GB1326831A - Method and apparatus for the transmission of binary information by means of two distinct frequencies - Google Patents

Abstract

1326831 Digital transmission; FSK modulators CHARBONNAGES DE FRANCE 24 June 1971 [9 July 1970] 29749/71 Heading H4P A fsk transmitter has a single oscillator switchable to two frequencies, each associated with separate information items from sources A, B, the output from which is passed to line through a filter which is switched synchronously to a corresponding frequency range; the oscillator is also blocked by a circuit operative, when incoming information is switched to the opposite state e.g. "1". The oscillator comprises a transistor T1 associated with a winding Trp 1 and capacitor C1 normally tuned to a higher frequency, and switched to a lower frequency by capacitors C2, C3 by a transistor T2. The output filter comprises a transformer Tr2 normally tuned to the lower frequency is switched to the higher frequency by capacitor C5 when a transistor T4 is conductive. Because coupling and amplifying transistor T5 causes a rectified parasitic current to pass through winding Trp 2, this is opposed by correcting winding Tra 2 on the same core. When the incoming signal or signals change state to a "1" this is detected by coincidence circuits CA, CB which render a transistor T3 conductive which blocks the oscillator. The arrangement may also be utilized to indicate the positions of mechanical elements by contact or magnetic sensors or other switching device, or indicate whether an electric motor is running or not, modifications to the arrangements being described with reference to Figs. 3 and 4 (not shown). Generator (G) Fig. 1 (not shown), provides a repetitive series of signals (s1, s2) which switch the oscillator and filter continuously between the two frequency values as long as the information sources A, B are emitting "O" signals.