Abstract
Stepped impedance resonator (SIR) has been widely used for applications in wide stopband bandpass filters. However, the reported SIR wide stopband bandpass filters are limited in either stopband performance or design complexity. Here we propose an approach to improve the stopband performance and reduce the design complexity simultaneously. Two in-resonator open stubs are adopted in a SIR (with tap-feeding) to stagger the frequencies of higher-order modes (compared with other SIR) and produce two adjustable transmission zeros without affecting the fundamental properties. For a SIR filter employing such a structure, the spurious passbands are doubly deteriorated by the staggering and up to four transmission zeros. At the same time, the fundamental passband is not affected, which significantly reduces the design complexity. As a result, not only wide stopband but also high-suppression stopband can be obtained in a simple design. A prototype based on the substrate integrated coaxial line (SICL) is experimented showing an impressive stopband performance. Agreeing with the simulation well, the measured stopband is extended up to about 12 f 0 with the suppression of 40 - 50 dB (f 0 is the center frequency). The proposed technique could be very useful to design a bandpass filter with a wide and high-suppression stopband efficiently.
Highlights
The wide and high-suppression stopband is important for a bandpass filter in the microwave/RF circuit and system
The spurious passband is doubly deteriorated by staggering the high-order modes and introducing multiple transmission zeros, which is implemented by using multiple in-resonator open stubs without affecting the fundamental passband
PROPERTIES This section investigates the properties of a Stepped impedance resonator (SIR) with two inresonator open stubs in order to facilitate the wide stopband bandpass filter design
Summary
The wide and high-suppression stopband is important for a bandpass filter in the microwave/RF circuit and system. P. Chu et al.: Wide Stopband Bandpass Filter Implemented by SIR and Multiple In-Resonator Open Stubs. In [18], staggering the higherorder modes is realized by using in-resonator open stubs with the fundamental frequency unaffected, which realizes the spurious passband deterioration in a very simplified design. By cascading lowpass and bandstop filter, the spurious passband could be suppressed without affecting the passband response in a sense [19] It will inevitably increase the circuit area and deteriorate the insertion loss. The spurious passband is doubly deteriorated by staggering the high-order modes and introducing multiple transmission zeros, which is implemented by using multiple in-resonator open stubs without affecting the fundamental passband. In a simple design, wide stopband and high-suppression stopband can be obtained
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