Abu Nasar Ghazali* and Srikanta Pal Planar UWB Filter with Multiple Notched Band and Stopband with Improved Rejection Level Abstract: Analysis and realization of a microstrip-based planar ultra-wideband (UWB) filter with integrated multi- ple notch elimination property and simultaneously extended upper stopband is proposed. Initially, a UWB filter based on back-to-back microstrip-to-CPW technol- ogy is designed. Later, multiple spiral defected ground structures (DGS) are embedded to obtain multiple pass- band notches. Further, double equilateral U (DEU)-type DGS are used to improve upon the rejection level in upper stopband. The multiple passband notches are results of embedded spiral-shaped DGS (SDGS), while extended upper stopband is the outcome of suppressed higher- order spurious harmonics. The flexible dual-attenuation poles of DEU-shaped DGS suppress the stopband harmo- nics and widen the stopband. An approximate lumped equivalent circuit model of the proposed filter is modelled. The filter is compact and its layout measures 25.26 mm  11.01 mm. The measured result is in good agreement with the full-wave electromagnetic (EM) simu- lation and circuit simulation. Keywords: double equilateral-U (DEU), extended stop- band, microstrip filter, microstrip-to-CPW, spiral defected ground structure (SDGS), ultra-wideband (UWB) DOI 10.1515/freq-2014-0194 Received October 23, 2014 1 Introduction The emission mask (–75 mW/MHz) imposed by the Federal Communications Commission (FCC) on ultra- wideband (UWB) systems prevents them from being source of interference to other in-band wireless services [1]. However, these in-band radio services like WLAN, C band, X band, etc. being high-energy transmitters at their frequency of operation act as intentional radiators, thereby causing significant interference to the UWB sys- tems. In order to circumvent this problem UWB filters with integrated band notch property became necessary. Necessity invoked exhaustive amount of research and development in the design and modelling of UWB filters with interference proof property [2]–[7]. Single narrow notched band is created by embedding parasitic coupled line to the UWB bandpass filter (BPF) [2] and by loading the input feed-lines with stepped impedance microstrip stub [3]. The notch in [2] is function of parasitic coupled line dimensions, whereas stepped impedance microstrip stub introduces its first transmis- sion zero at 5.5 GHz in [3]. UWB filter with dual notch bands using asymmetric coupling strip is reported in [4] and a simplified composite right/left-handed (SCRLH) resonator-based UWB filter generates dual notches in [5]. The structure in [4] provides multiple paths for signal flow, which enables it to generate multiple transmission zeroes, whereas the SCRLH is equivalent to two shunt- connected series LC resonance circuit which generates the dual notch. A dual notched band UWB filter with two electromagnetic bandgap (EBG) structure is reported in [6]. A UWB filter with triple notched band using triple mode stepped impedance resonator is reported in [7]. Here the triple mode resonator is equivalent to three shunt-connected series LC resonance circuit which gen- erates the triple notch. Another triple notched band UWB filter on an E-shaped multiple mode resonator (MMR) with three parallel U-shaped defected microstrip slots is reported in [8]. However, none of the above shows sig- nificant stopband extension. Here we propose a microstrip-based UWB filter with triple notched band and simultaneously extended upper stopband. In this research we have chosen the back-to- back microstrip-to-CPW [9]-based UWB filter because of its inherent strong coupling, smooth passband response and good selectivity. Later, multiple defected ground structures (DGS) are embedded to the UWB filter so as to obtain triple passband notch and simultaneously extended upper stopband. Spiral-shaped DGS (SDGS) generate the passband notch. The notch frequency posi- tion is function of SDGS profile dimension, whereas notch bandwidth and notch number are related to SDGS cas- cading. Extended stopband is obtained by suppressing *Corresponding author: Abu Nasar Ghazali, Department of Electronics and Communication Engineering, BIT Mesra, Patna Campus, Patna 800014, India, E-mail: anghazali@gmail.com Srikanta Pal, Department of Electrical Engineering, Shiv Nadar University, Gautam Buddha Nagar, Uttar Pradesh – 201314, India, E-mail: pal_srikanta@yahoo.co.uk Frequenz 2015; 69(5-6): 207–218 Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/21/15 2:32 AM