The importance of secondary technical indicators in test oscilloscopes

In recent years, with the steady improvement of semiconductor integration and functions, the continuous improvement of simulation models, the continuous change of structure, etc., the performance of electronic systems is constantly improving. However, the signaling speed and technology between devices have not changed significantly. why? Because the I / O signaling structure in the past is sufficient to complete the work, the underlying technology to implement the changes is not yet in place.

In the past five years or so, engineers have been focusing more on low-voltage differential signaling to significantly improve system performance. The data rate has been increased by geometric progression, which has driven the communication between devices to more widely adopt complex serial protocols, such as PCIExpress, Infiniband, XAUI, etc. These environments cover various data rates and transmission structures, but all these data rates and transmission structures require rigorous design and inspection methods.

This greatly increases the importance of test equipment such as oscilloscopes. Engineers rely on oscilloscopes to analyze the performance of serial device designs to support inspection and debugging. Their tasks include accurate parameter measurement, maintenance and signal integrity analysis. Later in the development process, they turned to an oscilloscope to generate eye diagrams for conformance testing.

Engineers who choose oscilloscopes often only consider the technical indicators listed in product manuals and magazine advertisement headlines. The most well-known indicators are bandwidth, sampling rate, and record length. Although these indicators to measure the performance of the oscilloscope are also very important, but they can not fully indicate the effect of the instrument in the actual daily use environment. For example, the bandwidth specification only indicates the general frequency range of the oscilloscope, and has almost nothing to do with the instrument's ability to reliably detect and capture fast abnormal events.

Therefore, when evaluating an oscilloscope, it is important to understand the implication of the main indicators. This suggestion actually has two meanings: first, it is best to deeply analyze the nuances hidden behind the technical indicators hyped by the manufacturer; second, remember to study certain functions, which may not be as popular as the most frequently touted functions on the market That is dazzling, but they may significantly affect the effectiveness of the designer ’s work and even the effectiveness of the work.

Bandwidth definition

The bandwidth specification is of course very important. For designers who continue to challenge the limits of high-speed serial bus architectures, bandwidth has always been the most important consideration when purchasing an oscilloscope.

However, the bandwidth itself is only an indicator to describe the frequency response of the instrument (sine wave roll-off -3dB frequency). Two oscilloscopes with the same rated bandwidth may have very different rise times and respond completely differently to complex waveforms. Is it necessary to carefully consider some indicators or functions in order to better promote buyers' decision-making?

There are two ways to answer this question, one is the real rise time performance of the oscilloscope, and the other is the behavior of the instrument in digital signal processing (DSP) mode.

The analog rise time is a function of the oscilloscope bandwidth. It attempts to use the formulas in the textbook to simply calculate the rise time from the bandwidth, which is the basis of some published rise time indicators. The observed rise time provides a better basis for measurement, including with or without DSP enhancements. Every engineer understands the importance of rise time response. To measure the difference between the measured rise time and the calculated rise time is to understand the implication.

DSP filtering can be used to extend the oscilloscope's net bandwidth, flatten its frequency response, and provide better matching between channels. These are key functions when the device under test uses a high-speed multi-channel serial transmission environment. However, DSP introduces certain errors, which generally increase in proportion to the frequency range that exceeds the actual analog bandwidth.

When should I use DSP? When measuring rise times or eye diagrams below nanoseconds (Figure 1), it is critical to obtain the maximum bandwidth from the oscilloscope.

Obviously, this is beneficial to the DSP method. The fastest measurement almost always requires the highest bandwidth.

But sometimes the DSP extension technology can be bypassed in some way, using only the analog bandwidth and rise time of the instrument itself. For example, some researchers use dedicated DSP algorithms and need to process the raw data in the oscilloscope. In this case, the DSP bypass function is very important. Such indicators may not be hyped by manufacturers, but it is an important consideration when choosing a high-performance oscilloscope.

Oscilloscope triggering and signal complexity

The term "high-speed measurement" has various meanings in terms of sub-nanosecond edges and fast clock rates. Sometimes people ignore that these high-speed measurements are usually very complex measurements. Capturing a code in the data stream involves judgment, luck, estimation, guessing ... or the correct choice of trigger function.

Oscilloscope triggering determines the items that can be captured, viewed, and measured using the instrument. This function is as important as the bandwidth and sampling rate. The trigger system has its own different set of technical indicators. The trigger path is generally a branch of the main input signal path and should reflect many of the same environmental characteristics, such as sensitivity, jitter, and so on. Another indicator of trigger performance is the range of trigger types, that is, the conditions that can be defined when a trigger occurs.

Of course, the trigger system has its own main indicators. Designers who choose an oscilloscope to measure fast serial signals may think that the bandwidth of the trigger path is the same as the bandwidth specified by the instrument. In fact, the relevant indicator is trigger sensitivity. This indicator presents a simple question: What are the amplitude requirements when capturing signals near the top of the frequency range? In many oscilloscopes, the trigger sensitivity does not match the analog acquisition bandwidth.

Even if the normal component of the signal falls completely within the performance index of the trigger, if the trigger sensitivity at high speed is insufficient, narrow glitches or truncated pulses may still not be detected. Fortunately, innovative technologies such as silicon germanium (SiGE) trigger circuit topologies have begun to overcome this limitation.

Engineers usually view the oscilloscope's triggering function as "certain" and think that the edge and glitch triggering they have been using is sufficient. But in fact, in order to effectively complete the actual work, the trigger sensitivity is also the main indicator of the instrument.

Each oscilloscope has an edge trigger function, and most high-end instruments also have an "advanced" trigger function. Edge triggering technology simply detects events that exceed the voltage threshold, while advanced triggering uses more indicators related to voltage, timing, or logic conditions. In the field of digital signals transmitted serially, advanced triggering is becoming increasingly important.

In some cases, advanced trigger settings may be the only way to trigger the actual signal of interest. For example, designers dealing with multi-channel Infiniband equipment must ensure that the channel time falls within a specific tolerance range, not only to meet the standards, but also to be able to operate normally.

The common way to deal with this measurement challenge is to trigger a feature in a data stream and then measure the offset or time shift between different channels. The measurement results will summarize the offset value at a certain point in time, which provides useful information, but it is usually not enough to ensure the stable operation of the instrument in the long term.

Recently, oscilloscopes with full-featured dual trigger technology have significantly simplified the complex task of observing these offset changes at different times. Two advanced trigger functions can be defined, and each function can be selected from the complete trigger condition menu. When the first trigger is triggered by the data characteristics, the second trigger can find the offset error within the set period, or re-equip the first trigger, and start the search again, as shown in Figure 2. When necessary, it can be set to wait a few days for error combinations to occur.

When evaluating an oscilloscope, trigger indicators are rarely given priority.

However, the trigger system is an important supporting indicator in detecting and capturing complex or intermittent events. It can save a lot of time to carry out offset measurement in an unattended manner in the long term, which is much stronger than carefully considering the trigger indicator!

Related "secondary" indicators

So far, the technical indicators we have discussed are generally inferior to the main indicators such as bandwidth and sampling rate. But in fact, many other parameters that are often regarded as secondary issues during oscilloscope evaluation may promote or hinder tight engineering schedules.

For many serial standards, embedded clock recovery is the basis for oscilloscope eye diagram analysis. It also provides support for measurements such as clock-to-data recovery (CDR shown in Figure 3). Designers dealing with embedded clock signals should not only examine the main indicators, but also consider how the oscilloscope can make the clock recover faster, easier, more flexible, and more repeatable.

Application requirements have always guided the choice. Can the oscilloscope be used for maintenance or consistency measurement? What are the clock recovery mechanisms? Can the oscilloscope recover the clock in real time and display the characteristics of the dynamic eye diagram?

Most high-end oscilloscopes provide one of two clock recovery methods, namely software-based clock recovery or hardware-based clock recovery. Software clock recovery is generated from stored collected data. For conformance testing using TDSRT-Eye automated conformance testing and analysis software and other procedures, the software method is recognized as the preferred tool.

You can use PLL-based clock recovery to collect real-time eye diagrams, but here you also need to carefully consider the indicators: Can PLL (which can be software recovery or hardware recovery) adapt to the evolving clock frequency in the current serial standard? Some can and some cannot, so you must understand the differences.

Eye diagram measurements are some of the most complex procedures designers need to use an oscilloscope. Another example is jitter measurements. In both cases, designers can benefit from the professional experience of the application software running on the oscilloscope. The software tool minimizes learning time and significantly reduces setup, measurement, and analysis time, as shown in Figure 3. However, these tools have never appeared in the list of main indicators. Engineers must work hard to ensure that appropriate tools are provided through the main indicators.

Probe index

Need to discuss probe specifications. All acquisition and analysis functions introduced here depend on the true transmission of the signal between the device under test and the oscilloscope itself. Many new high-speed interface standards are based on differential signaling, rather than the single-ended communication that people are more familiar with.

Although the probe solution has its own main specifications, especially in terms of bandwidth and load, you should also understand the impact of the oscilloscope and probe as a system. Does the oscilloscope system provide true differential probe tools? If not, then it is necessary to use two single-ended probes and on-board mathematical operations to exclude certain measurement types. In addition, common-mode rejection, sensitivity, response accuracy, and noise floor all affect the probe's effect on the signal. When measuring current high-speed signals, small differences in these parameters may cause large probe load distortion.

Probe connection methods are rarely included in the main specifications of the oscilloscope, but they are very important in every measurement. Some devices under test are equipped with SMA test points, while others require contact with the pins on tiny surface-mounted devices. Can oscilloscope series probe solutions meet all these needs?

to sum up

The main index has always been the accepted standard for comparing oscilloscopes. However, savvy engineers will carefully examine the underlying performance that affects daily tasks through the main indicators. Taken as a whole, some of the less obvious metrics in acquisition, triggering, analysis tools, and probing may be as important as the "primary" metrics describing bandwidth, sampling rate, and record length. Everything should be centered on work, and even the most secondary indicators sometimes determine success or failure.

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