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05 August 2014

Back to Basics: History Mode

Initial setup of WaveSurfer 3000 oscilloscope
Figure 1: Initial setup of WaveSurfer 3000 with a
2-MHz pulse waveform fed into Channel 1
Back in the day, one of the biggest deficiencies of early digital oscilloscopes was their lack of memory depth. A memory of 500 or 1000 points was about as good as it got, and this didn't provide much in the way of detailed waveform capture. Today's instruments are very different animals; for example, Teledyne LeCroy's recently introduced WaveSurfer 3000 oscilloscopes offer up to 10 Mpoints of memory per channel.


All that memory comes into play when one invokes History Mode, a debug and analysis tool found in many Teledyne LeCroy oscilloscopes. History Mode lets users scroll back in time to isolate anomalies or other waveform events. Then, they can be measured with parameters or cursors to quickly turn up the sources of signal problems.

Often, when we view waveforms on an oscilloscope's display, unexpected events may occur and we'll want to stop the trigger and view these interesting little behaviors. The great thing about History Mode is that it automatically stops the trigger and displays a list of previously captured waveforms. This shortens the amount of time it takes to identify problems, enabling us to view, quantify, and analyze those anomalies in our signal.

Here's an example, in which we fed a 2-MHz pulse into Channel 1 of a WaveSurfer 3000 (Figure 1). The best way to access History Mode is directly from the front panel; it's also available in the Timebase drop-down menu. Pressing the History button stops the trigger and displays a list of previously captured waveforms alongside the waveform grid. Here's how the History Mode dialog box looks along with the waveform list (Figure 2).

Invoking History Mode with the front-panel button opens the related dialog box at bottom.
Figure 2: Invoking History Mode with the front-panel button
opens the related dialog box at bottom. 
A neat feature of History Mode is its interactivity; tapping any line in the waveform table automatically brings up that acquisition. Scrolling up and down the table gives users direct access to any acquisition.

The Waveform History Navigation controls in the History Mode dialog give users a flexible means of traveling backward and forward through all of the acquisitions stored in memory. The controls offer slow/fast scrolling, single stepping, and a second scroll bar (Figure 2, again).

The top row of controls regulates scrolling with the two outer buttons starting a fast scroll and the two inner buttons a slower rate of scrolling. On the bottom row, the two outer buttons take the user to the oldest and newest acquisitions while the inner pair step through one at a time. You can tap the index box to directly enter a specific acquisition. The Relative Times check box toggles between the table displaying the absolute times of acquisitions and the relative time from the last acquisition.

Users can perform measurements and/or math operations on any History Mode acquisition.
Figure 3: Users can perform measurements and/or math
operations on any History Mode acquisition.
Any displayed acquisition in History Mode is saved by opening the File menu and selecting Save Waveform. Users may also apply measurements or math operations to any selected History Mode acquisition (Figure 3).

History Mode, then, is a great debug and analysis tool, automatically saving all acquisitions and giving users a convenient means of navigating amongst them with a tabular list.

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