The design of microstrip bandpass filters using stepped-impedance resonators (SIRs) is examined. The passband center frequency for the WCDMA-FDD (uplink band) Japanese cellular system is 1950 MHz with a 60-MHz bandwidth. The SIR physical characteristic can be designed using a SIR characteristic chart based on second harmonic suppression. In our filter design, passband design charts were obtained through the design procedure. Tchebycheff and maximally flat bandpass filters of any bandwidth and any number of steps can be designed using these passband design charts. In addition, sharp skirt characteristics in the passband can be realized by having two transmission zeros at both adjacent frequency bands by using open-ended quarter-wavelength stubs at input and output ports. A new even-mode harmonics suppression technique is proposed to enable a wide rejection band having a high suppression level. The unloaded quality factor of the resonator used in the proposed filters is greater than 240.

A novel class of microstrip bandpass filter is configured using the impedance transformers and an improved stepped impedance resonator (SIR). This SIR is composed of a central narrow strip section with an aperture on ground and two wide strip sections at the two sides. This low-high-low SIR resonator has a promising capability in achieving an extremely large ratio of first two resonant frequencies for design of a bandpass filter with ultra-broad stopband. The two quarter-wavelength transformers with low and high impedances, referred as to impedance- and admittance-inverters, are modeled and utilized as alternative types of inductive and capacitive coupling elements with highly tightened degrees for wideband filter design. After extensive investigation is made on the two transformers and the proposed SIR, the two novel bandpass filters are constructed, designed and implemented. Two sets of predicted and measured frequency responses over a wide frequency range both quantitatively exhibit their several attractive features, such as ultra-broad stopband with deep rejection and broadened dominant passband with low insertion loss.

A four-pole quasi-elliptic function bandpass filter is designed and fabricated using a miniaturized microstrip square Stepped Impedance Resonator (SIR). The Nonuniform Finite Difference Time Domain (NUFDTD) method is used to design the resonator and to calculate the coupling coefficients of three basic structures. Theoretical and experimental results are presented. This filter is not only compact size but also has a wide upper stop band.