An active tracking band-pass filter

A frequency-agile tracking filter, which operates over a wide frequency range with a high immunity to noise and interference, has been designed. It consists of a second-order bandpass filter which is continuously tunable over the 5 to 140-kHz band. The resonant frequency of the filter is adjusted by means of a feedback loop to the frequency component of greatest amplitude in the input signal. The loop includes a fast AGC amplifier, a fast frequency-to-voltage converter and a low-pass filter. The components of the tracking filter are described and analyzed. Analysis of the tracking filter, as regards its immunity to noise and interference, its frequency agility and the stability of its control loop, is also presented to optimize system parameters. Finally, the performance of the processor is shown by experimental measurements.