GB2274752A

GB2274752A - Interference suppressor

Info

Publication number: GB2274752A
Application number: GB8427442A
Authority: GB
Inventors: Kevin Malcolm Kelly; John Roland Sharman
Original assignee: Northern Telecom Ltd; Standard Telephone and Cables PLC
Current assignee: Nortel Networks Ltd; STC PLC
Priority date: 1984-10-30
Filing date: 1984-10-30
Publication date: 1994-08-03
Anticipated expiration: 2004-10-30
Also published as: GB8427442D0; GB2274752B

Abstract

An arrangement for suppressing interference signals in a radar receiver wherein the input signals including an unwanted large amplitude signal and a smaller amplitude wanted signal are split in an in-phase power divider 10 one output of which is fed to a limiter circuit 11 which makes a phase estimate of the input while the other output is adjusted in amplitude and phase 12 and applied to a mixer 13 where it is mixed with the output of the limiter circuit, the mixer output being filtered 14 to obtain the difference-frequency signal. The filtered signal is fed to a first 90 hybrid splitter circuit 15 to form a set of two components in phase quadrature, the output of the limiter circuit being also applied to a second 90 hybrid splitter circuit 16 to form a set of two quadrature components, the components of one set of quadrature components being separately mixed in upper and lower balanced mixers 17, 18 with respective components of the other set and the mixer outputs being combined 19 either in-phase or out-of-phase so as to suppress the unwanted image sideband component generated by the two mixers which is in antiphase with respect to its other sideband. <IMAGE>

Description

INTERFERENCE SUPPRESSOR This invention relates to an arrangement for suppressing interference signals in a radar receiver.

Apart from the target returns a radar receiver can also receive clutter returns and jamming signals.

One way of reducing the level of interference is to obtain, by mixing a limited amplitude version of the received signals with the original signals, a signal which is a baseband version of the difference signal between the wanted and the unwanted signals. This pre-supposes that the wanted signals have an amplitude which at least exceeds that of the unwanted signals, if only by a small amount. This difference signal can then be up-converted to the original frequency band. However, in doing this a so-called "image" signal is created with equal power to the wanted signal but spread by twice the unwanted signal bandwidth.This image signal can be partially filtered out if the unwanted signal bandwidth is larger than the wanted signal bandwidth, but this is increasingly difficult as the unwanted signal bandwidth reduces, especially in the case of a C.W. interferer and a spread spectrum (wideband) information system.

According to the present invention there is provided an arrangement for suppressing an unwanted large amplitude interference signal in a receiver wherein the input signals, including the unwanted signal and a smaller amplitude wanted signal, are split to provide one output fed to a limiter which makes a phase estimate, the other output being adjusted in amplitude and phase if necessary and applied to a mixer where it is mixed with the output of the limiter, the mixer output being filtered to obtain a difference signal, and wherein the filtered signal is split to form quadrature components and the output of the limiter is also split to form quadrature components, the quadrature components being so intermixed and combined as to suppress an unwanted image sideband component of the wanted signal generated by the two mixers which is in antiphase with respect to its other sideband.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of an arrangement for suppressing interference signals in a radar receiver, and Fig. 2 illustrates the operation of the arrangement of Fig. 1.

In the arrangement of Fig. 1 the r.f. input to a radar receiver is split into two signals in an in-phase power splitter, 10. One signal is fed to a limiter circuit 11 which makes a phase estimate of the input wave. The other signal is fed to an amplitude and phase adjuster circuit 12 after which it is applied to a balanced mixer circuit 13, where it is mixed (as a local oscillator) with the output of the limiter circuit 11.

The resultant output spectrum is passed through a filter 14 to obtain only the difference (baseband) frequency between the two inputs to the mixer 13. The filtered resultant signal is fed to a 900 hybrid splitter 15 to form two components in phase quadrature. The limiter 0 circuit output is also fed to a 90 hybrid power splitter 16 to form quadrature components. The quadrature components of the limiter signal are then mixed with respective quadrature components of the filtered signal in upper and lower balanced mixers 17, 18. Finally the outputs of mixers 17, 18 are combined either in-phase or out-of-phase in combiner 19, so as to suppress the unwanted image sideband component generated by the two mixers, which unwanted sideband is in antiphase with the wanted sideband.

Reference to Fig. 3 illustrates the operation of the arrangement. in a swept frequency radar a powerful jammer signal J will usually have a slight frequency offset Af from a wanted target signal S, Fig. 3(a). When the combined input signal is fed to the limiter the effect is to concentrate on the larger jammer signal J only, Fig. 3(b). Mixing the limiter output with the other output of the in-phase power splitter results in a loss of the jammer signal 5 at its original frequency but creates well spaced resultants, one at baseband frequency d and one at 25, Fig. 3(c). The 25 signal is filtered out. Mixing of the quadrature component of the bf signal with the quadrature components of the limiter signal results in translating the baseband signal into sidebands, upper and lower respectively in-phase from one mixer, Fig. 3(d), and upper and lower respectively out-of-phase from the other mixer, Fig. 3(e). Combining these mixer outputs results in cancellation of the out-ofphase sidebands leaving the wanted low amplitude input component only, Fig. 3(f), which is the dominant signal.

The loss incurred by this component with respect to its amplitude at the input is approximately 0.5.

Claims

CLAIMS:

1. An arrangement for suppressing an unwanted large amplitude interference signal in a receiver wherein the input signals, including the unwanted signal and a smaller amplitude wanted signal, are split to provide one output fed to a limiter which makes a phase estimate, the other output being adjusted in amplitude and phase if necessary and applied to a mixer where it is mixed with the output of the limiter, the mixer output being filtered to obtain a difference signal, and wherein the filtered signal is split to form quadrature components and the output of the limiter is also split to form quadrature components, the quadrature components being so intermixed and combined as to suppress an unwanted image sideband component of the wanted signal generated by the two mixers which is in antiphase with respect to its other sideband.

2. An arrangement for suppressing interference signals in a radar receiver substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. An arrangement for suppressing an unwanted large amplitude interference signal in a receiver wherein the input signals, including the unwanted signal and a smaller amplitude wanted signal, are split to provide one output fed to a limiter the output of which is dominated by the unwanted signal, the other output being adjusted in amplitude and phase and applied to a mixer where it is mixed with the output of the limiter, the mixer output being filtered to obtain a difference signal, and wherein the filtered signal is split to form quadrature components and the output of the limiter is also split to form quadrature components, the quadrature components of the filtered signal being separately mixed with respective quadrature components of the limiter output, the resultant mixed quadrature components being combined to suppress an unwanted complementary sideband component of the wanted signal generated by the quadrature mixers which is in antiphase with respect to a wanted sideband component of the wanted signal.