Dual-Band filter for WiMAX and WLAN with improved upper stop band performance

Abstract

In this paper, a novel and compact dual-band filter with enhanced upper stop characteristics has been presented. Dual band pass filter characteristics are achieved by introducing transmission zero (TZ) in pass band of band pass filter (BPF). The wide band pass filter (BPF) is implemented by combining low pass filter characteristics (i.e., stepped impedance resonator) and high pass -lter characteristics (i.e., short stubs). Closed rectangular ring resonator (CRRR) and open loop rectangular ring (OLRR) combination is used to produce two transmission zeros (TZs). One TZ is placed on the pass band of BPF such that resultant filter characteristic consists of two pass band. However, the second TZ is placed at edge of the pass band in BPF to improve skirt selectivity. The two pass bands are designed to cover two popular wireless bands namely WiMAX (center frequency f1 (3.5 GHz) and WLAN (center frequency f2 (5.7 GHz)) bands, i.e., 3.35{3.65 GHz and 5.5{5.85 GHz respectively. Equi-ripple low pass stepped impedance resonator (SIR) filter response is responsible for improved and spurious free upper stop band (> 20 GHz, i.e., > 6f1) and also provides sharp skirt attenuation at upper stop band. The proposed filter is implemented on an RT/Duroid 5880 (εr = 2:2) substrate with thickness of 0.785mm and surface area of 19 × 12 sq · mm. Good agreement between simulated and measured results ensures that the proposed filter is a suitable candidate for modern dual band communications.