Scope Measurement Hints

Use Holdoff to Stabilize Complex Digital Waveforms

Scope users typically start out using edge triggering. In this mode, the scope will trigger on any edge that meets the setup criteria, for instance any rising edge on channel 1 at 1.5 V. The trigger event becomes the time reference for displaying the data. This presents a problem when there are many edges at different positions in the waveform that meet this criteria, as is the case with complex waveforms. The scope triggers on multiple edges and overlays the waveforms with different time references. The result is a complicated display that looks untriggered. See Figure 1.

Try using your scopeís holdoff feature to stabilize such a waveform. When holdoff is used, the scope triggers on the first rising edge it sees, but then waits the holdoff time before re-arming the trigger to look for the next trigger event. Use a holdoff time just less than the period of the repetition of the waveform. For this example, the holdoff time is 28 microseconds. See Figure 2. The scope now triggers on the same edge every time, so the data has the same time reference each time it is displayed. It is now evident that there is an intermittent glitch.