Fast Fourier Transforms: Automatic Edition

Figure 1: Shown is the user interface for Teledyne LeCroy's
Spectrum Analyzer software option

Many motorists love the experience of driving a sporty car with its convertible top down and a stickshift manual transmission. Others don't want the hassle of a clutch and prefer the user-friendly feel of a slick automatic that does some of the work for them. Oscilloscopes can provide the same sort of choice for many measurement tasks in the same instrument.

Going From FFTs to Spectrum Analysis

Figure 1: Spectrum Analyzer software for the HDO series
oscilloscopes provides an intuitive user interface

In earlier posts, we looked at a) the basics of fast-Fourier transforms (FFTs) and b) how to set up an FFT on a modern digital oscilloscope. In this post, we'll take a brief look at what that modern scope can do with an FFT, provided that scope is outfitted with software that will let it take full advantage. After all, the object of an FFT is to transform a time-domain waveform into the frequency domain. Sounds kind of like a spectrum analyzer, no?

Oscilloscope Basics: Setting Up FFTs

Figure 1: Capture time determines the
frequency resolution, Δf.

For most of their history, oscilloscopes have been thought of chiefly as a time-domain instrument. That is, an oscilloscope facilitates the observation of changes in a signal's amplitude over time. However, many modern digital and mixed-signal oscilloscopes provide spectral analysis capabilities based on fast Fourier transforms (FFTs) that convert a time-domain waveform into the frequency domain. There are lots of good reasons for taking advantage of this capability. Perhaps the most important is to gain insight into characteristics of the signal that simply are not apparent from a time-domain perspective.