Center frequency error detecting circuit for FSK signal receiver

Abstract

In the system according to the prior invention, in order to shape the demodulated base-band signal of an FSK signal from a frequency discriminator 1 into a rectangular waveform, a waveform shaping circuit which may comprise a comparator 2 is typically provided. Any error is detected by finding a difference between the input and output signal of this comparator after the amplitudes thereof were made equal and smoothing its result. The reason why with this system the above-mentioned disadvantage occurs is that in the case of a bit synchronization signal, for example, at the point of time when its center level becomes equal to the shaping circuit output level a zero output is provided by the operation of the subtraction. In this invention, a correcting signal deriving circuit 6 is provided which generates such waveform portions of the base-band signal waveform that they protrude from the shaping circuit output waveform or a waveform corresponding to such protruding portions, said protruding waveform portions or corresponding waveform being superimposed on the output of an addition/subtraction circuit 11 to avoid the occurrence of the disabled point of error detection.

Dependencies are inferred from the claim text.