CONTROLLING THE BANDWIDTH OF AN ANALOG FILTER

摘要

A digital tuning system ( 250 ) for changing a cutoff frequency of an analog filter ( 132 ) includes digital synthesizers ( 292 and 294 ) for producing a two-tone calibration signal ( 196 ) 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 ( 304 and 306 ) for changing the quality factor and arrays ( 308 and 310 ) of capacitors for changing the cutoff frequency. The amplitude of the magnitude responses ( 409 and 411 ) of the filter to each tone ( 405 and 407 ) is measured by a two discrete Fourier transform single-frequency bin power detection circuits ( 253 and 254 ) 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.