PURPOSE: To obtain a variable pass band characteristic filter device acting excellently against a broad band input by providing a low pass filter having a cut-off frequency lower than the internal clock frequency of a switched capacitor filter (SCF) to the pre-stage of the SCF having a required characteristic.
CONSTITUTION: The cut-off frequency of a switched capacitor low pass filter LPF4 changes depending on the clock frequency fed from a frequency divider 5. The frequency divider 5 frequency-divides the 1st clock frequency fed from a frequency divider 2 to a prescribed ratio to generate the 2nd clock frequency signal. Through the constitution above, the 1st clock fed from a clock oscillator 1 is changed to shift the center frequency of an LPF3, then the cut-off frequency fc of the LPF4 is shifted accordingly. Since a signal of output frequency f3 of the LPF4 produced by an input frequency f4 is apart from the cut-of frequency f1 by 2∼3 octaves, the gain is attenuated by -24dB∼-36dB by the LPF3.
KUNUGI YOSHIRO
KATO MASAYUKI
