2 Low-Pass Filtering

A Fourier analysis of a typical PWM signal shows a peak at the carrier frequency, with higher order harmonics present at the integer multiples of the carrier (see Figure 2-1). These signals add unwanted noise to the system and can be reduced or eliminated using a simple low-pass filter.

The bandwidth of the desired signal should be less than or equal to the PWM frequency (see Figure 2-2). If the bandwidth of the desired signal is equal to the PWM frequency, a brick-wall type of filter may be used. The brick-wall type of filter transitions from no attenuation to complete attenuation almost instantly, but is a very expensive and complex filter to create. If that type of precision is necessary, it might be less expensive to use an external DAC than to build an expensive filter. For practical purposes, an external RC low-pass filter can be used as shown in Figure 2-3. If the simple RC filter is used, the bandwidth of the desired signal must be less than the PWM frequency.