GB2174575A

GB2174575A - Interference signal suppression in radio receivers

Info

Publication number: GB2174575A
Application number: GB08610031A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-04-29
Filing date: 1986-04-24
Publication date: 1986-11-05
Also published as: GB8610031D0; GB2174575B

Abstract

An interference suppressor which facilitates the reception of an SSB signal in the presence of an FM interfering signal comprises a single port mixer 4 or envelope detector and a limiter filter 14 through which a received SSB signal and interference signal are fed, an SSB generator 8 fed from the single port mixer via a d.c. blocking capacitor 6 and a dual port mixer 12 fed with a signal from the SSB generator and with a signal from the limiter filter thereby to provide a demodulated output signal in which the FM 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 use in receivers.

Interference suppressors for suppressing the interference of a strong unwanted signal on a weaker signal at the same nominal frequency are known. Such suppressors may employ parallel signal paths, one of which incorporates a tuned trap circuit which locks onto the stronger unwanted signal due to the capture effect. However, as the signal paths must be accurately matched for signal levels and time delay, the second path includes a delay circuit, the delay of which is designed to match accurately the propagation delay of the tuned trap circuit. This matching is very difficult to achieve in practice, leading to a degredation of the interference suppression achieved.

Accordingly, there is provided an interference suppressor for facilitating the reception, from a composite signal, of a single sideband signal in the presence of a relatively stronger frequency modulated interference signal, the interference suppressor comprising a signal envelope detector for receiving the composite signal, an FM demodulator for receiving the composite signal, and a mixer circuit, coupled to the envelope detector via a single sideband generator and to the FM demodulator, for providing an output signal in which the relatively stronger interference signal has been suppressed.

The signal envelope detector may comprise a single port mixer, and a direct current (DC) blocking capacitor may be provided between the signal envelope detector and the single sideband generator. The FM interference signal may comprise a frequency shift keying 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 according to the present invention; and Figure 2 illustrates a series of frequency spectrums useful in explaining the operation of the frequency suppressor shown in Figure 1.

Referring to Figure 1, an interference suppressor 2 comprises a signal envelope detector 4, such as a single port mixer, arranged to receive a composite input signal comprising a single sideband (SSB) signal and a relatively stronger FM interference signal. The envelope detector 4 is coupled via a DC blocking capacitor 6 to an SSB generator 8. The output of the SSB generator 8 is coupled to a first input terminal 10 of a mixer 12. An FM demodulator, such as a limiter filter circuit 14 is arranged to receive the composite input signal, the output terminal of the limiter filter circuit being coupled to a second input terminal 16 of the mixer 12.

The operation of the interference suppressor will now be explained with reference to a specific example, as shown in Figure 2. For the purposes of explanation it is assumed that the composite input signal comprises a wanted signal consisting of an upper sideband SSB speech signal having a suppressed carrier frequency of 500 KHz and an unwanted signal consisting of an unmodulated carrier on a frequency of 503KHz. The frequency spectrum, of such a composite signal is shown in Figure 2a. This composite signal is envelope detected by the envelope detector 4. The SSB signal beats with the unwanted carrier signal located on the opposite side of the spectrum from the suppressed carrier, thereby generating a baseband signal. The resultant spectrum at the output of the envelope detector 4 is an incorrect inverted speech signal, as shown in Figure 2b.A standing DC signal is also produced by the envelope detector 4 in view of the rectification of the unwanted interference signal.

This DC signal is removed by the blocking capacitor 6.

The inverted baseband signal is then translated to an RF frequency at 500 KHz, the carrier frequency of the wanted signal, by the SSB generator 8, as shown in Figure 2c. The original speech baseband signal, as shown in Figure 2d can be recovered from the RF spectrum shown in Figure 2c by the mixer 12. The RF spectrum is demodulated by using the 503 KHz carrier frequency of the unwanted signal as a local oscillator reference signal for the mixer 12. In the interference suppressor shown in Figure 1 this reference signal is derived by limiting and filtering the composite input signal 2a in the limiter filter circuit 14. Capture effect in the limiter filter circuit suppresses the weaker SSB signal leaving the unwanted signal frequency of 503 KHz.

It can be shown mathematically that the interference suppressor illustrated in Figure 1 cancels the frequency modulation on the unwanted signal.

Hence, if the unwanted carrier signal consists of an unwanted frequency shift laying (FSK) signal the SSB signal will still be demodulated correctly.

Although the present invention has been described with respect to a particular embodiment, it should be understood that modifications may be effected whilst remaining within the scope of the invention.

1. An interference suppressor for facilitating the reception, from a composite signal, of a single sideband signal in the presence of a relatively stronger frequency modulated interference signal, the interference suppressor comprising a signal envelope detector for receiving the composite signal, an FM demodulator for receiving the composite signal, and a mixer circuit, coupled to the envelope detector via a single sideband generator and to the FM demodulator, for providing an output signal in which the relatively stronger interference signal has been suppressed.

2. An interference suppressor according to claim 1 wherein the FM demodulator comprises a limiter filter circuit for providing a signal to the mixer circuit having a frequency dependent upon

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 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 use in receivers. Interference suppressors for suppressing the interference of a strong unwanted signal on a weaker signal at the same nominal frequency are known. Such suppressors may employ parallel signal paths, one of which incorporates a tuned trap circuit which locks onto the stronger unwanted signal due to the capture effect. However, as the signal paths must be accurately matched for signal levels and time delay, the second path includes a delay circuit, the delay of which is designed to match accurately the propagation delay of the tuned trap circuit. This matching is very difficult to achieve in practice, leading to a degredation of the interference suppression achieved. Accordingly, there is provided an interference suppressor for facilitating the reception, from a composite signal, of a single sideband signal in the presence of a relatively stronger frequency modulated interference signal, the interference suppressor comprising a signal envelope detector for receiving the composite signal, an FM demodulator for receiving the composite signal, and a mixer circuit, coupled to the envelope detector via a single sideband generator and to the FM demodulator, for providing an output signal in which the relatively stronger interference signal has been suppressed. The signal envelope detector may comprise a single port mixer, and a direct current (DC) blocking capacitor may be provided between the signal envelope detector and the single sideband generator. The FM interference signal may comprise a frequency shift keying 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 according to the present invention; and Figure 2 illustrates a series of frequency spectrums useful in explaining the operation of the frequency suppressor shown in Figure 1. Referring to Figure 1, an interference suppressor 2 comprises a signal envelope detector 4, such as a single port mixer, arranged to receive a composite input signal comprising a single sideband (SSB) signal and a relatively stronger FM interference signal. The envelope detector 4 is coupled via a DC blocking capacitor 6 to an SSB generator 8. The output of the SSB generator 8 is coupled to a first input terminal 10 of a mixer 12. An FM demodulator, such as a limiter filter circuit 14 is arranged to receive the composite input signal, the output terminal of the limiter filter circuit being coupled to a second input terminal 16 of the mixer 12. The operation of the interference suppressor will now be explained with reference to a specific example, as shown in Figure 2. For the purposes of explanation it is assumed that the composite input signal comprises a wanted signal consisting of an upper sideband SSB speech signal having a suppressed carrier frequency of 500 KHz and an unwanted signal consisting of an unmodulated carrier on a frequency of 503KHz. The frequency spectrum, of such a composite signal is shown in Figure 2a. This composite signal is envelope detected by the envelope detector 4. The SSB signal beats with the unwanted carrier signal located on the opposite side of the spectrum from the suppressed carrier, thereby generating a baseband signal. The resultant spectrum at the output of the envelope detector 4 is an incorrect inverted speech signal, as shown in Figure 2b.A standing DC signal is also produced by the envelope detector 4 in view of the rectification of the unwanted interference signal. This DC signal is removed by the blocking capacitor 6. The inverted baseband signal is then translated to an RF frequency at 500 KHz, the carrier frequency of the wanted signal, by the SSB generator 8, as shown in Figure 2c. The original speech baseband signal, as shown in Figure 2d can be recovered from the RF spectrum shown in Figure 2c by the mixer 12. The RF spectrum is demodulated by using the 503 KHz carrier frequency of the unwanted signal as a local oscillator reference signal for the mixer 12. In the interference suppressor shown in Figure 1 this reference signal is derived by limiting and filtering the composite input signal 2a in the limiter filter circuit 14. Capture effect in the limiter filter circuit suppresses the weaker SSB signal leaving the unwanted signal frequency of 503 KHz. It can be shown mathematically that the interference suppressor illustrated in Figure 1 cancels the frequency modulation on the unwanted signal. Hence, if the unwanted carrier signal consists of an unwanted frequency shift laying (FSK) signal the SSB signal will still be demodulated correctly. Although the present invention has been described with respect to a particular embodiment, it should be understood that modifications may be effected whilst remaining within the scope of the invention. CLAIMS

2. An interference suppressor according to claim 1 wherein the FM demodulator comprises a limiter filter circuit for providing a signal to the mixer circuit having a frequency dependent upon the frequency of the interference signal.

3. An interference suppressor according to claim 1 or claim 2 wherein the signal envelope detector comprises a single port mixer for providing a baseband signal from the composite signal.

4. An interference suppressor according to any one of claims 1 to 3 comprising a DC blocking capacitor coupled between the signal envelope detector and the single sideband generator.

5. An interference suppressor according to any one of the preceding claims wherein the unwanted frequency modulated interference signal comprises a frequency shift keying signal.

6. An interference suppressor substantially as hereinbefore described with reference to the ac companying drawings.