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A Spice Model of Transmission Lines Transformer (TLT)
Keywords: Broadband Transformers, Transmission Lines, Spice Modeling
The transmission line transformer (TLT) is widely used in RF broadband matching circuits and measuring circuits in EMC. There are many constructive versions but being interested in a principle aspect we have used the simpler TLT - the inverting type for the presentation. In literature the TLT is considered as a transmission line (having conductors (1) and (2)), and some design equations are provided. The paper propose to model this circuit not as a single line, but as a two coupled lines assembly, line 1 composed by (1) and ground G, and line 2 by (2) and ground G. In this case we have to study the coupling between two symmetrical lossless lines with common return. The aim of the paper is to analytically and experimentally validate such an approach, and then, based on this analysis, to find a SPICE model for the transmission line transformer. The validity of this explanation is based on the analysis of the closed-form response of the inverting TLT, and on experimental data. Starting from these results, a SPICE model for the inverting TLT is proposed, and the simulations are compared to classical results and experimental data. From these simulations, all the important conclusions of the classical theory can be refound, without using different approaches at LF and HF, and the experiments show a good agreement with the proposed model. The influence of the core can be easily explained with the new model, and this unitary approach. In our opinion, this model, which can be extended for other types of TLTs, allows a better physical understanding of the TLT behavior, and allows us to include the TLT in larger RF circuit simulations. Using the SPICE model, all the influences (core, terminations, lines) can be easily evaluated.
Sergiu Radu, Senior Staff Engineer, Ph.D.
Sun Microsystems
Menlo Park, CA
USA


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