High-Selectivity Lowpass–Bandpass Filters With Flexible Passband Allocation

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
$0.139\lambda _{g} \times 0.046\lambda _{g}$
.