GB2132432A

GB2132432A - Radio receiver arrangements

Info

Publication number: GB2132432A
Application number: GB08331967A
Authority: GB
Inventors: Andrew Dexter Barrett
Original assignee: Mcmichael Ltd
Current assignee: Mcmichael Ltd
Priority date: 1982-12-17
Filing date: 1983-11-30
Publication date: 1984-07-04
Also published as: GB8331967D0; GB2132432B

Abstract

A communications receiver arrangement for receiving both FM and AM signals, in which a muting or squelch circuit 12 is arranged to operate in dependence upon both FM "noise quieting" and A.G.C. signal level, obtained via 14, 15 and 6, 7 respectively, so that the muting is not disturbed by noise spikes or by noise levels higher than normal, to which an A.G.C. circuit would respond but an FM discriminator would not. <IMAGE>

Description

SPECIFICATION Radio receiver arrangements In radio communication systems in which carrier signals at any particular channel frequency are transmitted and received only intermittently as required for the traffic on that channel, it is common practice to mute the audio output of the receivers using that channel in the absence of a received carrier signal of above a predetermined threshold value at the respective channel frequency.

According to one aspect of the present invention in a radio receiver arrangement including means for demodulating both amplitude-modulated and frequency-modulated incoming signals and means for muting the audio frequency output of said arangement in dependence upon the amplitude level of an incoming carrier signal, the operation of said muting means is arranged to be dependent also upon an output signal from the means for demodulating frequency-modulated incoming signals.

According to another aspect of the present invention in a radio receiver arrangement including a variable-gain intermediåte-frequency amplifier, means to demodulate frequencymodulated incoming signals, and means to derive a gain-control signal for said variable-gain amplifier in dependence upon the amplitude level of signals passed by said amplifier, there are provided means to mute the audio-frequency output of said receiver arrangement in dependence upon the level of said gain control signed and upon the level of a signal derived from said means to demodulate frequency-modulated incoming signals.

According to another aspect of the present invention in a radio receiver arrangement including a variable-gain intermediate frequency amplifier, automatic gain control means to provide a gain-control signal for said intermediate frequency amplifier, and frequency discriminator means to demodulate frequency-modulated incoming signals passed by said amplifier, there are provided means to mute the audio-frequency output of said receiver arrangement in dependence upon the level of said gain-control signal and upon the level of a signal derived from an output of said frequency discriminator.

Preferably the level of the signal derived from an output of said frequency discriminator is dependent upon the level of noise signal components in a band of frequencies above the band allotted to the audio-frequency output of said receiver arrangement. The arrangement may be such that said audio-frequency output is muted if the gain-control signal is below a first predetermined threshold, if the signal derived from an output of said frequency discriminator is above a second predetermined threshold, or both.

A radio receiver arrangement in accordance with the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 shows part of the receiver arrangement schematically, and Figures 2 and 3 show signal level graphs illustrating the operation of the arrangement of Figure 1.

Referring to Figure 1, the receiver arrangement, which may for example form part of a mobile transmitter-receiver equipment for a VHF communications system, utilises signals from a frequency synthesizer local oscillation source 1 to down-change VHF signals from a receiver input stage 2 initially to a first intermediate frequency of 21.4 MHz at a mixer stage 3 and subsequently to a second intermediate frequency of 9.0 MHz at a mixer stage 4. These intermediate frequency signals are amplified by respective variable gain amplifiers 5 and 6, and an output from the amplifier 6 is rectified in a circuit 7 to provide an AGC signal to control the gain of the amplifiers 5 and 6.

A frequency discriminator or demodulator 8 is connected directly to the output of the mixer 4, while an amplitude-modulation demodulator, or detector, 9 is connected to the output of the amplifier 6, the audio frequency output signals from these demodulators being applied by way of a mode selector switching circuit 10 to a common audio output path 11.

A muting circuit, or squelch circuit, 12 is provided to silence the receiver audio output if the incoming or received VHF signal is below a predetermined level. This muting circuit may achieve the desired silencing by, for example, disabling the mode select switches by which audio frequency signals from the demodulators 8 and 9 are applied to the audio output path 11.

The predetermined received signal level, or threshold, may require to be adjustable, either when setting up a receiver or as an operator control, between limits of, say, 0.8 y Volts and 8 Volts at the aerial. In order to provide control over this range the present muting circuit derives its control signal both directly from the AGC line 1 3 and, by way of an active filter circuit 14 and a detector circuit 1 5, from the output of the discriminator 8. The filter circuit 14 may be tuned to pass noise signal components in a narrow band of frequencies above the band allotted in the receiver arrangement to the audio output signal, this narrow band being centred for example on 12KHz.

As shown in Figure 2, a typical discriminator 8, in the absence of a received carrier signal or for received carrier signals of below say 0.1 u Volt will produce at its output a considerable level of random noise some components of which will be passed by the filter t4 to the detector 1 5. As the received carrier signal increases in level to, say 2 u Volts or more the level of noise produced falls sharply, and a threshold circuit 1 6 may readily be set to respond to the drop in the noise signal level when the discriminator 8 receives a coherent input signal of above a predetermined threshold value 17 (Figure 2) within this range.Thus the transistor 1 8 in the circuit 1 6 may be arranged to be substantially conducting for detected noise signal levels higher than that at the threshold 17, and to be substantially non-conducting for lower detected noise signal levels.

As shown in Figure 3 the gain control signal or AGC signal developed on the line 13 tends to remain at a constant low value for no carrier or for received carrier signals of less than, say, 0.8 4 Volts, whereas received carrier signals of greater value give rise to a progressively increasing AGC voltage. If muting is made dependent upon the AGC voltage alone there is a tendency for the audio output to be unclamped in response to noise "spikes" or to noise levels higher than normal, to which the AGC circuit 7 responds but to which the discriminator 8 is insensitive.

In the present arrangement the muting of the audio output is arranged to cease when the voltage at the inverting input of an amplifier 1 9 in the muting circuit 1 2 becomes more positive than a reference voltage at the non-inverting input of that amplifier. For this to occur the discriminator 8 must receive a coherent signal higher than the threshold 1 7 to render the transistor 1 8 substantially non-conducting, so that the voltage on the AGC line 13 can be applied to the inverting input of the amplifier 19. In this way the actual threshold carrier signal level above which muting is dependent on the AGC level, and at the same time is dependent on the "quieting" of the output of the discriminator 8.

Claims

1. A radio receiver arrangement including means for demodulating both amplitudemodulated and frequency-modulated incoming signals and means for muting the audio frequency output of said arrangement in dependence upon the amplitude level of an incoming carrier signal, wherein the operation of said muting means is arranged to be dependent also upon an output signal from the means for demodulating frequency-modulated incoming signals.

2. A radio receiver arrangement including a variable-gain intermediate-frequency amplifier, means to demodulate frequency-modulated incoming signals, and means to derive a gaincontrol signal for said variable-gain amplifier in dependence upon the amplitude level of signals passed by said amplifier wherein there are provided means to mute the audio-frequency output of said receiver arrangement in dependence upon the level of said gain control signed and upon the level of a signal derived from said means to demodulate frequency-modulated incoming signals.

3. A radio frequency arrangement including a variable-gain intermediate frequency amplifier, atuomatic gain control means to provide a gaincontrol signal for said intermediate frequency amplifier, and frequency discriminator means to demodulate frequency-modulated incoming signals passed by said amplifier, wherein there are provided means to mute the audio-frequency output of said receiver arrangement in dependence upon the level of said gain-control signal and upon the level of a signal derived from an output of said frequency discriminator.

4. A radio receiver arrangement in accordance with Claim 3 wherein the level of the signal derived from an output of said frequency discriminator is dependent upon the level of noise signal components in a band of frequencies above the band allotted to the audio-frequency output of said receiver arrangement.

5. A radio receiver arrangement in accordance with Claim 3 or Claim 4 wherein said audiofrequency output is muted if the gain-control signal is below a first predetermined threshold, if the signal derived from an output of said frequency discriminator is above a second predetermined threshold, or both.

6. A radio receiver arrangement substantially as hereinbefore described with reference to the accompanying drawings.