| 摘要 |
In a radio receiver, a voltage-controlled crystal oscillator of a radio frequency section produces a local carrier for mixing it with a received radio signal to produce an analog output signal. A threshold decision circuit comprising an average circuit and a comparator compares this output signal with a decision threshold and produces a digital signal. A synchronization detector produces an in-sync signal when a synchronization bit pattern is present in the digital signal. Otherwise, it produces an out-of-sync signal. The radio-frequency section is periodically activated and deactivated by power saving pulses. Frequency control circuitry varies a control voltage applied to the voltage-controlled crystal oscillator by a predetermined amount if a predetermined number of the power saving pulses is produced after the instant the out-of-sync signal is produced. The control voltage will be varied if the same number of power saving pulses is produced again in the presence of the out-of-sync signal after the control voltage was varied previously. The control voltage is maintained at a level which is obtained at the instant the in-sync signal is produced. |
