An analog signal can have an amplitude of any voltage within a range, unlike a digital signal, which can be in one of two defined voltage states (either high or low). Figure 2-1 shows a typical analog signal (in this case, a sine wave).

Figure 2-1. An analog waveform

The voltage of a signal may vary over time, or it may be constant. If the voltage varies, it may repeat at regular intervals, in which case the signal is said to be periodic. The period is the interval of time that it takes the signal pattern to repeat (for example, from one wave crest to another). The frequency of the signal is the number of times per second that the pattern repeats.

Frequency is measured in Hertz (Hz) and relates to the period in the following way:

Frequency = 1 / Period

Thus, a signal with a period of 1ms has a frequency of 1kHz.

A unipolar signal (Figure 2-2) has component voltages that are either all positive or all negative. A bipolar signal (Figure 2-3) has both positive and negative voltages.

Figure 2-2. Unipolar signal

Figure 2-3. Bipolar signal

A typical analog signal will have both an AC component and a DC component (Figure 2-4). The DC component is the fixed voltage of the signal. The AC component is a varying voltage imposed upon the DC component. The AC component is sometimes referred to as the peak-to-peak amplitude of a signal and is denoted with the suffix pp. For example, an AC component of 5V would be written as 5Vpp.

Figure 2-4. DC and AC components of an analog signal