Patent GB-1245890-A (GB)
Title: A continuous tone controlled squelch radio system
IPC: H04B1/16
Filed: 1969-09-30
Abstract: 1,245,890. Radio signalling. COMPUTER EQUIPMENT CORP. 30 Sept., 1969 [2 Oct., 1968], No. 48117/69. Heading H4L. In a tone-operated squelch radio system in which a sub-audible tone is transmitted simultaneously with voice signals to render the receiver operative, solid state tunable active bandpass filters are used in the encoder and decoder to facilitate retuning to different squelch frequencies. The arrangement may also be used in repeaters which are muted in the absence of a received signal. An arrangement operating as an encoder or decoder for transmission and reception respectively is described, Figs. 1A, 1B, 1C. Reception.-Part of the output of the discriminator of the radio receiver is supplied via terminals 5 to an active low-pass filter including transistors Q1, Q2 and amplifier Q3, which passes only sub-audible tone signals, to a limiter CR1, CR2 and the limited signal is fed via line 9 to the active bandpass filter 11. In the filter 11 transistors Q4 to Q7 form a high-gain operational amplifier and a null frequency selective network 13 is connected by lines 51 and 28 in the negative feedback loop between the collector of Q7 and the base of Q4. The output from Q7 is fed to the base of a normally cut-off transistor Q8 causing it to conduct on negative half-cycles and discharge a capacitor 14. The resulting voltage developed across resistor 16 causes normally cut-off transistor Q9 to conduct and apply a voltage via lead 15 to unmute the receiver. To enable the operator to listen for anything on the channel before transmitting, to avoid interference, a hook switch on the microphone applies a voltage to leads 17, 19 to cause a transistor Q10 to conduct causing transistor Q9 to conduct. Transmission.-When the transmitter is in operation a positive voltage is applied to lead 25 causing normally cut-off transistor Q11 to conduct and reverse bias a diode CR4. This causes CR5 to conduct and a positive feedback path is established from the collector of Q7 to the emitter of Q4 so that the circuit oscillates at the null frequency of the network 13. The base-emitter junction of Q8 in conjunction with a diode CR3 limits the amplitude of oscillation. To ensure a quick oscillation build-up, the negative-going pulse at the collector of Q11 when Q11 conducts is coupled via capacitor 29 and the circuit comprising resistor 31 and diode CR5A to the base of transistor Q4. The desired tone appears at lead 51 and is supplied via terminals 33, 35 to the transmitter. A diode CR6 conducts when Q11 is conducting and cuts-off transistor Q3 to block any signal from the discriminator. Frequency selective network 13. In one arrangement, Fig. 4 (not shown), the output on lead 51 is supplied via a twin T network tunable by a potentiometer, to the gate of a field effect transistor and the output 28 is taken from the source of the transistor. In a modification, Fig. 3 (not shown), this circuit is preceded by an impedance transforming amplifier and a plurality of tunable pre-set circuits are provided by switching one of a number of different variable resistors into circuit.