Compact bandpass filter with multiple transmission zeros using modified stepped-impedance resonators

Abstract

A modified quarter-wavelength stepped-impedance resonator (SIR) is proposed and analyzed in this article. In the modified SIR, the low characteristic impedance stub of a conventional SIR is replaced by a closed rectangular loop. Based on the modified SIRs, a compact third-order bandpass filter (BPF) with multiple transmission zeros is presented. The filter consists of two novel SIRs and one half-wavelength embedded spiral-line resonator locating in the closed rectangular loops. Mixed-cross (M–C) coupling and source–load (S–L) coupling are introduced to the filter. Owing to the M–C coupling and S–L coupling, multiple transmission zeros and the suppression of harmonics can be obtained in the stopband for the enhanced frequency selectivity. A new BPF centered at 2.4 GHz with 25% fractional band width for wireless local area networks application has been designed, fabricated and measured to verify the validity of the above method.

Keywords:

• bandpass filter (BPF)
• modified stepped-impedance resonator (SIR)
• mixed-cross coupling
• source–load coupling
• transmission zeros (TZs)
• embedded spiral-line resonator (ESR)

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported in part by the National Natural Science Foundation of China [grant number 61101038], [grant number 61471086]; Fundamental Research Funds for the Central Universities [grant number ZYGX2012J031].