Microstrip Lowpass Filter with Ultra-Wide Stopband Using Folded Structures

Abstract

Abstract In this article, a new microstrip lowpass filter with ultra-wide stopband using T-shaped resonators and folded structures is described. The structure of this filter is very simple, so that only a T-shaped resonator is used as main resonator without any particular changes in these resonators. The designed T-shaped main resonator has a −3 dB cut-off frequency of 1.4 GHz. On the other hand, T-shaped resonators with low base length are utilized as suppressing cells. To provide an ultra-wide stopband, two pair of T-shaped resonator and two high frequency suppressing cells are employed. The structure of this filter is folded to have compact size. The measured results shows −3 dB cut-off frequency of 1.4 GHz. The transition band is equal to 0.25 GHz from 1.4 to 1.65 GHz, with corresponding attenuation levels of −3 to −20 dB. The ultra-wide stopband with −22 dB suppression level is from 1.68 to 40 GHz. Also, the total size of the proposed lowpass filter is 16.8×11.3 mm2.