Varactor-tuned active miniature notch filter

In this paper a novel tunable miniature active notch filter based on short circuited quarter-wavelength resonator is presented. This simple filter topology uses tunable Active Capacitance (AC) circuit and serves for trial purposes which are described below. Thanks to the use of a varactor diode in parallel within the AC circuit which allows a simultaneous tuning of the notch central frequency and depth whilst maintaining an electrically stable filter. Experimental prototypes based on conventional discrete surface mount components and assembled on a low-cost FR4 substrate have been built up in order to prove this new concept.