PURPOSE: To obtain a sine wave of a fixed P-P level regardless of the frequencies by using a switched capacitor low-pass filter to a variable low-pass filter.

CONSTITUTION: A switch capacitor low pass filter SCF6 has a cutoff frequency fc corresponding basically to a clock frequency and is attained by producing a desired clock frequency with use of a programmable counter. The fc of the SCF 6 is set at 700, 800, 900 and 1,000Hz respectively in the case of four waves of a double frequency tone of a pushphone. For this purpose, the clock frequency is set at 70k, 80k, 90k and 100kHz respectively (fc=100.fCLK). Thus a low-pass filter LPF of a variable frequency fc is attained with switching carried out among division ratios 86, 75, 67 and 60 in the case of an original oscillation clock 6MHz of a programmable counter 7.