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FSK Demodulator employing a switched capacitor filter and period counters

  • US 4,517,519 A
  • Filed: 11/05/1981
  • Issued: 05/14/1985
  • Est. Priority Date: 11/07/1980
  • Status: Expired due to Term
First Claim
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1. In an FSK demodulation circuit including a zero-cross detection circuit to demodulate a received binary signal, said binary signal being produced by frequency shift keying techniques whereby marks and spaces are transmitted at different selected frequencies, the improvement comprising:

  • a signal period counter, said signal period counter including a plurality N of counter sections;

    a counter section selector circuit, said selector circuit selecting said counter sections in sequence for reading out a count at every detected zero-cross of said received binary signal, said selector circuit selecting said counter sections in sequence for resetting each of said counter sections, counting pulses from a clock and holding the count accumulated in each of said counter sections between said resetting and said reading out, said count being detected to determine whether said received frequency signal represents one of a mark and a space, said counting period for each of said counter sections equalling N zero-cross detection periods,whereby the signal-to-noise ratio of said demodulation circuit is enhanced by an increase in the number of accumulated counts by a factor of N, relative to a substantially unvarying noise input.

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