Alias Protection for Signal Measurements

Alias protection is critical to accurate signal analysis. Aliasing is an effect associated with digitizing an analog signal. Aliasing can cause gross errors in measurement and testing. The only way to protect against aliasing errors is by using high quality anti-alias filters on the input channels.

An everyday experience with aliasing comes from viewing movies and videos. In many scenes the wheels of a vehicle appear to actually rotate in reverse to the direction of travel. This effect is caused by optical aliasing. The frame speed of the camera is too slow to capture reality. In data acquisition systems, aliasing occurs when a high frequency signal is digitized at a sampling rate that is too slow to accurately represent the signal frequency. Digitization is performed by an Analog to Digital Converter (ADC). An ADC converts the analog signal from a sensor into a digital signal that can be read by the acquisition system's computer. Digital sampling theory states that, at a minimum, a signal must be sampled at a rate equal to twice its frequency. In other words, there must be at least two samples for every cycle of the signal.