3.9: Summary

This chapter considered the most important planar transmission lines: microstrip, CPW and stripline. These lines can be produced using printed circuit board techniques. At frequencies below \(1\text{ GHz}\), economics require that standard FR4 circuit boards be used. The weave of a conventional FR4 circuit board can be a significant fraction of critical transmission line dimensions and so affect electrical performance. Nonwoven substrates and sometimes hard substrates such as alumina ceramic, sapphire, or silicon crystal wafers are often required. These also provide higher-dimensional tolerance than can be achieved using conventional woven FR4 substrates. In general, once a design has been optimized in fabrication so that the desired electrical characteristics are obtained, microwave circuits using planar transmission lines can be cheaply and repeatably manufactured in volume. This is true even with ceramic substrates that shrink when they are fired in a process performed after the transmission lines have been patterned. Microstrip and CPW are ideal transmission lines enabling surface modification in design optimization. Buried transmission line mediums such as stripline are difficult to rework, but of course allow more compact designs and thus lower unit costs. However, the rework difficulty results in higher design cost. Stripline does have an advantage over microstrip. Microstrip radiates, but with the fields confined between ground planes, stripline does not. Also stripline design enables multilayer circuit boards to be used, and thus results in smaller overall circuit size, at the price of higher design cost since design on-the-bench optimization are much more difficult.