Reconfigurable Filter for Bandpass-to-Absorptive Bandstop Responses

A frequency tunable filter with a bandpass-to-absorptive bandstop reconfigurable characteristic is studied, designed and validated theoretically and experimentally. To control the filter bandwidth and impedance matching performance, variable capacitors are used in each external and internal coupling structure, and, by using equivalent circuits, the required frequency responses of two different modes are obtained. Two single-pole double-throw switches embedded in the transmission lines running from the source to the load accommodate external coupling structures to control the operational mode of the proposed filter. All commercially available electronics such as switches and varactor diodes are placed on the microstrip line layer, and the frequency-tunable substrate integrated waveguide (SIW) resonators are coupled to the transmission line through coupling slots. In the absorptive bandstop mode, the stopband is tuned from 1.83 GHz to 2.49 GHz, while the passband can be tuned from 1.86 GHz to 3.3 GHz in the bandpass mode. In both modes, more than 10 dB in return loss over the frequency tuning range of interest has been achieved. Theoretical, simulated, and measured results are in good agreement, which validate the proposed filter structure.