High-Selectivity Lowpass–Bandpass Filters With Flexible Passband Allocation
Yuefeng Yuan, Wei Xie, Jinhao Dai, Chi Zhang, Runbo Jiang, Jia Wang, Chunguang Li, Yun Wu, Xu Wang, Liang Sun
Abstract
This letter presents a dual-band lowpass–bandpass filter utilizing dual-mode unit cells. The first mode can be tuned without affecting the second mode. The I/O coupled lines for the bandpass response introduce a transmission zero (TZ) to improve the selectivity of the lowpass response. Indeed, the developed improved unit cell provides a frequency ratio ranging from 1 to 4 and a fractional bandwidth of up to 60%. Additionally, a seven-section lowpass–bandpass filter is designed and fabricated, affording a flexible passband allocation, where the lowpass response’s cutoff frequency and roll-off rate are 0.917 GHz and 1055 dB/GHz, respectively. The center frequency and fractional bandwidth of the bandpass response are 1.718 GHz and 13%, respectively, and the filter measures only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.139\lambda _{g} \times 0.046\lambda _{g}$ </tex-math></inline-formula> .