Catalog of tunable heterodyne filters

A simple heterodyne process is used to convert very complicated fixed-coefficient digital filters into tunable digital filters that adjust the center frequency or the bandwidth of a filter by adjusting a single heterodyne frequency. This paper provides a complete catalog of practical tunable heterodyne filter for low-pass, high-pass, band-pass, and band-elimination (notch) filters. Tunability is provided from DC to the Nyquist frequency, although full-tunability of the low-pass and high-pass filters requires a certain amount of over design for the prototype filter. The tunable parameter can easily be made adaptive by using a simple adaptive algorithm discussed in the paper.