FILTER AUTOMATIC ADJUSTMENT CIRCUIT AND METHOD FOR ADJUSTING CHARACTERISTIC FREQUENCY OF FILTER, AND WIRELESS COMMUNICATION APPARATUS PROVIDED WITH THE SAME

摘要

A filter automatic adjustment circuit is provided for adjusting a characteristic frequency of a main filter whose characteristic frequency is adjustable with a reference signal frequency served as a target frequency. A reference filter has modes selectively changed over, and filters an inputted reference signal. A phase difference detector detects a phase difference between an input signal inputted to the reference filter and an output signal from the reference filter, and outputs a signal having a duty ratio corresponding to a phase difference caused by the reference filter. A counter counts a duty ratio corresponding to the phase difference caused by the reference filter based on the output signal from the phase difference detector and the reference signal, and outputs a signal representing a counted duty ratio. A decoder decodes the output signal from the counter into a control signal for variation correction on the main filter.