Compact tunable low-pass filter using variable mode impedance of coupled structure

A compact and tunable low-pass filter utilising a coupled structure is presented. A closed-form design method is used to estimate the required design parameters so that the filter can have a tunable cutoff frequency across wide range of values. The filter also has an extremely wide stopband,sharp cutoff response and compact size. The theory of operation for the proposed filter is explained, and its design procedure and equivalent circuit are derived. The tuning capability of the filter is enabled by using two varactor diodes connected between the coupled lines to change their pi-mode impedance. The simulated and measured results of a prototype with dimensions 15 mm x 20 mm designed to have a cutoff frequency tuning range from 1.5 to 2.5 GHz and a stopband that extends to 25 GHz prove the validity of the presented design method.