Abstract
Branch-line couplers (BLCs) are important components of wireless communication systems. Conventional BLCs are often characterized by large footprints which make miniaturization an important pre-requisite for their application in modern devices. State-of-the-art approaches to design compact BLCs are largely based on the use of high-permittivity substrates and multi-layer topologies. Alternative methods involve replacement of transmission-line sections of the circuit, with their composite counterparts, referred to as compact cells (CCs). Due to the efficient use of available space, CC-based couplers are often characterized by small footprints. The design of compact BLCs is normally conducted based on engineering experience. The process is laborious and requires many adjustments of topology followed by manual or, semi-automatic tuning of design parameters. In this work, a framework for low-cost automated design of compact BLCs using pre-defined CCs is proposed. The low cost of the presented design technique is ensured using equivalent-circuit models, space mapping correction methods, and trust-region-based local optimization algorithms. The performance of the framework is demonstrated based on three examples, concerning the design of unequal-power split coupler, comparison of automatically generated compact BLCs, as well as rapid re-design of the coupler for different substrates. Furthermore, the approach has been benchmarked against the state-of-the-art methods for low-cost design of circuits.
Highlights
Branch-line couplers (BLCs) belong to the key components of many microwave devices including beamforming networks [1,2], balanced mixers [3,4], amplifiers [5,6], multiplexers [7,8], and others.Conventional BLCs are characterized by large dimensions as they consist of two orthogonal pairs of 90◦ transmission lines (TLs)
The methodology is demonstrated based on three examples concerning (i) design of a compact coupler with unequal power-split, (ii) comparison of BLCs characterized by different topologies w.r.t. size/performance trade-off, and (iii) rapid re-design of structures for various substrate parameters
The starting points for compact cells (CCs) optimization were obtained using procedure of Section 3.1 and the designs xH* = [2.49 0.41 2.29 0.49 0.66 0.48]T and xV* = [3.38 0.2 0.2 0.82 1 1]T were found through minimization of (17)
Summary
Conventional BLCs are characterized by large dimensions as they consist of two orthogonal pairs of 90◦ transmission lines (TLs). Conventional approaches to couplers miniaturization include utilization of high-permittivity substrates [15], or replacement of TLs with their lumped-element equivalents [16]. An interesting alternative is in the replacement of conventional TLs by so-called compact cells (CCs). The latter are normally implemented using high-impedance lines [17], or a combination of high- and low-impedance sections [16], and as appropriately folded TLs [18], coupled sections featuring unequal length [19], or fractal-based topologies [20]. A small size is often achieved at the expense of degraded electrical properties
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