US2311696A - Interference suppressing circuit - Google Patents

Description

2 sheets-sheet 1 Filed Dee.

68 MWD... @52% k blk l0 T z, 3 OUTPUT 20 Feb. 23, 194s. H. B1 RUBI'N 2,311,696

INTERFERENCE SUPPREssING CIRCUIT Filed Dec.l 5', 1941 2 Sheets-Sheet 2 Patented Feb. 23, 1943 entete UNETED STATES PATENT OFMCE {NTERFERENCE SUPPRESSING CIRCUIT Hyman B. Rubin, ew Haven, Conn.

Application December 3, 1941, Serial No. 421,515

5 Claims. (Cl. Z50-20) 'I cuit elements which I employ, behave quite dif- My invention relates to a circuit and means for suppressing interfering voltages, such as aperiodic disturbances on received radio signals, Whether such interference be from natural or man-made sources. Circuits for this purpose are often called static eliminators. My circuit is adapted for the reception of voice or code or tone or picture modulated signals.

In the past, many devices and arrangements have been tried for this purpose but all have been found to have serious drawbacks. A common fault of such arrangements is that the method employed for reducing the strength of the interfering voltage also greatly reduces the strength of the desired signal voltage. These dilculties are avoided in my circuit.

In my circuit I separate the interfering voltage component from both the carrier-frequency and the audio-frequency components of a derived portion of the desired signal, and apply the separated interfering voltage in phase opposition to the interference component of the total received voltage, thus erasing the interfering voltage component applied to the conventional receiving circuit, whose output delivers the de sired signal substantially free from interference. My circuit is entirely responsive to small values, as well as large values, of ratio of static to signal.

An object of my invention is to provide an interference eliminator which will substantially eliminate all interfering voltage components without materially reducing the desired signal component voltage.

Another object of my invention is to provide an interference eliminator which separates the interference voltage component from the signal.

modulation and the carrier-frequency component of a derived portion of the total received signal, and applies the interference voltage component in phase opposition to the total received voltage signal which is being transmitted through the receiver. thus cancelling out substantially all of the interference component.

Still another object of my invention is to provide a static eliminator which employs multigrid tubes to apply dii-ferent voltage components of the total received signal in phase opposition to balance out certain components.

'I'he interference voltage wave is not of closely dened frequency characteristics but is substantially aperiodic, while the regular desired signal modulation and the carrier frequency and side bands have well dened frequency characteristics and hence after passing through tuned cirferently than do the interference Yvoltage compcnents of the wave.

'Ihe interference voltage component has a high decrement, which makes possible the ready separation, by suitable discriminating circuit elements, of the interference component from the desired portions of the signal.

I find that the interference elimination cir-l cuits which I provide work, if anything, better withv very sensitive receivers, than with a relatively insensitive receiver, whereas the opposite condition usually obtains with static eliminators heretofore known.

Y With the Aabove and other objects in View? which will more readily appear as the nature of the invention is better understood, the same consists in the novel construction, combination and arrangements of parts hereinafter more fully described, illustrated and claimed.

In the accompanying drawings, wherein like characters of reference denote corresponding parts in the different views:

Figure 1 is a block diagram showing sche- .V matically the basic operative yrelations between the different units of my circuit, and

' I may employ is known as the 6SK7 which is a applying a voltage representing the same com,

ponent in phase opposition to another grid 'ofv the same tube. One standard type of tube which super-control tube. Another type of standard tube which I may employ with certain increased advantages is the GSA?, Both these tubes are pentodes. The 6SA7 has the advantage that one grid is tied to the shell of the tube structure and hence is at ground potential, but this tube has the disadvantage that it has a much greater tendency to oscillate, since it was originally designed as an oscillator. In the operation of my circuit, as a static eliminator, I do not desire that these tubes oscillate.

I derive a portion of the total voltage of the incoming signal, including carrier frequency and desired modulation, and interference voltage components, separate out two adjacent side bands, and then rectify the side bands, separate the rectified side bands respectively into the modulation component and carrier frequency component, feed back as bias the rectified modulation component in phase opposition to the modulation component of the unrectified side band, and apply the rectified carrier-frequency component to the unrectiiied side band in phase opposition to its carrier-frequency component. The resultant output of` the tubes after these feed-back operations have been consummated is simply the interference voltage component, which.

is adjusted to be in opposite phase to the interference component of the total received signal passing through the receiver, and is applied to the received signal to cancel out substantially al1 of the interference component and deliver a clear understandable signal with the desired transmitted modulation.

Figure 1 shows the different paths traversed by the derived portion of the total received signal, its separation into side bands, the separatey rectification of the side bands, their respective separation into carrier-frequency and modulation components, and the feed-back paths of each of these components in phase opposition to the respective component of the total received signal.

Referring particularly to Fig. 2, a source of modulated radio-frequency signal is shown at 2 and is applied through suitable conventional means to the signal grid of a receiving tube I, which is shown as a multi-grid tube. For lpurposes of illustration the receiving circuit here described may be considered to have tube I constitute an intermediate stage of a super-heterodyne. The input signal from 2 may be applied through the primarywinding 4a of an input transformer 4 whose secondary 4b is shunted by variable condenser l which is connected to sig-nal grid II of tube I. The secondary 4b of transformer 4 is tuned to the carrier frequency of the desired incoming signal. Tube I has cathode I4, suppressor grid I2, -plate I3, oscillator grid 9 and shield grid I0, and this cathode I4 is connected to ground through resistor I6- shunted by condenser I5. Oscillator grid 9 is connected to ground through resistor I1 which is conveniently about 20,000 to 50,000 ohms. Oscillator grid 9 is connected to suppressor grid I2 through condenser I9 which is conveniently 0.1 microfarad. The output of tube I is connected to the primary of transformer 20 which is tuned by shunted condenser 2I, and the secondary of transformer 20 is connected to the output terminals 3 of theusual main signal line of a superheterodyne: receiver.

A wire 23 in which variable condenser'5 is'in'- serted in series, is connected to input terminal 2-of the total received signal, and is applied to one terminal of `primary winding 25 of transformer 24 which is shunted by variable condenser 28. The other terminal of primary 25 may be connected as shown to the low terminal of primary winding 4a. The circuit including primary 4a and primary 26 may be tuned by'variable condenser 5 and is tuned to the carrier frequency of the desired incoming signal. While I have shown one condenser 5 for tuning both primary 4a and primary 26, it is sometimes Vpreferable to use two separate variable condensers for tuning these two primaries separately. Condenser 8 is conveniently about 0.1 microfarad. Primary 4a and primary 25 are not mutually inductively coupled. Transformer 24 is provided with two secondary windings 26 and 21 which are respectively tuned by variable condensers 29 and 30. Transformer 24 may advantageously have a oneto-one winding ratio.

The primary winding 25 together with primary winding 4a is tuned by variable condenser 5 to the carrier frequency of the desired incoming signal, which for purposes of illustration may be taken as 450 kilocycles. Secondary winding 26 would in this case be tuned to include or peak at a mean value of an upper representative side band frequency a little above the main carrier frequency, such as 5 kilocycles thereabove, which in the case taken for illustration would be 455 kilocycles. Secondary winding 21 is tuned by variable condenser 30 to include or peak at a mean value of a lower side-band frequency a little below the main carrier frequency, such as 5 kilocycles therebelow, which in the case taken for illustration would be 445 kilocycles. In this way the circuits connected to transformer 24 do not constitute a substantial drain on the main received desired signals of the principal carrier frequency, but the side bands areV respectively transmitted through the two secondary windings together with side band modulation components, as Well as the aperiodic interference. The .part of the derived portion of the signal, which is taken up by secondary winding 26 and is delivered from the tuned circuit constituted by secondary winding 26 and condenser 29 is delivered by wire 3| to signal grid 36 of the tube 32 which is desirably a pentode of the type heretofore described. Tube 32 has a cathode 33 and anoscillator grid 34 which, however, in my circuit is not used asv an oscillator. Tube 32 also has suppressor grid 31 and shield grid 35, and plate 38. Oscillator grid 34 is connected. to ground through resistor 39. Cathode 33 is connected to ground through resistor 33a shunted by condenser 33D. Suppressor grid 31 is connected to ground.

The output circuit from plate 38 of tube 32 is connected through wire 53 to one primary winding of a transformer 54 which will be hereafter described. Plate 38 is also connected through wire 53 and wire 68 and condenser 69 to an anode 13 of a double diode 10. Instead of double diode 10 there may be employed two single diodes. To anode 13 of diode 10 corresponds cathode 1I. Double diode 10 also has another anode 14 to which corresponds cathode 12. The output of anode 13 is connected to ground through resistor 15 andresistor; 16 in series, constituting anoutput circuit. A tap 11 is provided on resistor 16 to provide proper negative bias on signal grid 3E with respect to cathode, and signal grid 46 is similarly biased. Resistor 16 is about 0.1. to l megohm. The. variable tap 11 is connected, as shown, to one terminal of secondary winding 26 of: transformer 24, so that any rectified output from diode: anode 13 which isv delivered to wire 3| and signal grid 36 passes through secondary winding 26 in series, which will not pass carrier frequency from a voltage thus applied, and only the modulation gets through. This results in continuously applying to signal grid 36 of tube 32 a varying bias which varies in magnitude exactly in accordance with the modulation of the received signal. Because of the inherent relationships between output and input of tube 32, and by adjustment of the circuit elements, this varying bias is exactly out of phase with the modulation of the received unrectied modulated radio-frequency wave which is applied to signal grid 36 from primarywinding 25, and in this manner I` find that the resultant instantaneous voltage on signal grid 36 cancels out the modulation vfrom the output of tube 32. I nd, however, that the interference voltages of relatively high decrement do not thus cancel themselves out from the output of tube 32.

From the common connection of resistor and resistor 16, the wire 8I leads to resistor 82 shunted by condenser 83. Resistor 82 may, in actual practice, have a resistance from about 0.1 megohm to 1.0 megohm. Condenser 83 may in practice have a value of about 100 micromicrofarads. This arrangement of resistor 82 shunted by the condenser 83 blocks the modulation component of the output from anode 13, but permits the radio-frequency component to pass through to connecting wire 84 and may be considered to act as a phase corrector and timer. The arrangement constituted by resistor 82 and condenser 83 is adjusted to provide proper phase relation in the radio-frequency output which is delivered by wire 84 to oscillator grid 44 of tube 42.

'Ihe electron stream passing from cathode 43 of tube 42, to its plate 48, is thus subjected to a radio-frequency voltage which corresponds to the carrier frequency but is adjusted to be exactly out of phase therewith, so that no carrier frequency appears in the output delivered from plate 48 of tube 42 through output winding 56. Oscillator grid 44 is connected to ground through resistor 49 which is conveniently about 20,000 to 50,000 ohms, as is also resistor 39. tube 42 is connected to ground. Cathode 43 of tube 42 is connected through resistor 43a shunted by condenser 43h to ground. Resistor 43a. as well as 33a are conveniently 500 to 1,000 ohms. Condenser 43h, as well as 33h, is conveniently about .A

0.1 microfarad. It will be seen from Fig. 2 that the circuit through which the. diverted signal passes is symmetrical about the center of transformers 24 and 54 and diode 10, and that there are cross-connections between certain portions of,

each of the symmetrical halves of the circuit.

Thus, the other'anode 14 of double diode 10 is connected through resistors 18 and 19 in series to ground, and resistor 19 is provided with variable tap 80 which is connected to one end of secondary winding 21 of transformer 24, whereby only the modulation component is applied through wire 4I to signal grid 46 of tube 42. Likewise wire 85 leading from the common point of resistors 18 and 19 delivers through resistor 86 shunted by condenser 81 and through wire 88 only the carrier-frequency component to oscillator grid 34 of tube 32.

Screen grid 45 is by-passed to ground and also connected to B+ potential through resistor 50 which is conveniently about 20,000 ohms. Screen grid is similarly connected through resistor 30. Screen grid 45 is also connected to suppressor grid 41 through condenser 5| which is convent iently about 0.1 microfarad. Suppressor grid 31 is similarly connected. In this manner Athe electron stream passing from cathode 33 to 'anode 38 of tube 32 is subjected to a modulated radio-frequency voltage applied from primary 25 which includes the interference voltage, and is also subjected to the out-of-phase rectified modulation bias component delivered from wire 11 through secondary 26 and wire 3l, and also to the outof-phase rectied carrier-frequency component delivered through wire 88 and resistor 86 and condenser 81 to oscillator grid 34 of tube 32. 'I'his causes the erasing or cancellation in tube 32 of both the carrier frequency side band component and the desired signal modulation periodic com- Suppressor grid 41 of A ponent of the'total received signal, whereby the 75 net resultant output from anode 38 of tube 32 as delivered through wire 53 to one of the input windings 55 of transformer 54, consists entirely of the interference voltage. I

The one input winding 55 of transformer 54 is adjusted in impedance to match the high output impedance of tube 32. Input Winding 55 is tuned by condenser 58 Vto the side-band frequency through wire 3l, Input winding 56 is tuned by condenser 59 to the side-band frequency through wire 4|. The other input winding 56 of transformer 54 is similarly adjusted in impedance to match the high output impedance of tube 42.

One end of input Winding 55 is connected to ground through condenser 6 I, conveniently about 0.1 microfarad, and is also connected to vpositive plate battery through resistor 62. One endof input winding 56 is connected to ground through condenser 64, which is conveniently about 0.1 microfarad, `and is also connected to positive plate battery through resistor 63. Resistors 62', 63, are conveniently 1,000 to 5,000 ohms.

Transformer 54 in actual practice advantageously has a one-to-one winding ratio. The phase relations of the voltages applied to input windings 55 and 56 are suitably adjusted to apply to output winding 51 of transformer 54, a combined voltage, which as previously explained consists almost entirely oi the interference voltage which is being applied to input terminal 2. Output winding 51 is here lshown as shunted by Variable condenser 60, and may be tuned in desired manner, and conveniently is tuned to the carrier frequency of the desired signal. It is, however, possible to get good results while omit-"- ting condenser 60 vand not tuning winding 51.

YBy adjustments including phasing of output Winding 51 with respect to windings 55 and 55, the voltageV on wire 22, as applied to grid 9 of tube I, is out of phase with the voltage on signal grid II of tube I. Since, as has been explained, the output energy from output winding 51 through wire 22 consists almost entirely of the interference voltage, the net result is that the output of tube I delivered from this anode I3 has erased therefrom substantially all of the interference voltage component, and contains only the main carrier frequency with its desired signal modulation. Transformer winding 26 of transformer 24, as above stated, has a resonance curve which in cludes vor peaks at a mean value of an upper representative side-band frequency a little above the main carrier frequency such as 5 kilocycles thereabove. Similarly secondary winding 21 of transformer 24 is tuned by variable condenser 30 to include or peak at a meanv value of a lower side-band frequency a little below the main car-` rier frequency suchv as 5 kilocycles therebelow. In operation, the interference eliminator is connected to the input terminal 2 by applying the connecting wire 23 thereto, and eliminator tuning condensers 5, 29, 30, are adjusted in the manner above described to respectively tune to the main carrier which may be the intermediate frequency and the upper and lower side bands. Ordinarily, in a superheterodyne receiver the interference eliminator circuit will be permanently connected and the variable tuning elements will be permanently tuned to the inter-v mediate frequency and its side bands respectively. The necessary adjustments are then made in the circuit elements as described to adjust for phase oppositionof thecOmpOnents which are to be Abalanced-out. In-this manner the output of the tube lV has substantially all of the interference component removed therefrom, so that the output of output transformer 20 as delivered to output terminal 3 consists substantially entirely of the desired modulated signal.

My invention is of great importance and utility in the reception of all knids of intelligence modulated radio-frequency signals, and has been very successfully employed for eliminating interference in reception of broadcast, telegraph, highfrequency, and video modulated radio-frequency signals.

While I have described my invention as applied for purposes of illustration to the intermediate stage of a superheterodyne receiver, it will be obvious to those skilled in the art that my invention is susceptible of application to substantially any kind of radio receiving circuit With equally good results, and also that the details which I have shown and the particular form of tube which I have illustrated in the auxiliary balancing circuit which I have specifically described, are susceptible of various modifications, without in any Way departing from the disclosure or the spirit of my invention, and all such modifications thereof which are comprehended within the scope of the appended claims I consider to be included Within my invention.

I claim:

l. The method of eliminating undesired interfering voltage components from a total received low-frequency modulated radio-frequency signal which consists in deriving a portion of the total incoming signal, rectifying a part of the so derived portion, separating the low-frequency signal modulation from the unmodulated carrier frequency of the so rectied part, and separately feeding back and separately applying said signal modulation and said carrier frequency of said rectified part to said unrectied derived portion in phase opposition to the signal modulation and carrier-frequency components thereof respectively, and applying the residual interfering voltage component of said derived portion in phase opposition to said total received signal.

2. The method of eliminating undesired interfering voltage components from a total received llow-frequency modulated radio-frequency signal,

which consists in deriving a portion of the total incoming signal, selecting two symmetrically closely spaced side bands thereof, rectifying each of said side bands separately, separating the lowfrequency signal modulation from the carrier frequency of each of said rectified side bands, separately feeding back and applying the rectified low-frequency signal modulation of each said side band to the unrectied same said side band in phase opposition to the low-frequency signal component thereof, separately feeding back and applying the so rectified carrier-frequency component of each said side band to the unrectied other said side band in phase opposition to the carrier-frequency component thereof, and applying the residual unrectied interfering voltage component of said derived portion in phase opposition to said total received signal.

3. In radio reception employing an auxiliary lateral circuit comprising a pair of multi-grid control tubes, the method of eliminating undesired interfering voltage components from a total received low-frequency modulated radio-frequency signal which `consists in deriving a portion of the total incoming signal, selecting two symmetrically closely spaced side bands thereof,

amplifying each of said side bands by one of said control tubes respectively, rectifying each of said so amplied side bands separately, separating the low-frequency signal modulation from the carrier frequency of each of said rectified side bands, separately feeding back and applying the rectified low-frequency signal modulation of each said side band to one grid of the said tube which ampliies the unrectied same said side band in phase opposition to the low-frequency signal component thereof, separately feeding back and applying the so rectified carrier-frequency ccmponent of each said side band to a different grid of the other of said tubes which amplifies the other said side band in phase opposition to the carrier frequency component thereof, and applying the residual unrectifed interfering voltage component of said derived portion as delivered from the outputs of said tubes in phase opposition to said total received signal.

4. In an interference eliminator, a source of incoming received modulated radio-frequency total signal containing interference voltage components, a radio-receiving circuit for receiving said signal, an auxiliary circuit for adjusting the components of said received signal and comprising a pair of translating means, the input of said auxiliary circuit being connected to said source for deriving a portion of said received signal, means in said auxiliary circuit for selecting a pair of spaced side bands from said derived portion and applying them respectively to each of said translating means, rectifying means connected for rectifying the output of each said translating means, means for separating the modulation component from the carrier frequency component of the output of each of said rectifying means respectively, means for feeding back and applying the modulation component of the output of each said rectifying means to the input of the same said translating means which is connected thereto in phase opposition to the modulation component of the unrectied side band delivered thereto, means for feeding back and applying the so separated carrier frequency component of the output of each said rectifying means to the input of the other said translating means which is not connected thereto in phase opposition to the carrier frequency component of the unrectied side band delivered thereto, and means for combining the outputs of said translating means containing interference components and applying the same to said receiving circuit in phase opposition to the interference voltage component therein.

5. In an interference eliminator, a source of incoming received modulated radio-frequency tota-l signal containing interference voltage components, a radioreceiving circuit for receiving said signal, an auxiliary circuit for adjusting the components of said received signal and comprising a pair of multi-grid tubes, the input of said auxiliary circuit being connected to said source for deriving a portion of said received signal, a pan` of adjustable reactance means in said auxiliary circuit for selecting a pair of spaced side bands from said derived portion and applying the same to a first grid of each of said tubes respectively, a pair of rectifying means connected for rectifying the output delivered from the plate of each of said tubes, the output of each of said rectifying means being respectively connected to the one of said reactance means through which the portion of the signal thereby rectied had passed before rectification, said reactance means through which the portion of the signal thereby rected did not pass before rectiiication, said -connecting means being respectively adapted to adjust the phase of the carrier frequency rectiiied component passing therethrough, and output means for combining the outputs of said tubes containing interference components and applying the same to said receiving circuit, said output means comprising phase adjusting means for adjusting the phase of said interference component.

HYMAN B. RUBIN.

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