Synthesis of ultra-wideband bandpass filter employing parallel coupled SIRs of one-wavelength

A transmission-line-theory-based novel synthesis theory is proposed to design an ultra-wideband bandpass filter to meet Federal Communications Commission (FCC)’s spectrum limit. One-wavelength parallel coupled stepped-impedance resonators (SIRs) are used to achieve a fraction passband from 3.1GHz to 10.6GHz (greater than 100%). An unsymmetrical SIR is employed to replace the conventional centrally-symmetrical one, so that the coupling gap is successfully enlarged to alleviate the requirement on fabrication precision. A simpler equivalent circuit for the parallel coupled-line is used in this synthesis to avoid calculation of parameter J [7]. As an example, a filter with two-stages is synthesized, simulated and fabricated. The good agreement between the measured and predicted results validates the effectiveness of newly proposed synthesis theory. Meanwhile, the designed filter exhibits good characteristics of low insertion loss, sharp skirts, flat group delay and good stopband (especially in lower one) as well.