Filter automatic adjustment apparatus, filter automatic adjustment method, and mobile telephone system

A filter automatic adjustment method for adjusting a characteristic frequency of a main filter to a target frequency, comprising the steps of:

• inputting a signal of the target frequency to a reference filter;
• detecting a phase difference between an input signal and an output signal of the reference filter to output a phase difference detection signal having a duty ratio corresponding to the phase difference;
• converting the phase difference detection signal into a rectangular wave to output the rectangular wave;
• consecutively counting a length of a predetermined interval of the rectangular wave a plurality of times based on a reference signal;
• selecting a count value satisfying a predetermined criterion from the count values of the plurality of times;
• decoding the control signal for adjusting the characteristic frequency of the main filter to the target frequency based on the selected count value;
• storing the control signal in a register;
• and
• adjusting the characteristic frequency of the main filter based on the control signal.