Digital vs. analog triggering in oscilloscope: What’s the difference?

Oscilloscopes provide critical signal visibility for debugging, testing, and ensuring that electronic systems match developers’ imaginations. Specifying when the oscilloscope captures information enables effective debug and test. Oscilloscope triggering determines when the oscilloscope captures information. Triggering has the same degree of importance as a photographer determining when to open the shutter to capture an image.

Historically, oscilloscopes have incorporated analog trigger circuits. An incoming signal is split with part of the signal going to an acquisition sub-system and part of the signal going to a trigger circuit. The trigger circuit determined when to tell the acquisition sub-system to capture information. With the advent of more sophisticated CMOS chips about a decade ago, some oscilloscopes began incorporating the industry’s first digital triggers. For oscilloscopes with digital triggers, the trigger and acquisition paths are the same. Acquisition data is digitized and compared against a user trigger term to determine when to trigger.

Figure 1 shows block diagrams comparing the analog and digital trigger circuits. As can be seen, oscilloscopes with digital triggers have, for instance, the advantage that triggering can be applied to signal processing tasks such as de-embedding and filtering. That brings us to the question: what is the difference and what an oscilloscope user should know about using digital versus analog triggers. The following article will provide some answers.

Figure 1 Oscilloscopes with analog trigger circuits split incoming signals with a portion of the signal going to a separate trigger circuit. Oscilloscopes with digital triggers have an advantage as triggering can be applied to signal processing tasks like de-embedding and filtering. Source: Rohde & Schwarz

How to know if oscilloscope has a digital or analog trigger?

Every oscilloscope family incorporates a trigger sub-system that is either digital or analog. Unfortunately, it’s not always easy to tell by looking at a product brochure. Asking your oscilloscope vendor is a good way to find out. Or more definitively, users can determine if the trigger system is digital or analog by looking at a number of triggering attributes in a datasheet or the scope user interface.

For instance, can the user set the trigger hysteresis manually? Or can the user apply techniques like flexible bandwidth limit filters and de-embedding correction figures to the signal that goes through the trigger circuit?

Is digital triggering better than analog triggering?

It depends on the specific attributes a user wants, but generally, digital triggering has a number of advantages. One key advantage, for instance, is the ability to trigger on very small signals. Triggering on small signals is a challenge for most oscilloscopes, yet there’s an increasing trend in electronic use of small signals in the presence of larger signals. Examples include triggering on a low-power signal, a sleep state signal, small audio signals, looking at ripple on a power rail, and bio-electronic signals which can be in range of a few millivolts.

Edge triggering is the dominant trigger mode for most oscilloscope users, and with the increasing need to trigger on small signals, the edge trigger sensitivity value gains importance. Oscilloscope manufacturers specify a certain signal amplitude that must be achieved in order for the oscilloscope to recognize an edge. This parameter, specified in triggering section of a datasheet, describes how many vertical divisions in height an edge must be for the scope to successfully trigger.

The edge trigger sensitivity value can be larger than expected. For example, one vendor specifies edge trigger sensitivity as “4.5 div from DC to instrument bandwidth” for the scope’s 1 MΩ path at vertical settings 0.5 mV/div to 0.99 mV/div. For the 50 Ω path with the same vertical settings, the specification is “3.0 div from DC to instrument bandwidth.” This particular oscilloscope has 10 vertical divisions, meaning that a signal amplitude must be 30% of the display for the oscilloscope to trigger.

But what happens if a signal is smaller? Then an oscilloscope with digital triggering is the better choice. While oscilloscopes with analog trigger circuits require bigger signals for triggering, oscilloscopes with digital triggers consistently and reliably trigger on signals that are just a fraction of a vertical division. Since the triggering circuitry is based on digitized data, the digital triggering systems can recognize a small event much better than their analog equivalents.

Oscilloscopes with a digital trigger can trigger on signals that have just 0.01 divisions of amplitude. This means digital triggers are 100 times to 300 times better than analog triggers at recognizing small signals. For this reason, engineers working with small signals in the presence of large signals often seek oscilloscopes with digital triggers. Figure 2 shows an exemplary screenshot of an oscilloscope with digital trigger that manages to trigger on a very small anomaly.

Figure 2 The R&S RTO6 oscilloscope’s digital trigger can trigger on this very small anomaly that oscilloscopes with analog triggers can’t isolate. Source: Rohde & Schwarz

Another advantage of digital triggering as compared to analog triggering is when it comes to signal processing. Oscilloscopes with analog triggers perform post-processing like de-embedding after the trigger. Therefore, the trigger doesn’t know what a signal looks like when de-embedding has been applied and doesn’t have the ability to trigger with these attributes. Oscilloscopes with digital triggering have the advantage as de-embedding can be done before the trigger, allowing the oscilloscope to trigger on a de-embedded signal.

Coming back to edge triggering, oscilloscopes with digital triggering offer another advantage over analog triggering. Higher bandwidth oscilloscopes with analog triggers often have restrictions on the upper bandwidth limit for edge triggers. For example, an oscilloscope with an 8-GHz acquisition bandwidth may have a triggering limit of 2 GHz. This may not be a significant issue for working with digital signals as users often pick an oscilloscope that has three times to five times the bandwidth of fundamental frequencies in order to capture 3rd and 5th harmonics.

Triggering on the fundamental frequency might be sufficient. However, users lose the ability to trigger on anomalies related to harmonics. Users can see these on scopes via the acquisition system, but can’t trigger on anomalies they see if the anomalies are above the trigger frequency. For oscilloscopes with digital triggering, edge triggering bandwidth typically is equal to the maximum acquisition bandwidth. For example, on the R&S RTP 16 GHz bandwidth oscilloscope, the digital trigger system also supports 16 GHz triggering.

Another advantage has to do with triggering hysteresis. Oscilloscopes with digital triggering incorporate a user-adjustable trigger hysteresis setting. A user can determine how much hysteresis the scope should apply when determining what to count as noise versus a true signal transition. An example of this is shown in Figure 3, where on the display of the oscilloscope, users can see a hysteresis line that gets thicker when users apply more hysteresis and thinner when a smaller hysteresis value is selected.

Figure 3 An example of an oscilloscope with digital triggering, the R&S RTO6 oscilloscope user interface shows the control for trigger hysteresis. Source: Rohde & Schwarz

Here, users have the option: setting a 1 division hysteresis triggers on the falling edge of the bigger pulse, while setting a hysteresis of 0.1 division allows the digital trigger to trigger on the very small falling edge of the small pulse. Visually seeing the trigger hysteresis simultaneously with the signal can be very helpful, especially for signals that have some noise. For oscilloscopes with analog triggering, a hysteresis value is built into the analog circuitry and is not adjustable by the user. Hence, when it comes to triggering hysteresis, oscilloscopes with digital triggering are superior.

Both, analog triggering and digital triggering circuits have the ability to suppress noise using bandwidth limit adjustments. For oscilloscopes with analog triggers, trigger settings typically include a high-frequency and/or a low-frequency reject. However, these are fixed values with no user variation to whatever was initially developed in the analog triggering circuit. Oscilloscopes that have digital triggering offer a far richer set of triggering path frequency filters. Additionally, the bandwidth limit for the trigger path can typically be different or the same from the bandwidth limit of the acquisition path. This allows users to more precisely trigger on events of interest, while seeing a signal representation with or without bandwidth limitation.

Zone triggering

While oscilloscope users most often choose a simple edge trigger to tell the oscilloscope when to capture, oscilloscopes offer a number of less common trigger modes. Triggering types and variety of triggering modes are typically the same for oscilloscopes with digital triggering or analog triggering. These include glitch pulse width, runt, window, timeout, slew rate, setup and hold, state, pattern, and serial bus triggering. All of these triggering modes are offered on oscilloscopes with analog or digital triggering. Many oscilloscopes incorporate an additional triggering capability known as zone triggering and scopes with digital triggering often have enhanced zone triggering capabilities.

So, how does zone triggering work? Users graphically draw one or more zones on the oscilloscope display as shown in Figure 4. Each zone can be parameterized with a “must intersect” or a “must not intersect” condition. With each new oscilloscope acquisition, the scope looks through the acquired record. If the acquisition matches the zone conditions set by the user, the scope displays the data. If the acquisition does not meet the zone condition, the scope discards the data. So, only acquisitions that meet user-specified zone conditions are displayed on the oscilloscope.

Figure 4 The R&S RTO6 oscilloscope incorporates digital triggering and offers zone triggering as a standard trigger feature. Users can trigger on frequency events such as a specific zone exceeding a specified power level. Source: Rohde & Schwarz

While the number of triggering modes is typically the same, zone triggering is another area where oscilloscopes with digital triggering offer some additional capabilities. Oscilloscopes with analog triggering have historically allowed zones to be created exclusively on channel sources. Oscilloscopes with digital triggering have furthered the zone triggering capabilities by allowing zones to be created on channel sources, as well as using math including fast Fourier transform (FFTs). As a consequence, a user can trigger exclusively on a specific frequency hop, or when a particular zone exceeds a power level as shown in Figure 4.

Where analog triggering is superior to digital triggering?

An area where analog triggers are better than digital triggers is triggering on signals that are off the oscilloscope display. Digital triggering requires that the signal is visible on the scope display, as this is where the analog-to-digital converter (ADC) is digitizing bits. Analog triggers can be developed to trigger on events outside of the oscilloscope display. So, for users who want this flexibility, analog oscilloscopes have a specification called trigger level range. This specification lets users know how much offscreen triggering range is available. Typically, it ranges from none to a couple extra divisions outside of the vertical display range.

Digital triggering oscilloscope architectures have multiple advantages over their analog equivalents. Understanding these attributes makes it easier to determine how important these advantages are for a team when choosing an oscilloscope. As the need to trigger on ever-smaller signals increases, the desire by engineers to have triggering bandwidth equal to full acquisition bandwidth, having user-adjustable trigger hysteresis, and having the ability to employ processing techniques to the trigger path is better understood. So, expect to see more new oscilloscopes incorporate digital triggering architectures.

Joel Woodward is strategic planning manager for oscilloscopes at Rohde & Schwarz.

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