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Offset QPSK demodulator and receiver

  • US 4,501,002 A
  • Filed: 02/28/1983
  • Issued: 02/19/1985
  • Est. Priority Date: 02/28/1983
  • Status: Expired due to Term
First Claim
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1. An offset QPSK receiver for receiving an offset QPSK signal including data at a predetermined data rate modulated on a suppressed carrier wherein the offset QPSK signal is approximately the sum of a transmitted inphase data stream multiplied by a first suppressed modulating singal at approximately the frequency of the suppressed carrier plus a transmitted quadraphase data stream mutiplied by a second suppressed modulating signal at approximately the frequency of the suppressed carrier and approximately in phase quadrature with said first suppressed modulating signal, the transmitted inphase data stream having a substantial proportion of data transitions offset from the data transitions in the transmitted quadraphase data stream, said offset QPSK receiver comprising, in combination,(a) means for effectively multiplying the offset QPSK signal by a locally-generated first modulating signal at approximately the frequency of the suppressed carrier and a locally-generated second modulating signal at approximately the frequency of the suppressed carrier and approximately in phase quadrature with said locally-generated first modulating signal, the phase of said locally-generated modulating signals relative to the phase of the suppressed modulating signals being responsive to a carrier phase control signal, to thereby generate respective inphase and quadraphase signals from said offset QPSK signal responsive to said carrier phase control signal,(b) respective means for generating hard limited inphase and quadraphase signals from said inphase and quadraphase signals,(c) a data clock for generating a periodic data clocking signal in response to a data phase control signal, and(d) digital circuit means responsive to said respective hard limited inphase and quadraphase signals and said data clocking signal for(1) obtaining respective received inphase and quadraphase data streams,(2) generating said data phase control signal so that said received data streams become substantially phase synchronized to said transmitted data streams, and(3) generating said carrier phase control signal so that said locally-generated modulating signals become substantially phase synchronized to said suppressed modulating signals and so that said carrier phase control signal is unresponsive to the amplitudes of the inphase and quadraphase signals and thereby unresponsive to the amplitude of the offset QPSK signal.

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