Design of Broadband Interdigital Bandpass Filter for the Tunable of Transmission Zero

This paper proposes the design of an interdigital bandpass filter in the C-band and introduces a structure that shifts the transmission zero in the stopband. The designed filter achieves a wide bandwidth of 1.9 GHz with a center frequency of 5.05 GHz. The simulated filter shows an insertion loss of 2.66 dB and a return loss of 11.59 dB in the passband, with a transmission zero located at 6585 MHz. The fabricated filter exhibits an insertion loss of 2.43 dB and a return loss of 13.78 dB in the passband. The overall size of the filter is 28.5 X 10.5 X 4.4[mm³] . Based on the final bandpass filter design, a modified structure is proposed to shift the high-frequency transmission zero.