Novel Rat-race Hybrid Coupler for RF Front-end Module
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Novel Rat-race Hybrid Coupler for RF Front-end Module Fei Zhang & Chengfang Li
Received: 17 October 2006 / Accepted: 13 August 2007 / Published online: 14 September 2007 # Springer Science + Business Media, LLC 2007
Abstract In this paper, a novel compact rat-race hybrid coupler incorporating an improved beeline compact microstrip resonant cell(IB-CMRC) is presented firstly. The size of the coupler is reduced by 46.5% than conventional one due to IB-CMRC’s excellent slow-wave effect. The novel coupler possesses strong ability of high-order harmonic suppression (above 30 dB) in addition. The measured insertion loss is comparable to that of a conventional microstrip hybrid coupler. Great potential applications of this IB-CMRC can be explored in MICs and MMICs to reduce the circuits size, suppress harmonics and improve the overall performance of the RF front-end module. Keywords Rat-race hybrid coupler . Beeline compact microstrip resonant cell . Slow-wave effect . Miniaturization . Harmonic suppression
1 Introduction Rat-race hybrid coupler finds numerous applications in microwave circuits and antenna systems for its simplicity in design, wide bandwidth in power division and high isolation between different ports. The rat-race coupler has dimensions a half-wavelength by quarterwavelength [1]. Due to these large electrical lengths of the transmission-line elements, ratrace coupler consumes a significant amount of circuit area. This problem worsens with decreasing frequency. Modern wireless communication systems usually require smaller hybrid coupler size so as to meet the cost reduction and circuit miniaturization, especially in the radio frequency(RF) and low microwave frequency range. Thus, size reduction is becoming major design considerations in practical applications. Lots of attempts are continually contributed to minimize the occupied area of hybrid coupler [1–5]. Defected ground structures(DGS), as one type of PCs, were utilized to reduce the size of hybrid-ring couplers [2]. But they are patterned on the ground plane and F. Zhang (*) : C. Li Department of Physics, Wuhan University, Wuhan 430072, China e-mail: [email protected]
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Int J Infrared Milli Waves (2007) 28:937–943
the components should be suspended from ground plane avoiding the invalidation of DGS function, which severely limits its practical applications. The meander-line technique [3] has also been used for the miniaturization of a rat-race coupler structure. However, the existence of sharp corners may limit the use of such configuration at higher frequencies. In addition, as the meander line is based on a single design construction, the degree of freedom in the design parameters is limited. The coupler is often realized in a circular rather than a rectangular ring geometry, in which case, even more circuit area is occupied. In either case, the interior of the ring is unoccupied. Another way to reduce the size of the hybrid ring is adopting periodical open stubs directed inside the ring. Periodical open stubs produce a slow-wave effect i
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