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A radio-frequency tunable active filter

A design approach for a tunable active filter of constant bandwidth and center-frequency gain is described. The approach is based on a feedback configuration with a complex zero pair in the feedback transfer function. Center frequency tuning is achieved with the variation of only one element, a potentiometer, which controls the feedback loop gain. A circuit realization using two commercial video amplifiers with RC embedding is presented. A tuning range of 25 percent at 5 MHz, with a nominal bandwidth of 90 kHz or 180 kHz, and a variation of center-frequency gain of less than /spl plusmn/1 dB over the tuning range, is obtained.