A simple balanced bandpass filter is presented. It is constructed mainly by a loop-type resonator with loaded shorted/opened stubs. The resonator is fed by the balanced coupled-line structure. In the loop-type resonator, three approaches can be simultaneously utilized to achieve high common-mode suppression. One is that the loop-type resonator has different differential/common-mode resonant frequencies, which results in a good in-band common-mode suppression. The second is that the loaded short stubs with different lengths will make the input/output port couplings to have different coupling strengths, which will deteriorate the common-mode bandpass response. The third is that loading the grounded resistors can effectively dissipate the common-mode signal. Meanwhile, loading the grounded resistors in the balanced coupled-line structure can effectively dissipate the reflective common-mode signal. A detailed description about its structure, operational mechanism and design method is given. For demonstration, a prototype balanced bandpass filter working at 2.4 GHz is designed, fabricated and measured. A high in-band common-mode suppression of 49 dB is achieved. The measured and simulated results can verify the effectiveness of the proposed balanced bandpass filter and the design method.
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