A Ferroelectric Tunable Combline Filter With Improved Stopband Transitions

In this work the design of a microstrip line based frequency agile bandpass filter with improved selectivity is presented. The original combline bandpass performance is improved, by a transmission zero which is inserted in the lower stopband, resulting in steeper stopband transitions. The frequency agile behavior is obtained by using ferroelectric thin-film varactors as tuning elements at the resonator ends. The implemented prototype filter can be tuned continuously in the frequency region from 1.8 GHz to 2.1 GHz exhibiting a worst case insertion loss of less than 3 dB and passband return loss better than 20 dB at all operating bias states. Excellent agreement between simulation and measurement results is obtained for the proposed topology at all tuning states. An output referred 3 rd order intercept point of 36.5 dBm was measured by a two tone setup.