FM digital transmission system - uses phase inversion to provide synchronisation without separate channel

摘要

Digital signals modulate a carrier with two frequencies corresponding to 0 and 1 bits. A phase reversal is superimposed on mobulated signal by a square wave, the phase change occurring at each transition between bits. After detection the received signal is applied to filters (12, 11) and the outputs applied to a comparator after further detection. The steep edge of the sawtooth corresponds to the bit transition. A full-wave rectifier (16) converts the waveform to one which is compared with a DC level. A signal falling below the level triggers a succeeding monostable circuit, until the signal returns to DC level. The leading edge of the monostable output monostable provides synchronising pulses.