A tunable bandpass-to-bandstop filter using stub-loaded resonators and PIN diode

A tunable bandpass-to-bandstop filter using stub-loaded resonators is proposed in this letter. The proposed filter employs two open-circuited stub-loaded resonators with six varactors and a PIN diode. The center frequency of the proposed filter can be flexibly controlled by adjusting both odd-mode and even-mode resonant frequencies of the resonators. The bandpass-to-bandstop transformation can be achieved by adjusting the PIN diode. In the bandpass mode, a tuning range of 680–985 MHz (36.6%) with return loss better than 13.5 dB can be achieved. In the bandstop mode, a tuning range of 715–995 MHz (32.7%) can be achieved. The rejection level of the bandstop mode is higher than 13 dB.