GB2174574A - Interference signal suppression in radio receivers - Google Patents

Abstract

An RF interference suppressor which facilitates the reception of a weak wanted AM or FM signal in the presence of a stronger RF interference signal comprises - a tracking oscillator 4, the tracking oscillator 4 being fed with a received signal comprising the wanted signal plus the interference signal and being arranged to track the stronger interfering signal, a phase quadrature demodulator comprising including a splitter 6 and a pair of mixers 8, 10 fed from the tracking oscillator with phase quadrature related signals corresponding to the interference, and with the received signals thereby to produce a pair of baseband signals. The baseband signals are fed to a pair of further mixers 20, 22 which are also fed in phase quadrature from the tracking oscillator 4 thereby to provide a pair of RF signals. The RF signals are combined to afford an output signal corresponding to the wanted signal in which the interference is suppressed. <IMAGE>

Description

SPECIFICATION Improvements in or relating to receivers The present invention relates to improvements in or relating to receivers and in particular to interference suppressors for facilitating the reception of a wanted signal in the presence of a relatively stronger unwanted interference signal.

Accordingly, there is provided an interference suppressor for facilitating the reception of a wanted signal from a received composite signal comprising the wanted signal and at least one relatively stronger interference signal, the interference suppressor comprising a tracking oscillator arranged to receive and track the strongest interference signal, a phase quadrature demodulator comprising a pair of mixers arranged to receive the composite signal and phase quadrature components of the strongest interference signal, thereby to produce a pair of baseband signals, a pair of further mixers arranged to receive the phase quadrature components of the strongest interference signal and the baseband signals, and a combiner for combining the output signals of the further mixers for providing an output signal in which the strongest interference signal has been suppressed.

The interference suppressor may be provided with a hard limiter circuit arranged to receive and hard limit the received composite signal afforded to the tracking oscillator and the mixers of the phase quadrature demodulator for facilitating the reception of a wanted amplitude modulated signal or a wanted frequency modulated signal from a received composite signal comprising the wanted amplitude or frequency modulated signal and at least one relatively stronger amplitude modulated unwanted interference signal.

The interference suppressor may comprise a cascade of serially coupled interference suppressors, the first interference suppressor of the cascade being arranged to receive the composite signal comprising of the wanted signal and a plurality of relatively stronger interference signals, and the subsequent interference suppressors in the cascade being arranged to receive, as a composite input signal, the output signal of the immediately preceding interference suppressor in the cascade.

Each interference in the cascade may comprise a tracking oscillator arranged to receive the composite input signal and to track the strongest interference signal therein, a phase quadrature demodulator comprising a pair of mixers arranged to receive the composite input signal and phase quadrature components of the strongest interference signal thereby to produce a pair of baseband signals, a pair of further mixers arranged to receive the phase quadrature components of the strongest interference signals and the baseband signals produced by the mixers of the phase quadrature demodulators, and a combiner for combining the output signals of the further mixers for providing an output signal in which the strongest interference signal has been suppressed.

The cascade thereby facilitates the reception of a wanted signal from a composite signal comprising the wanted signal and a plurality of interference signals, each interference signal being relatively stronger than the wanted signal.

The present invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates a schematic block diagram of an interference suppressor in accordance with the present invention; and, Figure 2 illustrates a schematic block diagram of a hard limiter circuit which may be coupled to the input of the interference suppressor shown in Figure 1 to facilitate the reception of a wanted frequency modulated signal or amplitude modulated signal in the presence of a relatively stronger unwanted amplitude modulated signal.

Referring to Figure 1, an interference suppressor 2 comprises a tracking oscillator 4, which may comprise a phase lock loop circuit, arranged to receive a composite input signal which comprises a wanted input signal, which may be modulated using any mode of modulation, together with one or more relatively stronger frequency modulated signals on the same channel. The output of the tracking oscillator 4 is coupled to a phase quadrature demodulator which comprises a 900 splitter 6 and a pair of mixers 8 and 10. The mixers 8 and 10 are also coupled to a delay circuit 12 which is arranged to receive the composite input signal. The delay circuit 12 is designed to provide a delay period which is substantially equal to the propagation delay of the tracking oscillator 4.The outputs of the mixers 8 and 10 are coupled via AC coupling capacitors 12, 14 and band pass filters 16, 18 to the input of a pair of further mixers 20 and 22. The mixers 20 and 22 are also coupled to the 90 splitter 6 of the phase quadrature demodulator. A combiner 24, coupled to the outputs of the further mixers 20, 22, is provided for producing an output signal from the interference suppressor 2.

In operation, the tracking oscillator 4 locks onto and tracks the strongest signal present in the composite input signal and generates the local oscillator clock signal for the interference suppressor 2, which is afforded to the 900 splitter 6. Phase quadrature components of the clock signal are generated by the 900 splitter 6 which are fed to the mixers 8 and 10. The composite input signal is also fed directly to the mixers 8 and 10 via the delay circuit 12. As previously stated the delay circuit 12 is arranged to provide a signal delay which compensates for the propagation delay of the tracking oscillator 4.

The output signals from the mixers 8 and 10 consist of the instantaneous difference frequency between the local oscillator signal generated by the tracking oscillator 4 and the composite input signal. Since the tracking oscillator 4 is locked onto and tracks the strongest interference signal, the local oscillator signal is at the same frequency as the strongest interference signal. Hence, the instantaneous difference fequency between the local oscillator signal and the input signal appearing at the output of the mixers 8 and 10 is zero, or a DC level. The strongest interference signal is, therefore, converted to a DC level which is removed by the AC coupling capacitors 12 and 14.The output signals from the capacitors 12 and 14 comprise a baseband signal, corresponding to the modulation on the wanted signals together with any high frequency signals which may be generated by the mixers 8 and 10. These unwanted high frequency signals are removed by the band pass filters 16 and 18. The baseband signals are received at inputs 26, 28 of the further mixers 20, 22. The mixers 20, 22 also receive the phase quadrature components of the clock signal from the 90 splitter 6.

Hence, the baseband signals are converted back to the same RF frequency as the weaker components of the composite received input signal by the mixers 20 and 22. The RF frequency output signals of the mixers 20 and 22 are combined in the combiner 24. Because the output signals from the mixers 20 and 22 are in phase quadrature, when these are added in the combiner 24, one sideband will cancel and the other sideband will be cumulative. Hence, the spectrum of output signal from the combiner 24 corresponds to the received composite signal except that the strongest interference signal has been suppressed.

As the output signal spectrum is the same as the input signal spectrum except for the suppression of the strongest signal the input and output signal frequencies are the same. It can be seen therefore that two or more interference suppressors as described above can be serially connected to form a cascade whereby the strongest signal is suppressed by the first stage, the next strongest signal by the second stage; and so on until the signal remaining is the weakest signal in the composite input signal.

The interference suppressor 2 may also be used to facilitate the reception of a wanted FM or AM signal 30 in the presence of a stronger unwanted AM signal 32 by the provision of a hard limiter 34 as shown in Figure 2. The hard limiter 34 acts to produce at its output a substantially square wave signal at the frequency of the stronger unwanted AM signal 32. This output signal from the hard limiter 34 is phase modulated by the weaker wanted signal 30. Hence, the signals received by the mixers 8 and 10 via the delay circuit 12 will be phase shifted with respect to the signals received via the 90 splitter 6. This phase shifting of one input signal causes the mixers 8 and 10 to act as phase sensitive detectors, thereby producing a pair of baseband signals at the output of the mixers 8 and 10. These baseband signals are processed by the further mixers 20, 22 and the combiner 34 in the manner as previously described.

It can be seen, therefore, that the present invention enables an efficient and relatively simple interference suppressor to be achieved which permits the suppression of a wanted FM or AM signal in the presence of an unwanted relatively stronger FM or AM signal.

Although the present invention has been described with respect to a particular embodiment it should be realised that modifications may be effected within the scope of the invention. For example, the interference suppressor may be fabricated as an applique unit or fitted as an internal module to existing radio receivers.

Claims (8)

1. An interference suppressor for facilitating the reception of a wanted signal from a received composite signal comprising the wanted signal and at least one relatively stronger interference signal, the interference suppressor comprising a tracking oscillator arranged to receive and track the strongest interference signal, a phase quadrature demodulator comprising a pair of mixers arranged to receive the composite signal and phase quadrature components of the strongest inteference signal, thereby to produce a pair of baseband signals, a pair of further mixers arranged to receive the phase quadrature components of the strongest inteference signal and the baseband signals, and a combiner for combining the output signals of the further mixers for providing an output signal in which the strongest interference signal has been suppressed.

2. An inteference suppressor according to claim 1 comprising a band pass filter coupled between the mixers of the phase quadrature demodulator and the further mixers.

3. An inteference suppressor according to claim 1 or claim 2 comprising AC coupling means between the mixers of the phase quadrature demodulator and the further mixers.

4. An inteference suppressor according to any one of claims 1 to 3 comprising delay means for delaying the received composite signal afforded to the mixers of the phase quadrature demodulator.

5. An interference suppressor according to any one of the preceding claims comprising a hard limiter circuit arranged to receive and hard limit the received composite signal afforded to the tracking oscillator and the mixers of the phase quadrature demodulator for facilitating the reception of a wanted amplitude modulated signal or a wanted frequency modulated signal from a received composite signal comprising the wanted amplitude or frequency modulated signal and at least one relatively stronger amplitude modulated unwanted interference signal.

6. An interference suppressor according to any one of the preceding claims wherein the tracking oscillator comprises a phase lock loop oscillator.

7. An inteference suppressor comprising a cascade of serially coupled interference suppressors according to any one of the preceding claims, each interference suppressor in the cascade being arranged to suppress the strongest unwanted interference signal in the input signal afforded thereto thereby to facilitate, from the final interference suppressor in the cascade, the reception of a wanted signal from a composite signal comprising the wanted signal and a plurality of interference signals, each interference signal being relatively stronger than the wanted signal.

8. An inteference suppressor substantially as hereinbefore described with reference to the accompanying drawings.