DIRECT MODULATION FM DATA RECEIVER

摘要

13 A direct modulation FM data receiver comprising a mixer (14) having a first input for receiving a directly modulated FM signal (fc ? .DELTA.f), where .DELTA.f is the deviation and fc the carrier, and a second input for a local oscillator (20) signal (fL) having a frequency within the signal channel but offset from the input signal carrier (fc) by an amount (.delta.f). Demodulating means (22 to 30) distinguish between the signalling tones (.DELTA.f + .delta.f) and (.DELTA.f - .delta. f) and derive a data output signal therefrom. The implementation of the demodulating means depends upon the demodulation strategy. However for a strategy based on comparing the energies in the two signalling tones then this can be done by connecting the output of a mixer (14) to two bandpass filters (22, 24) having centre frequencies .DELTA.f + .delta.f and .DELTA.f - .delta. f. The outputs therefrom are amplitude or envelope detected in detectors (26, 28) and applied to a comparator (30) from which the data output signal is derived. Such a direct modulation FM receiver may be fabricated as an integrated circuit and is suitable for use in a paging receiver.