GB1331111A - Electrical circuits - Google Patents

Abstract

1331111 Digital transmission; FSK systems; phase locking and detecting; detecting whether signal is within a given amplitude POST OFFICE 22 April 1971 [29 April 1970] 20534/70 Headings H3A H3T and H4P A circuit for comparing two input trains of pulses comprises responsive to the trains a bistable arrangement giving a mean output signal representative of the relative times of rest in each state. As applied to a F.S.K. system wherein marks and spaces are representative by plus and minus frequencies relative to a carrier, the mean signal causes adjustment of an oscillator automatic tuning circuit (VFO, Fig. 1, not shown) to the mid point between the mark and space received frequencies via a signal assessor (1) which comprises a pair of differentiating networks. The bi-stable circuit comprises transistors VT13, VT14 with separate inputs from mark and space trigger circuits (Fig. 3, not shown) connected to the bases. The bi-stable circuit output is connected to a circuit comprising a diode D7, capacitor C19 and resistor R38 producing an output representative of the bi-stable mark-space ratio, which is examined by detector 4. The upper and lower threshold detector comprises an emitter follower VT8 connected to a Schmitt trigger circuit including transistors VT9, VT10 with a potentiometer RV1 setting the trigger level. Separate outputs, connected through input circuits comprising Zener diodes D5, D6 and C.R. networks to series transistors VT11, V12 form an AND network giving the following effects:- D.C. level below lower threshold; VT11 ON, VT12 OFF, hence output equals "1" D.C. level between lower and upper thresholds; VT 11 ON, VT12 ON, hence output equals "0"; D.C. level above upper threshold; VT11 OFF, VT12 ON, hence output equals "1". The 1 and 0 signals start and stop the motordriven tuner to restore equality to the signals emitting from the demodulator (8). It is stated that a similar arrangement may be employed for phase difference indication between two waveforms of the same frequency.