FREQUENCY MODULATING CIRCUIT

PURPOSE:To stable the carrier frequency, by correcting the oscillating frequency while no modulated signal is supplied and having a control so that the corrected frequency is maintained as a center frequency even when the modulated signal is supplied. CONSTITUTION:Switches 2 and 9 are turned off and on respectively with a switching signal W. Then the output of a voltage control oscillator 5 is compared with that of a reference oscillator 7 by a phase controller 6. Those compared outputs are connected to the control input terminal of the oscillator 5 through a filter 8 via a buffer part 10. Under such conditions, the oscillating frequency of the oscillator 5 is corrected to the frequency of the oscillator 7. When the switches 2 and 9 are turned on and off respectively with the signal W, a modulated signal S is applied to the control terminal of the oscillator 5. In this case, the output voltage of the filter 8 obtained during correction is held at a capacitor 11 and then applied via the buffer 10. Therefore a carrier frequency is equal to the frequency of a reference signal.