GB2076603A - Radio Receiver Squelch Circuit - Google Patents

Abstract

In a radio receiver which is continuously in an "ON" condition, in order to reduce power consumption a squelch circuit is employed to suppress any audio output in the absence of a received signal. The receiver has an IF amplifier 22 and discriminator 24 incorporating a resonant circuit normally tuned to the intermediate frequency. The output from the discriminator is proportional to differences between the frequency of the input signal and that of the tuned circuit and the resultant audio tone output on reception of a signal is used to switch the output stage 34 of the receiver ON. A tone filter 28 feeds a varactor diode whose capacitance is modulated by the output tone when an IF carrier is present. An audio output tone is produced by the quadrature detector 24 even if the IF carrier is unmodulated. Provided the gain and phase shift are correct through amplifier 26 and tone filter 28 the circuit becomes regenerative when an IF carrier is present and oscillation builds up at the tone filter frequency. The output from the tone filter is then used to unmute the output stage. A tone rejection filter 36 removes the tone from the audio output at 38, 40. <IMAGE>

Description

SPECIFICATION Radio Receivers This invention relates to radio receivers and to public address systems of the kind where the apparatus is sometimes required to be in an 'on' condition regardless of whether an audio output generated at any given instant.

The requirement of communications receivers such as are used by the services and by taxis that the receiver should be continuously in a condition to give an audio output whenever a signal is received presents the problem of continuous power consumption, often under conditions where power is not readily available, and hitherto a substantial background "noise" generated in the absence of a received signal has been suppressed by 'mute' or 'squelch' circuits. This background "noise" gives rise to the substantial risk that a radio operator will reduce the gain to such a low level that when an input signal is received, no audio signal is produced.

Such mute or squelch circuits are operative to keep the receiver at a quiescent level of operation until a signal is received and the receipt of this signal automatically triggers the receiver to give a preset audio output. Such circuits not only have the advantage of conserving power supplies but avoid the continuous generation of irritating background noise with the consequential risk referred to.

Previously proposed 'mute' or 'squelch' circuits have been unsatisfactory in practice from one viewpoint or another and have generally, although not always, required an additional control requirement which calls for understanding and sensitive operation by the radio operator.

In more detail, AM receivers have incorporated a 'squelch' circuit which detects the increase in signal at the output of the IF amplifier following the presence of a received carrier wave. This circuit has the disadvantage that variations in the RF or IF gain of the receiver affect the operating threshold and an increase in background noise may cause the receiver 'mute' circuit to be lifted in the absence of a signal. This kind of circuit is generally known as 'carrier squelch'.

For FM receivers a circuit has been proposed in which the presence of a signal causes a reduction in the noise level from the discriminator of the receiver. One disadvantage of this circuit is that changes in amplitude and spectral composition of the input noise can also cause a reduction in the noise output of the discriminator, which is then mistaken for the FM silencing effect of an incoming signal. This type of 'squelch' circuit is normally known by the term 'noise squelch'.

A third form of 'squelch circuit is used for selective calling systems in which'the presence of a low-level tone of the correct frequency in the audio signal causes the receiver 'mute' to lift or in other words become inoperative. This type of squelch circuit' is normally known as 'tone squelch' and may be used in conjunction with either carrier squelch or noise squelch.

The novel features of the present invention will be readily apparent from the following description, which is given by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a circuit diagram of an integrated circuit IF amplifier and quadrature discrimination of a radio receiver; Figure 2, is a fragmentary circuit diagram of a radio receiver embodying the invention; Figure 3, is a diagram illustrating the principle of the invention; and Figure 4 is a block diagram of the radio receiver of Figure 1.

Squelch circuits in accordance with the invention are particularly suitable for use with low-consumption integrated circuit IF amplifiers and discriminators incorporated in communications ratio receivers and paging systems.

Referring now particularly to Figure 1, an integrated circuit IF amplifier has incorporated therein an external resonant circuit normally tuned to the IF. The discriminator circuit output is proportional to the difference between the frequency of the input signal and that of the tuned circuit L1, C1.

Figure 2 shows an embodiment of a squelch circuit incorporating the integrated circuit of Figure 1 but the fixed capacitor C1 in the external tuned circuit is replaced by a varactor diode CA biased so that the circuit is resonant at the IF.

Components L2, L3, C2, C3, couple the output of the tone filter F across the varactor diode without affecting the operation of the quadrature detector incorporated in the integrated circuit as shown in Figure 1.

The discriminator output is proportional to the difference between the input frequency and that of the tuned circuit. If the varactor diode capacitance is modulated by an output tone from the filter when an IF carrier is present, an audio output tone is produced by the quadrature detector even if the IF carrier is unmodulated.

Provided that the gain and phase shift are correct through the amplifier A and tone filter F, the circuit becomes regenerative when an IF carrier is present at the input and oscillation builds up at the tone filter frequency. The tone frequency may be below, within, or above, the audio frequency range of the receiver. The final magnitude of the oscillation thus generated is dependent upon circuit non-linearity, but is high enough to be readily detected and provide a precise indication of the presence of an IF carrier.

In general terms, the squelch circuit in accordance with the invention makes use of a fundamental property of a limiting IF amplifier and quadrature detector as illustrated in Figure 1. For a high signal to noise ratio the output voltage is related to the input frequency by the well-known 'S' curve as illustrated in solid lines in Figure 3.

At low signal to noise ratios the slope of the characteristic diminishes (see broken line) and the output noise increases. It is this change in the slope of the V/F (voltage/frequency) characteristic with change of input signal to noise ratio which is used to operate the receiver muting at a given signal to noise ratio.

In order to determine a measure of the slope of the V/F characteristic another parameter needs to be varied. In the preferred embodiment illustrated in Figure 2 this further parameter is the tuned circuit frequency, but it could also be the local oscillator frequency of the receiver. By frequency modulating the local oscillator frequency an audio output would be produced only in the presence of a received signal.

Essentially, therefore, the invention makes use of variations in a third parameter to assess the slope of the U/F characteristic as shown in Figure 3, which in turn is related to the signal to noise ratio of the received signal.

It follows that receivers in accordance with the present invention have the advantage over previously proposed receivers in that muting is not dependent upon the constancy of gain or noise characteristics in the RF and IF sections of the receiver. It is, therefore, possible to dispense with any manual control for the squelch threshold thereby lowering the cost of the receiver and eliminating the need for adjustment by an unskilled operator which is not always correct.

Low power consumption circuitry of the integrated kind may be employed since only a small degree of amplification is required at audio frequency.

Finally interferring signals which produce an IF outside the centre portion of the V/F characteristic cannot operate the squelch circuit regardless of the intensity of such interfering signals. This contrasts with both carrier and noise squelch circuits either of which can be triggered by interfering signals.

The radio receiver shown in Figure 4 includes an antenna 10 for receiving a radio signal. The control of the antenna is amplified by a radio frequency amplifier 12 and mixed in a mixer 14 with the output signal from a local oscillator 14.

The output from the mixer 14 is fed through an intermediate frequency band-pass filter 1 8 to a limiting intermediate frequency amplifier 22 and a discriminator 24 (see also Figure 1). The output from the discriminator 24 is fed to an amplifier 26 and a tone band-pass filter 28 (see also Figure 2).

A tone detector 30 detects a tone signal output from the filter 28. A trigger circuit 32 is tripped when a tone is detected and this in turn actuates a voltage-controlled audio switch 34 to feed the output of the discriminator 24 through a tone rejection filter 36 to an audio power amplifier 38 driving a loud speaker 40.

Claims (12)

Claims (Filed on 15 May 1981)

1. A squelch circuit for a radio receiver having an intermediate stage and an output stage, the circuit comprising means for producing a control signal related to the slope of the output voltage versus frequency characteristic of the signal appearing at the output of the intermediate stage, and means responsive to the control signal to inhibit the output stage from responding to the signal appearing at the output of the intermediate stage while the control signal lies within a predetermined range.

2. A circuit according to claim 1, wherein the control signal producing means comprises a tone filter for passing any noise or other component i the demodulated output of the intermediate stage having a predetermined tone frequency and a feedback loop for increasing the level of the tone frequency component in the output of the intermediate stage.

3. A circuit according to claim 2, wherein the intermediate stage is arranged to receive a carrier and the feedback loop is arranged to modulate the carrier with a signal of said predetermined tone frequency.

4. A circuit according to claim 2 or to claim 3, wherein the intermediate stage includes a demodulator with a resonant circuit and the feedback loop is arranged to vary the resonant frequency of the resonant circuit.

5. A radio receiver comprising an intermediate stage having an IF amplifier and a demodulator for receiving and demodulating a carrier whn present; an output stage; switch means coupling the output of the intermediate stage to the input of the output stage; and a squelch circuit having a tone filter for filtering out a tone signal of predetermined frequency from the output of the demodulator of the intermediate stage and operative in response to the presence of the filtered tone signal to cause the switch means to connect the output of the intermediate stage to the input stage, the squelch circuit incorporating a feed-back circuit to feed the filtered tone signal back to a part of the receiver to modulate the carrier and thereby superimpose on the carrier the tone signal.

6. A radio receiver, comprising: an intermediate stage arranged to receive and demodulate a carrier when present and having a limiting intermediate frequency amplifier and a quadrature detector incorporating a variable tuned circuit normally resonant at the intermediate frequency; an output stage; switch means coupling the output of the intermediate stage to the input of the output stage; a squelch circuit having a tone filter connected to the output of the intermediate stage to produce a tone signal from any tone frequency component in the detected modulation and being arranged to feed the tone signal to vary the resonant frequency of the tuned resonant circuit to cause the quadrature detector and produce an output having an amplified'tone signal component; and control means for controlling the switch means to effect said coupling when the level of the output from the tone filter exceeds a predetermined threshold level.

7. A receiver according to claim 5, wherein the tuned circuit comprises an inductor and capacitor connected in parallel.

8. A receiver according to claim 7, wherein the tuned circuit includes a varacter diode, and the squelch circuit includes capacitance and inductance means for coupling the output of the tone filter to the varacter diode to vary the capacitance thereof, and thus the resonant frequency of the tuned circuit, without affecting the operating characteristic of the quadrature detector.

9. A receiver according to any one of claims 5 to 8, wherein the switch means comprises a voltage-controlled switch and the means responsive to the output of the tone filter comprises a tone detector and a trigger circuit set at said predetermined threshold.

10. A receiver substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

11. A receiver substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

12. A receiver substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.