Compact multi‐order bandpass filter with high stopband rejection performance

Abstract

This Letter presents a multi-order bandpass filter (BPF) based on unit cells, which are constructed by coupled lines and microstrip stubs. This BPF unit cell features ultra-wide upper stopband and excellent stopband rejection performance. The external quality factors and coupling coefficients of unit cells are analysed in detail for multi-order BPF design, where transmission lines are adopted as a coupling structure for easy post-fabrication tuning. Following filter coupling matrix theory, two- and three-pole Chebyshev-response BPFs are designed and measured, centred at 1.0 GHz with 3-dB fractional bandwidth of 10%. Experimental results demonstrate excellent out-of-band rejection performances. In two-pole BPF, the upper stopband can reach 4.8 GHz (4.8 f 0 ) with the rejection level of 37 dB. In three-pole BPF, within the stopband frequency of 3.4 GHz (3.4 f 0 ), the rejection level is better than 55 dB, while the upper stopband can be extended to 6.5 GHz (6.5 f 0 ) with the suppression level of above 26.8 dB.