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.

Waveform Generators, Arbitrary and Otherwise

Figure 1: Teledyne LeCroy's ArbStudio 1104 is a four-channel,
16-bit arbitrary waveform generator with a maximum interpolated
sampling rate of 1 GS/s.

Next to an oscilloscope and perhaps a digital multimeter, a waveform generator is one of the most versatile and useful pieces of test equipment on the bench. Often, a device or circuit under test will require some kind of signal stimuli with which to confirm proper function and/or ferret out faults. This can run the gamut from a simple swept sine or pulse waveform for purposes of characterizing signal response, to advanced serial-data protocols, and even to the playback of analog signals captured in the real world. A waveform generator is your ticket to creating the required stimuli for your device or circuit under test.

Back to Basics: Differential Probing

Figure 1: Emitter voltage measurement
in simplified schematic view

Whether or not we think of it in such terms, any voltage measurement taken with an oscilloscope or voltmeter is, in reality, a differential voltage measurement. A voltage is, by definition, the difference in electrical potential between two points in a circuit. It's impossible to take a voltage measurement with only one voltmeter lead. One lead must be attached to the point of interest while the other must be connected somewhere else as a reference point.

The Realities of Oscilloscope Probes

Figure 1: Teledyne LeCroy's ZS2500

high-impedance active probe

Looking for a good, albeit very expensive, paperweight? Try using an oscilloscope without probes! Probes are often taken for granted but they are one of the most critical elements of the signal chain in any test scenario.

Back to Basics: Sequence Mode

Figure 1: Sequence mode enables fast trigger rates
and optimizes memory usage by
ignoring dead time.

Now and again, an oscilloscope user may need to capture either a large number of fast pulses in quick succession, or a small number of events separated by relatively long periods of time. Either of these scenarios are challenging with typical acquisition modes. Fortunately, most modern oscilloscopes offer what we call "sequence mode" (other oscilloscope makers refer to similar acquisition modes as "fast-frame" or "segmented memory" mode).

Back to Basics: Random Interleaved Sampling

Figure 1: This image illustrates the
general principle underlying RIS.

Modern oscilloscopes come with all kinds of bells and whistles, and users might be tempted to invoke them for all sorts of situations. But not every whiz-bang feature of an oscilloscope is applicable all the time. Rather, some features are great in the right applications but disastrous in others.

Oscilloscope Basics: Sampling Rate

In a recent overview post on oscilloscope banner specifications,
one of the topics covered is sampling rate. Let's do a somewhat deeper dive on that topic and look at what sampling rate means to oscilloscope users.