Back to Basics: What is an FFT?

Figure 1: An FFT of a 300-kHz square wave.

In an earlier post, we discussed the basics of setting up a fast-Fourier transform (FFT) on an oscilloscope, and why you'd want to use an FFT to get a frequency-domain view of a time-domain signal in the first place. It might be a good idea to take a step back and dig into just what an FFT is (Figure 1).

Back to Basics: Creating Pulsed Waveforms

Figure 1: The Pulse
waveform dialog box.

Many test applications call for the creation of pulsed waveforms ranging from clock signals to logic control to trigger signals, among others. Often, these waveforms are used in the characterization and debug of digital devices and circuits. Stand-alone pulse generators provide one way to generate pulsed waveforms but in many cases, a general-purpose waveform generator will do a fine job.

Don't Just Trigger, But Trigger Smart

Figure 1: The runt pulse and non-monotonic edge anomalies
in this signal are not apparent with a simple edge trigger.

In an earlier post, we looked at some of the basics of oscilloscope triggering and noted that there are two broad classes of triggers: simple triggers that sense particular characteristics of the input signal (transition edges, pulse widths, and so on), and more complex triggers that let you zero in on more specific attributes of the signal based on timing and amplitude parameters.

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.