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Quadrature amplitude modulation type digital radio communication device and method for preventing abnormal synchronization in demodulation system

  • US 5,438,591 A
  • Filed: 07/30/1992
  • Issued: 08/01/1995
  • Est. Priority Date: 07/31/1991
  • Status: Expired due to Fees
First Claim
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1. A demodulation device of a quadrature amplitude modulation type digital radio device having an oscillator means with an oscillation frequency controlled by a control signal, wherein identifying means for identifying levels of demodulated base band signals to derive a value corresponding to the levels, phase difference detection means for deriving a phase difference signal of a reproduced carrier wave based on the value, the phase difference signal is used as the control signal to control the oscillation frequency so as to derive an oscillation frequency corresponding to the phase difference signal, an output signal having the thus derived frequency is used as the reproduced carrier wave, the reproduced carrier wave is subjected to a 90°

  • phase-shifting process to create two reproduced carrier waves with 90°

    phase difference which are separately multiplied with a reception signal to create demodulated base band signals, clock reproducing means for reproducing a transmission clock signal from the demodulated base band signals to define timings for identification of the levels of the demodulated base band signals, and the reception signal is restored based on the value derived by the identification, comprising;

    monitoring means for monitoring the carrier pull-out based on the demodulated base band signals and outputting a carrier pull-out alarm signal when the carrier pull-out occurs;

    abnormal synchronization detecting means for sampling the reception signal level at a speed twice the transmission speed of the reception data based on the identification value derived by the identification and for detecting abnormal synchronization when a detection area of the sampled data derived by abnormal synchronization detecting means corresponds to a predetermined area of a plurality of areas;

    abnormal synchronization signal generating means for generating a signal for a preset period of time, when the abnormal synchronization is detected and the carrier pull-out alarm signal is not generated; and

    selection means for receiving a reference signal used for generating a signal having a reference carrier frequency nearly equal to the frequency of the carrier and the phase difference signal, selecting the phase difference signal as the control signal in the normal operation state, selecting the reference signal as the control signal and supplying the same to the oscillator means while receiving an output signal of said abnormal synchronization signal generating means;

    wherein said identifying means is operated at a speed twice the transmission clock to effect the identification.

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