Tunable sallen-key filter stage and filter assembly

Disclosed is a filter circuit assembly that includes a filter stage with a variable resistor and a resistor/capacitor (RC) oscillator. A controlling output of the RC oscillator controls the value of the variable resistor. The RC oscillator itself also includes a variable resistor. The controlling output of the RC oscillator also controls the value of the variable resistor of the RC oscillator. The structure of the variable resistor of the filter stage is substantially the same as the structure of the variable resistor of the RC oscillator.