FREQUENCY MODULATING CIRCUIT

摘要

PURPOSE:To stabilize a center frequency by using an adder provided to a pre- stage of a voltage controlled oscillator of a phase locked loop so as to add a modulation signal via a band pass filter with an output of a low pass filter of the phase locked loop. CONSTITUTION:When a modulation signal S3 is fed to an adder 7 provided to the input side of the voltage controlled oscillator 5 in the phase locked loop comprising a phase comparator 1, a low pass filter 3 and the oscillator 5 via a band pass filter 9, an output of the adder 7 is a waveform V2 and the output signal of the oscillator 5 is an output in response to the waveform V2. Since an output (e) of the comparator 1 is inputted to the filter 3 having a filter characteristic not responding to the frequency of the signal S3, the output is a value V1 before the modulation signal is added. Then the DC component and the low frequency component of the signal S3 are cut off by a filter 9, the output of the adder 7 remains the value V1 the same as the value when the carrier signal S1 only is applied.