A Tunable Dual-Band Bandpass-to-Bandstop Filter Using p-i-n Diodes and Varactors

In this paper, a tunable dual-band bandpass-to-bandstop filter and its analysis are proposed. The bandpass-to-bandstop transformation could be obtained by changing the state of the p-i-n diode. Using two sets of half-wavelength resonators loaded with varactors, the center frequency of each band can be tuned independently by adjusting the varactors. In order to achieve the controlling of the external quality factors (Q(e)) for the bandpass mode, the feeding lines are loaded with two varactors and wide tuning range of Q(e) can be obtained. In the bandpass mode, the lower passband tuning range is from 1.7 to 2.2 GHz (25.6% tuning range) with return loss better than 10.5 dB, and the tuning range of higher passband is from 2.2 to 2.7 GHz (20.4% tuning range) with return loss better than 12.9 dB. For the bandstop mode, the lower stopband tuning range is from 1.7 to 2.3 GHz (30% tuning range) and the higher stopband tuning range is from 2.3 to 2.9 GHz (23.1% tuning range), while the rejection level is higher than 16.4 dB in the stopband.