GB2169477A - Frequency agile communication receiver - Google Patents

Abstract

A communication receiver operable in a frequency agile or hopping mode, the receiver including two receiver aerials 2, 4 and an interference cancellation circuit. The interference cancellation circuit includes two weighting circuits W1, W2 operatively associated with two switches SW1, SW2 for switching the two weighting circuits W1, W2 alternately into operation so that whilst one of the weighting circuits W1, W2 is used at one frequency, the other of the weighting circuits W2, W1 can be set as appropriate for the next frequency to be switched into operation synchronously with the next frequency hopping operation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to communication receivers The present invention relates to a communication receiver and more particularly to a frequency agile communication receiver incorporating interference cancellation means.

Frequency agile radio receivers incorporating meansforcancelling an interfering signal are known.

One type of known receiver includes two receiver aerials, signals from which are fed to an interference cancellation circuit where they are differentially weighted and combined, so that any interfering signal is suppressed. When such a receiver is in its frequency agile or hopping mode with a short period between frequency changes the performance ofthe interference cancellation circuit may become degraded by virtue of its own transient response to each change in frequency.

An objective of the present invention is to overcome the difficulty due to the transient response time ofthe interference cancellation circuit by providing a communication receiver having an interference cancellation ci rcuit which is adapted to suppress interference on a frequency prior to that frequency being used for wanted signal reception.

According to the present invention there is provided a communication receiver operable in a frequency agile or hopping mode, the receiver including two receiver aerials and interference cancellation means, the interference cancellation means comprises an interference cancellation circuit including two weighting circuits operatively associated with switching means, the switching means being operative for switching the two weighting circuits alternately into operation so that whilst one weighting circuit is used at one frequency, the other weighting circuit can be set as appropriateforthe next frequency to be switched into operation synchronously with the next frequency hopping operation.

In one embodiment of the present invention the switching means includes two switches, each weighting circuit being coupled alternately by operation of the first switch to an input of a first combiner circuit, anotherinputofthefirstcombinercircuitbeing coupled to one ofthe aerials for receiving a signal at the tuned frequency ofthe communication receiver, the output signal of the first combiner circuit being an interference suppressed signal.

In a preferred embodiment each weighting circuit is associated with a respective sample/hold circuit connectible by the second switch to the output of a correlatorwhich is coupled to the output of a second combiner circu it.

Advantageously the output ofthe correlator is connectibletoeachoftheinputsofthesample/hold circuits alternately by operation ofthe second switch.

The outputs of the respective weighting circuits being switchable alternately by the first switch into connec tion with an input of the second combiner whose output is coupled to a n input an the correlatorthereby completing respective negative feedback loops.

In oneembodimentthesecondcombinercircuithas two inputs one ofwhich is coupled to the aerial for receiving a signal at the next wanted frequency and the second input being connectible by operation of the first switch to each ofthe outputs alternately of the weighting circuits, the negative feedback loops when formed each serve to provide an output signal at the respective weighting circuit which when fed as a signal to the second input ofthe second combiner circuit suppresses the interference in the output signal from the second combiner circuit. On the next switching operation ofthe first switch the same interference suppressed output signal will appear at the output ofthe first combiner circuit, the interference suppression being at the samefrequencyto which the receiver is tuned.

The present invention will be described further, by way of example, with reference to the accompanying drawings in which: Figure lisa schematic block diagram of a conventional interference cancellation circuit; Figure 2 is a schematic block diagram of an interference cancellation circuit according to one embodiment ofthe present invention; and Figure 3 is a time diagram illustrating the various conditions ofthe weighting circuits W1 and W2 of Figure2with respect to the frequency reception bythe radio receiver.

Referring to Figure 1 a primary receiver aerial 2 and an auxiliary receiver aerial 4 are coupled to a known type of interference cancellation ci rcuit which dif ferentially weights and combines the signals from the aerials 2,4 so as to suppress any interference signal.

Each ofthe r.f. signals received bythe aerials 2,4 may consist of a wanted signal Sand an interference signal I both at a tuned frequency Fc. The signal received by the auxiliary aerial 4feeds an auxiliary receiver channel comprising a mixer 5 which is fed from a local oscillator L02 operating at a frequency Fo.

The r.f. signal received by the aerial 4 is also fed to a first input of a weighting circuit. The intermediate frequency i.f., which equals Fo - Fc, from the mixer 5 is fed via an amplifier 6 to a first input of a correlator circuit C.

The r.f. signal from the primary aerial 2 is fed to a first input of a combinercircuit 8the second input of which is connected to the output ofthe weighting circuit W. The output of the combiner circuit 8 is fed to an output terminal O/P ofthe interference cancellation circuit and also fed to a primary receiver channel comprising a mixer9fedfrom a local oscillatorLOl operating at frequency Fo. The intermediate frequency i.f., which equals Fo - Fc,fromthe mixer9 is fed via an amplifier 10 to a second input ofthe correlator circuit C.The nature ofthe input signalsto the correlator circuit C is dependent on the method used for discriminating between the wanted and unwanted signals, the method used being arranged so that the input signals are predominantly interference. The correlator circuit C comprises an arrangement of splitters and phase detectors and theirfunction in providing an outputsignal dependent on the interference signal I is well known in the electronic field and so will not be described in detail herein. The correlator circuit C's output signal is fed via an amplifier 12 to a second input ofthe weighting circuit.

Theweighting circuitWservesasavectormodula- torto weightthe signal received at its first input by the signal received at its second input. The weighting adjusts the signal received atthefirst input in both phase and amplitude to give an output signal from the weighting circuitW,which when fed as the signal to the second input ofthe combiner 8, suppressesthe interference component ofthe output signal from the combiner 8. The weighting circuit W comprises an arrangement of a splitter, PIN diode attenuators and a combinerthefunction ofthe arrangement being well known in the electronic field and so will not be described in detail herein.

In a known way the combiner8, the amplifier 10, the correlator C, the amplifier 12 and the weighting circuit Wtogetherdefine a negative feedback loop which suppresses the component ofthe interferencesignal included in the output signal transmitted from the outputterminal O/P. An inherent characteristic of the known interference cancellation circuit of Figure 1 is that when tuned to a newfrequencythere is a transient response before the signal atthe output O/P has settled two an interference suppressed signal.

If the radio receiver is in a frequency agile or hopping mode having a period between frequency changes ofthe same order as or less than the transient response of the interference cancellation circuit then the performance ofthe interference cancellation circuit is degraded.

An embodiment ofthe present invention will now be described with reference to Figure 2, identical features to Figure 1 being designated by identical reference numerals and letters where possible.

Figure 2 has two weighting circuits W1 , W2 which can be coupled alternately by a switch SW1 with the second input ofthe combiner circuit 8 each weighting ci rcuit W1,W2 being associated with a respective sample/hold circuit 22. Consider initially the situation in which the r.f. signal being received in aerial 2, which signal includes the wanted signal Sand an interference signal I, is at a first frequency F1 and is fed to the first input ofthe combiner 8. The interfer ence signal I at the frequency F1 has already been weighted and the output signal ofthe weighting circuit W1 set by a voltage from the sample/hold circuit 20.

The switch SW1 is in a first position with its terminals A and B connected and its terminals C and D connected, the output of the weighting circuitW1 being coupled to the second input ofthe combiner circuit 8, the output signal of the combiner circuit 8 being the interference suppressed signal which is fed totheoutputterminal O/P.

Whilstthe interference on frequency F1 is being suppressed by the output signal from the weighting circuit the local oscillators L01 and L02 are switched to a newfrequency such that the primary and auxiliary receiver channels are tuned to the next r.f.

frequency F2 to be used by the radio receiver. A second switch SW2 is in the position shown in Figure 2 so completing the interference cancellation circuit comprising the weighting circuitW2, a second com binercircuit8',the primary tuning channel,the amplifier 10, the correlatorC, the sample/hold ciruit 22, an amplifier 12', the auxiliary tuning circuit andthe amplifier 6. The signals received by both the aerials 2 and 4 on frequency F2 will atthis stage be made up of interference only as the wanted signal on frequency F2 is not yet present.The completion ofthe interfer encecancellation circuit thereby provides at the output ofthe weighting circuitW2 a signal, which when eventually fed as the signal to the second input ofthe combiner 8, will suppress the interference component atfrequency F2 of the output signal from the combiner 8when the radio receiver is tuned to the frequency F2.

When it is desired to tune the radio receiverto the frequency F2the second switch SW2 is operated to connect the output ofthe correlator C with the input terminal 1 ofthe sample/hold circuit 20. At the same time the switch SWI is operated to form electricai connections between the terminals A and D, and the terminals B and C. The weighted signal at frequency F2 is maintained atthe output ofthe weighting circuit W2 by a voltage from the sampie/hold circuit 22. In this way the combiner8 receives simultaneouslytwo input signals, one being the r.f. signal direct from the primary aerial 2 and the otherthe weighted signal from the weighting circuitW2.The output signal from the combiner8 is the interference suppressed signal at frequency F2.

Whilst the interference on frequency F2 is being suppressed by the weighting circuit W2 the local oscillators LO1 and L02 are switched to a new frequency such thatthe primary and auxiliary receiver channels are tuned to the next r.f. frequency F3 to be used by the radio receiver. The signals received by the aerials 2 and 4 on frequency F3 will be made up of interference only as the wanted signal on frequency F3 will not be present. With the terminals B and C connected the interference cancellation circuit comprisestheweighting circuitWl,the second combiner circuit 8', the primary tuning circuit,the amplifier 10, the correlator C, the sample and hold circuit20, the amplifier 12, the auxiliary tuning circuit and the amplifier 6.The output signal from the weighting circuit W1 is weighted so that, when the radio receiver is tuned to frequency F3 and the switch SW1 operated to close terminals A and B and the switch SW2 reverts to its position as shown in Figure 2, the output signal is fed to the second input ofthe combiner 8 resulting in an interference suppressed signal at frequency F3 from the output ofthe combiner 8.

The above process involving leapfrogging ahead to suppress interference on a frequency priorto that frequency being used forwanted signal reception is illustrated in a time diagram in Figure 3.

Figure 3(a) illustrates the periods during which the radio receiver is tuned to and is receiving signals at frequencies F1, F2, F3 and F4. Figure 3(b) illustrates periods atwhich data instructionsforthe next tuning frequency to be received are transmitted in a known mannerto ancillary equipment associated with the radio receiver. Figures 3(c) and 3(d) illustrate the conditions of the weighting circuits W1 andW2 respectively.

Itwill be seen from Fig u re 3 that after the data instructions for the frequency F2 have been received the weighting ci rcuit W2 tu nes for the freq uency F2 so that its output signal is set for the frequency F2 before the radio receivertunesto frequency F2. Similarly the weighting circuit W1 tunes for the frequency F3 immediately afterthe data instructionsforthefrequency F3 have been received so that its output signal is setforfrequency F3 before the radio receivertunes to frequency F3. This is a continued process which in some conditions will proceed when there is no wanted signal.

Immediately after the end of each period during which the weighting circuits W1 and W2 have been set for a particularfrequencythere is a period during which the weighting circuits W1 and W2 are resetting in preparation for turning to the next frequency. These resetting periods are illustrated by the crossed sections in Figures 3(c) and 3(d).

The embodiment described above with reference to Figure 3 has two weighting circuits, and the interference cancellation circuit adapts in conjunction with one of these weighting circuits to suppress interference signals. Other embodiments of the present invention make use oftwo or more interference cancellation circuits, which in total adapts to provide interference suppression of two or more interference signals. Each additional interference cancellation circuit is associated with its respective aerial, its respective auxiliary receiving channel, its respective correlator, its respective pairs of weighting circuits and sample-hold circuits etc.

Claims (7)

CLAIMS:

1. A communication receiver operable in afrequency agile or hopping mode, the receiver including two receiver aerials and interference cancellation means, the interference cancellation means comprises an interference cancellation circuit including two weighting circuits operatively associated with switching means, the switching means being operative for switching the two weighting circuits alternately into operation so that whilst one weighting circuit is used at one frequency, the other weighting circuit can be set as appropriate for the next frequency to be switched into operation synchronously with the next frequency hopping operation.

2. Acommunication receiveras claimed in claim 1 wherein the switching means includes two switches, each weighting circuit being coupled alternately by operation ofthe first switch to an input of a first combiner circuit, another input ofthefirst combiner circuit being coupled to one of the aerials for receiving a signal at the tuned frequency ofthe communication receiver, the output signal ofthe first combiner circuit being an interference suppressed signal.

3. A communication receiver as claimed in claim 2 wherein each weighting circuit is coupled to a respective sample/hold circuit couplable by the second switch to the output of a correlator which is coupled to the output of a second combiner circuit.

4. Acommunication receiver as claimed in claim 3 wherein the correlator is couplable to each of the inputs ofthe sample/hold circuits alternately by operation of the second switch, the outputs ofthe respective weighting circuits being switchable alter nately by the first switch into connection with an input of the second combinerwhose output is coupled to an input of the correlatorthereby completing respective negative feedback loops.

5. A communication receiver as claimed in claim 4 wherein the second combiner circuit has two inputs one of which is coupled to the aerial for receiving a signal at the next wanted frequency and the second input being couplable by operation of the first switch to each ofthe outputs alternately of the weighting circuits, the negative feedback loops when formed each serving to provide an output signal at the respective weighting circuit which when fed as a signal to the second input ofthe second combiner circu it suppresses the interference in the output signal from the second combiner circuit.

6. A communication receiver as claimed in any one of claims 1 to 5 wherein further interference cancellation circuits are provided, each interference cancellation circuit being associated with a respective aerial, a respective auxiliary receiving channel, a respective correlator, respective pairs of weighting circuits and sample/hold circuits.

7. A communication receiver substantially as hereinbefore described with reference to, and as illus- trated in, Figure 2 ofthe accompanying drawings.