Controlling the bandwidth of an analog filter

A digital tuning system for changing a cutoff frequency of an analog filter includes digital synthesizers ( and ) for producing a two-tone calibration signal applied to an input of the filter after a quality factor of the filter is increased. The filter includes at least one R/C circuit with two resistors ( and ) for changing the quality factor and arrays ( and ) of capacitors for changing the cutoff frequency. The amplitude of the magnitude responses ( and ) of the filter to each tone ( and ) is measured by a two discrete Fourier transform single-frequency bin power detection circuits ( and ) while the filter is sequenced through a plurality of capacitance settings. An optimal capacitance for the R/C circuit is selected by comparing, to a pre-selected value, a difference between the responses of the filter to each tone, for each capacitance setting.