Making Sense of High SpeedFollow article
The modern world depends on data. So many innovations that affect our daily lives, from the latest 5G network to the rise of Smart Factories, depend on the transmission of huge amounts of data.
The Speed Limit of Communication
For many years, data had a speed limit. For centuries, the speed of data was dictated by the method used to send it. How quickly a message was delivered depended on the speed of the ship, the horse or (in some famous examples) how fast a man could run. Even after the invention of the steam engine, data could only move as quickly as it could be delivered.
The invention of the electric telegraph ushered in the information age. The speed limit of data changed subtly. No longer would information need to be carried physically, but instead was transmitted almost instantaneously over miles of cable. However, there was still a limit. The speed of transmission was limited to how quickly an operator could create the message using Morse code, and how quickly the receiver could decipher the information.
The true revolution came with the electronics age and the automation of the sending and receiving functions. Suddenly, information could be sent without the need for human intervention. However, the speed limit remained, as anyone who lived through the 90s and the joy of dial-up internet services will remember. At this time, the internet was delivered using the existing telephone network, a system that simply was not designed for the volume of data it was being asked to handle.
Entering the Digital Age
As we enter the third decade of the 21st Century, the demand for data from every element of society means that we can no longer rely on existing infrastructure. Whether we are streaming movies to our smartphones or updating the operating system of our autonomous vehicles, our data use will continue to grow over the next 10 years.
As part of the digital infrastructure, connectors are having to evolve in parallel with the growth of data services. In fact, with the development of PAM4 transmission techniques, for the first-time connectors are capable of greater data rates than the processors generating it. The speed limit of data has changed again.
But what is PAM4? Many high-speed connectors from Samtec and others define their channel performance with a number of Gigabits-per-second (Gbps), followed by strange acronyms. To truly understand what these connectors are capable of, we need to understand what these acronyms mean.
Understanding Acronyms - NRZ
Data is transmitted between computers as a series of ones and zeros. In order to send more data in a given time, the switch between the one and the zero needs to be quicker. In reality, the difference between the two is not a true on-off relationship but is more accurately the difference between the high and low signal levels. This is known as NRZ which stands for non-return-to-zero.
The challenge for transmitting data is to establish a clear difference between the high and the low signal levels, and the quicker the states change, the higher the potential for problems.
This is where the eye-graph comes in. This is the trace from an oscilloscope which shows the quality of the signal. In this instance, we are looking for a clearly defined eye - the interval “A” in figure 1.
Figure 1: NRZ Eye Diagram
NRZ is a perfectly capable technique for transmitting data, especially over relatively short runs. However, to transmit data at a greater rate, it is necessary to look at different techniques.
Understanding Acronyms – PAM4
PAM4 stands for Pulse Amplitude Modulation 4-level. In contrast to the simple high-low nature of the NRZ technique, PAM4 seeks to double the amount of information that can be transmitted over a given time by using 4 signal levels. Here is the eye chart for a PAM4 channel.
Figure 2: PAM4 Eye Diagram
As you can see, there is a lot more going on. The signal is formed from four different voltage levels, designated 00, 01, 11 and 10. One of the critical things to note is that the chart shows three eyes, not one. For a good signal, each of the eyes needs to be clearly defined, and this highlights one of the pitfalls of PAM4 channels. As the eye walls are closer together, it is easier for a poor signal to result in the loss of data. So, while PAM4 is ideal for sending data at a higher rate, it is more vulnerable to interference.
A Greater Understanding
Samtec has a wealth of information to help you learn more about the comparative performance of NRZ and PAM4 channels. For a more in-depth introduction and to see the inspiration for this article, visit Brian Niehoff’s blog, where he explains the differences from a Mechanical Engineer’s point of view.
If you are ready to make the leap into high-speed signal, Samtec also has an online simulation tool called the Samtec Channelyzer® which will help understand how your circuits will perform in the real world.
And of course, you can take a look at some of the Samtec products that will make your NRZ or PAM4 design a reality - the SEARAY™ SEAFand SEAM series, along with the Edge Rate® ERF8 and ERM8 series.