A lot of this book has been concerned with time-harmonic excitation and behavior (frequency domain) of passive and distributed elements. This is convenient – as the differential equations simplify to algebraic ones – and it is also a very common situation. Most RF circuits, for example, are narrowband, and therefore for all practical purposes the input signal is a sinusoid at the carrier frequency. But there are a host of situations where the signals of interest are wideband. This includes digital switching circuits, high-speed data circuits, and ultra-wideband circuits and systems. Furthermore, digital circuits are switching increasingly faster, now into the multi-GHz regime. There are also many high-speed links, such as chip-to-chip communication on a PCB. These high-speed interface circuits drive pulses or spectrally rich waveforms on long board traces. We will treat these different cases in a uniform manner by generically calling these circuit applications “high-speed” switching circuits.

Transmission lines and high-speed switching circuits

The focus of the chapter is switching waveforms on transmission lines. You may wonder why we would use transmission line analysis for switching circuits, especially small on-chip or on-board circuits. Let's take a practical example to get a feel for the problem. Let's say a digital chip has a dimension of less than 1 cm. For a time-harmonic circuit, we know that we may treat most structures as lumped as long as their dimension does not exceed 1/10λ.