In this paper, a high selectivity quadruple-mode bandpass filter (BPF) with source-load coupling is proposed. This filter uses two short-circuited stubloaded stepped-impedance resonators (SIRs) which have the same type but different size. Two SIRs can generate four operating modes, which can be approximately adjusted individually. Owing to the special design of the filter, the coupling of two resonators is weak. In each resonator, the even-mode frequency can be flexibly controlled by changing the length of the short stub, whereas the odd-mode one remains stationary. Due to the source-load coupling, two transmission zeros are close to the cut-off frequencies of the passband, which leads to high selectivity. Simulated results show that central frequency is 2.27 GHz with 3-dB fractional bandwidth of 22.9%. The measured and simulated results are well complied with each other.
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