Demodulator.

摘要

In a demodulator, a delay detection means receives a pi /4-shift QPSK signal and performs delay detection of a signal at an interval of symbols. An averaging circuit respectively averages two channel quadrature signal components of a signal. A preamble detection means detects a preamble having a specific pattern in which a phase shift of pi /4 of the phase of a received symbol from the immediately preceding symbol and a phase shift of -3 pi /4 of a phase of a next symbol from the immediately preceding symbol are alternately repeated on a phase plane of the two channel quadrature signal components. A phase angle calculating means calculates the phase angle of an output signal from the averaging circuit. A frequency offset estimating circuit calculates a carrier frequency offset. A voltage-controlled oscillator has an output oscillation frequency variably controlled by an output signal from the frequency offset estimating circuit. A frequency converter performs frequency conversion of the output oscillation frequency of the voltage-controlled oscillator and the input pi /4-shift QPSK signal. A reproduction circuit reproduces a carrier and a clock from an output pi /4-shift QPSK signal from the frequency converter, and obtains demodulated data by coherent detection. <IMAGE>