AUTOMATIC FREQUENCY CONTROL DEVICE IN COHERENT DEMODULATORS FOR M-QAM MODULATION SYSTEMS

摘要

Automatic frequency control device in a coherent demodulator for an M-QAM modulation systems comprising a phase locked loop and a detector of phase lock-up or release condition, which measures the oscillation frequency of the voltage controlled oscillator (VCO) and compares it with a value memorized upon the creation of a loss of lock-up condition. The comparison value serves to control direction of counting of an up-down counter (CNT2) for a period of time fixed after beginning of the loss of lock-up. The counter feeds a digital/analog converter (DAC) which supplies a control signal to the voltage controlled oscillator.