GB644106A - Improvements in or relating to two-way radio communication systems - Google Patents

Abstract

644,106. Duplex radio signalling; keying carrier frequencies; radio receiving circuits. STANDARD TELEPHONES & CABLES, Ltd. Feb. 6, 1948, No. 3545. Convention date, March 1, 1947. [Class 40 (v)] In a two-way frequency-modulated communication system between a fixed and a mobile station, each station has separate transmitting and receiving circuits, and the mobile unit only has a separate keying oscillator for rendering its transmitter and receiver alternately operative. The fixed receiver is muted whenever the mobile transmitter is inoperative. The mobile unit, Fig. 3, which consists of a frequency-modulated transmitter 19 operating on frequency f 2 and a frequencymodulation receiver 8 operating on frequency f 1 has the keying oscillator 25 operating on a frequency of 8000 c.p.s. The oscillator output is applied to the third frequency multiplier 45, the final drive 46, and the power amplifier 47 of the transmitter. A tap on the grid resistor 55 of the fourth multiplier stage 56 provides a keying voltage of opposite phase which controls the receiver by being applied to the first mixer valve 58, the first I.F. valve 59, and the second limiter 60. A diode 65 prevents the bias developed by the limiter from appearing at the other keyed stages. A diode 67 prevents the keyed grids from going positive. The receiver is coupled to the common aerial by an isolating stage 10 to prevent the absorption of appreciable power from the transmitter. At the fixed station, the transmitter, operating on frequency f 1 , is independent of the receiver and of conventional design. The receiver, Fig. 4, is muted not only during those periods when the mobile transmitter is keyed off, but also when the mobile transmitter is not operating. The grid circuit of the first limiter valve 74 includes a resistor 75 and condenser 76 having a time constant such that the bias voltage developed by grid rectification during that portion of the cycle when a signal is being received is maintained during the period when no signal is received, thus reducing the gain and eliminating noise in the output. The valve is rendered conducting by the positive loops of the waves applied during the receive period. A circuit 88, 89, tuned to the keying frequency, is connected to the output of the first limiter 74 so that when a correctly keyed signal is received, a bias voltage, rectified by a diode 91, is applied to a thyratron oscillator 85 to prevent oscillation and allow relay 99 to complete the audio output circuit. In the absence of a correct signal, not only does the relay 99 open, but since the output of the oscillator is also coupled to a diode 100, a bias voltage is developed across resistor 101 which cuts off the audio output valve 83.