Published online by Cambridge University Press: 15 January 2016
A compact microstrip multi-stub loaded bandpass filter (BPF) with ultra-wide stopband is introduced and investigated. Lumped-equivalent circuit (LEC) method is adopted to analyze the characteristics of the in-band response based upon the low frequency LEC and even-odd mode analytical method. Besides, equivalent transmission line circuit method with even-odd mode analytical method are adopted to discuss the mechanism of the out-band rejection based upon the transmission poles and the transmission zeros analysis of the even-/odd-mode equivalent circuits. Accordingly, the design and the optimization procedures of this type of filters are summarized. To experimentally validate the proposed method, the filter is fabricated and measured. The measured results illustrate this BPF centered at f 0 = 2.8 GHz, with −3 dB fractional bandwidth of 109.3%, the insert loss (IL) of less than 1 dB from 1.4 to 4 GHz, and the rejection level of 20 dB from 5.1 to 33.5 GHz (1.82f 0 to 11.96f 0 ).
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