GB2212686A - Phase adjustment circuit - Google Patents

Abstract

In a closed loop interference cancellation system for a signal receiver channel, main (6 Fig. 1) and auxiliary (7) signal paths are connected to a vector modulator 13 and correlator 14 combination such that output limbs from the correlator 14 carry feedback information for automatically controlling the vector modulator 13. The auxiliary channel 7 contains a representation of the interference which after modulation 13 is combined (8) with the main path 6 to reduce the interference to a minimum residue. One of the inphase and quadrature input limbs of said vector modulator 13 is connected to a generator 19 for applying a test signal to said limb, a phase sensitive detector 21 being connected to the opposite limb from said correlator 14 effective to produce a phase error signal, said error signal being applied to a counter 23 controlling the setting of a phase shifter 26 connected such that the phasing of signals entering said correlator 14 is adjusted to minimise said error signal. The system can be incorporated in a circuit module which thus becomes self- adjusting and it can be used and replaced in a radar receiver without demanding separate phase adjustments of the whole receiver. <IMAGE>

Description

PHASE ADJUSTMENT CIRCUIT.

This invention relates to a phase adjustment circuit. It relates particularly to such a circuit which can be used in a sidelobe interference cancellation system for example for a radar signal receiver.

Sidelobe interference cancellation systems of the closed loop feedback type require precise initial adjustment to ensure that the feedback phase is close to 1800. The presence of even a small phase error acts to degrade the speed of response and in an extreme case it can result in instability of the circuit.

In the manufacture of apparatus for a radar system, the initial phase adjustments of the circuit have to be carried out by skilled engineering staff using test equipment. If the replacement of any components or sections of the feedback loop should become necessary later, then these adjustments will have to be repeated and this will be a costly and time consuming operation.

One way of constructing a radar system would be to provide the circuit sections in the form of replaceable modules. In order to avoid the need for a lengthy manual adjustment procedure following the replacement of a module, it would then be convenient if each feedback loop included its own phase adjustment system which could be used to make any necessary adjustment after any module had been replaced.

The present invention was devised to provide a phase adjustment system which was self-contained and which would simplify the operation of interchanging circuit modules.

According to the invention, there is provided a closed loop interference cancellation system for a signal receiver channel, the system comprising main and auxiliary signal paths which are connected to a vector modulator and correlator combination such that output limbs from the correlator carry feedback information for controlling the vector modulator, in which one of the inphase and quadrature input limbs of said vector modulator is connected to a generator for applying a test signal to said limb, a phase sensitive detector being connected to the opposite limb from said correlator effective to produce a phase error signal, said error signal being applied to a counter controlling the setting of a phase shifter connected such that the phasing of signals entering said correlator is adjusted to minimise said error signal.

The phasing adjustment may be effected to reduce said phase error to a minimum threshold level. The said test signal may be of a symmetrical squarewave type.

The invention also comprises a circuit module for a radio receiver including the said interference cancellation system. The radio receiver may be a radar signal receiver.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a single sidelobe interference cancellation loop of the analogue type, Figure 2 is a similar diagram of the interference cancellation loop of the invention, and, Figure 3 is an alternative circuit diagram.

As depicted in Figure 1, the circuit includes a single sidelobe interference cancellation loop of the analogue (inphase and quadrature) type. In this context, the word "cancellation" means the removal of interference from a radar or communications receiver channel, by feeding into the channel an independent sample of the same interference in antiphase and with equal amplitude.

Interference, which may be due to jamming or the presence of an interfering signal in the receiver band, is present with the wanted signal in the main channel. An independent sample of the same interference waveform is therefore obtained, for example by directing an auxiliary aerial towards the interfering source, and this is adjusted in amplitude and phase by the vector modulator and injected into the main channel to achieve cancellation.

In order to adapt to any changes in the situation it is necessary to control the vector modulator automatically. This is achieved by using a feedback technique whereby a small amount of interference remains after cancellation, information from this residue signal is fed back to the modulator which then operates to keep the residue at a low level. It is not possible to remove the residue signal completely and this would not be desirable since this signal forms the error signal for the feedback loop. The term "error signal" is not of course to be confused with the phase error that is intended to be reduced by the system.

To obtain the required phase and amplitude information, a correlation is performed between the residue and the interference in the auxiliary channel. The correlator basically measures the phase angle between the residue and the auxiliary channel interference, which forms a phase reference. The correlator outputs are proportional to the sine and cosine of this phase difference and also to the amplitudes of its input signals, averaged over times that are long compared with the interference modulation but short compared with the changing interference problem, for example, in the radar case, due to the aerial sidelobes sweeping past the interference direction.

As shown in Figure 1, the circuit arrangement comprises a main channel fed from an aerial 6 and an auxiliary channel fed from an aerial 7. Signals received by the aerial 6 are fed via a signal combiner 8 to a signal splitter 9. The signal splitter 9 is arranged to feed a receiver (not shown) via a line 11.

In an auxiliary receiver channel, signals received by the auxiliary aerial 7 are fed via a further signal splitter 12 which delivers part of the signal to a vector modulator 13 and part to a correlator 14. At the vector modulator, a quadrature signal splitter 16 divides the signals into portions for Inphase (I) and Quadrature (Q) channels of the vector modulator 13.

Similarly at the correlator 14, a further quadrature signal splitter 17 divides the incoming signal into portions for Inphase (I) and Quadrature (Q) channels of the correlator.

The correlator 14 Quadrature channel provides a control signal for the vector modulator across points 1 and 2 of the loop. Similarly, the Inphase channel provides a control signal for the vector modulator across points 3 and 4 of the loop.

In operation of the circuit, a RF signal is applied to the auxiliary input terminal in place of the aerial 7. The primary input is terminated. The effect on the circuit if the loops are broken at the points 1-2 and 3-4 will now be considered.

If the phasing of the loops is correct, a small positive going voltage applied to the vector modulator at point 2 will result in a negative going voltage appearing at point 1. A similar result will apply to points 3 and 4 in the Inphase channel. However, no change in voltage will be observed at point 3 due to applied voltage at point 2, or at point 1 due to applied voltage at point 4.

If the phasing of the loops is in error, there will be crosstalk between the Inphase and Quadrature channels. Thus changing the voltage at point 2 will change both correlator 14 outputs, at points 1 and 3. This effect may be made use of as a measure of the phase error.

Figure 2 shows a sidelobe interference cancellation loop circuit incorporating the modified loop arrangement of the invention. In this circuit, a clock pulse generator 18 serves to control a signal generator 19 which applies a perturbing signal in the form of a small bipolar squarewave at low frequency to point 2.

The resulting perturbation results in a small squarewave output appearing at point 3 from the inphase correlator due to the phase errors present in the system. The squarewave output signal is applied to a phase sensitive detector 21 to which an input from the clock pulse generator 18 is also fed. This results in a DC output which after passing through a threshold detector 22 is applied to a binary counter 23. A digital output from the binary counter 23 is converted to analogue form in a converter 24 and the resulting signal serves to drive a voltage controlled phase shifter 26. The phase shifter 26 is located in the line feeding the signals from the signal splitter 12 to the correlator 14.

In operation of this circuit, at switch-on of the circuit, the counter 23 is set to a count of zero, and this results in a minimum phase control voltage and minimum phase shift from the phase shifter 26. This sets the phase error to a known polarity. The counter 23 then counts upwards, resulting in a slowly incrementing phase shift. As soon as the output from the detector 22 is below a preset threshold, representing an acceptably low phase error, the counter 22 is stopped. The count is then frozen and the sidelobe cancellation circuit reverts to normal operation until the phasing operation is re-initiated (normally this occurs at the time of system power up).

In a practical construction, the system was found to be capable of coping with phase errors of up to plus or minus 60 in the complete loop. The use of symmetrical squarewave signals and a phase sensitive detector 21 allows AC coupling, thus rejecting any DC offset voltages which may be present and which would otherwise lead to the occurrence of inaccurate results.

In the present embodiment where the system is applied to a radar sidelobe interference cancellation circuit, the cancellation loop can have its component parts distributed throughout several circuit modules. A single phase adjustment system is thus able to cater for the phase tolerances of all of these replaceable modules.

Figure 3 shows an alternative construction arranged for digital rather than analogue circuit use. In this instance, the phase shifter 26 is digitally rather then analogue voltage controlled and the digital-to-analogue converter 24 is not necessary.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope to the invention as defined in the appended claims. For instance, it is not essential that the interference cancellation system should be applied to adaptive cancellation in a radar receiver channel, in a different embodiment it could be applied to a communications receiver.

Claims (6)

1. I. A closed loop interference cancellation system for a signal receiver channel, the system comprising main and auxiliary signal paths which are connected to a vector modulator and correlator combination such that output limbs from the correlator carry feedback information for automatically controlling the vector modulator, in which one of the inphase and quadrature input limbs of said vector modulator is connected to a generator for applying a test signal to said limb, a phase sensitive detector being connected to the opposite limb from said correlator effective to produce a phase error signal, said error signal being applied to a counter controlling the setting of a phase shifter connected such that the phasing of signals entering said correlator is adjusted to minimise said error signal.
2. 2. A circuit as claimed in Claim 1, in which the said phasing adjustment is effected to reduce said phase error to a minimum threshold level.
3. 3. A circuit as claimed in Claim 1 or 2, in which the said test signal is of a symmetrical squarewave type.
4. 4. An interference cancellation system substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.
5. 5. A circuit module for a radio receiver including an interference cancellation system as claimed in any one of Claims 1 to 4.
6. 6. A radar signal receiver including an interference cancellation system as claimed in any one of Claims 1 to 4.