|Figure 1: The runt pulse and non-monotonic edge anomalies|
in this signal are not apparent with a simple edge trigger.
Typically, oscilloscope users know what shape they should be seeing in their signal, but want to trigger on abnormalities in that shape. That's where these complex triggers are especially useful. One tool that helps pin down issues is exclusion triggering based on characteristics such as signal timeout, glitch/pulse width, pattern width, or signal/pattern intervals. The exclusion trigger gives oscilloscope users the best likelihood of isolating rare events by keeping the triggering circuit active nearly 100% of the time.
Let's take a look at an example of a signal with a runt pulse and a non-monotonic edge anomaly (Figure 1). Here, a simple positive-going edge trigger is in effect. Thus, the runt and non-monotonic edge are not obvious.
|Figure 2: The first of two pulse anomalies found|
using an exclusion trigger.
Let's see how applying an exclusion trigger nails this little critter. What we'll do is set up a width trigger that excludes a small range of values around the nominal value. By doing so, we should get the oscilloscope to trigger on pulses that don't fit this "nominal" criteria.
|Figure 3: The second of two pulse anomalies in our|
square-wave example: a non-monotonic edge.
This is only one example of how triggers can be tweaked beyond the simple category into much more powerful tools. Learning how to use them well goes a long way toward finding elusive problems in signals.