Design of Low-Pass Tunable Complex Heterodyne Filters

Low-pass filters with tunable cut-off frequencies are useful in variable-bandwidth communication systems, zero-crossing detection circuits and many other applications. The complex heterodyne approach that has been applied to band-pass and notch filters, can also be applied to design low-pass and high- pass tunable filters. Using a half-band fixed filter as the prototype, the complex heterodyne approach yields a fully tunable low-pass filter that can tune the cut-off frequency from nearly DC to the Nyquist frequency.