Triggers Reel in Waveform Abnormalities

Knowing which triggers you have, how they work, and which to use can help you hook that elusive signal.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2000

Troubleshooting electronic circuits can resemble a fishing trip—you try to catch something as it passes by. In fishing, you drop your line into the water and, with the right bait and a little luck, you catch a fish. When you use a trigger, you probe the depths of a signal and try to capture a specific characteristic of the waveform. As when fishing, though, you’ll get better results if you know what you want to catch; then, you’ll know which bait—or scope trigger—to use.

Digital storage oscilloscopes (DSOs) give you more than just the simple edge trigger you get on analog scopes. A DSO’s advanced triggers—including glitch, runt, pattern, dropout, qualified, or slew rate—let you capture anomalies in signals. But you can’t select a trigger until you have an idea of what you’re looking for. Furthermore, you should understand how a trigger in a DSO differs from a trigger in an analog scope.

When you think of a scope trigger, you think of an event in a signal that initiates another event. In analog scopes, when the signal meets the trigger’s condition (an edge exceeding the trigger voltage level, for example), the scope’s beam begins to sweep across the screen from left to right.

DSOs work differently. A DSO continuously captures and displays data. When you set a trigger, the DSO looks for the conditions you set. If it finds a match to those conditions, the DSO plots data from all or part of its acquisition memory on the screen. In other words, a trigger doesn’t initiate a signal capture, nor does it initiate a trace on the scope screen. Instead, a trigger marks a place in the scope’s memory. That place need not be at the beginning of the scope’s memory, either. The trigger condition can occur anywhere in memory, which means you can view events that occur before, during, and after the trigger point. The term "trigger" isn’t quite accurate in the realm of DSOs, but it’s the term we all know so we use it.

With the assortment of triggers available in DSOs, you can view just about any characteristic of a digital or analog signal, including

• pulses that are too short, too long, or just right,

• pulses that are too high or too low in amplitude,

• conditions that occur only after an initial condition, and

• combinations of levels and edges across channels.

Each of the available triggers can help you view error conditions and diagnose circuit problems.

Figure 1. When you first use an edge trigger, you may get an unstable display, indicating that you’ll need a more sophisticated trigger.

Figure 2. Stopping the scope lets you see the waveform, which can show pulses of several widths, indicating the need for a pulse-width trigger.

Figure 3. A pulse-width trigger can find a glitch.

Try stopping the scope. You’ll get a static display and you can scroll through it to look for conditions that you can use to trigger the scope. In Figure 2, a stopped scope reveals a waveform with several pulse widths. No wonder the edge trigger didn’t work; the edges occur at irregular intervals. Now, you have a clue as to which trigger to use.

In this case, a more useful trigger would be a pulse-width trigger. In a signal with pulses of three different widths, a pulse-width trigger gives you three options: You can trigger on the widest pulse, the narrowest pulse, or the middle-width pulse. In this case, triggering on the narrowest pulses wouldn’t be a good choice, because the signal has two identical conditions.

Suppose you decide to trigger on the widest pulse. You would first need to measure the width of the pulses using the scope’s cursors. Most DSOs calculate the time difference between two cursors. In the pulse in Figure 2, the widest pulse measured 6 ms and the middle-width pulse measured 3.8 ms. You could, for example, set the scope’s pulse-width trigger to trigger the scope on any positive-going pulse more than 5 ms wide.

You can also use a pulse-width trigger to uncover intermittent glitches. Suppose the trace in Figure 2 occasionally had a pulse you could see while the scope was running but that didn’t appear often enough for you to see it with the scope stopped—even with 1 Msample of memory. To capture that pulse, you could set the scope to trigger on any pulse of a width less than the minimum width of a good pulse. For the trace in Figure 2, for example, the minimum width of a good pulse was 2 ms. Setting a pulse-width trigger to capture narrower pulses would capture a glitch such as that shown in Figure 3.

Some scopes let you trigger on out-of-range pulse widths rather than just on in-range pulse widths. Suppose you wanted a scope to trigger on any pulse that didn’t meet timing specs—that is, a pulse that’s either too wide or too narrow. To do that, you specify an acceptable range and set the trigger to find any pulses outside that range. The scope screen in Figure 4 displays a signal triggered when the pulse width is less than 0.995 ms or greater than 1.01 ms.

Glitch Trigger
Another way to find a glitch is to use a glitch trigger. A glitch trigger is a specialized pulse-width trigger in which the scope triggers on a pulse of width that is narrower than acceptable pulses.

Figure 4. The scope triggers on the pulse that doesn’t meet the trigger qualifications. (Courtesy of LeCroy.)

Figure 5. A pulse in the upper trace that doesn’t reach the upper threshold (dashed line) triggers the scope. (Courtesy of Tektronix.)

Window Trigger
A window trigger complements a runt trigger. Finding use in analysis of disk-drive signals, window triggers capture signals that fall outside both high and low voltage levels. The scope triggers when the signal’s amplitude exceeds V_H, and it also triggers when the signal drops below V_L. Typically, you set V_H and V_L to the threshold limits of logic levels for a given logic family.

Setup-and-Hold Trigger
In addition to voltage, you can also use a signal’s timing to trigger a DSO. A setup-and-hold trigger monitors two signals—clock and data—and evaluates their edge-to-edge timing relationship. You must define a "setup/hold" violation zone relative to the clock. Data that changes state within this zone triggers the scope, indicating a timing problem.

Dropout Trigger
Rather than triggering when an event happens, a dropout trigger captures a signal when it is inactive for a specified period of time. Many telecom signals, for example, encode a clock signal into the data. Clock encoding requires a maximum amount of time between transitions so a receiver can extract the clock. You could use a dropout trigger to monitor the data stream and trigger a scope when the time between transitions exceeds the period set in the trigger. In other words, each transition resets the dropout trigger’s clock so that the absence of a transition allows the dropout trigger’s timer to time out, triggering the scope.

You can use divide-by-N triggers when you want to measure signals on rotating devices relative to position. Rotating devices such as disk-drive platters and shaft encoders produce pulses that indicate rotational position instead of time. Most rotating devices initiate a pulse indicating the start (or completion) of a revolution, but they also may produce a pulse every 1/Nth revolution. To capture the pulse that begins (or ends) each revolution, you could trigger the scope on the same relative angle for each revolution regardless of rotation speed, if you set your value of N equal to the number of pulses per revolution.

Figure 7 shows an example of a qualified trigger that’s delayed by N events. Here, the scope triggers on the fourth rising edge in the second trace following a rising edge in the upper trace. The two lower traces show the area around the trigger in more detail.

A qualified trigger also can use time delay instead of events to decide when the scope triggers. Take, for example, a serial bus application. You may qualify the trigger on an initial event but delay the trigger by several milliseconds to skip over bursts of activity. After the delay time expires, the scope will trigger on the next burst of activity.

You also could use a qualified trigger to "filter" bounces in a mechanical switch. Assume that a switch can bounce for up to 2 ms after changing states. You could set the DSO to trigger on the first transition that occurs at least 2 ms after the previous one.

Slew-Rate Trigger
What if a pulse occurs at the right time but takes too long to rise or fall? Digital signals don’t always rise or fall as you’d like them to. A pulse with a risetime or falltime that is too fast or too slow might cause a data error or might cause a circuit component to operate improperly. That’s where slew-rate triggers, sometimes called transition triggers, can help. In Figure 8, the falling edge of one pulse is more sloped than the falling edge of the other. To catch that slowly falling edge, you could set the scope to trigger on a falling edge whose slope exceeds some pair of voltage levels. In this case, the falling edge must fall from 1.402 V to 0.128 V in 10 ns or less, which corresponds to a slew rate of 127 mV/ns or faster. If a pulse falls more slowly, it triggers the scope.

Figure 8. Using a slew-rate trigger, the scope triggers when a falling edge exceeds a preset falltime. (Courtesy of LeCroy.)

Figure 9. A pattern trigger looks at the states of two or more signals and triggers the scope when the pattern matches a specified set of conditions. (Courtesy of LeCroy.)

Pattern Trigger
When you need to catch a specific pattern across two or more channels, you can turn to a pattern trigger. In effect, a pattern trigger lets you use a scope like a logic analyzer.

Pattern triggers wait for a specified combination of events before activating. On some scopes, you have two options when setting a pattern trigger: You can set a scope to trigger as soon as a specified pattern occurs, or you can set it to trigger when the pattern ceases.

A pattern trigger’s events are high levels, low levels, rising edges, and falling edges. Figure 9 shows a typical pattern trigger. The trigger activates when channel 1 contains a low-level signal and channels 2, 3, and 4 contain high-level signals.

Unlike a logic analyzer, which has just two levels (high or low), scopes can resolve a signal to at least 250 levels. Therefore, you must define digital highs and lows on your scope with respect to voltage levels. Typically, you should set those voltage levels consistently with the threshold for the logic family whose signals you’re measuring.

Before scopes had pattern triggers, you may have had to build a logic circuit that generated a pulse based on a set of input conditions and then connected that pulse to the scope’s external-trigger input. Pattern triggers may eliminate that, depending on your scope’s number of channels. If you need to trigger on a pattern of more signals than you have scope channels, you’ll have to either design your own trigger circuit, use a logic analyzer, or use a scope that combines logic channels with oscilloscope channels.

Select Your Bait
So, now you know many of the forms of bait your scope may use to catch signals, but how do you know which one to use? Finding a signal error is often the difficult part of troubleshooting a circuit. Unfortunately, if you have an intermittent problem, the scope may not have captured the error.

A DSO’s persistence feature helps you look for glitches and temporary dropouts. Many scopes now come with a color-graded or intensity-graded persistence that clue you as to how often an error occurs.² Of course, you can still use an analog scope, which also may give you a view of the signal error, and then use a DSO to reel in the problem.

If you have a DSO without persistence and don’t have an analog scope, don’t give up. You’ll just have to fish in the dark for a while. Mike Wadzita, product planner at Tektronix (Beaverton, OR), suggests the following procedure for locating a signal error:

1. Check that your scope probe has enough bandwidth for the job. A passive probe, for example, may filter glitches on a signal so your scope can’t catch the error.
2. Stop the scope and manually look for anomalies by adjusting the horizontal position control.
3. Look for stable logic levels. Ringing or slow transitions can cause data errors.
4. Use the runt trigger, which will help you find a signal condition that falls between the logic levels of its logic family.
5. Use the glitch trigger, which will catch any pulse that’s shorter than what your system should have.
6. Use the slew-rate trigger to find any fast or slow edges you may have missed in step 1.
7. If you suspect that more than one line driver is driving a bus at the same time, use a pattern trigger.

Overall, the type of trigger you use depends on the nature of the problem. Triggers can help you snag that elusive signal error, but expect to spend some time fishing. T&MW

FOOTNOTES
1. To create the images for Figures 1, 2, and 3, I used an Agilent HP 54622D DSO, which the company lent me for several weeks.

2. Rowe, Martin, "DSO Displays: Almost as Good as Analog," Test & Measurement World, February 2000. p. 41.   

FOR FURTHER READING
Coombs, Clyde F., Electronic Instrument Handbook, 3rd. ed., McGraw-Hill, New York, NY, ISBN 0-07-012618-6, 2000.

DL7100 Digital Oscilloscope: Important Functions and their Specifications, Bulletin 7014-01E, Yokogawa of America, Newnan, GA, 1999.

Fundamentals of DSOs. LeCroy, Chestnut Ridge, NY, 1998. www.lecroy.com/Tutorials/Fundamentals/fund.html.

You can contact Martin Rowe at m.rowe@tmworld.com