Abstract-On the basis of impedance steps and coupled-line sections as inverter circuits, novel wide-band and very compact filters are presented. The application of alternately high-and low-impedance lines presented to the connected transmission-line resonators partly reduces their lengths to a quarter-wavelength only. In addition, effective techniques are demonstrated to reduce spurious stopband resonance resulting from a remaining half-wavelength resonator. Both suspended stripline (SSL) and microstrip filters were designed, fabricated, and tested, proving this concept in an excellent way. For the prototype filters, center frequencies around 6 GHz were selected. Bandwidths are between 2.5-3.25 GHz, and insertion-loss amounts to around 0.25 dB for the microstrip filters and 0.5 dB (including the transitions to coaxial line) for the SSL filters, respectively. For the selected center frequency and on a substrate with a dielectric constant of 10.8, the smallest microstrip filter is only 15 mm 5 mm in size.Index Terms-Broad-band planar bandpass filter, impedance inverter circuits, multipole resonator.

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In this paper, a novel ultra-wideband (UWB: 3.1~10.6 GHz) bandpass filter on coplanar waveguide (CPW) is presented, designed and implemented. At first, an openended nonuniform or multiple-mode resonator with three distinctive sections is constructed and investigated toward generating the first three resonant modes occurring around the lower-end, center, and higher-end of the UWB band. Then, a CPW interdigital capacitor element with enlarged ground-to-ground distance is characterized to excite two additional resonant poles below and above the UWB's center. As a result, a five-pole UWB bandpass filter with only one full-wavelength is constituted. Its performance is studied on the basis of a simple cascaded transmission-line network, whose parameters are extracted from our selfcalibrated method of moments. After the optimized results are confirmed by full-wave simulation over the filter layout, a UWB filter sample is fabricated to demonstrate the actual UWB passband behavior with the 2.8-10.2 GHz bandwidth, where the insertion loss is less than 1.5 dB and variation in group delay is less than 0.33 ns. V V C 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE 17: 225-232, 2007.

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