Twin-T dual notch filter

A communication filter circuit useful in a dual-mode receiver comprises a cascade of an active twin-T filter and a passive twin-T filter sections defining sharp notches at the center of second adjacent and of third adjacent channels, respectively, and specifically providing most attenuation at 60 kHz and at 90 kHz, for operation in the ISM136 band which has 30 kHz channel spacing. Since communication is channelized and limited in bandwidth, an active/passive twin-T notch filter structure is effective for signal attenuation. In a circuit in which real and imaginary signal components are processed, active-passive twin-T filters are provided in each of four signal paths, namely I and -I, Q and -Q.