Application of mode transformers for characterization of symmetrical bi-modal planar transmission lines

A new approach for characterization of electrically symmetrical bi-modal planar transmission lines is presented. The method relies on the application of standard two-port vector network analyzer (VNA) and mode transformers used to exert single mode propagation in the line under test. Verification of practical usability of developed method has been performed for coupled coplanar waveguides (C-CPW). Good coherence between results of measurements of both modal propagation coefficients and characteristic impedances with solutions obtained from the full-wave computations has been achieved. Acquired time domain reflectometry (TDR) waveforms have demonstrated that discontinuities introduced to the investigated structure response in case of the use of mode transformers (single-end driving) are comparable or smaller than in case of differential driving. Moreover, TDR measurements have demonstrated that, dependently on the nature of used transformer, either power-current or power-voltage definition of the modal characteristic impedance provides the best coherence with measurement.