~tion to a second loc~tion, said system comrricing an optical ~ nsll~ includin~ means for l~iving signals col~ ~n~ling to each c~nnPl of info~ n, means for genP~ting a plurality of radio frequency carrier signals, means for geu~-.,.l;ng a plurality of optical ~ig~,~lc, means for modul~tinE each of said info.... ~I;on signals onto at least one of said radio carrier frequencies to produce ,~..~;tivc m~~ t~ radio 30 fic~lu~ ~ carrier ~ig~ls, means for ~lecti~ a first group of said mod~ t~l radio frequency carrier signals, none of which have the same radio carrier frequency, means for ~l~ting a second group of said mod~ t~ radio - 7b 2069944 frequency carrier signals, none of which have the same sdio frequency carrier, means for modnl~ting a first optical signal with said first group of mod sdio rlc~luency carrier signals, and means for m xlul~ting a second optical signal with said second group of mod~ tp~ sdio frequency carrier signals;
an optical rccci~r in~ ling means for demodl~1~ting each said optical signal into its l~ iVC modul~t~ sdio frequency carrier sign~l~, and means for combining said modlll~tpd sdio frequency carrier signals into a single signal; and a pluslity of optical links c4nnF~ g each of said optical signal modlll~tinf! means with a l~s~ive one of said optical signal demod~ ting means.
A method for lCCCiVillg a plurality of optical sign~l~, each of which is m~lll~t~ with at least one rnodlll~t~d radio frequency carrier signal, said method compri~ing the steps of:
~emod~ ting each optical signal into its Lci.~æ~ive m~lll~t~pd sdio rrc~lueacy carrier signals, wLc~ at least one of said rn~lll~ted sdio frequency carrier signals has a frequency band that u~c.laps with another of said modul~tPd sdio frequency carrier signals;
con~.ling the at least one of said modul~tP~ sdio frequency carrier signals to a dirr"~nl frequency band of carrier signals which does not overlap with any other modlll~t~ sdio frequency carrier signal mod~ ting said optical si~ ; and colllbi~ g the res lting mo~ tP~ radio frequency carrier signals in a single signal path.
A t~n~mittP~ for t~n~mittin~ a plurality of cl-~nn~l~ of inform~tion compri~ing:
means for lcceiving signals coll~onding to each c~nnel of info....~l;on;
means for gencldting a plurality of sdio frequency carrier .~ign~
means for g,~ne.,-t;ng a plurality of optical signals;
7c 2069944 means for modul~ting each said inform~tion signals onto at least one of said radio frequency carrier signals to produce a l~L,e~;live modu radio frequency carrier signal;
S means for sP1~tinp a first group of said modul~tP~ radio frequency carrier signals wherein none of the signals of the first group have the same carrier frequency as any other;
means for sel~ting a second group of mod~ tP~d radio r,~u~nc~
carrier signals whe~ein none of the signals of the second group have the same carrier frequency as any other;
means for m~dnl~ting one of said optical signals with the m~lll~ted radio frequency carrier signals of said first group; and means for modlll~ting al~lh~. of said optical signals with the m~lll~tP~ radio frequency carrier signals of said second group.
A method for ~ ;~ a plurality of radio frequency ~h~nmPlc covering more than one octave of bandwidth from a first location to a second location, said method compricinp the steps of:
sPl~p~ting two or more groups of contiguous radio frequency ch~nnels, each of said groups covering a bandwidth of less than one octave;
gen~dlii g a plurality of optical signals of dirr~,ent wavelPngths at said first location wherein each of said optical signals co~ ds to at least one group of radio frequency ch~nnPlc;
mod~ ting the intensily of each optical signal with the radio frequency ch~nnpl~ of its l~xclive group;
I.~n~.,;ll;,~g each res~ltin~ m~l~l~t~d optical signal to the second l~tion via a s.~ ~e optical link; and demodlll~ting each l.~n~."ill~ optical signal at the second location into its l~;live group of radio frequency ch~nnPls A method of l~eiving inlcnsil~-m xlul~t~P~d optical signals over optical links to produce a plurality of radior~u~cy ch~nnPlc covering more than one octave of bandwidth, compri~ing:
. ;~, 7d 2069944 receiving at least three intensity-modl-lqtPd optical sign_ls over optical links;
dçm xl-llqting each received int~,~s,l~-m~dl-lqtP,d optical signal to produce a co"~onding group of mr~lllqt~Pd radiof~u~ncy carrier signals;
filtP.ring each said group of modulqtçd radiorl_luel ey carrier signals to roll off noise and distortion products, ll,e,~)~ produc.ing l~,~CIiVegroups of modlllqtçd radiofrequency carrier sig~qls, each group of modlllqtPd radior~equellcy cqrrier signals having a bandwidth of contiguous radiofrequency10 ch~nnPl~ covering less than one octave; and combining the groups of modlllat~d radiofrequency carrier signals to produce a single signal having a plurality of radioL~u~ r cll~nnel~ covering more than one octave of bandwidth.
An optical l~C~ . for ~iving inte,~sily-modulqt~d optical signals over optical links to produce a plurality of radioL~u~l~cy cll~nn~
covering more than one octave of bandwidth, complicing:
at least three opt~~ unic recei~,s for lc~iving at least three h~ sil~-modulqt~d optical signals over optical links and for dçmod~ q-ting each lcceived hllensil~-m~l-lqt~d optical signal to produce a coll~ ng group of0 mo~ul~qted radior~uenc~ carrier signals;
at least three filters, each said filter c~ ,~n~l;ng to one of said optoelectronic lCC~iV~ i, for filtP.ring the group of modulqtP.d radiofrequency carrier signals produced by its coll~onding optoPl~onic recei~ to roll off noise and distortion products lh~ producing ~ eclive groups of mod 25 radiofrequency carrier signals, each group of modlllqtP~l radiofrequency carrier signals having a bandwidth of contiguous radiofrequency c~l~nn~l~ covering less than one octave; and a signal cu,l,biner, couFlcd to said filters, for co",bh~ing the groups of modulqt*~ radiorl~uency carrier signals to produce a single signal 30 having a plurality of radiofrequency c~qnnpl~ covering more than one octave of bandwidth.
7e 2069944 It is an object of an aspect of the present invention, th~ forc, to provide a method for gen~r~ting and receiving light signals mo~ul~tP~ with bro~Ab~nA
radio frequency signals in a manner which m~simi7P the signal to noise ratio of 5 the rcceived radio frequency signals.
It is an object of an aspect of the invention to minimi7~ h~rmnnic and il-lf~ ion distortion products introduced by the optical t~ncmiccion system into the radio frequency signals.
It is an object of an aspect of the invention to accomplish reducti-~n of 10 the in~ tinn and h~..-onic distortion products in a --am ~r which minimi7~S the nu~l,bel of optical signal paths n~fc*s~ for t~nQmiQQion of a given number of radio frequency signals.
It is an object of an aspect of the invention to achieve a re~uctin~ in the number of optical signal paths or links nPc~s~aly to ~n~mit a given nl~.llbf r of 15 b~QPb~nd television ch~nnel~ without collvclling the carrier frequencies of the television ch~nn~Plc to ~_s~ætivc higher ~u'nc;~Ps~
BRIEF DESCRIPIION OF THE DRAVVINGS
The above and further objets and advantages of the invention will baco,,,c a~ t with reference to the det~il~ d;Q~10S-~ of the invention set 20 forth below and the accG",panying illllstrative figures in which:
Fig. 1 is a block tlia~m of a h~dend optical tr~nsmit-tpr for a system o~ldting in accor~ce with the 8 206~944 present invention;
Fig. 2 is a block diagram illustrating how optoelectronic nodes may be connected in an existing CATV system to provide an interface between optical signal paths and a previously 5 existing coaxial cable network;
Fig 3 is a block diagram of an optoelectronic receiver node for a system operating in accor~A~ with the present nventlon;
Fig. 4 is a block diagram of an alternative embodiment 10 of an optoelectronic receiver node operating in accordance with the present invention;
Figs. 5a through 5h are schematic diagrams of presently preferred filters which are incorporated in the optoelectronic receiver nodes;
Fig. 6 is a schematic diagram of an illustrative directi,onal coupler which may be incorporated in the receiver nodes of the invention;
Fig. 7 is a schematic diagram of a combiner which may be incorporated in the receiver node of the invention;
Fig. 8 is a schematic diagram of a diplex filter which may be incorporated in the receiver node of the present invention; and Figs. 9a and 9b are schematic diagrams of presently preferred receiver trimmers (frequency signature correction 25 networks) for use in the present invention for equalizing radio frequency carrier amplitudes at the outputs of the receiver ro~C.
MODE(S) FOR CA~K~N~ OUT T~E I~v NTION
Referring now to Fig. 1, there is shown an illustrative embodiment of a CATV headend configured in accordance with the invention. At this headend there exists a number of channels of information (channel 2 through ch~nnel n) of information 35 to transmit through the CATV network fed by the headend. In this illustrative example the channels are television channels which occupy 6 MHz of bandwidth at h~5~hA~ (4.5 MHz of video, ~ 9 206~944 audio and other information and a 1.5 MHz guard band). It will be appreciated, however, that the present invention would be equally applicable for the transmission of any type of information through a network. Each of the baseband channels 5 of information is supplied to a respective modulator of the type normally found at CATV headend installations (e.a., a Scientific Atlanta Model 6350). Each modulator serves to impose the baseband channel on a radio freguency carrier supplied by the modulator. Standard plug-in modulators are 10 in common use at CATV headends which can produce modulated radio frequency signals in the range of, for example, 54-550 MHz and higher in 6 MHz increments. In the conventional CATV
systems in use today, each baseband signal must be modulated onto a carrier frequency which is distinct from carrier 15 frequencies associated with any other baseband signal since all of the carrier frequencies must be mixed together to form a single broadband signal for transmission over a coaxial cable network wherein the trunk and distribution links of the network comprise a single 75 ohm coaxial cable path. In 20 accordance with present invention, however, several optical links may be used to replace or supplement portions of a trunk of a CATV system. In view of the plurality of optical links employed, a hA~^hAn~ signal transmitted over one link may, if desired, be modulated onto a radio 25 frequency carrier having the same frequency as the carrier of a baseband signal transmitted over a different link.
Thus, as shown in Fig. 1, a first group of basehAn~
signals designated as chAnnels 2 through k may be fed to 30 a first group of modulators having carrier frequencies in, for example, the 300-550 MHz range. Similarly, a second group of baseband signals designated as channels k+1 through m may be fed respectively to a second group of modulators which may, optionally, have the same or 35 some of the same carrier frequencies as the first group of modulators. Finally a third group of baseband signals designated as channels m+l through n may be fed to their respective group of modulators which may also have carrier frequencies in the same range as the 5 modulators of the first two groups (e.a., 300-500 MHz). It will be noted that if each of the group of bas~hAnA signals is modulated with the highest possible set of carrier frequencies within the radio frequency band of interest a ~ um number of channels may be accommodated in a single 10 signal path while maintaining the total bandwidth of the group of channels within less than an octave range. As discussed above, this objective of keeping the bandwidth below an octave is important in order to minimize problems associated with intermodulation distortion-and harmonic~distortion produced 1~ when the radio frequency signal is used to amplitude modulate the light output of a light emitting device such as a laser.
It is particularly noteworthy, that the present invention achieves this objective without resorting to up-conversion of one or more groups of modulated radio frequency signals since 20 such up-conversion can cause degradation and distortion of the signals upon which it operates.
In the illustrative example set forth herein three groups of baseband signals are designated. It will be appreciated that the invention is equally applicable to systems in which 25 two groups of baseband signals or greater than three groups are defined. In the illustrative embodiment of Fig. 1, each of the three groups of baseband signals are combined into respective broa~h~n~ signals in the range of 300-500 MHz by respective signal combiners 1, 2, and 3. Each of these 30 broadband radio frequency signals may be passed through respective buffer amplifiers and equalizers in order to provide equivalent signals to amplitude the light output of respective lasers 1, 2, and 3. In the illustrative embodiment of Fig. 1, each of the lasers 1, 2, and 3 may emit light of 35 the same wavelength since their respective optical outputs are transmitted through the network over individual fiber optic links Fl, F2, and F3. Separate physical links for 20699~4 transmitting the respective optical signals are not nec~cs~ry to practice the present invention however. Referring to Fig.
la the lasers 1, 2, and 3 may each be selected to emit light of a unique wavelength. The three amplitude modulated optical S signals may then be combined in an optical combiner device and transmitted over a single fiber strand or other type of optical link.
Fig. 2 shows the manner in which an optical signal transmission network may be interfaced with an existing 10 coaxial CATV network so as to complement the existing network.
Fig. 2a shows a portion of an existing network consisting of a cascade of trunk amplifiers. Fig. 2b shows an interfacing scheme called a reverse cascade in which two nodes or interfaces between the optical electronic systems are utilized 15 to feed signals carried by the trunk to forward and reverse cascades of amplifiers. It will be noted in this configuration that it is required to reverse the direction of some of the existing amplifiers in the CATV system. Fig 2c shows a system in which the optical signal transmission system 20 bypasses portions of the existing CATV trunk. In this system each node receives both an optical signal and a radio frequency signal from the cascade of trunk amplifiers between the node and the headend of the system.
Fig. 2d shows a system similar to the system set forth 25 in Fig. 2c which requires one-half the number of nodes. The question of whether the system of Fig. 2c is more or less reliable than the system of Fig. 2d depends in great part on the reliability of the node circuitry. Thus, if the node circuitry is relatively reliable compared to trunk amplifier 30 circuitry, the circuitry of Fig. 2c may be more reliable on a system from a system standpoint. On the other hand, if the node circuitry proves to be relatively unreliable when compared with the trunk amplifier circuitry or the likelihood that the existing coaxial trunk system will suffer outages for 35 other reasons, the network illustrated by Fig. 2d will tend to have a better system reliability. Fig. 3 illustrates an embodiment of a node which is adapted to serve as an interface between an existing coaxial signal distribution system and an optical signal distribution system such as is set forth in Fig. 1. The node circuitry set forth in Fig. 3 is particularly adapted for nodes operating in accordance with 5 the requirements of the systems illustrated in Figs. 2c and 2d.
Referring to Fig. 1, it will ,be noted that the light signals amplitude are modulated with radio frequency carriers at the radio frequency carriers are themselves be modulated 10 with video or other types of information. The modulated light signals are delivered to the node via optical fiber strands F1, F2, and F3. In this connection it should be noted that the node of Fig. 3 is equally applicable to a system in which only one physical fiber strand delivers multiple optical 15 signals having different wave lengths to the node. In this case, the optical signals delivered by the single fiber may be physically separated by optical means to enable each optical signal to be conveyed to a corresponding optical signal receiver/demodulator.
In either of the above cases, each amplitude modulated optical signal is provided as an input to a corresponding optical receiver/demodulator. In the illustrative embodiment of Fig. 3, optical receivers 51, 52, and 53 serve this purpose. These receivers may be any type of receiver which 25 has characteristics which are suitably linear to adapt it to serve as a device for demodulating an analog radio frequency signal from an amplitude modulated light beam. One such type of receiver is called a high imrD~nCD receiver and another type of receiver suitable for this purpose is a transimpedance 30 receiver. A type of receiver which is particularly adapted to implement receivers 51, 52, and 53 is the receiver described in U.S. Patent Application Serial No. 481,436 entitled "Push-Pull Optical Receiver" which is also assigned to the assignee of the present application. In addition to 35 the above receivers a number of other types of optoelectronic receivers are currently available.
One or more of the receivers 51, 52, or 53 may feed down-converters such as down-converters S4 and 55 if the band of radio frequency carriers assigned to these receivers overlaps with the bands of radio frequency carriers assigned to another receiver such as receiver 53. It will be recalled that in the 5 illustrative signal transmitter illustrated by Fig. 1, the radio frequency carriers carried by optical links Fl, F2, and F3 were all in the range of 300-550 MHz so as to ~x;~ize the number of channels which could be carried by the link and still contain the channels of each link within one octave.
10 Assuming that the transmitter of Fig. 1 feeds the receivers of Fig. 3, it will be necessary for suitable oscillators 56 and 57 to feed converters 54 and 55 so as to down-convert or up-convert the radio frequency signals at the outputs of receivers 51 and 52 so that they do not overlap the frequency 15 band at the output of receiver 53 or with each other. The outputs of receivers 51 and 52 and their respective optional down-converters 54 and 55 are connected respectively to filter circuits 60 and 61. The ouL~uL of receiver 53 is connected to filter 62. Each of these filters is designed to attenuate 20 frequencies outside of the frequency band of interest on a particular signal path. Circuits for presently preferred embodiments of these filters are discussed in more detail on connection with the discussion of 5a through 5h below. Each of the filters 60, 61, and 62 is connected to a respective 25 radio frequency test point 63, 64 or 65. These radio frequency test points are directional couplers which are described in greater detail in connection with the discussion of Fig. 5 below. The radio frequency test points 63, 64, and 65 are in turn connected to Pads 66, 67, and 68 which are of 30 conventional design. Pads 66 and 67 are connected to a combiner 69 and the output of the combiner 69 and of the Pad 68 are connected to a combiner 70. The output of this combiner 70 is connected to an equalizer trim circuit 71 which in turn feeds a hybrid amplifier 72. The u~u~ of the hybrid 35 amplifier feeds a diplexer or diplex filter 73, illustrative circuitry for which is illustrated in Fig. 8. The diplex filter in turn feeds a return laser or LED 74 via a test point 75. The common output of the diplex filter 73 goes to a relay 76. The common point 77 of the relay is connected to a 75 ohm radio frequency output port of the node via an AC bypass circuit 78 and to another port of the node via another AC
bypass circuit 79. When the common arm 77 of the relay is in its other position 80 the optical signal path is bypassed and the radio frequency output port is connected to a radio frequency input port via a Pad 81, an equalizer 82, and an AC
bypass circuit 83 and 84.
The operation of the circuitry described above is as follows. Filters 60, 61, and 62 are set up to roll off noise and distortion products outside the band of interest of their respective receivers 51, 52, and 53. In a typical arrangement receiver 51 and down converter 54 will produce about 6 MHz radio frequency video carrier channels beginning at channel 2 beginning at 54 MHz. The receiver 52 and down-converter 55 may accommodate the next 30 radio frequency channels in the frequency band and the remainder of the channels would be handled by receiver 53. In such a configuration, filter 60 would be a low pass filter allowing only the frequencies up to and including the highest frequency channel in the frequency band of interest to pass. Filter 61 would be of the band-pass variety, attenuating noise and distortion at frequencies above and below the band of interest. Filter 62 would be a high pass filter allowing all frequencies above the lowest frequency channel of the band of interest through.
Alternatively a two-path system is possible utilizing only optical links F1 and F2. In such a system, approximately one-half of the channels are accommodated by each of the receivers 51 and 52. Here filter 60 would be a low pass filter with a break point at about 330 MHz and filter 61 would be a high pass filter with a similar break point.
The respective filters 60, 61, and 62 feed combiners 69 and 70 through test points 63, 64, 65 and Pads 66, 67, and 68. The pads are pluggable units which are selected to equalize the amplitude of the signals fed to the inputs of combiners 69 and 70. The attenuation required of each pads may be determined by monitoring the outputs of filters 60, 61, and 62 via radio frequency test points 63, 64, and 65.
The purpose of the equalizer trim circuit 74 is to flatten out the output signal from combiner 70. In this 5 regard two types of adjustments may be required to the signal.
Firstly, the signal from the combiner may be "tilted." That is, there may be differences between the amplitudes of the higher frequency signals and lower frequency signals which vary across the frequency band in a linear manner. Secondly, 10 at certain points in the frequency and, especially at frequencies at the interfaces between the sub-bands carried by the various optical links, there may be notches in the response wherein some channels or group of channels have a higher or lower amplitude than the channels surrounding them.
15 It is desireable to smooth out these notches or peaks in the frequency response. Fig. 9a illustrates a presently preferred embodiment of a equalizer trim circuit which is particularly adapted for systems utilizing two optical links. Fig. 9b illustrates a presently preferred embodiment of a circuit 20 particularly adapted to provide equalizer trim in a system with three optical links.
In the presently preferred emho~ nt of the invention, the equalizer trim circuit 71 feeds a hybrid amplifier 72 such as a Model SHW 571 manufa~L~ed by Motorola.
At this point it should be noted that in many of today's coaxial CATV systems television signals are transmitted from the headend to subscriber locations in a frequency band of 54-550 MHz. Some systems also have the capability of transmitting signals from subscriber locations back to the 30 headend. In most such systems, these return signals are carried in a frequency band ranging from about 5-30 MHz.
Referring to Fig. 3, the output of the hybrid amplifier 72 feeds a 54-550 MHz broadband signal into port A of the diplexer 73. This signal exits the diplexer 73 at port B
35 thereby passing through the relay 76 and the AC bypass 78 to the radio frequency output. It will be noted that a return signal in the band of 5-30 MHz enters the node from the radio ,.~
2069~44 frequency output, passes through the AC bypass and the relay 76 and enters the diplexer 73 at its port B. This signal exits the diplexer 73 at port C and is supplied to the return laser or LED 74 where it modulates the optical signal 5 generated by this device. This modulated optical signal may be transmitted back to the headend of the system via the optical link F4 which may, for example, be an additional fiber optical strand.
It should also be noted that the relay 77 is conditioned 10 to go to its other position when a problem occurs in signal transmission along the optical path. Under these conditions the broadband 54-550 MHz signal enters the node through radio frequency input port 90 via AC bypass 83, equalizer 82, and a Pad 81. The signal is routed through relay 76 and AC bypass 15 78 to reach the radio frequency output port 91. Similarly, the 5-30 MHz return signal entering at radio frequency output port 91 is routed through AC bypass 78, the relay 76, and the AC bypass 84 so as to exit the node in the return direction at radio frequency input port 90. A relay which is suitable 20 for use as the relay 76 of Fig. 3 is the OMRON~M Model G5Y-154P
(catalog No. K37-E3-1).
Fig. 4 illustrates another embodiment of an optoelectronic bridger node which is similar in most respects to the optoelectronic bridger node of Fig. 3 except for the 25 differences which are set forth below. It will be noted that in the node of Fig. 4 the output of the combiner is fed directly to the hybrid amplifier 72 rather than feeding the hybrid amplifier through an equalizer/trim circuit 71. In Fig. 4, the hybrid amplifier has a separate trim adjustment 30 101 and equalizer 103 which are connected to each other through a selectable Pad 102. This separation of the trim and equalizer adjustments provide more flexibility for trimming the frequency response of the system to eliminate minor deviations caused by the split band technique of the present 35 invention. The Pad 102 provides means for adjusting the level of the entire 54-550 MHz band prior to its being fed to the diplexer. The equalizer 103 adjusts for tilt in the signal .~:",.
, . . :~, ~
response. Another feature of the optoelectronic bridger node of Fig. 4 is that it includes a transponder 106 which has a capability of monitoring all of the important operating modes of the node by monitoring key analog parameters, including 5 power supply voltages. Each of the test points illustrated in Fig. 4 may provide signals to the transponder 106 indicative of the status of the radio frequency signals at the corresponding test point. This transponder also allows remote control of a relay 108 which can bypass the optical path 85 10 and return the node to the original radio frequency path 80.
In this regard, the transponder may receiver messages from the headend to, for example, acti~ate the back up mode by switching to the radio frequency path 80 or to perform tests on the node and to return data back to the headend reporting 15 on these tests.
Referring to Figs. 5a through 5h there are illustrated a number of radio frequency filters which can be used as the radio frequency filters 60, 61, and 62 of Figs. 3 and 4. As noted previously, these filters may be selected by the user 20 as appropriate depending upon the manner in which the frequency band of interest is split among the various optical signal links. In preferred embodiments of these filters the components of the filter may be plugged into a standard circuit board. Thus, in certain of the filters below a 25 particular leg of the filter may either be jumpered to ground or may not be used (i.e., no components plugged in).
With reference to the figures, Figs. 5a and 5b represent 330 MHz low pass and high pass filters respectively. Figs.
5c and 5d represent 270 MHz low pass and high pass 30 respectively. Figs. 5e and 5f represent 234 MHz low pass and high pass filters respectively. Finally, Figs. 5g and 5h represent 174 MHz low pass and high pass filters respectively.
It will be noted, of course, that various combinations of these high pass and low pass filters may also be used to 35 obtain bandpass characteristics for middle h~nAC when the total frequency band of interest is split into more than two bands.
,, , Fig. 6 is a schematic diagram of a directional coupler which is the presently preferred embodiment of the test points (e.a., test points 63, 64, and 65 of Fig. 3) which are utilized in the optoelectronic receiver nodes of the present 5 invention. This directional coupler comprises a radio frequency transformer 203 which is adapted for sampling a signal 17db down from the signal which is applied to the transformer. A radio frequency input signal is applied to terminal 3 of the transformer and exits at terminal 5 where 10 this radio frequency signal is passed on to the next component in the circuitry. A secondary winding of the transformer is connected between terminals 1 and 4. Externally connected to terminal 4 is a resistor 202 which preferably is a 75 ohm 5%
1/8 watt resistor. This resistor is connected to ground.
15 Also connected to ground from terminal 4 and parallel with resistor 202 is a capacitor 201 which, preferably, has a value of 2pf and is rated at 100 volts, 5%. Terminal 1 of the transformer is the output which supplies a sample of the radio frequency input signal to a conventional Pad 204 and thence 20 to the physical test point 205.
Fig. 7 is a circuit diagram of a radio frequency combiner circuit which may be used, for example, to implement the radio frequency combiners 69 or 70 of Fig. 3. Referring to Fig. 7, the circuit includes a pair of input terminals 320 and 321 25 which are adapted to be connected to respective radio frequency input signals. A resistor R301 in series with a capacitor 302 are connected across the input terminals 321 and 322. The resistor R301 is preferably 130 ohm and the capacitor 302 is preferably 510pf. Resistor R301 is connected 30 through a three-turn radio frequency choke to a point 323.
Capacitor 302 is also connected through a three-turn radio frequency choke to a point 323. It will be noted that there is ferrite coupling between the chokes 303 and 304. Point 323 is connected through a capacitor 305 to ground. This 3S capacitor is preferably .3pf. Point 323 is connected through a radio frequency choke 307 to the output terminal 322.
Output terminal 322 is also connected through a capacitor 306 to ground. ~his capacitor 306 is pr~ferably a .lpf capaeitor.
~oint 323 is further connected through an R~ chokQ (preferably 2 turns) to ground.
~hQ above circuit opQrates in a ~anner t~ combine two 5 radio frQquQncy input signals of diffe~ent frequency bands appl~Qd at ~nput t~r~inAl~ 320 and 321 into a single broadband at output terminal 322. This bro~bA~ signal constitute~ all of the frequency channels contained in ~oth of the input signals.
0 RefQrrin~ now to Fig. 8 ther~ is shown a schematic diagram o~ a presQntly preferred but ne~ertheless illustrative embodiment o~ a circuit which i~ suitable for use as the dipl~xer 73 of Fig. 3.
Referring to Figs. 9a and 9b there are ~hown illustrat~ve 15 schematic diagrams of trim circuitQ which may ~Q used for smoothing out the o~u~s o~ the combinsr~ of Figs. 3 and 4.
Fig. 9a illustrates a trim circuit which is applicable to systsm~ in which the freguency b~nd of interest ~8 split into two sub-bands ~or tran~ ion ovQr two optical linXs. ThQ
20 circuit diagram of Fig. 9b i~ ustrativ~ of a trim circuit which may be particularly useful for systQms ~n wh~ch the band o~ intere~t has b~en split into threQ sub-bands ~or transmis~ion ovQr threQ optical links ~nd then recombined.
It will bQ apparent that thQre has b~en described above 25 sevQral preferred but ne~Qrth~less, illu~trativQ embodiments o~ apparatus for transmitting br~h~ amplitude modulated radio frequency ~gnals o~er opt~cal l~nks and recei~ing such signals. MQthod~ hav~ al80 b~en ~escr~Qd for utilizing the illustrati~e app~r~tus ~or tran~tt~ng the signals in a
3 o m~nner which maximlzes thQ slgnal to noise ratio of ~he signals while ~ini~izing distortion introduce~ by the apparatus. Variations and mod~cations o~ the apparatus describQd above wlt~ln th~ scope o~ t~e in~ention w~ll undoubtedly suggQs~ themsel~e~ to those skillQd in the art.
35 Accor~ngly, the foregoing desctipt~on should be ~aken a6 m~rely illustrati~e.
35 Accor~ngly, the foregoing desctipt~on should be ~aken a6 m~rely illustrati~e.
Claims (34)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of transmitting a plurality of radio frequency channels covering more than one octave of bandwidth from a first location to a second location, said method comprising the steps of:
selecting two or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave;
generating a plurality of optical signals at said first location, each said optical signal corresponding to one of said groups;
modulating the intensity of each optical signal with the radio frequency channels of its respective group;
transmitting each resulting modulated optical signal to the second location via a separate optical link; and demodulating each transmitted optical signal at the second location into its respective group of radio frequency channels.
selecting two or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave;
generating a plurality of optical signals at said first location, each said optical signal corresponding to one of said groups;
modulating the intensity of each optical signal with the radio frequency channels of its respective group;
transmitting each resulting modulated optical signal to the second location via a separate optical link; and demodulating each transmitted optical signal at the second location into its respective group of radio frequency channels.
2. The method of claim 1 wherein said optical signals generating step comprises:
generating a plurality of optical signals of different wavelengths at said first location wherein each of said optical signals corresponds to one group of radio frequency channels.
generating a plurality of optical signals of different wavelengths at said first location wherein each of said optical signals corresponds to one group of radio frequency channels.
3. A method of transmitting a plurality of channels of information from a first location to a second location said method comprising the steps of:
defining two or more groups of channels wherein each channel is assigned to at least one of said groups;
modulating a radio frequency carrier signal with each channel in each group, wherein each radio frequency carrier signal uniquely identifies one channel in a group;
generating a plurality of optical signals, each corresponding to one of said groups of channels;
amplitude modulating each of said optical signals with the modulated radio frequency carriers corresponding to its respective group of channels;
transmitting each of said optical signals to the second location;
demodulating each of said optical signals at the second location into its respective modulated radio frequency carrier signals;
converting at least one of said modulated radio frequency carrier signals to a different frequency band of carrier signals which does not overlap with any other transmitted radio frequency carrier signals; and combining the resulting modulated radio frequency carrier signals in a single signal.
defining two or more groups of channels wherein each channel is assigned to at least one of said groups;
modulating a radio frequency carrier signal with each channel in each group, wherein each radio frequency carrier signal uniquely identifies one channel in a group;
generating a plurality of optical signals, each corresponding to one of said groups of channels;
amplitude modulating each of said optical signals with the modulated radio frequency carriers corresponding to its respective group of channels;
transmitting each of said optical signals to the second location;
demodulating each of said optical signals at the second location into its respective modulated radio frequency carrier signals;
converting at least one of said modulated radio frequency carrier signals to a different frequency band of carrier signals which does not overlap with any other transmitted radio frequency carrier signals; and combining the resulting modulated radio frequency carrier signals in a single signal.
4. The method of claim 3 wherein said optical signals generating step comprises generating a plurality of optical signals of different wavelengths.
5. In a system for communicating information signals with a plurality of optical signals, each of which being modulated with a plurality of carrier signals, a method of receiving said optical signals comprising the steps of:
demodulating each of said optical signals into its respective carrier signals, wherein a frequency of one of said carrier signals is substantially equal to a frequency of another of said carrier signals; and converting the frequency of the one carrier signal to a frequency different than that of the another carrier signal to thereby produce carrier signals having different frequencies.
demodulating each of said optical signals into its respective carrier signals, wherein a frequency of one of said carrier signals is substantially equal to a frequency of another of said carrier signals; and converting the frequency of the one carrier signal to a frequency different than that of the another carrier signal to thereby produce carrier signals having different frequencies.
6. A system for transmitting a plurality of channels of information from a first location to a second location, said system comprising:
an optical transmitter including means for receiving signals corresponding to each channel of information, means for generating a plurality of radio frequency carrier signals, means for generating a plurality of optical signals, means for modulating each of said information signals onto at least one of said radio carrier frequencies to produce respective modulated radio frequency carrier signals, means for selecting a first group of said modulated radio frequency carrier signals, none of which have the same radio carrier frequency, means for selecting a second group of said modulated radio frequency carrier signals, none of which have the same radio frequency carrier, means for modulating a first optical signal with said first group of modulated radio frequency carrier signals, and means for modulating a second optical signal with said second group of modulated radio frequency carrier signals;
an optical receiver including means for demodulating each said optical signal into its respective modulated radio frequency carrier signals, and means for combining said modulated radio frequency carrier signals into a single signal; and a plurality of optical links connecting each of said optical signal modulating means with a respective one of said optical signal demodulating means.
an optical transmitter including means for receiving signals corresponding to each channel of information, means for generating a plurality of radio frequency carrier signals, means for generating a plurality of optical signals, means for modulating each of said information signals onto at least one of said radio carrier frequencies to produce respective modulated radio frequency carrier signals, means for selecting a first group of said modulated radio frequency carrier signals, none of which have the same radio carrier frequency, means for selecting a second group of said modulated radio frequency carrier signals, none of which have the same radio frequency carrier, means for modulating a first optical signal with said first group of modulated radio frequency carrier signals, and means for modulating a second optical signal with said second group of modulated radio frequency carrier signals;
an optical receiver including means for demodulating each said optical signal into its respective modulated radio frequency carrier signals, and means for combining said modulated radio frequency carrier signals into a single signal; and a plurality of optical links connecting each of said optical signal modulating means with a respective one of said optical signal demodulating means.
7. The system of claim 6 wherein said optical signal modulating means comprises:
means for generating a plurality of optical signals of different wavelengths.
means for generating a plurality of optical signals of different wavelengths.
8. The system of claim 6 wherein said optical link comprises:
a unique strand of optical fibers connecting each optical signal modulating means to its corresponding optical signal demodulating means.
a unique strand of optical fibers connecting each optical signal modulating means to its corresponding optical signal demodulating means.
9. The system of Claim 6 wherein said receiver further comprises means for converting at least one of said modulated radio frequency carrier signals to a frequency which is different from the frequencies of any other of said radio frequency carrier signals.
10. A method for receiving a plurality of optical signals, each of which is modulated with at least one modulated radio frequency carrier signal, said method comprising the steps of:
demodulating each optical signal into its respective modulated radio frequency carrier signals, wherein at least one of said modulated radio frequency carrier signals has a frequency band that overlaps with another of said modulated radio frequency carrier signals;
converting the at least one of said modulated radio frequency carrier signals to a different frequency band of carrier signals which does not overlap with any other modulated radio frequency carrier signal modulating said optical signals; and combining the resulting modulated radio frequency carrier signals in a single signal path.
demodulating each optical signal into its respective modulated radio frequency carrier signals, wherein at least one of said modulated radio frequency carrier signals has a frequency band that overlaps with another of said modulated radio frequency carrier signals;
converting the at least one of said modulated radio frequency carrier signals to a different frequency band of carrier signals which does not overlap with any other modulated radio frequency carrier signal modulating said optical signals; and combining the resulting modulated radio frequency carrier signals in a single signal path.
11. The method of Claim 10 wherein said different frequency band is a lower frequency band.
12. A transmitter for transmitting a plurality of channels of information comprising:
means for receiving signals corresponding to each channel of information;
means for generating a plurality of radio frequency carrier signals;
means for generating a plurality of optical signals;
means for modulating each said information signals onto at least one of said radio frequency carrier signals to produce a respective modulated radio frequency carrier signal;
means for selecting a first group of said modulated radio frequency carrier signals wherein none of the signals of the first group have the same carrier frequency as any other;
means for selecting a second group of modulated radio frequency carrier signals wherein none of the signals of the second group have the same carrier frequency as any other;
means for modulating one of said optical signals with the modulated radio frequency carrier signals of said first group; and means for modulating another of said optical signals with the modulated radio frequency carrier signals of said second group.
means for receiving signals corresponding to each channel of information;
means for generating a plurality of radio frequency carrier signals;
means for generating a plurality of optical signals;
means for modulating each said information signals onto at least one of said radio frequency carrier signals to produce a respective modulated radio frequency carrier signal;
means for selecting a first group of said modulated radio frequency carrier signals wherein none of the signals of the first group have the same carrier frequency as any other;
means for selecting a second group of modulated radio frequency carrier signals wherein none of the signals of the second group have the same carrier frequency as any other;
means for modulating one of said optical signals with the modulated radio frequency carrier signals of said first group; and means for modulating another of said optical signals with the modulated radio frequency carrier signals of said second group.
13. The transmitter of claim 12 further comprising:
means for selecting a third group of said modulated radio frequency carrier signals wherein none of the signals of the first group have the same carrier frequency as any other; and means for modulating another of said optical signals with the modulated radio frequency carrier signals of said third group.
means for selecting a third group of said modulated radio frequency carrier signals wherein none of the signals of the first group have the same carrier frequency as any other; and means for modulating another of said optical signals with the modulated radio frequency carrier signals of said third group.
14. A method for transmitting a plurality of radio frequency channels covering more than one octave of bandwidth from a first location to a second location, said method comprising the steps of:
selecting two or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave;
generating a plurality of optical signals of different wavelengths at said first location wherein each of said optical signals corresponds to at least one group of radio frequency channels;
modulating the intensity of each optical signal with the radio frequency channels of its respective group;
transmitting each resulting modulated optical signal to the second location via a separate optical link; and demodulating each transmitted optical signal at the second location into its respective group of radio frequency channels.
selecting two or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave;
generating a plurality of optical signals of different wavelengths at said first location wherein each of said optical signals corresponds to at least one group of radio frequency channels;
modulating the intensity of each optical signal with the radio frequency channels of its respective group;
transmitting each resulting modulated optical signal to the second location via a separate optical link; and demodulating each transmitted optical signal at the second location into its respective group of radio frequency channels.
15. The method of claim 14, wherein said step of selecting comprises:
selecting three or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave.
selecting three or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave.
16. The method of claim 1 wherein said selecting step comprises:
selecting three or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave.
selecting three or more groups of contiguous radio frequency channels, each of said groups covering a bandwidth of less than one octave.
17. The system of claim 6 wherein said optical transmitter further comprising:
means for selecting a third group of said modulated radio frequency carrier signals, none of which have the same radio carrier frequency; and means for modulating a third optical signal with said third group of modulated radio frequency carrier signals.
means for selecting a third group of said modulated radio frequency carrier signals, none of which have the same radio carrier frequency; and means for modulating a third optical signal with said third group of modulated radio frequency carrier signals.
18. The method of claim 3 wherein said step of transmitting comprises:
transmitting each of said optical signals to the second location via an optical link unique to that optical signal.
transmitting each of said optical signals to the second location via an optical link unique to that optical signal.
19. The method of claim 18 wherein said optical signals generating step comprises generating a plurality of optical signals of different wavelengths.
20. The method of claim 11 further comprising the step of receiving the plurality of optical signals on a plurality of optical fibers.
21. The method of claim 20 wherein each optical signal is modulated with a plurality of modulated radio frequency carrier signals.
22. The method of claim 21, wherein:
a first group of said plurality of modulated radio frequency carrier signals modulates the optical signal received on a first optical fiber of said plurality of optical fibers, and a second group of said plurality of modulated radio frequency carrier signals modulates the optical signal received on a second optical fiber, said first group of modulated radio frequency carrier signals having the same frequency band as said second group of modulated radio frequency carrier signals; and said step of converting comprises converting each of the first group of modulated radio frequency carrier signals to a different frequency band as the second group of modulated radio frequency carrier signals.
a first group of said plurality of modulated radio frequency carrier signals modulates the optical signal received on a first optical fiber of said plurality of optical fibers, and a second group of said plurality of modulated radio frequency carrier signals modulates the optical signal received on a second optical fiber, said first group of modulated radio frequency carrier signals having the same frequency band as said second group of modulated radio frequency carrier signals; and said step of converting comprises converting each of the first group of modulated radio frequency carrier signals to a different frequency band as the second group of modulated radio frequency carrier signals.
23. The method of claim 21 wherein the plurality of modulated radio frequency carrier signals modulating the optical signal received on each optical fiber have a bandwidth of less than an octave.
24. The method of claim 23 wherein the plurality of modulated radio frequency carrier signals modulating the optical signal received on each optical fiber have a bandwidth of about 300-550 MHz.
25. The method of claim 23, wherein said combined modulated radio frequency carrier signals have a bandwidth of greater than one octave.
26. The method of claim 21 wherein n optical signals are received on n optical fibers and said step of converting comprises converting the modulated radio frequency carrier signals received from n-1 of said optical fibers to a different frequency band of carrier signals which does not overlap with any other radio frequency carrier signal modulating said optical signals or any other converted modulated radio frequency carrier signal.
27. The method of claim 5, wherein said carrier signals having different frequencies have a bandwidth greater than one octave.
28. The method of claim 27, wherein the carrier signals demodulated from each optical signal have a bandwidth of less than one octave.
29. The system of claim 1 wherein said means for demodulating comprises filtering means for filtering each said demodulated optical signal to roll off noise and distortion products and thereby produce the respective modulated radiofrequency carrier signals.
30. The system of claim 4 wherein said means for demodulating comprises filtering means for filtering each said demodulated optical signal to roll off noise and distortion products and thereby produce the respective modulated radiofrequency carrier signals.
31. A method of receiving intensity-modulated optical signals over optical links to produce a plurality of radiofrequency channels covering more than one octave of bandwidth, comprising:
receiving at least three intensity-modulated optical signals over optical links;
demodulating each received intensity-modulated optical signal to produce a corresponding group of modulated radiofrequency carrier signals;
filtering each said group of modulated radiofrequency carrier signals to roll off noise and distortion products, thereby producing respective groups of modulated radiofrequency carrier signals, each group of modulated radiofrequency carrier signals having a bandwidth of contiguous radiofrequency channels covering less than one octave; and combining the groups of modulated radiofrequency carrier signals to produce a single signal having a plurality of radiofrequency channels covering more than one octave of bandwidth.
receiving at least three intensity-modulated optical signals over optical links;
demodulating each received intensity-modulated optical signal to produce a corresponding group of modulated radiofrequency carrier signals;
filtering each said group of modulated radiofrequency carrier signals to roll off noise and distortion products, thereby producing respective groups of modulated radiofrequency carrier signals, each group of modulated radiofrequency carrier signals having a bandwidth of contiguous radiofrequency channels covering less than one octave; and combining the groups of modulated radiofrequency carrier signals to produce a single signal having a plurality of radiofrequency channels covering more than one octave of bandwidth.
32. An optical receiver for receiving intensity-modulated optical signals over optical links to produce a plurality of radiofrequency channels covering more than one octave of bandwidth, comprising:
at least three optoelectronic receivers for receiving at least three intensity-modulated optical signals over optical links and for demodulating each received intensity-modulated optical signal to produce a corresponding group of modulated radiofrequency carrier signals;
at least three filters, each said filter corresponding to one of said optoelectronic receivers, for filtering the group of modulated radiofrequency carrier signals produced by its corresponding optoelectronic receiver to roll off noise and distortion products thereby producing respective groups of modulated radiofrequency carrier signals, each group of modulated radiofrequency carrier signals having a bandwidth of contiguous radiofrequency channels covering less than one octave; and a signal combiner, coupled to said filters, for combining the groups of modulated radiofrequency carrier signals to produce a single signal having a plurality of radiofrequency channels covering more than one octave of bandwidth.
at least three optoelectronic receivers for receiving at least three intensity-modulated optical signals over optical links and for demodulating each received intensity-modulated optical signal to produce a corresponding group of modulated radiofrequency carrier signals;
at least three filters, each said filter corresponding to one of said optoelectronic receivers, for filtering the group of modulated radiofrequency carrier signals produced by its corresponding optoelectronic receiver to roll off noise and distortion products thereby producing respective groups of modulated radiofrequency carrier signals, each group of modulated radiofrequency carrier signals having a bandwidth of contiguous radiofrequency channels covering less than one octave; and a signal combiner, coupled to said filters, for combining the groups of modulated radiofrequency carrier signals to produce a single signal having a plurality of radiofrequency channels covering more than one octave of bandwidth.
33. The optical receiver of claim 32, wherein said at least three filter comprise a high pass filter, at least one bandpass filter, and a low pass filter.
34. The optical receiver of claim 33, wherein said at least three filters comprise plug-in filters that may be plugged into a circuit board.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/446,461 US5191459A (en) | 1989-12-04 | 1989-12-04 | Method and apparatus for transmitting broadband amplitude modulated radio frequency signals over optical links |
| US446,461 | 1989-12-04 |
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|---|---|
| CA2069944A1 CA2069944A1 (en) | 1991-06-05 |
| CA2069944C true CA2069944C (en) | 1996-08-13 |
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| CA002069944A Expired - Fee Related CA2069944C (en) | 1989-12-04 | 1990-11-21 | Method and apparatus for transmitting broadband amplitude modulated radio frequency signals over optical links |
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| US (3) | US5191459A (en) |
| EP (1) | EP0507799B1 (en) |
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| DK (1) | DK0507799T3 (en) |
| ES (1) | ES2118748T3 (en) |
| WO (1) | WO1991008631A1 (en) |
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| CN114070429B (en) * | 2021-10-28 | 2023-04-18 | 中国电子科技集团公司第二十九研究所 | Method for combining radio frequency internal and external correction and computer storage medium |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3535640A (en) * | 1966-06-16 | 1970-10-20 | Anaconda Wire & Cable Co | Reduction of distortion and losses in cable television distribution systems |
| US3704419A (en) * | 1971-01-14 | 1972-11-28 | Anaconda Astrodata Co | Automatic compensation of cable television systems |
| US4236243A (en) * | 1978-04-20 | 1980-11-25 | National Research Development Corporation | Telecommunication systems |
| DE3045876A1 (en) * | 1980-12-05 | 1982-07-08 | Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt | SERVICE-INTEGRATED, DIGITAL TRANSMISSION SYSTEM |
| GB2109197B (en) * | 1981-10-13 | 1985-12-04 | Standard Telephones Cables Ltd | Radio system |
| US4545075A (en) * | 1981-11-18 | 1985-10-01 | Times Fiber Communications, Inc. | Satellite block transmission using wideband fiber optic links |
| US4709418A (en) * | 1983-09-14 | 1987-11-24 | British Telecommunications Public Limited Company | Wideband cable network |
| JPS616942A (en) * | 1984-06-21 | 1986-01-13 | Tokyo Keiki Co Ltd | Optical multiplex communication system |
| DE3528252A1 (en) * | 1985-08-07 | 1987-02-12 | Standard Elektrik Lorenz Ag | FIBER OPTICAL DISTRIBUTION SYSTEM FOR BROADBAND SIGNALS |
| JPS62100037A (en) * | 1985-10-28 | 1987-05-09 | Hitachi Ltd | Optical transmission system |
| US4959862A (en) * | 1988-04-28 | 1990-09-25 | Catel Telecommunications, Inc. | Active multichannel video processing hub for optimum transition from fiber to coax |
| US4953156A (en) * | 1988-09-08 | 1990-08-28 | Gte Laboratories, Incorporated | Wideband subcarrier multiplexed optical communication system operating over more than one octave |
| JPH0286384A (en) * | 1988-09-22 | 1990-03-27 | Pioneer Electron Corp | Moving picture information service system and head end device for the system |
| US5016242A (en) * | 1988-11-01 | 1991-05-14 | Gte Laboratories Incorporated | Microwave subcarrier generation for fiber optic systems |
| US4891694A (en) * | 1988-11-21 | 1990-01-02 | Bell Communications Research, Inc. | Fiber optic cable television distribution system |
| US5153763A (en) * | 1989-12-01 | 1992-10-06 | Scientific-Atlanta, Inc. | CATV distribution networks using light wave transmission lines |
| US5191459A (en) * | 1989-12-04 | 1993-03-02 | Scientific-Atlanta, Inc. | Method and apparatus for transmitting broadband amplitude modulated radio frequency signals over optical links |
-
1989
- 1989-12-04 US US07/446,461 patent/US5191459A/en not_active Expired - Lifetime
-
1990
- 1990-11-21 EP EP91901147A patent/EP0507799B1/en not_active Expired - Lifetime
- 1990-11-21 JP JP3501501A patent/JPH05502563A/en active Pending
- 1990-11-21 ES ES91901147T patent/ES2118748T3/en not_active Expired - Lifetime
- 1990-11-21 AU AU69144/91A patent/AU647065B2/en not_active Ceased
- 1990-11-21 WO PCT/US1990/006783 patent/WO1991008631A1/en active IP Right Grant
- 1990-11-21 AT AT91901147T patent/ATE167968T1/en not_active IP Right Cessation
- 1990-11-21 DE DE69032454T patent/DE69032454T2/en not_active Expired - Lifetime
- 1990-11-21 DK DK91901147T patent/DK0507799T3/en active
- 1990-11-21 CA CA002069944A patent/CA2069944C/en not_active Expired - Fee Related
-
1992
- 1992-12-30 US US07/998,208 patent/US5379141A/en not_active Expired - Lifetime
-
1994
- 1994-10-11 US US08/321,234 patent/US5500758A/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| WO1991008631A1 (en) | 1991-06-13 |
| DE69032454D1 (en) | 1998-08-06 |
| US5191459A (en) | 1993-03-02 |
| EP0507799B1 (en) | 1998-07-01 |
| CA2069944A1 (en) | 1991-06-05 |
| DE69032454T2 (en) | 1999-03-25 |
| EP0507799A4 (en) | 1993-06-09 |
| ES2118748T3 (en) | 1998-10-01 |
| EP0507799A1 (en) | 1992-10-14 |
| ATE167968T1 (en) | 1998-07-15 |
| JPH05502563A (en) | 1993-04-28 |
| DK0507799T3 (en) | 1998-10-26 |
| US5500758A (en) | 1996-03-19 |
| US5379141A (en) | 1995-01-03 |
| AU647065B2 (en) | 1994-03-17 |
| AU6914491A (en) | 1991-06-26 |
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| EEER | Examination request | ||
| MKLA | Lapsed |