| 摘要 |
PURPOSE:To prevent out of synchronism at a rapid change in an input signal level by holding an output of a frequency phase comparator so as to control a VCO based on a control signal from a logic circuit when an output of a level detector changes rapidly. CONSTITUTION:A frequency difference signal between an input signal and a signal from a VCO 2 are outputted from a frequency mixer 1 through a variable resistance attenuator 3 or the like subject to feedback control by an AGC circuit 5 and the result is compared with a frequency of a reference signal at a frequency phase comparator 6 and the VCO 2 is controlled in response to the result of comparison. Simultaneously, a level of an output signal of the circuit 5 is detected by a level detector 13 and when an output of the circuit 13 is rapidly fluctuated, the result is held by an output holding circuit 11 of the comparator 6 via a logic circuit end the VCO 2 is controlled based on the holding value. Thus, even when the input signal level is fluctuated rapidly, an output signal frequency is kept constant and occurrence of out of synchronism is prevented. |
