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High selectivity balanced bandpass filter using improved parallel‐coupled line with intrinsic deep common‐mode suppression

In this article, a balanced bandpass filter (BPF) is developed by using the improved parallel‐coupled line (IPCL) and the parallel‐coupled‐line‐loaded resonators (PCLLRs). The proposed IPCL, which is realized by loading the short‐circuited or open‐circuited parallel‐coupled line section to the traditional one, behaves the bandpass response or bandstop property. Thus, feeding structure of balanced BPF constructed by IPCL can realize intrinsic common‐mode suppression. Besides, compared with traditional stub‐loaded resonator, PCLLR can provide two transmission zeros (TZs) on both sides of passband when short‐circuited parallel coupled‐line is applied. Moreover, short‐circuited stepped‐impedance stubs are installed at input and output feed lines for TZs to further enhance the selectivity of passband. Finally, a high‐selectivity balanced BPF operated at 2.5 GHz with fractional bandwidth of 11% is designed and fabricated. The measured results agree well with the simulated ones and show a desirable in‐band common‐mode suppression of higher than 44 dB.