Eye Diagram Basics and Embedded Systems

With each passing day, embedded system designs are becoming more complex as they require increasing data transfer rates, shorter cycle times, and other requirements.

Embedded systems engineers are required to sort out signal integrity issues quickly to speed up the development life cycle. You might already be aware that modern serial data links operate at very high frequencies in the range of gigahertz. At these high speed extremes,  a host of variables such as impedance mismatches, termination schemes, and transmission-line effects can affect the quality of the signals. All the above-mentioned factors can lead to errors when a system is trying to interpret the value of a bit.

One way to evaluate the performance of your system is using "eye diagrams" generated by an oscilloscope – these can help you can gain valuable insight into the workings of your system and all the channel imperfections that might have crept in while designing the system.

But what exactly is an eye diagram? How does it work? And more importantly, how can you generate one?

Well, let’s have a look at some eye diagram basics.

An Eye Diagram from an Oscilloscope

The eye diagram is a pattern that shows what the digital signal stream looks like from a holistic point of view. Ideally, an eye diagram should resemble a step function (rectangular boxes). However, real-life communication between digital systems is not free from imperfections;  the resultant signal samples look like an eye.

But, how does an oscilloscope generate the pattern?

The oscilloscope simply receives and samples the digital signals received from your system. By sampling small segments of a long data stream, the oscilloscope is able to superimpose them over each other and display the data on the screen, as an eye diagram. It reveals crucial information such as SNR, Jitter, noise, etc. that can be used to unearth issues with the design in early stages of development.

An Eye Diagram from the Total Phase Advanced Cable Tester

Typically, eye diagrams are used to measure transmitter performance, potentially through a link. The Advanced Cable Tester uses eye diagrams in a slightly different way:  visualize the performance of a cable by transmitting a known signal, capture that signal on the other end of the cable, then analyze the signal and building an eye diagram. A cable with better performance will have a more “open” eye. Compare the two images below captured with our cable tester:

An image of an eye diagram from an advanced cable tester
Figure 1

 

An image of an eye diagram from an advanced cable tester
Figure 2

Both were captured through the same channel (same differential pair) of the same cable, but Figure 1 was captured at 10 Gbps, and Figure 2 at 5 Gbps. You can see the first image shows significantly less opening. This cable is adequate for both speeds, but has much more margin for 5Gbit/s than 10Gbit/s. A lower quality cable would show a much smaller eye opening.

Click below if you would like a demonstration of the Advanced Cable Tester and see an eye diagram for yourself. You can also take a look at our datasheet and contact our technical sales team with your questions.

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