Patent ID: 7492216

Claim:
A filtering apparatus having a main filter, the filtering apparatus comprising: a variation detection circuit including a reference filter having at least one first resistor and at least one first capacitor, the variation detection circuit detecting a variation of a CR-product, which is a product of a resistance value of the at least one first resistor times a capacitance value of the at least one first capacitor, in response to each of a plurality of reference signals having different frequencies from each other, and outputting a variation detection signal indicating a detected result; and a variation correction circuit for receiving the variation detection signal from said variation detection circuit and for correcting frequency characteristics of the main filter based on the variation detection signal; wherein the variation detection circuit includes: a frequency divider for frequency-dividing the reference signal by a predetermined frequency division ratio, and outputting a frequency-divided signal to the reference filter; a reference signal frequency selection controller for generating a frequency selection signal for switching over group delay characteristics of the reference filter based on the frequency of the reference signal, and outputting the frequency selection signal to the reference filter, wherein the group delay characteristic is based on the at least one first resistor and the at least one first capacitor of the reference filter; a delay time detector for detecting and outputting a delay time of an output signal of the reference filter for the frequency-divided signal; and a counter for counting a multiple of a signal time cycle of the reference signal for the delay time detected by the delay time detector, and outputting the variation detection signal including a counted result to the variation correction circuit.