Adaptive filtering using heterodyne frequency translation

Both digital and switched-capacitor adaptive-sample-rate filters can achieve very robust frequency tracking with relatively simple circuitry. However, both of these adaptive sample rate filters are generally useful only with single notch and bandpass filters. If one tries to adapt a filter that has multiple band-passes or multiple notches, the distance between the notches or pass-bands is scaled as the sampling rate is scaled. While this may be desirable in some applications, many applications require a scheme in which the distance between notches or pass-bands remains fixed as the filter is adapted. In this paper, we present a technique in which a heterodyne unit is used to shift the input frequency to the fixed filters rather than adjusting the sampling rate. As a result, the input frequencies are translated, rather than scaled, to the filter allowing a fixed distance to be maintained between notches or pass-bands. The results of extensive simulations demonstrate the feasibility of this approach.