Finding Intermittent Events

Figure 1: Statistics for 1261 Width measurements
taken over 97 acquisitions on the Measure table.
Width statistics can help determine the set up
of a Glitch SmartTrigger.

Glitches, dropouts, runts, aperiodicity, missed cycles, slow edges—whatever you call them, they are irregular waveform elements that can wreak havoc with you circuit operation. Because they do not occur with regularity, they can be hard to find and correlate with whatever synchronous events may be causing them. How can you use your oscilloscope to easily find intermittent events where they occur? The answer is by judicious application of the oscilloscope’s measurement statistics and SmartTriggers®.

Glitches

Let’s start with an easy one, a glitch. A glitch is a spurious pulse or spike that is generally of much shorter duration than the other pulses in your signal. First, you have to detect the presence of a glitch. The best way to do that is to apply measurement statistics to a lot of acquired waveforms—which, by the way, is the best way to detect the presence of any type of anomaly. The Width measurement parameter with Statistics turned on will measure each pulse acquired and report the mean, min, max, etc. on the Measure table (Figure 1).

The minimum Width value recorded is then used to set the upper limit for a Glitch trigger. Our trigger limit was set to 200 ns, slightly higher than the 167 ns minimum value discovered by the statistics, so that the oscilloscope triggered when a pulse width around that minimum value occurred. A Width trigger could also be set up to catch glitches using the same formula. 

Figure 2: The Runt trigger captures pulses that
cross the lower amplitude limit but fail to cross
the upper limit within the specified time.

Take a Single acquisition, and the trigger should eventually “catch” one and display it on screen. Once the glitch is found, zoom around the trigger time to see it in more detail.

Runts

Setting up a trigger to catch runts, pulses that have lower than acceptable amplitudes, is done similarly, as shown in Figure 2. The Runt trigger captures pulses that cross the lower amplitude limit but fail to cross the upper limit within the specified time. 

The threshold levels for the Runt trigger are most easily discerned by using the oscilloscope Find Levels feature. In this example, they are set roughly 300 mV inside the base and top levels.

Slow Edges

Figure 3: The Slew Rate trigger finds edges with
slow rise/fall times. The Rise or Fall parameters
help detect the presence of slow edges.

The Slew Rate trigger and Rise (or Fall) measurement can be used to find edges with excessively slow rise or fall times. In Figure 3, the Rise measurement maximum of 103.6 ns is much greater than the mean risetime of 31.96 ns, indicating the occurrence somewhere of a slow edge. The Slew Rate trigger Lower Interval time of 50 ns is set to be higher than the mean Rise measurement value, but less than the maximum. Unfortunately, there is no "rule of thumb" as to exactly where you should set your trigger limits. Some trial and error will be needed to fine tune triggering thresholds.

Zoom trace Z1 in Figure 3 shows the slow edge caught at trigger time 0, which has a visible step compared to the Z2 zoom of a normal transition.

Drop Outs

Figure 4: The Dropout trigger initiates an
acquisition when there is no trigger-level crossing
within the preset time interval.

Our final trick is to trigger on nothing. Actually, it is to trigger the oscilloscope on a drop out—the loss of detectable pulses within a signal. The Dropout trigger can be used to capture times when the signal “disappears”.  This disappearance of the signal is determined by the lack of a trigger-level edge crossing within a predetermined time interval (Figure 4). 

The Period measurement is used to help determine the Dropout Condition time interval. In our example, the 45 µs timeout period is set at slightly greater than the minimum period of 41 µs, but less than the maximum value of 90 µs which reads the duration of the drop out.

Learn more about Finding the Root Cause of Intermittent Failures in our on-demand webinar.