Novel Low-Pass Filter Structures Using Spur-Lines to Generate Additional Attenuation Poles

We propose two novel low-pass filter (LPF) structures that have generated additional attenuation poles from applying spur-lines to typical open-stub LPFs. The first structure has the spur-lines added to the serial lines, and the second structure has the spur-lines added to the parallel open stub. From the characteristic analyses of the filters with the proposed design, it was confirmed that the length of the spur-line was an important variable for controlling the stopband bandwidth and attenuation depth. The two types of LPFs were fabricated and then validated by the agreement between the theoretical and measured results. From the measured results, the stopband bandwidths of the serial- and parallel-configuration spur-line LPFs were improved by approximately 4.7 times and 1.4 times of the defined reference level, respectively, compared to that of the typical LPF.