Patent ID: 7180364

Claim:
A filter apparatus comprising: a master circuit for receiving a reference frequency signal having a reference frequency to generate a control voltage; and a slave gm-C filter comprising at least one first operational transconductance amplifier and at least one first capacitor, the operational transconductance amplifier of said slave gm-C filter being controlled by said control voltage to tune one of a cut-off frequency and a center frequency of said slave gm-C filter, said master circuit comprising: a phase shifter comprising a second order am-C filter including at least two operational transconductance amplifiers forming an inductance, said phase shifter being adapted to receive said reference frequency signal and change a phase of said reference frequency signal in accordance with said control voltage; a phase comparator connected to said phase shifter, said phase comparator being adapted to compare a phase of an output signal of said phase shifter with said phase of said reference frequency signal to generate a phase error signal; and a loop filter, connected to said phase comparator, said loop filter being adapted to exclude an AC component from said phase error signal to generate a DC component thereof as said control voltage, a phase locked loop of said master circuit being operated so that a loop gain thereof is maximum.