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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 (<i>Q</i>_e) for the bandpass mode, the feeding lines are loaded with two varactors and wide tuning range of <i>Q</i>_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.