Signal Integrity Analysis Basics Series: Eye Diagram Measurement (Part 1)

Signal Integrity Analysis Basics Series: Eye Diagram Measurement (Part 1)

The history of eye diagram can be traced back to about 47 years ago. Before LeCroy invented the method based on continuous bits to measure eye diagram in 2002, the eye diagram was measured based on the traditional method of sampling oscilloscope for 40 years from 1962 to 2002.
In the long-term training and technical support work, we found that few engineers can fully and accurately understand the measurement principle of eye diagram. Many engineers are often satisfied with the measurement wizards provided by various standard authorities, step by step, and the Pass or Fail conclusion given by the eye diagram measured by the "universal" Sigtest software. This obsession with Sigtest even makes some engineers forget that eye diagram can be used as an important debugging tool.
Before I came to LeCroy for an interview in 2004, I had never heard of eye diagram. During the interview that day, the boss repeatedly emphasized LeCroy's advantages in eye diagram measurement, but I didn't understand what he meant. Later, I Googled "eye diagram" and saw a limited number of articles on the Internet, but I still didn't understand what he meant. Just now I Googled "eye diagram" again, and still couldn't find even one article that thoroughly explained eye diagram measurement.
The most frequently appearing texts about eye diagrams found on the Internet are as follows. They seem to be very professional, but they are refusing our interest in reading.
"In actual digital interconnection systems, it is very difficult to completely eliminate inter-symbol crosstalk, and the impact of inter-symbol crosstalk on the bit error rate cannot be found in a mathematically tractable statistical law, and cannot be accurately calculated. In order to measure the performance of the baseband transmission system, in the laboratory, an oscilloscope is usually used to observe the waveform of the received signal to analyze the impact of inter-symbol crosstalk and noise on the system performance. This is the eye diagram analysis method.
If the input waveform is input to the Y axis of the oscilloscope, and when the horizontal scanning cycle of the oscilloscope is synchronized with the symbol timing, the phase is adjusted appropriately so that the center of the waveform is aligned with the sampling moment, the figure displayed on the oscilloscope looks very much like a human eye, so it is called an eye diagram. Map).
The eye diagram has only one "eye" when binary signal is transmitted. When ternary code is transmitted, two "eyes" will be displayed. The eye diagram is formed by the superposition of each segment of the symbol waveform. The vertical line in the center of the eye diagram indicates the best sampling time, and the horizontal line located between the two peaks is the decision threshold level.
In the ideal case without inter-symbol crosstalk and noise, the waveform is undistorted, and each symbol will overlap. In the end, what is seen on the oscilloscope is an "eye" with a thin and clear trace. The "eye" is opened to the maximum. When there is inter-symbol crosstalk, the waveform is distorted, the symbol does not completely overlap, and the trace of the eye diagram will be unclear, causing the "eye" to be partially closed. If the influence of noise is added, the lines of the eye diagram will become blurred, and the "eye" will be opened smaller. Therefore, the size of the "eye" opening indicates the degree of distortion and reflects the strength of inter-symbol crosstalk. It can be seen from this that the eye diagram can intuitively show the influence of inter-symbol crosstalk and noise, and can evaluate the performance of a baseband transmission system. In addition, this graph can also be used to adjust the characteristics of the receiving filter to reduce inter-symbol crosstalk and improve the transmission performance of the system. Usually the eye diagram can be described by the graph shown in the figure below. From this figure, we can see that:
(1) The width of the eye diagram determines the time interval in which the received waveform can be sampled and regenerated without being affected by crosstalk. Obviously, the best sampling time should be selected when the eye is widest.
(2) The slope of the hypotenuse of the eye diagram indicates the sensitivity of the system to timing jitter (or error). The larger the slope, the more sensitive the system is to timing jitter.

Figure 1 Eye diagram
(3) The horizontal width of the shaded part of the left (right) corner of the eye diagram indicates the variation range of the signal zero point, which is called the zero point distortion. In many receiving devices, the timing information is extracted from the signal zero point position. For such devices, the zero point distortion is very important.
(4) At the sampling moment, the vertical width of the shaded area indicates the maximum signal distortion.
(5) At the sampling moment, half of the interval between the upper and lower shaded areas is the minimum noise tolerance. If the instantaneous value of the noise exceeds it, an erroneous decision may occur.
(6) The horizontal axis corresponds to the decision threshold level. "
It's time to write a special article to explain the eye diagram in detail! If there are any inaccuracies or deficiencies in the writing, please point them out so that this article can be continuously revised and improved, which will benefit the learning of engineers.

1. Background knowledge of serial data
There are many types of serial signals. As shown in Figure 2, there are PCI Express, Rapid IO, DVI, S-ATA, USB, SDH, XAUI, etc. In fact, there are far more popular buses now. Every year, some new and popular serial buses are released. Almost every bus has an authoritative organization to define the signal standards and test specifications of the bus. Most of the members of these organizations are part-time experts from different companies. Of course, the serial bus of PC is almost led by Intel. As shown in Figure 3, the various buses in the framework diagram of a laptop computer based on Intel Chipset, except for DDR and FSB, which are parallel data, are all serial data. In addition to defining specifications, these authoritative organizations will of course have some interest games. Therefore, when a new interest group (this is a neutral word) plans to promote it, a new bus specification may be issued, just like the three standards of 3G. You sing and I come on stage, making downstream manufacturers busy.
There are more and more serial data buses, and the test specifications defined by authoritative organizations are also complicated. I have always felt that so many authoritative organizations should be unified into one authoritative organization, called "Serial Bus International Engineers Association". If LeCroy first initiated and led this association, and then defined a series of serial signal test specifications that only recommended LeCroy oscilloscopes, then dear friends, what would be the final result of this Day Dream? The oscilloscope industry may be reshuffled. People always believe in the recommendations of authoritative organizations, for example, when we usually use toothpaste, we believe in the recommendations of the "Chinese Medical Association".
The signal rate keeps doubling. When I first came to LeCroy in 2004, the mainstream serial signal rate in the PC industry was 2.5Gb/s and in the communications industry was 3.125Gb/s. Today, the PC industry has doubled to 5Gb/s, and the communications industry has doubled to 6.25Gb/s. Moreover, 8Gb/s in the PC industry and 12.5Gb/s in the communications industry seem to be just around the corner. As the rate gets higher and higher, parallel data must give way to serial data. The typical structural block diagram of serial data transmission is shown in Figure 3. "No matter how it changes, it remains the same", which is "two differential lines". Compared with parallel data, the advantages of serial data are: 1. The number of signal lines is reduced. 2. The delay problem of parallel data transmission is eliminated.


Figure 2 Overall characteristics of serial data


Figure 3 Schematic diagram of a laptop computer architecture

3. Because the clock is embedded in the data, the transmission delay between the data and the clock is also eliminated.
4. The PCB design of the transmission line is also easier.
5. Signal integrity testing is also easier.

Figure 4 Serial signal example
The test points of serial data include different nodes such as the transmitter and receiver of the chip. The commonly used units to describe serial data are baud rate and UI. For example, 3.125Gb/s means that the data bits transmitted per second are 3.125G bits (byte). The corresponding unit interval (1UI) means that the width of a bit is the reciprocal of the baud rate, 1UI=1/(3.125Gb/s)=320ps. The more common serial signal code is NRZ code. Positive level represents "1" and negative level represents "0". Figure 3 shows a set of serial signals captured by an oscilloscope. The time interval between the dotted lines represents a UI. The corresponding code in the figure is 101100101010001.

2. Some basic concepts of eye diagrams
- "What is an eye diagram?"
- "An eye diagram is a graph that looks like an eye."
The eye diagram is the result of accumulating and superimposing the bits of the collected serial signal in a persistence manner. The shape of the superimposed graph looks very similar to an eye, hence the name eye diagram. The eye diagram usually displays a time window of 1.25UI. The shapes of eyes vary, and so do the shapes of eye diagrams. The shape characteristics of the eye diagram can quickly determine the quality of the signal. The eye diagram in Figure 6 has "double eyelids", which can be used to determine that the signal may have crosstalk or pre-emphasis (de-emphasis). The eye diagram in Figure 7 has "bloodshot eyes", which indicates that the signal quality is too poor, which may be due to errors in the test method or obvious errors in the PCB wiring. The eye diagram in Figure 8 is very beautiful, which may be an eye diagram measured with a sampling oscilloscope. Figure

5 Eye Diagram Definition

Figure 6 "Double Eyelid" Eye Diagram

Since the eye diagram fully represents the bit information of the serial signal with a single graph, it has become the most important tool for measuring signal quality. Eye diagram measurement is sometimes called "Signal Quality Test (SQ Test)". In addition, the result of eye diagram measurement is qualified or unqualified, and its judgment basis is usually relative to the "mask". The mask specifies the tolerance of the "1" level of the serial signal, the tolerance of the "0" level, the tolerance of the rise time and the fall time. Therefore, eye diagram measurement is sometimes called "mask test". The shape of the mask is also various. The template of the common NRZ signal is shown in the blue part of Figure 5 and Figure 8. At different nodes of serial data transmission, the template of the eye diagram is different, so when selecting the template, pay attention to the specific sub-template type. If the template of the transmitting end is used as the eye diagram template of the receiving end, the template may be touched all the time. However, signals such as Ethernet signals and E1/T1 are not NRZ code shapes, and their templates are relatively special. When a bit touches the template, we think that the signal quality is not good and the circuit needs to be debugged. Some products require that the template cannot be touched 100%, while some products allow the number of times the template is touched within a certain probability. (Interestingly, products with 85% eye diagrams passing the template often have no problems with functional testing. For example, the network port of the computer I use always fails the test, but I have no problem accessing the Internet. This makes many companies feel that they can still make good products without buying an oscilloscope for signal integrity testing. As for the copycat version, they will not buy an oscilloscope to test the eye diagram.) There are measurement parameters in the oscilloscope that can automatically count the number of times the template is touched. In addition, according to the position of the "invasion" of the template, you can know which aspect of the signal has problems and guide debugging. As shown in Figure 9, the problem of the signal is mainly that the falling edge is too slow, and Figure 10 shows that the 1 level and the 0 level have "collapsed", which may be caused by ISI problems.

Figure 7 "Eyes full of bloodshot" eye diagram

Figure 8 The most beautiful "eye"

Figure 9 Eye diagram with falling edge touching the template

Figure 10 Template with "collapse" of "1" level and "0" level
There are many eye diagram parameters related to the eye diagram, such as eye height, eye width, eye amplitude, eye crossing ratio, "1" level, "0" level, extinction ratio, Q factor, average power, etc. Figure 12 shows the definition of amplitude-related measurement parameters. The "1" level and "0" level indicate that the 20% UI part in the middle of the eye diagram is projected onto the vertical axis to make a histogram, and the center values ​​of the histogram are the "1" level and the "0" level respectively. The eye amplitude indicates the "1" level minus the "0" level. The 3sigm difference between the upper and lower histograms indicates the eye height. Figures 12, 13, 14, and 15 show the definitions of some other eye diagram parameters, which are clear at a glance and will not be described one by one here. However, experienced engineers know that when the eye diagram image is very bad, the results of the eye diagram parameter test appear very inaccurate. At this time, it is recommended that you can use LeCroy's custom eye height measurement method for measurement, as shown in Figure 16.


Figure 11 Eye diagram parameter definition

Figure 12 Eye diagram parameter definition Figure 13

Eye diagram parameter definition


Figure 14 Eye diagram parameter definition


Figure 15 Eye diagram parameter definition


Figure 16 Custom eye height measurement method