Patent ID: 7279962

Claim:
A tuning loop for tuning a frequency of an active RC filter, said tuning loop comprising: a differential amplifier having an inverting input terminal, a non-inverting input terminal, an inverting output terminal, and a non-inverting output terminal; a first current source having an inverting terminal and a non-inverting terminal; two variable resistances each having a first and a second terminals, wherein said two first terminals are respectively electrically coupled to said non-inverting and said inverting input terminals of said differential amplifier and said two second terminals are respectively electrically coupled to said non-inverting and said inverting terminals of said first current source; two capacitors each having a first and a second terminals, wherein said two first terminals are respectively electrically coupled to said non-inverting and said inverting input terminals of said differential amplifier and said two second terminals are respectively electrically coupled to said non-inverting and said inverting output terminals of said differential amplifier; two switches each having a first and a second terminals, wherein said two first terminals are respectively electrically coupled to said first terminals of said two capacitors and said two second terminals are respectively electrically coupled to said two second terminals of said two capacitors; a comparative device having a first, a second, a third, and a fourth input terminals and an output terminal, wherein said first input terminal of said comparative device is electrically coupled to said non-inverting terminal of said first current source, said second input terminal of said comparative device coupled to said inverting terminal of said first current source, said third input terminal of said comparative device coupled to said inverting output terminal of said differential amplifier and said fourth input terminal of said comparative device coupled to said non-inverting output terminal of said differential amplifier; an applied preset time configured for driving said two switches; and a signal comparative tuning device for comparing said applied preset time cycle with a time constant corresponding to an output signal in transition state from said comparative device and tuning resistances of said two variable resistors.