Patent ID: 7408417

Claim:
A filter automatic adjustment apparatus, which adjusts a characteristic frequency of a main filter to a target frequency, comprising: a reference filter; a measuring section that consecutively measures a characteristic frequency of the reference filter a plurality of times; and a determination circuit that determines the characteristic frequency of the reference filter based on measurement results of the measuring section; wherein the measuring section comprises: a phase difference detector which detects a phase difference between an input signal and an output of the reference filter to output a signal having a duty ratio corresponding to the phase difference; a comparator which converts the output signal of the phase difference detector into a rectangular wave to output the rectangular wave; and a counter which consecutively counts a predetermined interval of an output signal of the comparator a plurality of times based on a reference signal, wherein the determination circuit selects a count value satisfying a predetermined criterion from a plurality of count values outputted from the counter, and wherein the characteristic frequency of the main filter is adjusted according to a determination result of the determination circuit.