Matsushita Electric Industrial Co., Ltd.

Kazunao Urata

Hiroshi Saka

Shigeru Soga

A QPSK modulation signal is processed with frequency conversion into an IF signal by a channel selection circuit, and then is converted into a baseband signal by a quadrature demodulator. The baseband signal has a wave thereof shaped by DTFs and then sent to a carrier recovery circuit. An offset frequency of the IF signal detected by an AFC circuit is compensated for by a baseband frequency converter including a complex multiplier and a DVCO. The frequency tracking circuit detects whether an operating frequency of another DVCO in the carrier recovery circuit goes beyond a range. By tracking an oscillation frequency of the DVCO in the baseband frequency converter so as not to go beyond the reference value, frequency deviation of the spectrum of signals which are input to the DTFs is controlled not to go beyond the reference value. By offsetting a frequency dividing ratio of a variable frequency divider of the channel selection circuit by an amount corresponding to the offset frequency of the IF signal detected by the frequency detector, the frequency deviation of the QPSK modulation signal can be compensated for whichever channel is selected.

Demodulation apparatus performing different frequency control functions

A channel selecting apparatus simultaneously selects both the modulation signals, which are simultaneously received at different channels where modulation signals having a continuous phase and digital modulation signals having a discontinuous phase are located. As both the modulation signals are propagated on the same transmission path or approximately the same inclination of transmission path, modulation signals existing on different channel frequencies suffer approximately similar frequency shift from the transmission path before being inputted to the channel selecting apparatus. Therefore, the frequency of the digitally modulated IF signal can be controlled in accordance with the frequency shift information related to an IF signal having a continuous phase and the frequency of the digitally modulated IF signal may be focused into the comparatively narrow frequency range around the synchronous pull-in frequency of the digital demodulating circuit and the modulation signal having a continuous phase may be focused into the comparatively narrow frequency range around the optimum input frequency of the demodulation circuit.

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Tondabayashi, Japan

Kazunao Urata