In this article, a compact tri-band microstrip bandpass filter (BPF) using asymmetric stepped-impedance resonators (SIRs) is proposed. Only one set of asymmetric SIRs are used in designing this filter to achieve triple passband response with high selectivity and band-to-band isolation level. By properly selecting the impedance and electrical length ratios of the asymmetric SIRs, the tri-band BPF is designed. By using a cross-coupled configuration and 0˚ feed structure, high selectivity frequency responses with six transmission zeros are achieved. The three bands of the proposed tri-band filter are located at 1.57/3.9/7 GHz, respectively, and the circuit size is much smaller in comparison with previous works using the same substrate. Measured results are in good agreement with electromagnetic (EM) simulation.
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