Continuously Tunable Dual-Mode Bandstop Filter

A varactor-based tunable bandstop filter has been proposed in this letter. The proposed filter is based on a dual-mode circuit developed by introducing inductive and capacitive couplings into a notch filter. The frequency tunability is achieved by using varactor diodes instead of the lumped capacitors in the circuit. Next, the equivalent circuit model has been implemented in planar microstrip technology using thin inductive traces and varactor diodes. The fabricated filter prototype shows a continuous center frequency tuning range of 0.66-0.99 GHz with a compact size of 0.12 lambda(g) x 0.16 lambda(g), where lambda(g) is the guided wavelength at the middle frequency of the tuning range.