The time domain characterization of planar microwave transformers for monolithic RF power amplifiers using the SCN-TLM method is described in this paper. The primary objective of this study is to determine the influence of losses on the electrical properties of such transformers. For this a simplified model and a full model of the real transformer are investigated. We found that the principle loss mechanism originates from conductor losses of the windings. The influence of the lossy Si substrate can be neglected up to 5 GHz
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