RECEIVER HAVING AN ADJUSTABLE SYMBOL SLICE DEMODULATOR

摘要

An FSK zero IF receiver has means (DEM) for producing representations of demodulated modulating signals as a series of narrow pulses having a repetition rate greater than the symbol rate of the modulating signals and an adjustable symbol slice demodulator (54 to 66) for determining the data. The demodulator has means for adjusting a symbol slice threshold value in dependence of the pulse like representations of the deviation frequencies which may be different as a result of drifting between the local oscillator and nominal transmitter carrier frequencies (fL and fC). The means for adjusting the symbol slice threshold comprises means for measuring the peak excursions of the instantaneous representations of the frequency, the difference between the peak excursions being proportional to the transmitted deviations, means for calculating the mean deviation of the peak excursions, which mean deviation is proportional to the offset between the transmitted centre frequency and the local oscillator frequency, and means for setting the symbol slice level to the mean deviation in the case of a 2 level signal or to a plurality of symbol slice levels in the case of a M-ary signal.