Patent ID: 7446600

Claim:
A filter adjustment circuit, comprising: a filter capable of adjusting a cut-off frequency that is composed of a plurality of integrators connected to each other, each of which has an amplifier, a resistor connected to an input of the amplifier, a capacitor connected between the input and the output of said amplifier, and a variable resistor connected between the input and the output of said amplifier; a reference frequency generating circuit that outputs a first signal having said cut-off frequency and outputs a second signal that is out of phase with respect to said first signal to said filter; a phase comparator that compares the phase of a third signal, which is said second signal processed by and output from said filter, with the phase of said first signal to determine whether or not the frequencies of the signals are equal to each other; a reference voltage generating circuit that generates a reference voltage having a desired amplitude value for determining a Q factor; an amplitude comparator that compares the amplitude value of said third signal with that of said reference voltage and outputs the comparison result; and a control circuit that adjusts the Q factor by outputting a control signal to said filter, thereby adjusting the frequency of said third signal to said cut-off frequency based on the comparison result from said phase comparator and adjusting the resistance value of said variable resistor based on the comparison result from said amplitude comparator to adjust the amplitude of said third signal to that of said reference voltage.