A tunable bandpass-to-bandstop filter using PIN diode

A tunable bandpass-to-bandstop filter using PIN diode is proposed in this paper. The proposed filter employs two quarter-wavelength resonators and a half-wavelength resonator with six varactors and a PIN diode. The bandpass-to-bandstop transformation can be achieved by adjusting the PIN diode. The center frequency of the proposed filter can be flexibly controlled by adjusting the bias voltage of the four varactors at the end of resonators. By adjusting two varactors connected to feeding line, external quality factors can be flexibly controlled. So the proposed bandpass filter can be designed conveniently. In the bandpass mode, a tuning range of 650-1200 MHz (59.5%) with return loss better than 15 dB can be achieved. In the bandstop mode, a tuning range of 675-1200 MHz (57.5%) can be achieved. The rejection level of the bandstop mode is higher than 15 dB.