Importance of secondary technical indicators in th

  • Detail

The importance of secondary technical indicators in oscilloscope

in recent years, with the steady improvement of semiconductor integration and function, the continuous improvement of simulation model, the continuous change of structure and so on, the performance of electronic system is constantly improving. However, the signaling speed and technology between devices have not changed significantly. Why? Because the past i/o signaling structure is enough to complete the work, but the underlying technology to realize the change is not 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 geometrically, which promotes the communication between devices to adopt more complex serial protocols, such as PCI Express, Infiniband, XAUI and so on. These environments cover various data rates and transmission structures, but all these data rates and transmission structures need strict design and inspection methods

this greatly improves the importance of oscilloscope and other test equipment. Engineers rely on oscilloscope to analyze the performance of serial device design and support inspection and debugging. Their tasks include accurate parameter measurement, maintenance and signal integrity analysis. At the later stage of the development process, they turned to the oscilloscope to generate eye charts for consistency testing

engineers who choose oscilloscopes often only consider the technical indicators listed in the product manuals and magazine advertising titles. The most well-known indicators are bandwidth, sampling rate and recording length. Although these indicators to measure the performance of oscilloscope are also very important, they can not fully show the effect of the instrument in the actual daily use environment. For example, the bandwidth index only indicates the general frequency range of the oscilloscope, which is almost irrelevant to the ability of the instrument to reliably detect and capture fast abnormal events

therefore, it is very important to understand the implication of the main indicators when evaluating oscilloscopes. This suggestion actually has two meanings: first, it is better to analyze the subtle differences hidden behind the technical indicators that manufacturers hype; Second, remember to study some functions. These functions may not be as dazzling as those most often touted in the market, but they may significantly affect the effect of designers' work, and even affect the effectiveness of work

bandwidth definition

bandwidth index is of course very important. For designers who constantly challenge the structural limits of high-speed serial bus, bandwidth has always been the primary consideration when purchasing oscilloscopes

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

there are two aspects 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

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

DSP filtering can be used to expand the net bandwidth of the oscilloscope to flatten its frequency response and provide better matching between channels. These are key functions when the tested equipment adopts high-speed multi-channel serial transmission environment. However, DSP will introduce some errors, which will generally increase in proportion to the frequency range exceeding the actual analog bandwidth

when should I use DSP? When measuring the rise time or eye diagram below nanoseconds (Figure 1), it is very important to obtain the maximum bandwidth from the oscilloscope. Obviously, this is beneficial to the DSP method. The fastest measurements almost always require the highest bandwidth

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

oscilloscope triggering and signal complexity

"high speed measurement" has various meanings in terms of edges lower than nanoseconds 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 correctly selecting the trigger function

oscilloscope triggering determines the items that can be captured, viewed and measured by the instrument. This function is as important as bandwidth and sampling rate. The trigger system has its own set of technical indicators. The trigger path is generally the branch of the main input signal path, which 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, conditions that can be defined when triggering 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 that specified by the instrument. In fact, in terms of the foam wheel of the stroller, the relevant indicator is the trigger sensitivity. This indicator reflects a simple question: what is the amplitude requirement 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 completely falls within the performance index range of the trigger, if the trigger sensitivity at high speed is insufficient, narrow burrs or truncated pulses may not be detected. Fortunately, innovative technologies such as silicon germanium (SiGe) trigger circuit topology have begun to overcome this limitation

engineers usually regard the trigger function of the oscilloscope as "certain", and think that the edge and burr trigger they have been using is sufficient. But in fact, in order to effectively complete the actual work, trigger sensitivity is also the main index of the instrument

each oscilloscope has edge trigger function, and most high-end instruments also have "advanced" trigger function. Edge trigger technology simply detects events that exceed the voltage threshold, while advanced trigger applies more indicators related to voltage, timing or logic conditions. Advanced triggering is becoming more and more important in the field of digital signals that transmit the market reference price of titanium dioxide in a serial manner from 15950 yuan per ton at the beginning of the year

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 devices must ensure that the channel time falls within a specific allowable error range, which not only meets the standard, but also can operate normally

a 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 paths. The measurement results will summarize the offset value at a certain time point, which provides useful information, but is usually not enough to ensure the stable operation of the instrument in the long term

recently, the oscilloscope with full-function double trigger technology has 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 setting period, or re equip the first trigger and start the search again, as shown in Figure 2. If necessary, it can be set to wait a few days for the error combination to occur

when evaluating oscilloscopes, trigger indicators are rarely given priority. However, trigger system is an important supporting indicator in detecting and capturing complex events or intermittent events. In the long run, it can save a lot of time to carry out offset measurement in an unattended way, which is much better than carefully considering the trigger indicators

related "secondary" indicators

so far, the technical indicators we discuss are usually inferior to the main indicators such as bandwidth and sampling rate. But in fact, many other parameters, which are often regarded as secondary issues in the oscilloscope evaluation process, may promote or hinder the tight project schedule

for many serial standards, embedded clock recovery is the basis of oscilloscope eye diagram analysis. It also provides support for clock to data recovery (CDR as shown in Figure 3) and other measurements. In addition to the main indicators, the designers who deal with the embedded clock signal should also consider how the oscilloscope can recover the clock faster, simpler, more flexible and more repeatable

application requirements have always guided the selection direction. 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 dynamic eye diagram

most high-end oscilloscopes provide two high-end oscilloscopes! Tongling creates a method of clock recovery in the 100 billion level copper based material industry, that is, software based clock recovery or hardware based clock recovery. The software clock recovery is generated from the stored collected data. The software method is recognized as the preferred tool for the conformance testing of programs such as tdsrt eye automated conformance testing and analysis software

clock recovery based on phase locked loop (PLL) can be used for real-time eye image acquisition, but here we also need to carefully consider the indicators: can PLL (which can be software recovery or hardware recovery) adapt to the clock frequency evolved in the current serial standard? Some can and some cannot, so we must understand the differences between them

eye diagram measurement is some of the most complex procedures that designers need to use an oscilloscope. Another example is jitter measurement. In both cases, designers can benefit from the professional experience of the application software running on the oscilloscope. Software tools minimize learning time and significantly reduce setup, measurement and analysis time, as shown in Figure 3. However, these tools have never appeared in the list of key indicators. Engineers must work hard by themselves to ensure the provision of appropriate tools through the main indicators

probe index

probe index also needs to be discussed. All the acquisition and analysis functions introduced here depend on the true transmission of signals between the tested equipment and the oscilloscope itself. Many new high-speed interface standards are based on differential signaling rather than the familiar single ended communication

although the probe solution has its own main indicators, especially in terms of bandwidth and load, we should also understand the impact of oscilloscope and probe as a system. Does the oscilloscope system provide a real differential probe tool? If not, it is necessary to use two single ended probes and airborne mathematical operations to exclude some measurement types. In addition, common mode suppression, sensitivity, response accuracy and background noise all affect the probe

Copyright © 2011 JIN SHI