Design of tunable dual-band filter with multiple types of resonators

A microstrip dual-band filter, which is composed of two kinds of tunable resonators for flexibly tuning the operation frequencies of two passbands, is presented in this paper. This filter is constructed by using inductive coupled lumped resonator for the lower passband and ring resonators for the higher passband to minimize the size of the filter and flexibly control the characteristics of two pass bands; besides, both types of resonators all are tunable for varying the operation frequencies of the filter. A methodology that can determine the components of the filter with assigned transmission zeros as well as the bandwidths of passbands is proposed. To verify the proposed design concept, an experimental prototype with two pass bands at 0.65∼0.75 GHz and 1.75∼1.95 GHz was fabricated on PCB substrate. The size of the filter is about 38×40 mm2 and the measured insertion losses in the two passbands are less than 2.0 dB and 2.5 dB, respectively. The associated return losses are all greater 18 dB over the frequency bands of interest.