I Spy With My Little Eye A Signal Integrity Issue

2019-10-23  |  4 min read 

Signal integrity addresses two key aspects of high-speed digital design: signal timing and quality. Signal integrity analysis ensures that signals reach their destination in good condition. For example, a system with good signal integrity has data that arrives early enough to meet timing requirements and to avoid false clocking (where a clock crosses a logic threshold more than once during a transition).

In a system, signals travel through various kinds of interconnections. For example, from chip to package, package to board trace, and trace to high-speed connectors. Signal integrity problems arise from the physical nature of the interconnecting wires. Unlike a connection line drawn on a schematic, a real wire has resistance, capacitance to ground and to other wires, and inductance. At higher frequencies, capacitance and inductance cause the wire to act as a transmission line. The resulting antenna effects lead to crosstalk and electromagnetic interference (EMI).

Sufficiently significant signal-integrity problems cause systems to fail or work only intermittently, producing “bad” data. It is crucial to find signal integrity problems early in a design cycle. Intermittent failures are difficult to debug on prototypes.

You should take the following three steps approach to solve signal integrity problems:

  1. simulate the channel
  2. identify the root cause of degradation
  3. explore design solutions

To ensure high signal integrity for your device, start by determining the simulation and measurement results you expect to see and compare them to the measured results. Engineers use electronic design automation software to create a circuit simulation. Design automation software provides fast and accurate channel simulation using bit-by-bit and statistical simulation technology. An algorithmic modeling interface is a standard used in design software to provide easy simulation of multi-gigabit serial links (transmit to receive).

In addition to design automation software, engineers use signal analysis tools such as the eye diagram (hence the title of this blog), mixed-mode S-parameters, time-domain reflectometry (TDR), and single pulse response. While simulating data transmission from the transmitter to the receiver, the eye diagram that an oscilloscope displays is the analysis tool that helps evaluate a channel’s performance.

Eye Diagram

Within the diagram, eye width and height are key indicators of signal distortion. A wide-open eye means the transmitted data is good. A closed eye indicates signal integrity degradation.  If the eye diagram shows an open eye at the transmitter and a closed eye at the receiver, the next step is to determine which devices or interconnects in the channel are causing the signal degradation. View the eye diagram directly at the transmitter’s output and progress through each interconnect up to the receiver to determine which device is responsible for degrading the signal.

Once you have designed a solution to address the signal integrity issues, you will need to establish a robust signal integrity workflow with both the software and the hardware. To learn more about high-speed digital design challenges and solutions to your signal integrity issues, download the Signal Integrity Fundamentals white paper.