PURPOSE: To simplify the constitution, by driving a variable band filter by a clock signal obtained from the output of a voltage control oscillator (VCO).

CONSTITUTION: Under the state where a signal of a constant frequency is applied to an input terminal 1, when a switch 10 has terminals (a) and (c) connected and the voltage of a constant voltage generator 7 is adjusted by an attenuator 8 to make the output of a variable band filter (SCF)3 maximum, the frequency of the input signal coincides with the band of the SCF3. The output of the SCF3 is converted to a voltage proportional to the frequency of the input signal by an f/V converter 5 and is adjusted by an attenuator 6 to become equal to the voltage of the terminal (a). Next, if the switch has terminals (b) and (c) connected, the SCF operates as a filter following frequency which has the band changed in accordance with that input frequency. The input signal is branched and is applied to the second SCF13, and n-fold frequency components are generated in a terminal 14 if the driving clock fundamental frequency of the SCF13 is set to an n-fold frequency.