Chapter 6. Transmission Line Transformers

Summary

Impedance transformation is discussed again, but now with more careful attention given to the special problems and solutions required for RF frequency designs. The discrete element designs described previously can be used in RF designs with the understanding that element values will change as frequency changes. The alternative to discrete element circuits are transmission line circuits. The classical microwave quarter wavelength transformer can be used up to hundreds of GHz in the appropriate transmission line medium. However, at 1 GHz, a three-section quarter wavelength transformer would be a little less than a meter long! The solution lies in finding a transformation structure that may not work at 100 GHz but will be practical at 1 GHz.

The conventional transformer consists of two windings on a high-permeability iron core. Ideal transformers have perfect coupling and no losses. The leakage inductance together with the interwire capacitances limits the high-frequency response. The transmission line transformer avoids these frequency limitations.