The Role of Eye Diagrams in High-Speed PCB Design

In high-speed PCB design, signal integrity is very crucial. As the frequencies in digital devices increase, we have to ensure reliable communication between components on a high speed PCB. One of the most effective tools for evaluating signal integrity in high-speed designs is the eye diagram. It is a tool for the evaluation of the combined effects of channel noise, dispersion and inter-symbol interference on the performance of a baseband pulse-transmission system.

Eye diagrams provide a visual representation of a digital signal's performance, allowing engineers to assess factors like noise, jitter, and timing errors. From a mathematical perspective, an eye pattern is a visualization of the probability density function (PDF) of the signal. An open eye pattern in the eye diagram corresponds to minimal signal distortion. This blog explores the role of eye diagrams in high-speed PCB design, their importance, and how they help improve signal integrity.

Inter-symbol Interference of Signals

Inter-symbol interference is the condition which occurs when two successive signals interfere with each other due to signal integrity problems. Either the signals are differentiated by each other in frequency domain or in time domain. Both conditions can not occur at same time which leads to inter-symbol interference(unwanted noise in the signal due to other components). Moreover now we have some techniques to eliminate the effects of ISI by sampling the signal at zero crossovers in the time domain.

Ideally, we desire zero signal distortion, zero jitter, zero pulse spreading, and zero amplitude noise. In other words, the output signals exactly match the input signals. The ability to see this is what makes eye diagrams such a fundamental part of signal integrity.

Key Components of an Eye Diagram

An eye diagram is a graphical representation of a digital signal captured by repeatedly sampling the signal at different intervals. When plotted on an oscilloscope or simulation tool, the signal traces resemble an eye-like pattern. The term "eye diagram" comes from the characteristic shape created by superimposing multiple signal transitions over time.

The “eye” is formed by the intersection of the signal’s rising and falling edges. Usually a myth about high speed circuits is “they have a high frequency clock” but in actual high speed signals depend on rise time and fall time of a signal. The diagram provides key insights into the quality of the signal:

• Eye height: Represents the signal-to-noise ratio and indicates how well the signal differentiates between high and low logic levels.

• Eye width: Reflects the time margin (timing jitter) and shows how well the signal can be sampled at a precise timing interval.

• Eye opening: Refers to the overall clarity of the eye shape and indicates the available margin for signal reception. The more open the eye, the better the signal quality.

Metrics to be Calculated From an Eye Diagram

These diagrams show how a bitstream switches between different signal levels by plotting the signal level measured at the receiver end of a channel. Some of the information that can be extracted from an eye diagram includes:

• Signal level noise
• Edge transition noise (jitter)
• Skew
• Duty cycle distortion
• Bit error rate

Signal level variance: You can clearly observe how the signal level fluctuates, which is generally influenced by timing jitter and random noise. Impedance mismatches may also contribute to signal level variations.

Average rise/fall time: This is the time between the average 90% and 10% signal levels. It reflects both the channel response and noise in the system. Strong reflections, noise, or inter-symbol interference (ISI) can cause the rise/fall times to be uneven, potentially showing plateaus or significant variations.

Average symbol duration: This is the interval between the midpoints of successive signal crossings.

Bit error rate (BER): By comparing logic thresholds with the received bits shown in the eye diagram, you can estimate the bit error rate. This value depends on various factors, with an ideal rate being as low as 10-12 or lower.

These are some primary metrics that can be identified from an eye diagram in both simulation and measurement. The same set of metrics applies to single-ended channels and differential channels.

How to Read an Eye Diagram

The eye diagram illustrates the statistics of signal transitions between different voltage levels. This gives you a measure of noise that exists at the receiver due to inter-symbol interference, crosstalk, and any noise like jitter on the driver's I/O power rail. However, the typical metric used to read an eye diagram is its mask, or eye opening. The "eye" opening indicates signal quality. A wide and open eye means the signal has minimal distortion and good timing, while a closed or partially closed eye suggests issues like jitter or noise. Other some main factors to interpret are:

• Vertical Eye Opening: The vertical height of the eye represents the signal’s voltage margin. A tall eye indicates a strong, well-defined signal level, while a smaller vertical opening suggests a lower signal-to-noise ratio.

• Horizontal Eye Opening: The width of the eye reflects the timing margin. A wide horizontal opening means there’s sufficient time for the signal to be correctly sampled, while a narrow one could lead to timing errors.

• Crossing Points: Where the signal crosses the center of the eye diagram helps identify potential timing issues like clock skew.

• Noise & Jitter: Deviations from ideal signal edges reveal jitter (timing variations) and noise, which degrade signal integrity.

Why Eye Diagrams are Crucial for High-Speed PCB Design

Evaluate Signal Quality In high-speed designs, digital signals need to maintain a clear distinction between logic high (1) and low (0) levels. Noise, interference, and jitter can distort these signals, causing errors. These problems are usually encountered more in high speed designs, where rise/fall time is relatively smaller.

A narrow eye width is a clear indicator of timing jitter, helping engineers address issues related to clock skew or signal delay in the PCB layout. Crosstalk produce interference of signals degrades signal quality, making it harder to distinguish between logic levels. Eye diagrams can help detect this problem by highlighting areas where the signal is distorted.

Optimize PCB Layout Eye diagrams can be used to fine-tune and optimize high-speed PCB layouts. Engineers can simulate eye diagrams during the design phase. Two main types of simulations are listed below while making a high speed PCB design.

Types of Simulations in a High Speed Design

Eye diagrams are typically generated using oscilloscopes or signal integrity simulation tools. In the context of PCB design, they are often used in pre-layout and post-layout simulations to predict and validate signal performance.

Pre-layout simulation: Before finalizing the PCB design, engineers can simulate the signal performance based on schematic data, expected trace lengths, and routing parameters. Eye diagrams generated during this stage provide a preview of how well the design will perform under real-world conditions.

Post-layout analysis: Once the PCB is fabricated, actual signal performance can be tested using an oscilloscope. By capturing real-time signals and generating eye diagrams, engineers can compare the results with simulations to identify any deviations or areas requiring optimization.

Tips for Improving Eye Diagrams in PCB Design

To ensure clean, open eye diagrams and maintain signal integrity in high-speed PCB designs, consider the following tips:

1) Impedance control: Maintain proper trace impedance by controlling trace width and spacing according to the design's requirements. See our full guide on how impedance mismatch reacts to a signal.  

2) Minimize trace lengths and Via placement: Avoid unnecessary vias and large tracks, which can introduce delay and signal degradation.

3) Signal integrity: Use proper differential pair routing to minimize skew and maintain signal synchronization.

4) Proper decoupling: Implement robust grounding and decoupling strategies to reduce noise and crosstalk.

5) Power and Ground return planes: Ensure adequate power integrity by designing power planes with low impedance and using decoupling capacitors strategically.

Conclusion

Eye diagrams are invaluable tools in high-speed PCB design, providing a clear visual representation of signal quality and integrity. By using eye diagrams to evaluate and optimize your design, you can identify potential issues early, ensure reliable signal transmission, and enhance the overall performance of your high-speed PCBs. Proper use of eye diagrams helps engineers balance the competing demands of speed, reliability, and manufacturability, ensuring that modern electronic devices function as intended in even the most demanding applications.

Eye diagrams are a powerful tool in the field of electronics, offering invaluable insights into the quality and integrity of digital signals in communication systems. Whether you're an amateur looking to expand your knowledge or a seasoned professional seeking to troubleshoot complex issues, understanding how to interpret eye diagrams is an essential skill. By leveraging the information provided by eye diagrams, engineers can design, optimize, and troubleshoot communication systems with greater precision and efficiency.