A CAD Model of Triple Bandpass Filter Implemented with Mushroom Structure Seema Awasthi, Animesh Biswas, Mohammed Jaleel Akhtar Department of Electrical Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh, India 208016 Received 8 April 2013; accepted 8 July 2013 ABSTRACT: This article demonstrates the design of planar triple-band filter using mush- room resonator for significant size reduction of the order 80%. Here, mushroom type reso- nator is used to realize a triple passband filter using the coupling matrix approach. The electric and magnetic couplings between the mushroom resonators are achieved by proper positioning of corner via which have been analyzed and presented. Finally, the frequency transformation has been applied to design a highly selective 12-pole triple-passband cascaded-quartets filter. This symmetric triple-band filter is operative at center frequencies 3.41/3.56/3.72 GHz with 100 MHz bandwidth of each band. It is observed that the measured results are in good agreement with the simulated results. A significant size reduction has been achieved while realizing triple-passband filter, which is as small as 50 mm 3 25 mm 3 1.27 mm. VC 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:421–428, 2014. Keywords: triple bandpass filters; mushroom structure; electric and magnetic coupling; planar filters; coupling matrix synthesis I. INTRODUCTION The present modern communication system requires multi-band operation. This initiates the need for micro- wave devices capable of working on multiple frequency bands. In recent era, technology is more directed toward miniaturization and energy efficient systems. To minimize the size and cost of the circuit, there is a trend to design a single circuit that is capable of producing the different designated bands. Beside the multiband operation, high performance and compactness are desired features for multiband filters. Different techniques have been reported to provide passive component size reduction, among which application of the metamaterials is used widely [1]. There has been growing interest in research and develop- ment of the devices (antennas, filters, diplexer, power divider, etc) using the metamaterial concept and it has generated a great attention from the scientific and engi- neering community working in RF and microwave field [1]. In previous research of metamaterial filters, Martin and coworkers have demonstrated that split ring resonators (SRRs), complementary split ring resonators (CSRRs), and spirals can be applied to the design of compact nar- row bandpass filters and diplexers [2]. However, they have moderate selectivity performance attributed to the smooth transmission band edges and low out-of-band rejection levels above the pass band. The effective area reduction between metamaterial filters and microstrip open loop filters resonators has been compared by Martin and coworkers [3]. It has been shown that significant reduction in area takes place. Recently, a Sievenpiper mushroom structure is used to design a bandpass filter [4, 5]. The mushroom structure consists of a patch and via. These resonators are much smaller in size compared to the conventional Right-Handed (RH) resonators and have significant advantages in building filters with small size [4]. Single bandpass filter based on mushroom structure with corner via has been presented in [6]. Similar struc- ture is used in this article to implement triple-band filter. To generate a multiband filter, it is still a real chal- lenge to synthesize the coupling coefficients and the external quality factors accurately for multiple bands. The frequency mapping is a systematic procedure for design- ing symmetric and asymmetric multiple-band filters [7]. Frequency transformation for dual and triple passband fil- ter is presented in [8, 9]. Similar transformation for Correspondence to: S. Awasthi; e-mail: sema@iitk.ac.in. DOI: 10.1002/mmce.20780 Published online 24 October 2013 in Wiley Online Library (wileyonlinelibrary.com). VC 2013 Wiley Periodicals, Inc. 421