| 摘要 |
PURPOSE:To attain accurate frequency modulation by providing a demodulation means demodulating an output signal and an adder means adding a signal outputted from the demodulation means and an input signal in opposite phase to each other to a detection means detecting the center frequency of a modulated output signal and eliminating the frequency change component of the output signal so as to detect the center frequency. CONSTITUTION:After the output signal is demodulated by an FM demodulation circuit 2, the signal is added to the input signal in opposite phase by a comparator amplifier circuit 3. Thus, the output of the comparison amplifier circuit 3 is detected for the frequency change component of the output signal depending on the input signal and only the frequency change component generated in the inside of the frequency modulation circuit 1 due to temperature change or change in the lapse of time or the like is detected. Thus, the comparator amplifier 4 compares the output of the amplifier circuit 3 with a reference voltage to control the output center frequency of the frequency modulation circuit 1. Thus, the AFC circuit independent of the input signal is formed. Thus, the frequency is modulated accurately even to the input signal having a signal component slower than the response speed of the AFC circuit or the DC component. |
