Taixiang Test 001: Is shaking more healthy? ! Quickly crack the secret of "shaking"

Although the beginning of spring has passed, old man Winter still wants to show that he is still in good shape. Shaking is still the standard for workers who go out early and come back late. They shake when they get off the bus and when they leave the office. Workers say, "Shaking is healthier, ha~~ ha~~!" Brother Shui also shakes frequently. It's nothing, it's just a habit. This is the perseverance of us workers!

Jitter is a normal part of life, so when measuring the integrity of digital signals, it is normal for the signal to jitter a little. Please maintain stability! The key is to analyze the reasons behind the jitter, so as to become a different man like Shui Ge who knows the secrets behind the jitter!

Today I will teach you the password weapon to deal with jitter: eye diagram analysis . Yes, the eye of the eye, look!

When testing and verifying digital signals, especially high-speed serial digital signals, eye diagrams are a very successful method. Eye diagrams include almost all combinations from 101 to 010. Eye diagrams contain rich information. In addition to basic parameters such as rise time, fall time, overshoot, undershoot, and threshold level, they can also reflect the characteristics of digital signal integrity such as noise, jitter, and inter-symbol crosstalk. Therefore, eye diagram analysis is the core of integrity analysis.

What is an eye diagram

Let's first look at the definition from Wikipedia:

An eye pattern is an oscilloscope display in telecommunications systems that shows the digital signal on the receiver, with the data rate triggering the horizontal update. In many different coding systems, the eye pattern looks like several eyes side by side, hence the name. The eye pattern superimposes all possible changes in a specific time on a single screen and is an experimental tool for evaluating channel noise and inter-symbol interference in baseband square wave modulation systems.

By analyzing the eye diagram, we can see many system performance and problems, such as signals that are too long or too short, poorly synchronized with the clock, too high or too low signal voltage, too much noise, too slow changes, or overshoot or undershoot. The larger the "eye" of the eye diagram, the smaller the signal distortion. If the signal is distorted by noise or inter-symbol interference, the "eye" of the eye diagram will become smaller.

How is the eye diagram formed?

Simply put, all bit sequence waveforms are aligned and superimposed according to a certain reference point, and then a waveform similar to the shape of an eye is obtained, which is the eye diagram.

Actual eye diagram displayed on an oscilloscope

There are many parameters of the eye diagram, such as eye height, eye width, eye amplitude, eye crossing ratio, extinction ratio, Q factor, bit error rate, average power, etc.

Jitter and Jitter Analysis

Jitter is the deviation of important transient changes of digital signals from their ideal positions in a short period of time. Eye diagram analysis is intuitive and perceptual, while jitter analysis can further analyze and measure system performance to improve the timing redundancy of high-speed circuits.

The definitions of jitter classification will not be described in detail here. The following figure is a classification diagram for reference.

Jitter analysis often uses waveform histogram statistics, time interval error (TIE) histogram, TIE trend chart, and jitter spectrum chart. There are many steps, but the Tektronix DPO/MSO70000 series oscilloscope provides a one-button automatic jitter analysis function, which can be said to liberate engineers' hands and greatly save time.

For those who have difficulty watching the video, Brother Shui has kindly summarized the specific steps for you:

1. Open the One Touch Jitter submenu of Jitter and Eye Analysis under the Analyze menu to start the one-touch automatic jitter analysis function.

2. Wait for a few minutes while all relevant parameters of the oscilloscope are automatically configured and checked.

3. The oscilloscope automatically generates reports, including histograms, eye diagrams, etc.

4. If you need to measure the bit rate, you can click the Select button, select Data Rate, and then click the Recalc button to recalculate, and we will get the bit rate result.

5. We can also click the Save button to save the report locally. The report includes a detailed data list and graphic data such as histograms, trend charts, spectrum charts, eye diagrams, etc.

How about this ultimate trick? In the time it takes to drink a sip of tea, you can get all the chart data. Isn’t it shocking enough, my friends?