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Quadrature demodulation circuit with carrier control loop

  • US 5,905,405 A
  • Filed: 08/11/1997
  • Issued: 05/18/1999
  • Est. Priority Date: 08/09/1996
  • Status: Expired due to Fees
First Claim
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1. A quadrature demodulation circuit comprising an input terminal receiving a multilevel QAM signal, a local carrier oscillating means for generating a local carrier, a first mixing means multiplying said multilevel QAM signal by said local carrier to generate an analog in-phase signal, a second mixing means multiplying said multilevel QAM signal by a 90°

  • phase-shifted signal of said local carrier to generate an analog quadrature signal, a first A/D converting means receiving said analog in-phase signal for generating a digital in-phase signal, a second A/D converting means receiving said analog quadrature signal for generating a digital quadrature signal, a waveform equalizing means for waveform-equalizing said digital in-phase signal and said digital quadrature signal after conducting a low pass filtering, to generate a digital in-phase demodulated signal and a digital quadrature demodulated signal, a code convening means for code-converting said digital in-phase demodulated signal and said digital quadrature demodulated signal, an error detecting and means for conducting an error detection and correction to an output of said code converting means, and a carrier phase error detecting means generating a digital phase error signal which is supplied through a D/A converting means to said local carrier oscillating means, said carrier phase error detecting means being connected to receive said digital in-phase signal and said digital quadrature signal when a number of out-of-synchronisms detected by said error detecting and correcting means exceeds a predetermined value, and to receive said digital in-phase demodulated signal and said digital quadrature demodulated signal when a number of out-of-synchronisms detected by said error detecting and correcting means does not exceed the predetermined value.

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