4 skills need to be mastered to understand oscilloscope eye diagrams

What is the use of eye diagrams in PCB design?

The eye diagram is due to the afterglow effect of the oscilloscope, which overlaps the waveforms of each scanned symbol to form an eye diagram.

This article will lead you to understand what the eye diagram on the PCB is, how the eye diagram is formed, what information is contained in the eye diagram, and how to distinguish the signal quality based on the eye diagram.

To understand the eye diagram of an oscilloscope, you need to master the following four points:

1. What is an eye diagram?

The eye diagram is a graph in which a series of digital signals are accumulated and displayed on an oscilloscope. It contains a wealth of information. The effects of inter-symbol crosstalk and noise can be observed from the eye diagram, which reflects the overall characteristics of the digital signal, thereby estimating the system optimization. Therefore, eye diagram analysis is the core of signal integrity analysis of high-speed interconnection systems.

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.

Use an oscilloscope across the output of the receiving filter, and then adjust the oscilloscope scanning period so that the oscilloscope horizontal scanning period is synchronized with the period of the received symbols. At this time, the pattern seen on the oscilloscope screen is called an eye diagram.

The signals generally measured by oscilloscopes are waveforms of some bits or a certain period of time, which reflect more detailed information, while the eye diagram reflects the overall characteristics of all digital signals transmitted on the link.

The method to observe the eye diagram is: use an oscilloscope across the output end of the receiving filter, and then adjust the oscilloscope scanning period so that the oscilloscope horizontal scanning period is synchronized with the receiving symbol period. At this time, the graph seen on the oscilloscope screen looks like a human being. eye, so it is called "eye diagram".

The impact of inter-symbol crosstalk and noise can be observed from the "eye diagram" to estimate the quality of the 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.

2. How is the eye diagram formed?

For digital signals, there can be many sequence combinations of high-level and low-level changes. Taking 3 bits as an example, there can be a total of 8 combinations from 000 to 111. Align enough of the above sequences to a certain reference point in the time domain, and then superimpose their waveforms to form an eye diagram.

As shown below. For test instruments, the clock signal of the signal is first recovered from the signal to be measured, and then the eye diagram is superimposed according to the clock reference, and finally displayed.

3. What information is contained in the eye diagram?

For a real eye diagram, as shown below, first we can see the average rise time (RiseTime), fall time (FallTime), overshoot (Overshoot), undershoot (Undershoot), and threshold level (Threshold) of the digital waveform. /CrossingPercent) and other basic level transformation parameters.

Rise Time (RiseTime): The rise time of the pulse signal refers to the interval between the two instants when the pulse instantaneous value first reaches the specified lower limit and the specified upper limit. Unless otherwise specified, the lower limit and upper limit are respectively set at 10% and 90% of the pulse peak amplitude.

Fall Time (Fall Time): The fall time of the pulse signal refers to the time interval from 90% of the pulse peak amplitude to 10%.

Overshoot: Also called overshoot, the first peak or valley exceeds the set voltage, which mainly manifests as a sharp pulse and can cause the failure of circuit components.

Undershoot: refers to the next valley or peak. Excessive overshoot can cause the protection diodes to operate, causing premature failure. Excessive undershoot can cause spurious clock or data errors.

Threshold level (Threshold/CrossingPercent): refers to the lowest reception level that the receiver can achieve when the system transmission characteristics are worse than a certain bit error rate.

4. How to distinguish signal quality based on eye diagram conditions

It is impossible for a signal to maintain exactly the same voltage value of high and low levels every time, nor can it be guaranteed that the rising and falling edges of high and low levels are at the same time. Due to the superposition of multiple signals, the signal lines of the eye diagram become thicker and appear blurred.

Therefore, the eye diagram also reflects the noise and jitter of the signal: on the vertical axis voltage axis, it is reflected as voltage noise (VoltageNoise); on the horizontal axis time axis, it is reflected as time domain jitter (Jitter). As shown below.

When noise is present, it will be superimposed on the signal and the traces of the observed eye diagram will become blurry. If there is inter-symbol crosstalk at the same time, the "eyes" will open even smaller. Generally, the wider the eyes of the eye diagram are opened and the higher the eye height of the eye diagram is, the better the signal quality is.

By doing signal simulation, you can obtain the eye diagram, and then judge the signal quality based on the eye diagram. If the eye diagram is not good, you can adjust the hardware design or PCB design to make the eye height higher to ensure the signal quality of the produced products.