A digital signal has non-continuous and discrete value, with finite resolution in time and magnitude. Digital signal always has two states either completely on (high) or completely off (low) and magnitude of amplitude will always remain stable. A simple digital signal is represented as a square wave as shown in the following diagram:

Due to the shortcomings of analog signal in control application, we use digital signal to control the same analog circuits because of the stable and discrete nature of digital signal. And here PWM comes into the picture.

PWM is a technique of generating analog signals using a digital source. PWM signals consist of mainly two components that define its behavior, one is duty cycle (DC) and another one is frequency.

Duty cycle represents the amount of time signal remains in high (on) state as a percentage of total time it takes to complete one cycle. Frequency defines how fast a signal switches between high (on) and low (off) state. For example, if the frequency of a digital signal is 100 Hz the signal will switch its state between high and low 100 times.

The following diagram shows how the digital signal behaves when the duty cycle is varied:

So by applying appropriate duty cycle and frequency, we can create a digital signal which will behave like a constant voltage analog signal and can be used to operate our DC motor.

The main difference between a pure analog signal and a digitally created analog signal is that pure analog signal's curve is smooth (refer Figure 8.3) whereas curve of a digitally created analog signal is stepping, square, and discrete in nature (refer Figure 8.6):