| 摘要 |
A data referenced demodulator is provided for recovering differentially encoded multiphase modulated digital data such as QPSK modulated audio data. An analog carrier containing the differentially encoded QPSK data is converted to a digital waveform at an intermediate frequency that is a multiple of the QPSK bit frequency. The digital waveform is delayed in a shift register that samples the waveform at a clock rate which is a multiple of the intermediate frequency. Different stages of the shift register output the digital waveform one bit time earlier plus 45 DEG and one bit time earlier minus 45 DEG . These outputs of the shift register are multipled with the digital waveform using exclusive OR gates to provide differential QPSK detection. The shift register sampling clock is phase locked to a system master clock, which in turn is locked to the received data. In an illustrated embodiment, the sampling clock is 24 times the intermediate frequency, providing 15 DEG phase resolution.
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