Quad-wideband bandpass filter with high design flexibility based on independent triple-mode resonators

This work presents a quad-wideband bandpass filter (BPF) based on independent triple-mode resonators (TMRs). Multiple TMRs can be designed at any desired frequencies below 5 GHz without interfering the in-band performance of the other TMRs. Because of the intrinsic wideband property of TMR, four TMRs are applied and fully surrounded by feedline to produce the final quad-wideband BPF. The 1st harmonic responses of TMR 1 and TMR 2 are suppressed to less than 23 dB below 5 GHz because of the source-load coupling effect of the feedline. Therefore, no harmonic response is introduced to interfere the in-band performance of the followed-up TMR 3 and TMR 4. Furthermore, all TMRs can be independently tuned without interfering the others, which further proves the flexibility of our proposed work. The measurement results of the fabricated filter show a 3-dB fractional bandwidth (FBW) above 15% for each passband, which is notably superior compared to other reported works. Good in-band performance of 0.84 dB/0.84 dB/0.83 dB/1.12 dB is obtained at the operation band of 1.8 GHz/2.4 GHz/3.2 GHz/4.5 GHz with a compact circuit size of 0.29 (g) x 0.24 (g). The simulation is consistent with the measurement, which validates our work.