Dual-Band Ultra-Wideband Bandpass Filter with Eight-Resonant Modes and Quad-Transmission Zeros Employing Synchronous-Quasi-Resonance

This paper presented a dual-band ultra-wideband bandpass filter, which employs three sets of T-shape stub-loaded resonators (TSSLRs). The new generation approaches of resonant mode (RM), which was named here as the synchronous-quasi-resonance (SQR), to generate more RMs were firstly presented and discussed. Five intrinsic RMs were generated by the even-/odd-mode resonances of the TSSLRs, while the other three additional RMs were achieved by the SQR among different TSSLRs. The fractional bandwidths of the dual-band filter were 42% (or 1.7 GHz) and 22% (or 1.1 GHz) with center frequencies of 3.45 and 5.10 GHz, respectively. The insertion loss of the filter was lower than 0.9 dB, and the return loss was higher than 17 dB. The frequency response of the filter was lower than 20 dB from 5.9 GHz to 8.4 GHz. The high skirt selectivity and excellent out-of-band rejection were achieved by four controllable transmission zeros, which were generated by the virtual ground of the loaded-stub and the source-load coupling of the feed-lines. The proposed harmonic-suppressed dual-band ultra-wideband bandpass filter was fabricated on a Teflon substrate in order to verify the simulation results.