An eye diagram is formed by the persistence effect of an oscilloscope, which overlays each symbol waveform obtained from scanning to create the eye pattern.
This article will guide you through what an eye diagram on a PCB is, how it is formed, what information it contains, and how to assess signal quality from the eye diagram. To understand an oscilloscope eye diagram, you need to master the following four key points:
1. What Is an Eye Diagram?
An eye diagram is a graphical display accumulated on an oscilloscope from a series of digital signals. It contains rich information and allows observation of the effects of inter‑symbol interference (ISI) and noise, reflecting the overall characteristics of the digital signal and enabling estimation of system performance. Therefore, eye‑diagram analysis is the core of signal‑integrity analysis in high‑speed interconnect systems.
Additionally, this pattern can be used to adjust the characteristics of the receiving filter to reduce inter‑symbol interference and improve the system's transmission performance.
To obtain an eye diagram, connect an oscilloscope across the output of the receiving filter and adjust the oscilloscope's sweep time so that its horizontal sweep period is synchronised with the period of the received symbols. The pattern displayed on the screen is then called an eye diagram.
Oscilloscopes typically measure signals over a few bits or a certain time window, reflecting more detailed information, whereas an eye diagram represents the overall characteristics of all digital signals transmitted on the link.
Method for observing an eye diagram: Connect an oscilloscope across the output of the receiving filter, and adjust the sweep time so that the horizontal sweep period is synchronised with the symbol period. The resulting pattern on the screen resembles a human eye – hence the name "eye diagram."
From the eye diagram, one can observe the effects of inter‑symbol interference and noise, thereby estimating the quality of the system.
This pattern can also be used to adjust the receiving filter's characteristics to reduce ISI and improve system transmission performance.
2. How Is an Eye Diagram Formed?
For a digital signal, the high‑level and low‑level transitions can have multiple sequence combinations.
Taking 3 bits as an example, there are 8 combinations from 000 to 111. By aligning a sufficient number of such sequences in the time domain to a common reference point and overlaying their waveforms, an eye diagram is formed – as shown in the figure below. For test instruments, the clock signal is first recovered from the signal under test, and then the eye diagram is superimposed using the clock as the reference and finally displayed.
3. What Information Does an Eye Diagram Contain?
For a real eye diagram, as shown below, we can first observe basic level‑transition parameters such as average rise time, fall time, overshoot, undershoot, and threshold/crossing percentage.
Rise Time: The rise time of a pulse signal is the interval between the instants when the instantaneous value first reaches the specified lower limit and the specified upper limit. Unless otherwise specified, the lower and upper limits are defined as 10 % and 90 % of the pulse peak amplitude, respectively.
Fall Time: The fall time of a pulse signal is the interval taken to decrease from 90 % to 10 % of the peak amplitude.
Overshoot: Overshoot is when the first peak or valley exceeds the set voltage, appearing as a sharp pulse that can cause component failure.
Undershoot: Undershoot refers to the subsequent valley or peak. Excessive overshoot can activate protection diodes, leading to premature failure. Excessive undershoot can cause false clock or data errors.
Threshold/Crossing Percentage: This is the minimum received signal level at which the receiver can operate when the system transmission characteristics are worse than a specified bit error rate.
4. How to Assess Signal Quality from an Eye Diagram
It is impossible for a signal to maintain exactly the same voltage level for each high or low state, nor can it guarantee that the rising and falling edges of every transition occur at precisely the same instant. Because of the superposition of many signal transitions, the traces in the eye diagram become thicker and appear blurred.
Thus, the eye diagram also reflects signal noise and jitter: on the vertical (voltage) axis, it manifests as voltage noise; on the horizontal (time) axis, it manifests as timing jitter – as shown in the figure below.
When noise is present, it is superimposed on the signal, causing the observed eye‑diagram traces to become blurred. If inter‑symbol interference is also present, the eye will appear to open even less. Generally, the larger the eye opening and the higher the eye height, the better the signal quality.
Signal simulation can be used to obtain the eye diagram, which then allows assessment of signal quality. If the eye diagram is poor, adjustments can be made to the hardware design or PCB layout to increase the eye height, ensuring good signal quality in the final product.
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