Ultra-Wideband Bandpass Filter using Short Circuited Stubs

This paper presents a microstrip ultra-wideband (UW B) bandpass filter using short circuited stubs. The pr oposed filter is designed using combination of step impedance low pass filter (LPF) and the optimum distributed highpass f ilter (HPF). The UWB filter is deployed on FR4 Epoxy diel ectric substrate with thickness of 0.8mm and the filter oc cupies 26.4mm × 16mm die of area. This filter is operating the whole UWB passband of 3.1 GHz to 10.6 GHz. The des ign d filter has been optimized using CST microwave studi o software. The fabricated filter was tested and comp ared the result. The simulated and measured results shows the close agreement. Index Terms —Bandpass, microstrip, optimum distributed HPF, step impedance LPF, ultra-wide band (UWB). 1.INTRODUCTION ULTRA-WIDEBAND (UWB) filters have attracted much research interest and various design methods have b een reported. Because of its high-data rate capability, the UWB wireless technology with unlicensed frequency limit s from 3.1 GHz to 10.6 GHz [1]. Industries has attracted vario us devices such as antenna, RF amplifier, and power combiner/s plitter that support the technology have been investigated inclu ding a number of UWB filters which have been realized usin g some techniques such as microstrip and uniplanar circuit s [2]-[7]. Various methods and structures are being used to de velop these UWB filters [8]-[12]. Lumped element filter design is generally unpopula r due to the difficulty of its use at microwave frequencies along with the limitations of lumped-element values. Hence, micros trip filters being a popular choice. Bandpass filters (BPF) are key devices in communication systems. For ultra wideband (UWB) applications, the BPF is required to have low inser tion loss over the band 3.1 GHz to 10.6 GHz, and a flat group dela y performance within that band. In the design of ultr a-wideband (UWB) bandpass filter, the topology constructed fro m the step impedance lowpass filter (LPF) and the optimum dist ribu ed highpass filter (HPF) is employed [13]. By taking inspiration from [14][15] and focusing in the bandwidth improvement, the aim of this paper is to develop a UWB filter working in the bandwidth of 3.1 GHz to 1 0.6 GHz. The filter is designed by combining and cascading t he step impedance lowpass filter (LPF) and the optimum dist ribu ed highpass filter (HPF), which gives a good solution for a wideband performance. The design was properly calcu lated and design simulation was carried out using CST softwar e to verify the performance. 2.THEORY ANALYSIS AND DESIGN A. Step impedance low-pass filter A relatively easy way to implement low-pass filters in microstrip or stripline is to use alternating secti ons of very high and very low characteristic impedance. Such fi lters are referred to as step impedance and are popular becau s they are easier to design and take less space. The step impedance low-pass filter is designed by using chebyshev polynomial with ripple value of 0.05dB. T o determine the impedance values of the step impedanc LPF the tabulated element values given in [14] for lowpass filters are used. After doing some calculations and paramet rical studies to have optimum design, the number of react ive element is odd number in order to have a symmetrica l filter. The width of lines section for series inductor and shunt capacitor elements is 0.75mm and 3.2mm. These width s correspond with the high impedance value (Z high) of 72Ω for series inductor elements and with the low impedance value (Zlow) of 30Ω for shunt capacitor elements. The lumped elements of LPF are converted into distributed elem nts and their microwave integrated circuit (MIC) layout is shown in Fig. 1(a). The LPF MIC layout is simulated on CST microwave studio and simulated S-parameters are sho wn in Fig. 1(b). 1853 International Journal of Engineering Research & Technology (IJERT) Vol. 3 Issue 1, January 2014