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Aperture-Backed Microstrip-Line Stepped-Impedance Resonators and Transformers for Performance-Enhanced Bandpass Filters
The authors are with the School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798. E-mail: ezhul{at}ntu.edu.sg
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.
Key Words: microstrip bandpass filter, backside aperture, harmonic suppression, impedance transformer, stepped impedance resonators
Manuscript received July 8, 2005. Manuscript revised September 29, 2005.