Background

This section will cover the operation and theory of the FFT analyzer, which is the most commonly used piece of signal analysis equipment in the vibration field. Many workers think of the FFT analyzer as a "magic box," into which you put a signal and out of which comes a spectrum. The assumption usually is that the spectrum tells the truth -- the box cannot lie. We will see that this assumption is valid in many cases, but we will also see that we can be misled, for there are several pitfalls in the process of digital signal analysis. One of the purposes of this section is to help you avoid falling into any of the pitfalls, and if you do, how to crawl out smelling like a rose.

FFT analysis is but one type of digital spectrum analysis, but we will not concentrate on the other types because they do not apply directly to the VMS program.