Signal integrity engineers have good reasons for preferring to work in the time domain. There is no reason to abandon the time domain now, but there is good reason to add the frequency domain to your areas of competence. First we will concentrate on time domain, later we'll concentrate on frequency domain.

When functions are expressed as a function of time, they are said to be in the time domain. Examples are such things as voltage, v(t), or current, i(t). Similarly, an oscilloscope waveform is almost always a time-domain presentation. In SPICE, time-domain analysis is performed by the .tran statement. This statement tells the simulator to observe the circuit for some specified amount of time. The simulator is usually initialized at the beginning of this time period with a voltage step or a pulse.