PURPOSE: To reduce a circuit scale and power consumption, by generating a mean value proportional to an original signal on an amplitude axis from a pulse forming FM modulation signal by using a chopping wave generation circuit.

CONSTITUTION: At the chopping wave generation circuit 3, a potential rises successively in a period of the LOW level of a pulse forming FM signal when a switching circuit 2 is opened, and the state of the circuit is returned to a base state in a period of the HIGH level of the pulse forming FM signal when the switching circuit 2 is closed. At the output 4 of the chopping wave generation circuit 3, a waveform as shown in figure (b) can be obtained. And the output 4 of the circuit 3 is envelope-detected at an envelope detection circuit 5, and a demodulation signal with the waveform as shown in figure (c) can be obtained. As for the pulse forming FM signal, since the repeated frequency of a pulse is proportional to the amplitude of the original signal, however, the mean value is constant, it is required to generate the mean value proportional to the amplitude of the original signal for demodulation.