Design of Bandpass Filters Using Three Parallel Coupled-Lines with Open Stub Resonator

Bandpass filter with equal-ripple response in the passband and four transmission zeros in the stopband is synthesized in this letter. Equivalent circuit model is proposed. Based on even- and odd-mode theory, design formulas are derived to provide relationship between filtering characteristics (center frequency, bandwidth, and transmission zeros) and circuit parameters for the filter. To verify the design concept, a bandpass filter with a fractional bandwidth of 26.7% (ripple level 0.01dB) at the center frequency of 3.9 GHz is designed and fabricated. The measured insertion loss is less than 0.6 dB, and return loss is higher than 19.4 dB in the passband. The stopband extends to 10.4 GHz with the rejection level of 20 dB. Measured results show good agreement with simulations.