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26 June 2013

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).

Sequence mode is a fast acquisition mode that functions by dividing the instrument's acquisition memory into thousands of segments. Each of these memory segments stores a single acquisition, creating a single waveform of individual segments. Each segment is triggered, captured, and stored when a predetermined number of triggers is reached. Thus, sequence mode is ideal for capturing either fast pulses in quick succession, or events spaced over long periods.

Sequence mode enables fast trigger rates for capturing many fast pulses. It also optimizes memory usage by not capturing dead time (Figure 1). All triggers are time-stamped and displayed for the user's reference.

For example, consider a signal captured from an echo-ranging system in which a transmitted pulse is followed by a reflected pulse with position and amplitude that varies with distance. That is what is depicted in Figure 1 with, in this instance, the oscilloscope being set to make 10 acquisitions (with an HDO6000 oscilloscope, this number can be set as high as 65,000).

Sequence mode may be combined with complex triggers to isolate rare events. As a result, users can capture all instances over periods of hours or even days, viewing and/or analyzing each event at a later time. Further, there is a variety of ways in which the acquisitions can be viewed: adjacent (the default display mode), overlay, waterfall, perspective, and mosaic. Each of these views provides a different way to visually compare segments.

Not every measurement task involves acquisitions of lots of fast pulses or isolated events spread out over long periods of time. But when these sorts of measurements are called for, sequence mode is a great tool for capturing them in a way that's both useful and efficient.

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