GB2050118A - Improvements in or relating to space diversity receiving systems - Google Patents

Abstract

A space diversity receiving system comprises two vertically spaced antennae 10, 11. Signals from the antennae 10, 11 are fed to a receiver 16 via a phase modulator 12 and a phase changer 15. A feedback loop 20 extends from the receiver 16 to the phase changer 15. If the phase relationship of the signals from the two antennae changes from a predetermined value an error signal is produced at the receiver 16 and is fed via the feedback loop to the phase changer. The phase changer operates to maintain the two signals in a fixed phase relationship. A relatively high modulating signal is used for the phase modulator so that the frequency of the error signal is well above the response time of the circuits in the feedback loop 20. <IMAGE>

Description

SPECIFICATION Improvements in or relating to space diversity receiving systems This invention relates to space diversity receiving systems.

Space diversity receiving systems are used in microwave radio systems to counter the effects of multipath fading. Multipath fading can occur when one or more additional paths exist between the transmit and receive aerials due to reflection or refraction causing reduction of the receive signal level as a result of cancellation and distortion of the received signal as a result of the relative delay between the paths. These effects result in a degradation of the radio system performance. In a space diversity receiving system this problem is alleviated by using two vertically spaced antennae, the spacing of which is such that when destructive interference occurs at one additive interference occurs at the other.

In one known system the signals from the two antennae are combined in a waveguide coupler and fed to a receiver. The signal from one of the antennae is phase modulated so that when the signals are combined amplitude modulation results if the signals are out of phase. The amplitude modulation forms an error signal which is used in a feedback loop from the receiver to a phase shifter connected between one antenna and the coupler to maintain the antennae signals in a fixed phase relationship.

In this prior art arrangement a relatively low phase modulating frequency is employed but this is not satisfactory for high rate digital systems as it imposes limitations on the time constant of the automatic gain control loop of the receiver.

We have developed a space diversity receiving system which is not subject to this limitation.

According to the present invention there is provided a space diversity receiving system comprising two spaced antennae, receiving means arranged to receive signals from the antennae, phase modulating means and phase shifting means connected between the antennae and said receiving means, a feedback loop coupling the receiving means to said phase shifting means whereby an error signal generated at said receiving means is applied to said phase shifting means such that a fixed phase relationship is maintained between the signals from the two antennae, means for applying a relatively high frequency modulating signal to said phase modulating means, and means for modulating said relatively high frequency signal with a relatively low frequency signal.

By using a relatively high modulating frequency amplitude modulation is produced at a frequency which is substantially above the highest frequency to which the automatic gain control loop responds.

Generally the phase shifting means is driven by a servomotor. The present arrangment allows the very convenient frequency of 50Hz to be used for the motor. Typically the phase modulating frequency is 1 OkHz.

The phase shifting means and phase modulating means can be connected between either antenna and the receiving means. Preferably one is connected in the path from one antenna and the other is connected in the path from the other antenna so that their losses tend to balance.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 is a block schematic diagram of a space diversity receiving system in accordance with the present invention; Figure 2 is a vector diagram illustrating the operation of the system of Fig. 1; Figure 3 is a side sectional elevation of a phase modulator used in the system of Fig. 1; Figure 4 is a circuit diagram showing an automatic gain control detector and amplifier used in the system of Fig. 1; Figure 5 is a circuit diagram of the control circuits of the system; Figure 6 shows the power supply and associated control circuits for the system, and Figures 7 and 8 are circuit diagrams showing alarm circuits which can be used with the present system.

A space diversity receiving system for use with a 11 GHz digital radio network comprises a pair of vertically spaced antennae 10, 11.

The antennae are spaced apart a vertical distance which can be determined from principles well known in the art. The antenna 10 is connected via a phase modulator 1 2 to a - 3dB coupler 14. The antenna 11 is connected via a rotary phase changer 1 5 to the coupler 14. The coupler 14 is connected to a receiver 1 6 which has a detector 1 7 connected to an AGC amplifier 18.

A feedback loop 20 extends from the receiver to a servo-motor 21 which drives the phase changer 1 5. The feedback loop includes a pre-amplifier 22, a bandpass filter 24, a demodulator 25 and a control amplifier 26 which is connected to the control input of the motor 21.

A 1 OkHz oscillator 28 drives the demodulator 25 which produces a 50Hz control signal which is applied to the motor 21. The oscillator 28 also drives a balanced modulator 30 which receives a 90 phase shifted 50Hz sine wave signal from a mains supply 32. The mains supply is also connected directly as a reference input to the motor 21. The output of the balanced modulator 30 is applied to the phase modulator 1 2 via a line 33. The phase modulating signal comprises a 1 OkHz carrier wave on which is modulated a 50Hz signal.

In operation the signals from the antennae 10, 11 are combined in the coupler 14. The signal from the antenna 10 is phase modulated in the phase modulator 1 2 and the signal from the antenna 11 has passed through the rotary phase changer 1 5. The signals from the antennae are added in the coupler and thus half the received power passes to the receiver. In the case where the signal to one antenna fades partially then the power fed to the receiver falls until the worst case when one antenna has faded completely only the signal from one antenna is fed to the receiver. However, this fading may be compensated in that the signal picked up by the antenna that does not fade may be increased by the addition of multipath signals at that antenna.

The effect of the phase modulation in the phase modulator 1 2 is to produce amplitude modulation at the receiver if the signals are out of phase. There will be some very small amplitude modulation of the resultant signal even when the signals are in phase but in practice this is negligible. The amplitude modulation forms an error signal which is applied to the feedback loop 20. The error signal is used to correct the relative phase of the signals from the antennae 10, 11 by actuation of the motor 21. The arrangement is such that the signals applied to the coupler 1 4 are 90 out of phase. In this way the signals are held in phase at the receiver.

The sense of the error signal with respect to the phase modulator indicates whether one signal leads or lags the other. This is illustrated in Fig. 2 of the drawings. It will be noted that a relatively high phase modulating frequency is used with the servo-motor frequency modulated on to it. In this way amplitude modulation is produced in the coupler which is well above the response time of the AGC amplifier 18 and the very convenient frequency of 50Hz can be used for the servomotor. The 50Hz servo-motor control phase signal is shifted 90 with respect to the reference phase and amplitude modulates the 1OHz carrier from the oscillator 28 giving a double side band suppressed carrier signal which passes through RF circuits and is coherently demodulated giving a 50Hz control phase error signal.If the phase of the 1 OkHz double side band suppressed signal recovered from the detector reverses phase so also does the demodulated 50Hz signal from the demodulator 25. The demodulated 50Hz signal drives the servo-motor 21 via the control amplifier 26 which is a class B power amplifier.

As shown in the arrangement of Fig. 1 the phase modulator and phase changer are mounted in different paths to balance their losses. They may however both go in one path without effecting the functioning of the arrangement.

Some band limiting is applied to the 1 OkHz error signal to remove amplitude modulation noise emerging from the amplifier 22, especially in the region of received level where the full band carrier to noise ratio becomes low or even negative. It is conceivable that a strong 1 OkHz component in a data stream might produce an interfering signal within the 1 OkHz centre band but if this ever does cause problems it should be possible to find a carrier frequency away from the prevalent spectral lines of the various data streams which the 11 GHz links have to carry.

The phase modulator 1 2 is a circulator coupled varactor phase modulator and is illustrated in more detail in Fig. 3 of the drawings.

The phase modulator comprises a waveguide 40 with a short circuit at one end 42. A CAY 10 varactor 43 is mounted across a section of reduced height adjacent the short circuit. The guide carriers three capacitative screws 43, 44 and 45 which extend into the guide and by means of which the bandwidth operation can be adjusted. The modulator is designed to operate with a peak deviation of + 9" with the varactor 43 being swung over the majority of its capacitance range. This is a compromise to account for the fact that the more strongly the diode is coupled for a greater phase shift the greater is the loss in the modulator.The varactor used is a Mullard CAY 10 paramp varactor with a capacitance variation coefficient of at least 0.1 5 and a cut-off frequency of at least 250 GHz. -In one example the voltage applied was 2 volts peak-to-peak with a selfbiasing of the varactor using a 0.1 uF isolating capacitor and a 100 kilohm leakage resistor.

The AGC detector and error signal preamplifier 22 are shown in more detail in Fig.

4. The detector includes a pair of diodes 50, 51 and is connected to the amplifier 22. The detector is also connected via a line marked A to an automatic gain control amplifier 54 the output of which is applied to the base of the transistor 55. The output from the receiving arrangement appears on line 56.

Referring now to Fig. 5 the output from the amplifier 22 is applied to the bandpass amplifier 24 which includes an operational amplifier 60 and associated components. The output from the amplifier 60 is applied via a capacitor 61 and variable resistor 62 to the demodulator 25 which comprises an integrated circuit 70 and associated components. The modulator 30 comprises a similar integrated circuit 71 and associated components. The oscillator 28 is a phase shift oscillator which generates a 1 OkHz signal. The construction of the oscillator is indicated generally at 28 in Fig. 5. The oscillator includes a variable resistor 75 which allows the frequency of the oscillator output to be adjusted. The 1 OkHz output from the oscillator is fed via a buffer amplifier 76 to drive the balanced modulator 30 and the demodulaor 25.The modulator has a signal terminal 78 which receives the 90 phase shifted 50Hz signal on line 79 via a variable resistor 80. This signal is derived from the mains. The modulator 30 provides on output line 85 a 2 volt peak-to-peak suppressed carrier AM signal which drives the phase modulator 1 2 directly.

The error signal from the amplifier 22 is amplified in the amplifier 24 which has a midband voltage gain of 1 50 and a - 3dB bandwidth of 500Hz. The amplified signal passes through a sensitivity control comprising the variable resistor 62 to the demodulator 25 which has a voltage gain o 1.5. The demodulator 25 operates to provide a demodulated 50Hz signal on line 77 which is applied to a transformer coupled B class amplifer 26 which is illustrated in Fig. 6. The amplifier 26 includes a pair of operational amplifiers 90, 91 the outputs of which respectively are connected to transistors 92, 93.

The collectors of the transistors 92 and 93 are connected to the primary of a transformer 94 which couples them to the control winding of the servo-motor 21. The amplifiers 90, 91 constitute a phase splitter driver stage for the amplifier 26. The amplifier 26 and servomotor winding are tuned by a 3.2 uF capacitor 96 connected across the transformer secondary to obtain unity power factor. With a 35 volt d.c. supply the amplifier 26 saturates at a drive level of 11 6 volts rms to the servomotor control winding.

Fig. 5 also illustrates the way in which power for various parts of the system are derived from a 240 volt 50Hz mains supply 95. The servo-motor reference winding signal is derived from the supply 95 via a transformer 96. The 50Hz 90 phase shifted signal to the modulator 30 is derived via a transformer 97 which has connected in its secondary circuit a capacitor 98 and a resistor 99 to provide the necessary phase shift. A 35 volt d.c. unstabilised supply for the amplifier 26 is derived via a transformer 100 and rectifier 101, 102.

The phase changer 1 5 is based on a laboratory instrument model made by Flann Microwave Instruments type 1 6/6-2. The servo-motor is a Muirhead 15M10A4 servomotor which drives the rotating section of the phase changer by way of a straight cut spur wheel with 240 teeth of 96 D.P. involute form. The servo-motor is designed to run on a 60Hz supply with a maximum rms voltage of 11 5 volts on the reference and control windings. For 50Hz operaion it is derated to 100 volts rms although with the present design this may be exceeded for an instant on the control winding. The gear wheel has 1 5 teeth so that for a phase change of 180 , say, the motor revolves four times. The rotating section of the phase changer runs on substantial ball races at either end.Quarter and half wave plates are formed from dielectric sheet and the bandwidth is specified as 8.2 to 1 2.5 GHz with a maximum insertion loss of 2dB. A shaft angle encoder is driven from the main spur wheel via a wheel with 1 20 teeth so that the angle it encodes corresponds to the electrical angular position of the phase changer.

The angle encoder is a Muirhead CE 1002/1A shaft angle encoder which is driven from the body of the motor driven phase shifter with a 2:1 speed reduction so that its position corresponds to the electrical angular position of the phase shifter. The encoder is connected to a resolver/encoder which gives the angular position as a binary coded decimal signal on ten wires.

The system is designed to operate in an 11 GHz digital system and is capable of holding signals from the two antennae in phase with a negligible error over the full range of received signal levels.

Alarm circuits may be incorporated into the system. The alarm circuits may monitor the output of the amplifier 22 or the drive signal for the phase modulator or both to provide an alarm should a fault occur in the control circuits. one form of alarm circuit is shown in Fig. 7 and includes an input line 110 which is connected to an amplifier 111 which has a low pass network including a capacitor 11 2 and resistor 11 4 for reducing harmonic components. The amplifier 111 is connected to a further amplifier 11 5 the output of which is connected to a rectifier 11 6. The rectifier is connected via an emitter follower 118 to an inverter 120. The circuit shown in Fig. 7 is designed to monitor the error output from the IF amplifier to see that it does not stay too high for too long.The a.c. component of the detected IF error signal is amplified by the amplifier 111 with some reduction of the harmonic components. It is rectified by the rectifier 11 6 and drives the inverting gate 1 20 which squares it up to provide a decision whether the error signal is too high. The gain of the amplifiers is adjusted so that the gate output goes from + 5 volts to 0 volts if the error signal exceeds 50 with equal signal levels in the two paths. At this point the peakto-peak voltage at the point marked X is about 50mV. The output from the inverting gate 1 20 can be used to energise an alarm as will be described shortly.

Fig. 8 shows an arrangement which is similar to that of the previous Fig. but with rather less a.c. gain. In this Fig. the output from the phase modulator 1 2 is applied to an amplifier 1 25 whose output is rectified by a rectifier 1 26. The rectifier gives a positive output provided the drive voltage to the modulator is sustained. The gain is adjusted so that an alarm condition is indicated if the peak-topeak voltage at the phase modulator falls below 1.5 volts. The output from the rectifier 126 provides one input to a positive AND gate 130, the other input to the AND gate being from the inverting gate 120 of Fig. 7.

The AND gate 1 30 is connected to a further AND gate 1 32 whose output is connected to an alarm device. The arrangement is such that an alarm is energised whenever the signal on either input to the gate 1 30 is indicative of an alarm condition.

In brief the arrangement of Fig. 7 monitors the output of the IF amplifier while the arrangement of Fig. 8 monitors the drive to the phase modulator. In this way the functioning of the modulation generating components and the servo loop components under fading conditions are continuously checked. The phase modulator is not checked but it could be monitored by picking off the 20kHz second harmonic from the AGC detector through another band pass amplifier.

Claims (4)

1. A space diversity receiving system comprising two spaced antennae, receiving means arranged to receive signals from the antennae, phase modulating means and phase shifting means connected between the antennae and said receiving means, a feedback loop coupling the receiving means to said phase shifting means whereby an eror signal generated at said receiving means is applied to said phase shifting means such that a fixed phase relationship is maintained between the signals from the two antennae, means for applying a relatively high frequency modulating signal to said phase modulating means, and means for modulating said relatively high frequency signal with a relatively low frequency signal.

2. A space diversity receiving system as claimed in claim 1 wherein said phase shifting means is driven by-a servomotor.

3. A space diversity receiving system wherein the phase shifting means is connected between one antenna and the receiving means and the phase modulating means is connected between the other antenna and the receiving means.

4. A space diversity receiving system substantially as hereinbefore described with reference to and as shown in the accompanying dawings.