Signal Integrity and Jitter Analysis Using Eye Diagrams

The way a signal travels across a wire varies depending on the transmitter and receiver. This is where methods for error identification and correction are useful. However, how can we tell if a signal is different? It goes without saying that we need to use a device to track at both TX-RX ends and view the data bit by bit. And why is the signal shifting, and is it possible to stop the cause? Yes, it is somewhat feasible, but there is little we can do to protect a wireless channel from noise. Due to EMI and environmental factors, the channel (the medium) has a tendency to alter the signal. However, EYE Diagrams can be used as a tool to plot that data in a more representative manner. An eye diagram can provide a variety of information, some of which are as follows:

• Signal level noise
• Edge transition noise (jitter)
• Duty cycle distortion
• Bit error rate
• Inter-symbol interference (ISI)
• Clock-data skew
• Inadequate rise/fall times
• Crosstalk
• Power supply noise

It is a method for assessing how intersymbol interference, dispersion, and channel noise all affect a baseband pulse-transmission system's performance. The eye diagram's open eye pattern indicates less signal distortion. This article examines the ideas of jitter and signal integrity as well as how eye diagrams can be used to measure and diagnose these issues.

What is Signal Integrity?

Signal integrity (SI) refers to the quality of electrical signals as they travel through traces, interconnects, and components on a PCB or communication channel. Ideally the digital signals should transition sharply between high and low logic levels without distortion. However, due to parasitic capacitance, impedance mismatches and reflections the signal can become distorted. Which leads to reduced timing margins and data corruption.

Signal integrity analysis ensures that transmitted signals maintain their intended shape, amplitude, and timing throughout the system. It becomes particularly important in systems operating at frequencies above a few hundred megahertz. Or when dealing with long interconnects and serial links.

Understanding Jitter:

The departure of a signal's timing edges from their optimal locations is known as jitter. It impacts clock and data recovery (CDR) circuit accuracy and shows up as a horizontal displacement in time. There are two main categories into which jitter falls:

• Deterministic jitter (DJ): Repeatable and predictable jitter brought on by power supply noise, crosstalk, or interference.
• Random jitter (RJ): Unpredictable jitter brought on by stochastic (random) phenomena, such as thermal noise, is known as random jitter (RJ).
  

Excessive jitter can lead to sampling errors in systems with tight timing margins. Analyzing and minimizing jitter is therefore crucial in maintaining reliable data transmission. We have covered a blog on timing analysis 101 series about propagation delays.

Eye Diagram Analysis:

The statistics of signal transitions between various voltage levels are depicted in the eye diagram. This provides you with a measurement of the noise present at the receiver as a result of crosstalk, intersymbol interference, and any noise, such as jitter, on the driver's I/O power rail. But the mask, or eye opening, of an eye diagram is usually the metric used to read it. Signal quality is indicated by the "eye" opening. A closed or partially closed eye indicates problems like jitter or noise, whereas a wide and open eye indicates the signal has little distortion and good time. Other important elements to consider are:

Eye height: Shows how efficiently the signal distinguishes between high and low logic levels and represents the signal-to-noise ratio.

Eye width: Indicates how accurately the signal can be sampled at a specific timing interval and reflects the time margin (timing jitter).

Eye opening: Denotes the available margin for signal reception and describes the general clarity of the eye shape. The quality of the signal increases with eye openness.

The voltage margin of the signal is represented by the eye's vertical height. A smaller vertical opening denotes a lesser signal-to-noise ratio, whereas a tall eye implies a high, well-defined signal level. The timing margin is reflected in the eye's breadth. While a tiny horizontal opening could result in timing mistakes, a large one indicates there is enough time for the signal to be accurately sampled.

Potential timing problems, such as clock skew, can be found by looking at the eye diagram's center, where the signal crosses. Signal integrity is weakened by jitter (timing changes) and noise, which are shown by deviations from perfect signal edges.

Eye Diagram Tools and Measurement Setup

To generate and analyze eye diagrams effectively, you typically require:

high-bandwidth digital oscilloscope or a bit error rate tester (BERT) for capturing high-speed signal waveforms over multiple clock cycles. A clean clock reference or a pattern generator to ensure that the signal being analyzed is not distorted by jitter or phase noise. Proper probing techniques including the use of low-inductance probes and short ground leads.

More advanced eye diagram analysis may leverage software-based tools that offer deeper insight into signal integrity. These tools can perform jitter decomposition, breaking it down into random and deterministic components. Eye diagram tools often include eye mask testing, which checks whether the signal remains within acceptable boundaries. These limits are defined by communication standards such as USB, PCIe or Ethernet. Bit error rate (BER) contour plotting, which visualizes the probability of error across the eye opening, offering a statistical measure of signal quality and margin.

Importance in High-Speed Interfaces

Eye diagram analysis is a standard validation step for interfaces like HDMI, USB, PCI Express and Ethernet. These protocols define compliance masks and acceptable jitter levels to ensure interoperability and performance. Failing eye diagram tests can indicate serious design flaws that require re-routing traces. By adjusting impedance and adding termination we can improve PCB layout and then these abnormalities.

Conclusion

Eye diagrams are invaluable tools in high-speed PCB design. They provide a clear visual representation of signal quality and integrity. By using eye diagrams to evaluate and optimize your design, here is the same as what we have seen. Proper use of eye diagrams helps engineers balance the competing demands of speed. They are essential for reliability and manufacturability. And ensures that modern electronic devices function as intended in even the most demanding applications. Whether you're an amateur looking to expand your knowledge or a seasoned professional, understanding to interpret eye diagrams is an essential skill.