MEANS FOR RAPIDLY SWITCHING TUNED CIRCUITS OF VARIABLE-FREQUENCY OSCILLATORS FROM ONE STATE TO ANOTHER

摘要

1,237,204. Frequency modulation. FERRANTI Ltd. 23 Jan., 1970 [1 Feb., 1969], No. 5558/69. Heading H3R. A variable frequency oscillator comprises: a tuned circuit LC which includes a switch T3 ... T6 having a first state in which the tuned Circuit is completed, and a second state in which the tuned circuit is interrupted; and a switch control circuit operable in response to a control signal at X either to maintain the switch in one state, or to cause it to alternate between the two states at a frequency considerably greater than the resonant frequency of the tuned circuit. The oscillator is used for transmitting FSK telegraphy signals, the signal at X being a two-state signal corresponding to "0" or "1". In one state the signal X opens a diode gate D3, D5, D6 to connect a multivibrator M to switch switches T3 ... T6 between their two states, thus alternately connecting and disconnecting capacitor C and inductor L to form a resonant circuit. In the other state of signal X the diode gate is closed and transistors T5, T6 of the switch remain conductive to close the LC circuit. In the state in which the switch is alternately opened and closed, the frequency of oscillation depends on the mark/space ratio of the multivibrator signal. Fig. 3 (not shown) discloses an alternative arrangement of the switches to permit one side of capacitor C to be connected to earth.