What is signal integrity ? Why does it matter ? And what does it have to do with the PCB ?

Today, embedded electronics are using fast signals that travel along transmission lines at (almost) the speed of light. Now that wouldn’t be a issue if the electricity is used to light a bulb or start a brushed DC motor. But the transiant nature of digital circuits forces the designer to pay much more attention to the way signals are generated, transmitted and received if the embedded application were to function at all.

Digital signals are fast switching in their very nature. A long chain of ones and zeros that are represented by voltages that drop and then rise to a defined value. Usually 3,3 or 5 Volts. Nothing in the universe can exceed the speed of light, including the signal transitions. The time the signal takes to move from one bit state to the other is called the Rise Time. All integrated circuits use thresholds and numerous algorithms that make sure the device continues to correctly intepret the signals. So far so good.

Sometimes however, things don’t go as planned and a ‘One’ voltage may momentarily drop to a very low voltage that then causes the IC to interpret it as a ‘Zero’. Thankfully, semiconductor founders have already tackled this issue and already integrated error correction algorithms that make sure the integrity of the signal is preserved. All signal protocols use some kind of error correction technique. But those methods only work if the signal is reliable enough.

So what happens if each signal transition is distorted ? What if the PCB was made in such a way that all the signals are skewed ? In that case, the answer is very straightforward : all integrity algorithms fail and the whole system fails as a result.

The issue of signal integrity gains even more importance if the signal’s rise time used in your application is around 5ns or below. The reason behind this is that the Fourier Transform of the signal content will start to include very high frequency harmonics. Causing the system to start showing integrity failure artifacts if the hardware is not designed and tested correctly. Some of these artifacts may include : Signal reflection, Crosstalk or Ringing ..

In ordrer to avoid these issues, the signal’s electromagnetic wave that is sent down the PCB traces needs to travel in a Controlled Impedance medium. The industry standard for transmission line impedance is usually 50 or 70 or even 90 Ohms in the case of USB protocol. Some of the parameters that influence the impedance are : Trace thickness, PCB core thickness, FR4 material, distance between traces and VIA placements.

Most of these aspects can be anticipated during the Design phase. But the only way to make sure your embedded system will deliver its promise is to go through the trial phase by testing your embedded application in real operating conditions.

We can help you during your testing phase. Contact us to know more.

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