Analysis and design of planar, spiral-shaped, transmission-line transformers

Coupled microstrip line theory is applied to analyze planar Ruthroff transmission-line transformers (TLTs). The results demonstrate that the TLT's low frequency response improves as the microstrip line's even-mode impedance increases, whereas the ratio of load impedance to the line's odd-mode impedance controls the high frequency response. Analysis of an approximate model of coupled microstrip lines formed into a spiral leads to a technique for predicting the performance of planar, spiral-shaped, Ruthroff TLTs. Microwave measurements up to 10 GHz are compared with predictions from the model for regular and suspended Ruthroff TLTs fabricated through a multichip-module deposited (MCM-D) process.