# Circuit Theory/Terminology

## Basic Terminology

There are a few key terms that need to be understood at the beginning of this book, before we can continue. This is only a partial list of all terms that will be used throughout this book, but these key words are important to know before we begin the main narrative of this text.

Time domain
The time domain is described by graphs of power, voltage and current that depend upon time. This is simply another way of saying that our circuits change with time, and that the major variable used to describe the system is time. Another name is "temporal".
Frequency domain
The frequency domain are graphs of power, voltage and/or current that depend upon frequency such as Bode plots. Variable frequencies in wireless communication can represent changing channels or data on a channel. Another name is the "Fourier domain". Other domains that an engineer might encounter are the "Laplace domain" (or the "s domain" or "complex frequency domain"), and the "Z domain". When combined with the time, it is called a "spectral" or "waterfall."
Circuit response
Circuits generally have inputs and outputs. In fact, it is safe to say that a circuit isn't useful if it doesn't have one or the other (usually both). Circuit response is the relationship between the circuit's input to the circuit's output. The circuit response may be a measure of either current or voltage.
Non-homogeneous
Circuits are described by equations that capture the component characteristics and how they are wired together. These equations are non-homogeneous in nature. Solving these equations requires splitting the single problem into two problems: a steady-state solution (particular solution) and a transient solution (homogeneous solution).