Tips and Tricks: Stabilizing a Waveform Trace

One initial challenge to oscilloscope users is achieving a stable display of an input waveform. In almost all cases, the Auto Setup button on Teledyne LeCroy oscilloscopes (and most other modern instruments) will automatically set the oscilloscope's triggering system to get that jumpy trace to settle down.

However, Auto Setup may make other changes to your horizontal and vertical settings that you may not want made. Teledyne LeCroy oscilloscopes also provide a Find Scale function in the channel setup dialog that will adjust only the vertical scale and offset without affecting the horizontal and trigger settings. Sure, you could even hit the Stop button, but now you're not looking at a live acquisition. There is a neat little trick one can use involving Smart Triggers that will help you achieve stability without altering any other settings.

Here's the thing: When you apply an input signal to an oscilloscope channel and hit Auto Setup, triggering defaults to a positive edge trigger with DC coupling. If the input signal is, say, an amplitude-shift-keying (ASK) signal, the oscilloscope will trigger on positive-going edges, but there's lots of those to choose from. The result in terms of the visual waveform display will be a trace that appears to jump around wildly.

Figure 1: Vertical cursors tell us that the time period of the
dropout in the ASK signal is about 5 ms

In such a case, and in many others, it's not enough to tell the oscilloscope to simply trigger on an edge. You've got to give the triggering system a little more information about the signal so that it can find a stable point, or characteristic of the signal, on which to trigger.

The answer is in what Teledyne LeCroy calls Smart Triggers, which are triggers that can be set up to look for specific conditions, conditions within some range, or other signal attributes. Going back to the ASK signal example, you know there is going to be some period of time in the waveform in which the signal will drop out as the modulating signal rises and falls from high to low and back again. With Smart Triggers, it's easy to take advantage of that fact to give the triggering system something to grab onto.

Figure 2: Applying a Smart Trigger, in this case a Dropout
trigger, tames this jumpy ASK signal

First, hit the Stop button so that the oscilloscope uses a set of vertical cursors to determine how long the dropout period is in the signal. We can see that the time period of the dropout is around 5 ms (Figure 1).

Now that we know something about our signal that a Smart Trigger can use, let's apply one. In this case, we're using an HDO6054 oscilloscope. Opening the Trigger Setup dialog and touching Smart Triggers in the Type section provides a range of Smart Trigger varieties. Conveniently, there's a Dropout trigger, which will trigger when the signal disappears for longer than the specified dropout value. Armed with the knowledge that our signal drops out for 5 ms, we can insert any lesser value. Now our live ASK signal will be stable, enabling us to more easily use cursors and other measurement functions.

Another way to skin this cat is to use trigger holdoff. Turn off the Dropout trigger and select the Holdoff tab in the Trigger setup dialog. Select Holdoff by time and dial in a period of >5 ms and <10 ms. This will cause the oscilloscope to trigger and then ignore triggers for the specified holdoff time. In this specific case, the instrument will ignore the last four cycles in the initial burst and be armed when the next burst begins.

So we've covered two ways to provide stable triggering, one with Smart Triggers and one without. Have fun exploring these options on your troublesome signals.