Uniplanar bandpass filters based on multimodal immitance inverters and end-coupled slotline resonators

This paper presents the design and implementation of uniplanar bandpass filters that use end-coupled slotline resonators and a new kind of multimodal immitance inverter. These inverters are based on an asymmetric shunt-short-circuit coplanar waveguide (CPW) transition, and are modeled using a multimodal circuit model, which takes into account both fundamental modes of the CPW (the even and the odd modes). From this model, analytical design expressions for the inverters are obtained. These structures are used as input/output inverters of a new class of compact low-loss arbitrary-order easily reconfigurable uniplanar filters featuring half-wavelength and quarter-wavelength end-coupled resonators. A multimodal model is obtained for these filters, which allows an accurate analysis, design, and prediction of their behavior. By using these multimodal tools, three second- and fourth-order prototypes, and a second-order frequency-tunable prototype, were designed and implemented. The fourth-order filter has an insertion loss of 2.2 dB at 2 GHz with an out-of-band rejection better than 20 dB in an extremely wide frequency band. The frequency-tunable filter features a tuning range from 1 to 1.37 GHz with quasi-constant 3-dB bandwidth from 121 to 135 MHz. The measurements show a good agreement with the proposed multimodal model, thus validating the model and the design methodology.