Compact Interdigital Bandpass Filter, Diplexer, and Triplexer Based on Through Quartz Vias (TQVs)

An interdigital bandpass filter (BPF), diplexer, and triplexer are proposed based on the through-quartz via (TQV) technology in this article. TQVs are inserted between adjacent resonators to adjust the coupling coefficients, resulting in the reduction in the total area of the filter. Equivalent lumped circuit of the filter is researched. The proposed BPF operates at 28.56 GHz, and the 3-dB fractional bandwidth (FBW) achieves 10%. The out-of-band rejection is better than 20 dB with frequencies up to 2.59 <inline-formula> <tex-math notation="LaTeX">$f_{0}$ </tex-math></inline-formula>. Based on the proposed filter, interdigital diplexer operating at 28.85 and 37.18 GHz, respectively, is constructed, which exhibits an isolation better than 30 dB and a stopband up to 2.61 <inline-formula> <tex-math notation="LaTeX">$f_{1}$ </tex-math></inline-formula> of the lower channel. The triplexer operating at 22.24, 29.6, and 36.46 GHz is also designed in this article, which exhibits a wide stopband up to 2.96 <inline-formula> <tex-math notation="LaTeX">$f_{3}$ </tex-math></inline-formula> of the lower channel with lower insertion loss. Measurement results agree well with the simulations over the whole band.