GB2123257A - Arrangement for cancelling cross-talk between two communications signals - Google Patents

Abstract

A circuit for canceling unwanted components in two received signals has a first input 10 for one of the signals and a second input 11 for the other. The first input is connected by a first circuit path to an output 12 and the second input by a second circuit path to an output 14. Each circuit path has an associated feedback which includes a correlating device 30, 31 which is arranged to receive fractions of both output signals. Each correlating device forms a control signal which is applied to a mixer 22, 23. Each mixer 22, 23 operates under the control of the control signal to feed to its associated circuit path a signal which tends to cancel the unwanted component. <IMAGE>

Description

SPECIFICATION Improvements in or relating to correlation circuits This invention relates to the cancellation or reduction of unwanted components in received signals. In particular the invention relates to the cancellation or reduction of unwanted components in two received signals each of which includes a component of the other.

In radio systems such as microwave links, receiving equipment is sometimes required to receive two or more signals each of which includes a component of one or more of the others. In order to process the received signals it is necessary to cancel orsubstan- tially reduce these unwanted components. The present invention is concerned with a circuitforcancell- ing or reducing these unwanted components.

According to one aspect of the present invention there is provided an electrical circuit for receiving two or more input signals each of which includes a component of one or more of the others, the circuit being arranged to cancel or substantially reduce said components in each signal, said circuit including a first input coupled by a first circuit path for a first of said input signals to a first output, a second input coupled by a second circuit path for a second input signal to a second output, a feedback loop associated with each circuit path, each feedback loop having correlation means arranged to receive fractions of both output signals, said correlation means being arranged to form a control signal which is fed to a further circuit, said further circuit being operative in accordance with said control signal to feed to the circuit path associated with the feedback loop a signal which tends to cancel the unwanted component, wherein each correlation means comprises a multiplying means followed by an integrator, said multiplying means comprising modulation means for modulating a first of the signals to be correlated with a preselected signal, a multiplier for multiplying the modulated signal with the second of the signals to be correlated, and demodulating means for demodulating the multiplied signal.

Each further circuit may comprise a complex attenuator which is arranged to receive a fraction of the signal received at the input associated with the other feedback loop and to adjust the magnitude and phase of this signal so as to cancel the unwanted component.

The integrator may comprise a chopper stabilised amplifier.

Each circuit path may include a first splitting circuit for splitting each received signal into two parts, one of which is fed to a combining circuit and the other which is fed to the correlation means of the feedback loop associated with the other received signal. Each combining circuit may be arranged to receive the output signal of its associated complex attenuator and to combine that output signal with the signal received from said first splitting circuit. Each circuit path may include a second splitting circuit arranged to receive output signals from said combining circuit, said second splitting circuit being arranged to produce output signals for feeding to the correlation means of each feedback loop.

Each circuit path may include an automatic gain control circuit for stabilising the level of the signal strength at each output.

The invention also provides an improved correlation circuit which can be used in the circuit of said one aspect. A conventional correlation circuit for correlating high and low level signals comprises a double-balanced mixer in which a high level signal is used to switch diodes. Imperfections in the double balanced mixer give rise to a DC offset voltage at the output which is related to the high level input signal.

This DC offset voltage will cause an error in the control signal to the complex attenuators and will reduce the efficacy of the cancellation process. We have devised a correlation circuit in which this problem is reduced.

According to a second aspect of the present invention there is provided a correlation circuit for correlating first and second signals, said circuit comprising modulation means for modulating the first signal with a preselected signal, multiplying means for multiplying the modulated signal with the second signal, and demodulating means for demodulating the multiplied signal. The modulating frequency can be substantially lower than thefrequen- cy of said first and second signals. The effect is a frequency conversion to lower frequencies with a consequent reduction in the leakage problems.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure lisa block schematic diagram illustrating an intermediate frequency canceller in accordance with the present invention; Figure 2 is a block schematic diagram showing in more detail a canceller of the form illustrated in Figure 1, and Figure 3 is a schematic diagram illustrating a correlator which can be used in the canceller shown in Figures 1 and 2.

Referring to Figure 1 of the drawings an intermediate frequency canceller has two inputs 10, 11. The input 10 is arranged to receive a first input signal and the input 11 is arranged to receive a second input signal. The first input signal is designated H + 31V in Figure 1 and the second signal is designated V + 32H. It will be seen that the signal H contains a component 31V of the signal V and the signal V includes a component 82H of the signal H. The purpose of the circuit shown in Figure 1 is to reduce or substantially remove the components 61V and 32H so that the output signal at output 12 contains only signal H and the output signal at output 14 contains only signal V.

Each input 10, 11 is connected to a splitting circuit 16 and 18. Each splitting circuit is arranged to split the received signal into two parts, one part being fed to a combining circuit 20, 21 and the other being fed to complex attenuators 22, 23. Each combining circuit 20,21 is connected to a further splitting circuit 24,25, one output of the splitting circuit 24 being the output 12 and one output of splitting circuit 25 being the output 14. Each splitting circuit has two further outputs designated 26, 27 and 28, 29. The output 26 is connected to the complex attenuator 22 by a feedback loop which includes a complex correlator 30 whilst the output 28 of the splitting circuit 24 is connected by complex attenuator 23 by a feedback loop which includes a complex correlator 31.The output 27 of the splitting circuit 24 is connected to the correlator 31 in the feedback loop associated with the circuit path for the signal V whilst the output 29 of splitting circuit 25 is connected to the correlator 30 in the feedback loop associated with the circuit path for signal H.

In operation the correlator 30 receives an output signal from the output 26 of the splitting circuit 24 and multiplies that signal with the output signal received from the output 29 of the splitting circuit 25.

The multiplied signal is integrated and then applied as a control signal to the complex attenuator 22. The complex attenuator 22 receives a signal from the splitter 18 and the effect of the control signal applied from the correlator 30 is such that the complex attenuator 22 applies to the combining device 20 a signal which tends to cancel the component 61V in the signal H + 31V. The resulting output signal at the output 12 of the splitting circuit 24 is (1 - 8,82)H.

The correlator 31 in the feedback loop associated with the signal path of the signal V operates in a similar manner. The correlator 31 provides a control signal for the complex attenuator 24 so that the complex attenuator applies to the combining circuit 21 a signal which tends to cancel the component 82H in the signal V + 52H. The resulting output from the splitting circuit is (1 - 8,82)V.

It will be appreciated that Figure lisa schematic representation of a canceller constructed in accordance with one embodiment of the present invention.

The component parts of the canceller can be conventional devices which will be known to those skilled in the art. For example the splitting circuits 16 and 18 and the combining circuits 20 and 21 can be conventional hybrid devices. The mixers 23 and 24 can be double balanced mixers. The correlators 30 and 31 will be described in more detail below.

A more detailed version of the canceller described with reference to Figure 1 is shown in Figure 2. The signal 16a from splitter 16 is applied to a quadrature hybrid 40 which split the signal into two equal parts which differ in phase by 90 . Similarly the signal 18a from splitter 18a is split by a quadrature hybrid 41 into two equal parts which differ in phase by 90 .

Double balanced mixers 22A, 22B and 23A, 23B multiply these signals with control signals output from complex correlators 30 and 31 respectively.

The signal output from double balanced mixers 22A, 22B and 23A, 23B are combined with signals 16b and 18b output from splitters 16 and 18 in combining circuits 20 and 21. The effect is to adjust the signals 16a and 18a output from splitters 16 and 18 respectively in both amplitude and phase so that when added to the signal 18b and 16b in combining circuits 21 and 20 respectively the interfering signals 32H and 81V are substantially reduced.

In the complex correlator the signal output from splitter 24 is split into two equal parts which differ in phase by 90 in a quadrature hybrid 43. The two signals are applied to double balanced mixers 30A and 30B which multiply these with signal 29 obtained from splitter 25. Signal 29 may pass through a limiter 55 in order to suppress any interference present in the output 14, which would cause errors in the control signals to the complex attenuators. However, since signal 29 is used to switch the double balanced mixers, the presence of a small interfering signal can have little effect and render the use of limiters unnecessary.

Signals output from double balanced mixers 30A and 30B are integrated in operational amplifiers 32, 33 to produce control signals for the complex attenuator 22. A similar arrangement comprises signal 28 output from splitter 25, signal 27 output from splitter 24, splitter 44, double balanced mixers 31A,31Band operational amplifiers 34, 35 to provide control signals for the complex attenuator 23.

It will also be seen that in the arrangement of Figure 2 the path for the signal H includes an automatic gain control (AGC) circuit designated generally at 50 for controlling and maintaining constant the output level from the splitter 23a. The path for the signal V includes a similar AGC circuit 51 for controlling and maintaining constant the level of the output from the splitter 24a.

It is known to make use of complex correlators which comprise simple double balanced mixers.

These double balanced mixers correlate a higher level signal with a low level signal. Imperfections in the double balanced mixer give rise to a DC offset voltage at the output which is related to the high level input signal. This DC offset voltage will cause an error in the control signal to the complex attenuator and will reduce the efficacy of the cancellation process. Thus, we have designed an improved correlating circuit which is intended to overcome this problem. The improved correlating circuit is illustrated in Figure 3 of the drawings.

Referring to Figure 3 the improved correlator comprises a modulating stage which is constituted by a double balanced mixer 60. The double balanced mixer 60 is a three port device having an input 61 which receives the low level signal which is to be correlated, an input 62 which receives a square wave 500 KHz signal from an oscillator 63 and an output 63. The output signal at the output 63 is the low level signal to be correlated amplitude modulated by the 500 KHz oscillator signal. This signal is applied to an input 65 of a second double balanced mixer 66 which also receives a high level corrulating signal at input 68. The mixer 66 multiplies the signal at input 65 with the high level signal at input 68 and produces at output 69 a 500 kHz signal which is fed via suitable a.c. coupling which includes an amplifier 67 to a third double balanced mixer 70. The double balanced mixer 70 receives the 500 KHz square wave output of the oscillator 63 at input 71 and operates to demodulate the input from the mixer 66 to produce a d.c. signal at output 72. This d.c. output is the correlation signal and is fed via an integrator in the form of a chopper stabilised amplifier 73 and used as the control signal for the mixers 23 and 24 as described earlier.

The circuit shown in Figure 3 effectively frequency converts the signals to be correlated to a lower frequency and this reduces considerably the problem produced by leakage of the high level signal to the low level signal.

The circuit described above has been designed to operate with incoming signals having frequencies of the order of 140 MHz although it will be appreciated that the principle of the present invention could be applied over a very wide rangebf frequencies.

Also the invention can be used to cancel unwanted components in more than two input signals.

Claims (11)

1. An electrical circuit for receiving two or more input signals each of which includes a component of one or more of the others, the circuit being arranged to cancel or substantially reduce said components in each signal, said circuit including a first input coupled by a first circuit path for a first of said input signals to a first output, a second input coupled by a second circuit path for a second input signal to a second output, a feedback loop associated with each circuit path, each feedback loop having correlation means arranged to receive fractions of both output signals, said correlation means being arranged to form a control signal which is fed to a further circuit, said further circuit being operative in accordance with said control signal to feed to the circuit path associated with the feedback loop a signal which tends to cancel the unwanted component, wherein each correlation means comprises a multiplying means followed by an integrator, said multiplying means comprising modulation means for modulating a first of the signals to be correlated with a preselected signal, a multiplier for multiplying the modulated signal with the second of the signals to be correlated, and demodulating means for demodulating the multiplied signal.

2. A circuit as claimed in claim 1 wherein each further circuit comprises a complex attenuator which is arranged to receive a fraction of the signal received at the input associated with the other feedback loop and to adjust the magnitude and phase of this signal so as to cancel the unwanted component.

3. A circuit as claimed in claim 1 wherein the integrator comprises a chopper stabilised amplifier.

4. A circuit as claimed in any preceding claim wherein each circuit path may include a first splitting circuit for splitting each received signal into two parts, one of which is fed to a combining circuit and the other which is fed to the correlation means of the feedback loop associated with the other received signal.

5. A circuit as claimed in claim 4 wherein each combining circuit is arranged to receive the output signal of its associated complex attenuator and to combine that output signal with the signal received from said first splitting circuit.

6. A circuit as claimed in claim 5 wherein each circuit path includes a second splitting circuit arranged to receive output signals from said combining circuit, said second splitting circuit being arranged to produce output signals for feeding to the correlation means of each feedback loop.

7. A circuit as claimed in any preceding claim wherein each circuit path includes an automatic gain control circuit for stabilising the level of the signal strength at each output.

8. A correlation circuit for correlating first and second signals, said circuit comprising modulation means for modulating the first signal with a preselected signal, multiplying means for multiplying the modulated signal with the second signal, and demodulating means for demodulating the multiplied signal.

9. A correlation circuit as claimed in claim 8 wherein the modulating frequency is substantially lower than the frequency of said first and second signals.

10. An electrical circuit for cancelling unwanted components in received signals substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

11. A correlation circuit substantially as hereinbefore described with reference to and as shown in the accompanying drawings.