| 摘要 |
PURPOSE:To stabilize the oscillation frequency of a local oscillator to perform the frequency conversion of a reception television FM signal even when a power source is applied or a signal is interrupted transiently. CONSTITUTION:A signal processing circuit 6, when detecting the specific period of a demodulated video signal (a), outputs a pulse voltage signal (b) synchronized with the period and a control signal (c) representing the presence/absence of the signal (b). A switch 11 selects either the pulse voltage signal (b) or a fixed voltage by the control signal (c). When the pulse voltage signal (b) is selected by the switch 11, a switch 13 and a hold circuit 14 sample-hold the specific period of the video signal (a) and a switch 16 and a hold circuit 17 sample-hold a comparison voltage from a comparison voltage generation circuit 15, respectively. When the fixed voltage is selected, the switches 13, 16 are always turned on, and the hold circuits 14, 17 hold a mean voltage that is the integral value of the video signal (a) and the comparison voltage, respectively. A differential amplifier 18 controls the local oscillator 3 by the differential voltage of the hold circuits 14 and 17.
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