Multilayer RF Dual-Band Bandpass Filters With Quasi-Flat Group-Delay Behavior and Multiple Stopband Transmission Zeros

This paper reports a novel class of RF dual-band bandpass filters (BPFs) with quasi-flat group delay and multiple stopband transmission zeros (TZs). These filters mainly exploit microstrip-to-microstrip (MS-to-MS) vertical transitions on stepped-impedance slotlines. A two-layer dual-band BPF with quasi-flat group-delay response and seven stopband TZs is firstly conceived by properly determining the electrical-length and line-impedance parameters of the constituent slotline sections of its devised stepped-impedance slotline resonator. In order to increase the abruptness of the filter cut-off slopes and the stopband power-rejection levels, a sharper-rejection dual-band BPF composed of several cascaded units is also realized. It includes a stepped-impedance-slotline-based vertical transition, two matching circuits, and two half-wavelength-slotline-based vertical transitions. The RF operational principles of these dual-band BPFs are illustrated through their transmission-line-based equivalent circuit models. Moreover, for validation purposes, a proof-of-concept circuit of the first dual-band BPF is designed and simulated at the electromagnetic (EM) level. The EM-simulated lower and upper passbands are centered at 5.935 and 10.025 GHz with 3-dB fractional bandwidths (FBWs) of 13.65% and 6.98% and maximum group-delay variations of ±0.05 ns from 5.152 to 6.556 GHz and 9.325 to 10.752 GHz, respectively.