Tunable Bandstop Filter with Bandwidth Compensation

In this paper, a tunable bandstop filter with bandwidth compensation is proposed. The equivalent circuit model of the tunable capacitor network is presented to study the tunable mechanism. The electric coupling factor of the tunable capacitor network increases while the tunable capacitor increases. This mechanism can be used to compensate the bandwidth of the tunable filter. In our work, semiconductor varactor diode loaded microstrip LC resonator is adopted to design a tunable bandstop filter. Each resonator requires only one varactor diode for both central frequency and resonator coupling coefficient control. The S-parameters and group delays of the tunable bandstop filter are presented. The measurement shows that the -30 dB absolute bandwidth varies from 371 MHz to 305 MHz, while the central frequency of the stopband varyies from 3.195 GHz to 2.285 GHz.