| Nowadays, dual-passband filter plays an important role in a variety of microwavecircuits, high-frequency applications, and wireless communication system. In order to meetthe need, much research work has been performed and various design approaches have beenproposed. In these scenarios, dual-passband filter using step impedance resonator is veryrepresentative. The dual-bandpass tunable filter using step impedance resonator is difficult tofind a larger tuning range and a more compact structure under the premise of ensuring thegood performance, which is also herein trying to do.In this paper, a dual-passband tunable filter using step impedance resonator is designedand fabricated. The filter can be used as the RF receiver front-end electrically tunable filter.Theoretically, by determining the ratio of the physical length of the step impedance resonator,it is possible to determine the center frequency of the passband. By using a specialinput-output coupling structure, we can avoid unnecessary capacitance. Comparing with thetraditional coupling structure, the new one has a more tight coupling structure and a smallerinsertion loss, while making the volume of the microstrip filter reduced; by setting a varactoron the midpoint of the step impedance resonator, we can meet the requirements to obtain asuitable tuning range of the microstrip filter. Theory and simulation results show that bychanging the value of the bias voltage across the varactor centrally loaded on the stepimpedance resonator, the center frequency of the upper passband can be tuned from1.5GHzto1.9GHz, while the center frequency of the lower passband is fixed at900MHz~1GHz. Themeasured results of the filter agree with the full-wave simulation very well，the firstpassband center frequency is about970MHz and the second passband center frequency canbe tuned from approximately1.5GHz~1.9GHz,the relative bandwidth is4%, the rejection of10%f0is greater than25dB. The physical area of the filter is approximately5cm×7.5cm. |
PDF Full Text Request