An S-band bandpass filter based on quarter wavelength stepped-impedance resonator (SIR) is presented in this paper. Two SIRs are loaded to the BPF to obtain wide stopband suppression. The center frequency f0 of the bandpass filter is located at 2.105 GHz with 3-dB fraction bandwidth (FBW) of 11.9%. It shows that the spurs free upper stopband with 15 dB rejection level can extend to 40 GHz (19f0). The circuit size of this filter is extremely compact, which occupies only 7.35 mm × 7.5 mm.
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