Summary
International Symposium on Antennas and Propagation
2010
Session Number:4FD4
Session:
Number:4FD4-3
Design of Bandpass Filter with Wide Stopband
Yuan Chun Li, Xin Yin Zhang, Quan Xue,
pp.-
Publication Date:2010/11/23
Online ISSN:2188-5079
Summary:
The bandpass filter (BPF) is becoming more and more important in RF front-end because of its easy fabrication and integration [1]. However, microstrip BPF has undesirable harmonic response at frequency doubling and this is harmful to the whole microwave system. Thus, it is urgent to come up with a wide stopband BPF to meet the requirements of microwave system. In the past two decades, a lot of ideas have been presented to design wide stopband filters [2-9]. Due to the bandstop characteristic of photonic band-gap (PBG) [2] and defected ground structure (DGS) [3], they are often used to compress the harmonic response. Wiggly-line [4] can control wave impedance by tuning the coupling line, thus the harmonic is degraded. The overlay dielectric filter [5] makes the velocity of odd and even mode equal. Stepped-impedance resonators (SIRs) [6] are able to ensure the passband performance while push the harmonics to higher frequencies. By loading different elements at the center of resonator [7], harmonic energy can be assimilated effectively. Dissimilar resonators composed of half- and quarter-wavelength SIRs are studied in [8], but the design is complicated and labor intensive, with only 22.5 dB rejection, which is not enough for certain applications. Radiation can also be observed in the stopband, which will influence the performance of the wireless system. Recently, discriminating coupling of open-loop resonators is presented in [9] and the design procedure is simple. However, only the second harmonic can be eliminated. This paper presents a wide stopband BPF by utilizing different resonators and their coupling regions. The second harmonic of BPF can be eliminated with quarter and half wavelength resonators. By choosing different coupling region, the coupling coefficient of the third harmonic equals to zero. The coupling coefficient in the passband is still a desirable value. Meanwhile, the paper gives the analytical theory of this filter. A design example is made and tested. The presented filter is able to suppress the harmonic below 30 dB in a wide band from 6.8GHz to 8.6GHz. Good agreements have been found with simulated results and this validates the presented design.