| 摘要 |
An automatic fine tuning loop in a TV receive is controlled by phase comparison between a video intermediate frequency signal and a reference oscillator signal. The reference oscillator signal is used in synchronous detection of the video intermediate frequency signal. The frequency of the reference oscillator is controlled by a phase locked loop, of which it is a part, and which includes a low-pass filter having selectable high and low cut-off frequency. A lock-out detector selects the high cut-off frequency for rapid correction when lock-out is detected and selects the low cut-off frequency for frequency stability when lock-in is detected.
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