| 摘要 |
PURPOSE:To prevent deterioration in the demodulation characteristic even with the presence of a frequency offset in a transmission line by providing a loop comprising a bias distortion detection circuit, a distortion averaging circuit and a VCO so as to cancel the bias distortion. CONSTITUTION:When a modulation signal receives the effect of frequency off set fOFF at a transmission line, the mark frequency of a received modulation signal is fz+fOFF and the space frequency is fA+fOFF and a bias distortion is produced in the demodulated data. If the offset fOFF is positive, for example, the distortion where the space of the demodulation data is longer than the bit length of the mark takes place. the distortion is detected by a bias distortion detection circuit 5, an output 1 is outputted, the output of the distortion average circuit 6 is positive and the output of the VCO has a high frequency. Then the lock frequencies fl1(=fz-falpha), Fl2(=f +falpha) of the DPLL2 are increased accordingly. When the frequencies fl1, fl2 reach as fl1approx.=fz-falpha+fOFF and fl2approx.=fA +falpha+fOFF, the bias distortion is nearly zero and the control loop operation is converged. When the offset is negative, the similar control is also applied.
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