Design of tunable microstrip bandstop filter

What is it about?

A novel microstrip bandstop filter with tunable center frequency and bandwidth is introduced. A dual-mode square loop resonator and three varactor diodes are used to achieve the tunability operation. The dual-mode square loop resonator is coupled to a straight transmission line that connects input and output ports. One of the varactors is used as a perturbation element on the symmetrical axis of the resonator and even mode resonance frequency can be tuned by changing the capacitance. Remaining two varactors are located at the lateral arms of the resonator as reference elements and they have task on the tunability of the odd mode resonance frequency. Depending on the capacitance changes of the varactors, both of center frequency and bandwidth of the stopband can be tuned. The designed filter was also fabricated and measured for the experimental verification of the predicted results.

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Why is it important?

A novel tunable bandstop filter design has been introduced by using a dual mode square loop resonator. The designed resonator is coupled to a straight transmission line that also connects input and output ports directly. In order to achieve the tunability operation, varactor diodes are used instead of perturbation and reference elements. Thus, center frequency of the filter can be tuned by changing the capacitances of the varactor diodes. Bandwidth tunability can also be achieved in a narrow range by only tuning the perturbation or reference varactors. The designed filter was also fabricated and tested.