Zone Trigger in The Oscilloscope This Allows Complex Signal Anomalies to Be Graphically Isolated

Complex signal anomalies are often difficult or impossible to detect with conventional hardware triggers. With graphical zone triggers, errors can be isolated by simply marking them on the oscilloscope display. A guide for the time and frequency domain.

The oscilloscope is the measuring instrument for troubleshooting and problem detection in electronic and electrical systems. A central element is the trigger function for isolating certain events. Oscilloscopes usually have several trigger types for investigating different events such as edges, pulse widths, patterns and other parametric conditions.

These hardware-based triggers are suitable for isolating rare events, but sometimes reach their limits. For example, a user may see an anomaly graphically, but not be able to easily isolate the event with the available trigger options. A graphical trigger, also known as a zone trigger, supplements conventional hardware-based triggering and provides a remedy here. It brings more flexibility to the analysis of signals. As this function is implemented in different ways depending on the manufacturer, it is important to consider exactly which requirements are to be met before purchasing an oscilloscope with a zone trigger function.

What is A Zone Trigger?

If you understand how a zone trigger works, you can better judge in which cases its use makes sense. With zone triggering, the user draws one or more zones on the oscilloscope screen. Each of these marked zones (Fig. 1) can be assigned the condition "Cut" or "Do not cut". The oscilloscope then checks with each acquisition whether a recorded signal waveform enters these zones or not. If the signal fulfills the zone conditions defined by the user, it is shown on the display. If this is not the case, the acquisition is rejected by the device. As a result, only those signal sequences are displayed that correspond exactly to the specified conditions.

A zone trigger is usually used as a second trigger stage after a conventional hardware-based trigger condition. This is similar to an edge trigger. This allows users to first isolate certain event types and then refine the analysis with a zone trigger.

Typical Applications for Zone Triggers

With the help of a zone trigger, users can graphically define conditions to isolate a specific event in the recorded signal curve. In practice, this is often more intuitive than thinking about how such signal events could be detected with a conventional hardware-based trigger condition. However, zone triggers were mainly developed to isolate events for which classic trigger types are simply unsuitable. A typical use case is, for example, the definition of a zone in order to find a specific pattern of ones and zeros in successive clock periods of a signal. A zone trigger can also be applied to mathematically calculated signal curves. This can be the product of current and voltage, for example, in order to trigger a specific power level in watts.

Zone Trigger for Mathematically Calculated Signals

The conventional triggering of an oscilloscope takes place directly during signal acquisition on the analog or digital channels, i.e. on the physically present signals. However, as zone triggering is only carried out after the actual acquisition, there are virtually no limits to the possible applications. It is even possible to use mathematically processed signals (math channels) as trigger sources. Figure 2 shows several zones that have been defined on the basis of such a mathematical link. This opens up completely new possibilities for isolating events:

For example, if a user uses a current clamp and a voltage probe, the power can be displayed directly by multiplying the two signals. Zones can now be used to trigger specific power levels or anomalies in the power curve. If the user also applies an integral function to this power signal, a specific energy value in joules can even be used as a trigger condition. The new possibility of using mathematically processed signals as a source for zone triggering brings enormous flexibility to the isolation of complex events.

In the past, oscilloscopes could only trigger on events in the time domain. Even devices with an FFT function, where the transformation into the frequency domain only takes place after the actual acquisition, did not offer the option of triggering in the spectrum. Figure 3 shows how users can now even use a zone trigger to isolate events of a signal specifically in its frequency range. By drawing in corresponding zones, it is possible to graphically determine at which frequencies and levels the oscilloscope should trigger.

With a zone trigger in the frequency range, you can quickly create user-defined filters that only allow certain frequency components to be recorded. A (blocking) zone on the left-hand side, i.e. at low frequencies, can take on the function of a high-pass filter, for example. If, on the other hand, the zone is located exclusively on the right-hand side, the result is the effect of a low-pass filter. A bandpass filter can be defined by combining several zones (Fig. 4).

Faster And More Reliable Detection With ASIC-Based Zone Triggers

As is so often the case in technology, a conventional zone trigger does not only have advantages, as it is traditionally a pure software post-processing process. This has two undesirable consequences: Firstly, the high computational effort considerably reduces the update rate (waveform update rate) of the oscilloscope. This can easily drop by a factor of 1,000 when a software zone trigger is activated. On the other hand—and this is even more critical for measurement technology—the dead time between the individual signal acquisitions increases drastically. In practice, purely software-based zone triggering can therefore usually only be used effectively for strictly periodic, repetitive signals, as rare events could otherwise fall into the long blind time.

However, a few current oscilloscope models have a hardware or ASIC-based zone trigger. This applies, for example, to the oscilloscopes of the MXO 3, MXO 4, MXO 5 and MXO 5C series from Rohde & Schwarz. Its zone trigger can process up to 600,000 signal acquisitions per second and is therefore many times faster than a purely software-based approach. The ASIC-based zone trigger offers an extremely short trigger reactivation time of only around 1 µs. This value is outstanding even compared to the reactivation times of classic hardware triggers of most oscilloscopes on the market.

Selection Criteria for Zone Triggers

Anyone thinking about upgrading an existing oscilloscope or purchasing a new one with zone trigger should consider the following criteria: First of all, the device must of course support this function in principle. However, a closer look at the implementation is then crucial. The differences can be seen above all in the speed: an ASIC-based trigger is considerably faster and more reliable (shorter blind time) than a pure software trigger.

It is also important which signal sources are supported for the trigger function. Although every zone trigger can generally use an analog channel as a source, devices that also allow mathematically calculated signals and the signal spectrum (FFT) are much more flexible. A zone trigger in the spectrum display in particular opens up far-reaching analysis options that classic triggers in the time domain simply cannot offer.

Zone triggers also differ in many practical details. For example, some models only support rectangular zones, while others allow free shapes (polygons). Here, the respective application decides what is most suitable. Ease of use also plays a major role in everyday laboratory work: how intuitively can the size, shape, source and logic (cut/no cut) of the trigger zone be adjusted? The best way to test the devices on the shortlist is to see for yourself how smoothly zones can be added and removed using the touchscreen. In practice, this happens more often than originally expected. A final, equally important criterion is the maximum number of zones that can be defined simultaneously on the oscilloscope for a measurement task.

When Conventional Triggers Reach Their Limits

The zone trigger is a useful function that complements classic oscilloscope triggering and is becoming increasingly popular in practice. It is used precisely where conventional trigger types reach their limits. The most important technological advances in this area include the enormous increase in processing speed thanks to ASIC-based hardware triggers and the flexibility to apply zone conditions not only in the time domain, but also to mathematically calculated signals and in the signal spectrum (FFT). (heh)

Joel Woodward is strategic product manager for oscilloscopes at Rohde & Schwarz. He has more than 35 years of experience in the measurement industry and holds a degree in Electrical and Computer Engineering from Brigham Young University and an MBA from Regis University. He has also completed courses at Harvard Business School and holds a patent for FPGA debugging.

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