Compact multi‐order bandpass filter with high stopband rejection performance

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.