Tunable reject filter for radar warning receiver

摘要

Spaced conductive strips are formed on a substrate. A first transmission line transformer is connected between the input and one end of the first strip. A second transmission line transformer is connected between the other end of the first strip and the output. The second strip is connected to ground. A shunt tuning circuit is connected between the strips. The circuit includes first and second MOS capacitors mounted on and connected to the grounded strip. An RF choke interconnects the other side of each capacitor. A varactor diode is mounted on and connected to the first capacitor. An etched spiral inductor connects the varactor and the first strip. A bias signal is applied to the junction between the choke and the second capacitor in order to tune the filter.