Contents I. Introduction
Ultra-Wideband (UWB) technology has shown tremendous progress in recent years for high-speed wireless communication. Therefore, the UWB bandpass filter (BPF) with a pass band from 3.1–10.6GHz, sharp out-of-band rejection especially at a lower stop band due to its extensive use in other technologies, low insertion loss, small and flat group delay is highly required [1]. The UWB BPF with ultra-wide bandwidth and flat group delay was realized using lossy material but with a high insertion loss of 6.7 dB [2]. Cascaded ring resonator sections along with an open stub were used to realize the UWB filter but with low bandwidth and return loss of only about 10 dB [3]. Coupled lines using backside aperture on ground plane for enhancing coupling [4] and interdigital coupled lines [5] with multimode resonator were also used for realizing ultra-wide pass band. However, these filters have not shown good return loss and sharp out-of-band rejection. The presented UWB BPF using two sections 2 of open stub connected in shunt with series of microstrip lines was low pass and stop band bandwidth [6]. A broadside coupled microstrip to coplanar waveguide UWB BPF was realized in [7], [8], which is sensitive to the proximity of a metal enclosure when packaged with RF integrated circuits and is also complex to fabricate due to pattern on ground metal. A broad band filter using quasi lumped interdigital capacitor and quarter wavelength microstrip line resonator was presented in [9] with maximum bandwidth only up to 76%. We are presenting here a planar UWB BPF using three-interdigital-edge-coupled microstrip lines for enhancing coupling and stepped impedance open stub for realizing transmission zeros. Design of the filter is easy and bandwidth can be controlled in wide range from narrow to UWB by changing impedance ratio of stepped impedance open stub.
