WO1985002309A1

WO1985002309A1 - Diversity combiner

Info

Publication number: WO1985002309A1
Application number: PCT/SE1984/000375
Authority: WO
Inventors: Mats Ingemar Lennartsson
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1983-11-07
Filing date: 1984-11-05
Publication date: 1985-05-23
Also published as: DK304685A; FI852459L; DE3490533T1; GB2159374A; AU3612984A; FI852459A0; DK304685D0; GB8516636D0; SE8306109L; SE8306109D0; JPS61500405A; NO852728L

Abstract

A diversity combiner for adding, before detection, two or more received radio signals having defined carrier waves of the same frequency contains for each of the radio signals (S1; S2) a phase shifter (4) for dividing the radio signals into a sub-signal (S1i; S2i) forming an in-phase signal and a sub-signal (S1q; S2q) forming a quadrature signal, for each of the sub-signals a phase-sensitive detector (2) controlled by a reference signal (A) common to all the detectors, and having an output signal of zero frequency direct-voltage connected to a modulation input of a subsequent modulator (Z2-Z5) for converting the radio signal to an alternating voltage signal (C) of the same phase angle as that of the reference signal (A), and for all the radio signal together an adder (Z8) for adding the cophasal alternating voltage signals from all the means for conversion (Z2-Z5) to a sum-signal (E) in which the received radio signals are added, the sum signal (E) being connected to the control inputs of the phase-sensitive detectors (2) such as to act there as the common reference signal for all the detectors.

Description

DIVERSITY COMBINER

TECHNICAL FIELD

The present invention relates to a diversity combiner in accordance with the preamble to the following independent claim.

BACKGROUND ART

In a type of diversity reception of radio signals where the signals impinge onto separate receiver antennae, the signals being converted to cophasal signals which are aggregated to one signal for feeding to a demodulator, it happens that the aggregated signal is fed back to the means in which the received signals are converted the cophasal signals. The principle for such feedback is given by I. Granlund, in "Topics in the design of Antennas for scatter" Lincoln Lab, MIT, Cambridge, Techn. rep 135, Mass. USA, Nov. 1956.

In the diversity combiners which have been made according to the principle, -it has been customary that the incoming, received signals with their frequencies have been subjected to a multiple mixing process to give rise to the cophasal signals at some other frequency than that of the incoming signal. The property of the high-frequency mixer of giving an output signal in the form of an alternating voltage with a phase angle which is the sum or difference of the phase angles of the input signals is utilized in such a case. An example of an apparatus implemented in this way is described in the patent CA-A-1141437. The insertion of signals at more frequencies than that of the incoming signal has, however, brought with it the accompanying disadvantage that false signals rnay have been formed, which have interfered with the desired signal in certain conditions. Furthermore, a plurality of tuned bandpass filters have been required in the apparatus.

DISCLOSURE OF INVENTION

In accordance with the present invention, the high-frequency mixer for each received signal is implemented as a modulator, which is adapted for being controlled- by a signal which has zero frequency and which is direct-voltage

^• connected to the modulation input of the modulator, and besides that there is a phase shifter arranged for each of the radio signals before the phase-sensitive detector for dividing the radio signals into a sub-signal forming an in-phase signal, and a sub-signal forming a quadrature signal.

With the diversity combiner thus arranged there is achieved the effective prevention of false signals, caused by the occurrence in the apparatus of signals having several different frequencies.

A further advantage with the apparatus in accordance with the invention is that it is simpler in its construction than apparatuses previously made, since only a simple lowpass filter for the signal coming from the detector is required, while the previous apparatus required bandpass filters for corresponding high- frequency auxiliary signals.

BEST DESCRIPTION OF DRAWINGS

An apparatus in accordance .with the invention is described in the following. with reference to the accompanying drawing, on which

Figure 1 illustrates in principle the circuitry of the diversity combiner,

Figure 2 illustrates the characteristic of a phase-sensitive detector,

Figure 3 illustrates the characteristic of a modulator,

Figure 4 illustrates the characteristic of the detector and modulator in co- action,

Figures 5 and 6 are vector diagrams for the sub-voltages before and after processing in one of the detectors and modulators, and

Figure 7(a, b, c) is a wiring diagram for an implemented diversity combiner.

DESCRIPTION OF AN EMBODIMENT

The diversity combiner in accordance with the invention is intended for combining radio signals impinging on two separate receiver antennae. The radio signals have the same frequency and the same information content. A high- frequency amplifier is associated with each antenna, as- well as a number of mixing stages, with local oscillators and associated intermediate frequency amplification stages, for successively mixing down the antenna signal to a final intermediate frequency, in this case 460 kHz. The received radio signals are frequency-modulated with audio-signals or digital frequency shift signals (frequency shift keying, FSK). The apparatuses mentioned so far are of known embodiment and are therefore not described further. The intermediate frequency signals are the input signals to the diversity combiner.

The amplification from antenna to intermediate frequency signal is equal for both signals, this having been achieved by matching of the apparatus.

The input signals have such a low level that the linearly operating circuits of the diversity combiner are given a dynamic range of at least 40 dB above the noise level of the input signals.

Both input signals may have different levels and different phase angles due to fading. The task is to aggregate the signals with the aid of the diversity combiner to an output signal at a given level for feeding to a detector.

For this purpose the combiner is adapted to convert each of the input signals to a new signal with the same phase angle as that of a reference signal with the same frequency. The actual phase angle of the reference signal has no significance.

The principle of signal processing in the combiner is described first with reference to Figure 1. An input signal S, , also called S, . for reasons which will be perceived further on, is amplified in an amplifier 1 with limitation and is fed to a phase-sensitive detector 2, where its phase angle is compared with a reference signal on the wire A. The output signal at B has a magnitude which is a function ofd?, , which is the phase difference between both alternating voltage signals fed in, and in principle it is a direct voltage which is separated from the intermediate frequency but contains other superposed signals. The amplifier 1 and phase detector 2 are put together in an integrated circuit Z6, here of the type TBA 120S. The circuit is conventional and is therefore not described further.

A lowpass filter 3 is inserted for filtering the output signal from the phase detector 2.

The filtered output signal from the phase detector is used in a modulator Z2 to modulate the original signal S,. at the intermediate frequency so that the output signal of the modultor at C is an alternating voltage with a frequency equal to the intermediate frequency, with a magnitude of S, . cos ψ. , and with a phase angle equal to the phase angle of the reference signal at A. The modulator is an integrated circuit, here of the type MC 1596. The circuit is. conventional and is therefore not described further.

Turning now to Figure 2, the operation of the phase detector 2 in co-action with the modulator Z2 will be described. The output voltage u at B in Figure 1, depends on the phase difference Q)-. in the manner illustrated by the diagram.

At the modulator Z2 its slope, i.e. (output current )/(input voltage), measured in

Siemens (S), varies in response to the control voltage u in the manner illustrated in the diagram in Figure 3. A combination of both diagrams has the result that the slope G of the signal S, . varies in response to the phase angle d in a manner illustrated in the diagram in Figure 4. With good approximation the slope is proportional to cos ψ..

To enable recreating the input signal S.with unchanged amplitude when the information as to its phase angle is to be carried by ^"the direct voltage signal from the phase-sensitive detector, S, is divided into two sub-signals S, . which is equal to S, , and S, , which is phase shifted 90° in a positive direction by a phase shifter 4, and is called a quadrature signal, see Figure 1. The signals are illustrated in the vector diagram in Figure 5. The phase-shifted signal S, is processed in a phase detector Z7 and a modulator Z3, which are identical with the previously described units Z6 and Z2. The sub-signals are added after processing, at D in Figure 1, where the sum signal may be denoted S , . The following relationship is then applicable:

S^ = S_n cos <p_χ + S₁ cos <ψ₁ + 90°) = S_li . cos ψ_χ - S_χ sin Φ_χ

By vector addition of- both contributions to S , , see Figure 6, and since the magnitudes of S,. and S, are both equal to the magnitude of S, a sum vector is created with the magnitude S-, and with a phase angle coinciding with that of the reference voltage.

The second signal S is processed in exactly the same way in circuits adapted for it such that a sum vector is formed from it, as below;-

^S2 ^{= S}2i ^{* C03}P ^{+ S}2α ^cos^2 ^{+ 90}°^ ^{= S}2i ^{* C08} _ ~ ^S2q ^sinΦz

with the magnitude S_? and with a phase angle which likewise coincides with that of the reference voltage.

The signal voltages are now cophasal and are added in an adding amplifier Z8. The sum signal, at E in Figure 1, which contains all the frequency modulation which the input signals S, and S₂ contained, is fed out from the diversity combiner for processing in a subsequent frequency tletector 5. The final amplifier in Z8 for the sum signal is limiting so that the output signal has a constant, and fairly high level.

It is typical for the diversity combiner described here that the reference voltages used in all phase-sensitive detectors Z6, Z7, Z*9 and Z10 are takep from the output voltage at E, i.e. they are fed back. By avoiding a special oscillator ^' for the production of a reference voltage at the freuqency of the input signals it has been possible to avoid false signals which otherwise usually occur when alternating voltage from a special oscillator is mixed with the input signals. In this apparatus in accordance with the invention, the content of frequency modulation of the reference voltages being the same as that of the input signals the result is that the signals fed out from the detectors Z6, Z7, Z9, Z10 do not contain any frequency modulation, since the modulation in both voltages fed in have cancelled each other.

It may be mentioned that the feed-back circuit for the reference voltage has a bandwidth of 3 kHz, substantially determined by the lowpass filters 3 etc.

The implementation of the diversity combiner is illustrated in detail in the wiring diagram of Figure 7. The denotations of the integrated circuits in the diagram are the same as those in Figure 1. Buffering and phase-shifting 90° of the input signals S, and S~ are carried out by N-P-N transistors V 5, V6 and the oscillation circuits L1C2 and L2C4, which are tuned to the carrier frequency of the input signals. The phase-shifted S, , and the non-phase-shifted S, . sub- signals are fed to the amplifier input in limiters, input 14 on Z6 etc and in balanced modulators, input 1 on Z2 etc.

The aggregated output signal of the diversity combiner is formed by connecting the signal C from the outputs 6 of the four balanced modulators Z2-Z5 to a common load, the tuned circuit L3C46. The combined signal is limited in Z8 and fed out as the balanced signal E on the outputs 6 and 10 and is fed back as the reference signal A on the inputs 7 and 9 in the four phase detectors Z6, 7, 9, 10. The fed-out signal for modulation is obtained on the output 8. Resistors R35. etc determine the output signal level. The RC filter C15R21C27 etc, corresponding to the lowpass filter 3 in Figure 1, is inserted to filter off radio frequency and determine the control circuit bandwidth to about 3 kHz.

The modulating signal B is fed to the input 7 on Z2 etc. A bias voltage is taken from the potentiometer R66 and applied to the balancing input 8. The bias voltage is obtained by supply current to Z6-Z10 -flowing through the resistor R48.

The supply current varies as the temperature of the apparatus, and thus the bias on the input 8 also varies, and a certain amount of compensation for temperature change is obtained.

Bias voltage to the amplifier steps in Z2-Z5 is obtained by voltage division in R17R18. The limiters have built-in biassing networks.

Required bias voltages to the modulator inputs of Z2-Z5 are obtained by supplying these with +11 volts. The limiters are supplied with +8 volts.

The amplifier Z8, here also of the type TBA 120S, contains a superfluous detector, which is used as frequency detector for the diversity combiner output signal E. The quadrature reference voltage for this detector is obtained from the tuned circuit L4C40. The demodulated signal occurs at the output 8.

Claims

1 A diversity combiner for adding, before detection, two or more received radio signals having defined carrier waves of the same frequency, the combiner including for each of the radio signals (S.; S a phase-sensitive detector (2), controlled by a reference signal (A) common to all the detectors and having an output signal containing information as to the phase angle of the radio signal and a means (Z2-Z5) controlled by the output signal of the associated, phase- sensitive detector (2) for converting the radio signal to an alternating voltage signal (C) with the same phase angle as that of the reference signal (A), and for all the radio signals together, an adder (Z8) for adding the cophasal alternating voltage signals from all the means for conversion (Z2-Z5) to a sum signal (E), in which the received radio signals are added, the sum signal (E) being connected to the control inputs of the phase-sensitive detectors (2) for acting there as the reference signal common to • all the detectors, characterized in that the means for conversion (Z2-Z5) are modulators adapted for control by the output signal of the associated phase-sensitive detector (2), this signal having zero frequency and being direct-voltage connected to the modulation input of the modulator.

2 Combiner as claimed in claim 1, characterized in that a phase shifter (4) is arranged for each of the radio signals before the phase-sensitive detector (2) for dividing the radio signals into a sub-signal (S, .; S-.) forming an in-phase signal, and a sub-signal (S, ; S_ ) forming a quadrature signal.