A Dual-Band Bandpass Filter Using SIR Suitable for WiMAX Band Pankaj Sarkar 1 , Rowdra Ghatak 2 + and D. R. Poddar 3 1 ECE Dept., ITER, Siksha ‘O’ Anusandhan University, Bhubaneswar, Orissa, INDIA 2 ECE Dept., National Institute of Technology Durgapur, West Bengal, INDIA 3 ETCE Dept., Jadavpur University, West-Bengal, INDIA Abstract. In this paper a dual band bandpass filter with sharp rejection is proposed. The filter is realized by using two half wavelength stepped impedance resonator to operate at the passband 2.5 GHz and 3.5 GHz. To increase the band width further interdigital capacitors are introduced between resonator and input and output combining network. A finite transmission zero in between two pass band at 2.84 GHz is realized to improve the selectivity of the filter. The design procedure to get highly selective response of the proposed filter is explicitly explained. An equivalent circuit model of proposed filter is developed that matches well with full wave electromagnetic simulations. Keywords: Dual-band Filter, Stepped Impedance Resonator (SIR), Interdigital Capacitor (IDC). 1. Introduction Worldwide Interoperability for Microwave Access (WiMAX) technology has great potential in the development of modern communication system, for instance, mobile broad band connectivity, wireless connectivity, providing telecommunication services and also to facilitate machine to machine communications. Bandpass filter is a key component in receiver front end for frequency selection purpose. In the present scenario of multiband communication systems, single front end devices catering to different band simultaneously is desirable. The conventional topology of filter design fails to meet this requirement due to its large area requirements and also suffers from uncontrollable spurious pass band response [1-2] which restricts to design a multiband bandpass filter. Stepped Impedance Resonator (SIR) is widely used for designing dual band filter, where the first spurious mode is shifted to the second pass band [3-5]. SIRs with a new coupling scheme is used to realize a compact dual band BPF is reported in [6]. A new topology has been proposed to improve the response to some extent by using defected SIR [7-8]. A compact size dual band characteristic is realized by implementing Meander Loop resonator with CSRR DGS [9]. In [10] by inserting transmission zeros in either side of the passband a compact dual band BPF is realized using periodic stepped impedance ring resonator. However a dual band pass filter with highly selective response in conjunction with lower insertion loss with wide stopband is still a challenging task in microwave circuit design paradigm. Here a dual band pass filter is realized by using two SIRs of resonating frequency 2.5GHz and 3.5GHz. Impedance ratio is chosen to shift the spurious mode above 9 GHz. A new coupling scheme is also presented to combine both the resonators. Finally Interdigital Capacitors (IDC) is used to increase the bandwidth of the proposed dual band BPF. 2. Design of Proposed Dual-Band Band Pass Filter The filter is designed using half wavelength (λ/2) open end SIR. The basic structure of the λ/2 SIR is shown in Fig. 1. The resonator composed of high impedance (Z1) section of electrical length (θ1) followed by two low impedance (Z2) section of electrical length (θ2). The resonance condition of the resonator can be derived by + Corresponding author. Tel.: + 91-343-2755517; fax: +91-343-2547375. E-mail address: rowdraghatak@yahoo.com 2011 International Conference on Information and Electronics Engineering IPCSIT vol.6 (2011) © (2011) IACSIT Press, Singapore 70