| 摘要 |
PURPOSE:To prevent the deterioration in an error rate characteristic of the demodulator by compensating the deviation of a frequency of a digital modulation signal received by a waveform shape filter from an optimum input frequency. CONSTITUTION:A frequency error detector 21 detects a frequency error between a QPSK modulation signal and a signal from a fixed frequency oscillator 2 based on a phase difference signal obtained by a phase detector 12, the frequency error is smoothed by an AFC loop filter 22 and the result is given to a DVCO 6 via an adder 19 to control the frequency from the DVCO 6 thereby compensating the frequency error. A frequency correction circuit 10 uses a frequency comparator 16 to check whether or not the operating frequency of the DVCO 14 exceeds an upper reference frequency or is less than a lower reference frequency based on the signal given to the DVCO 14. A selection circuit 17 provides the output of a frequency correction signal controlling the oscillated frequency of the DVCO 6 corresponding to the result of comparison, and then the frequency correction loop of the demodulator is operated so that the frequency deviation in an input signal spectrum to DTFs 7, 8 does not exceed the upper limit reference frequency and is not less than the lower limit reference frequency. |
