| 摘要 |
PURPOSE:To stabilize a frequency with a high precision for a long time by a simple control. CONSTITUTION:A picture signal is inputted to an adder 3 through a clamp circuit 2, and an error signal is added to the picture signal, and this signal is subjected to pulse frequency modulation by a pulse modulator 1 and is outputted. Meanwhile, the picture signal is led to a synchronizing signal extracting circuit 4, and the synchronizing signal, namely, the time of the reference level signal is detected. The output of the pulse modulator 1 is led to a duty correcting circuit 5 and is converted to a pulse signal of duty ratio 1:1 and is normalized to a pulse signal indicating a frequency f1 of the said modulation signal and is compared with a pulse of a reference frequency fb in frequency by a digital phase comparator 8. In accordance with the comparison result, a discrimination output signal (a) or (b) is generated. The discrimination output signal is applied to a charge pump 10 through a switch circuit 9 whose switching is controlled by the synchronizing signal. A signal obtained by taking out the output of the charge pump 10 through a low pass filter 12 becomes the error signal. |
