Electromagnetic Interference (EMI)

Most courses and textbooks on electronics design would not take time to introduce the core concepts in routing and grounding alongside topics like electromagnetic interference (EMI). However, in PCB design, these conceptual areas are deeply related, and several design problems that generate excessive EMI are related to grounding and routing in a PCB. In today’s modern electronics, even simple designs like microcontrollers can experience EMI challenges due to the use of fast edge rates in digital logic.

We’ve mentioned multiple times in earlier lessons, and in various contexts, that there are some basic routing rules to follow when working with digital signals, and particularly with high speed signals. While we’ve focused on impedance and crosstalk in the previous lesson, we haven’t focused on the broader area of EMI. If you think about it, crosstalk is really a basic form of EMI; it is unwanted noise that is coupled into one of your circuits. Generally, the term “EMI” is broad enough to include noise induced from any source (internal or external to the board). This section will look more at EMI, particularly as it relates to routing and stackup design.

Why EMI is Important

Why do we need to worry about noise and EMI? The overarching answer is simple: it’s required by law. The majority of countries in the developed world have developed regulations that set limits on EMI generated by electronics, as well as PCBs under test and complete PCBAs. If a new product is to be released for sale on the open market, it must pass electromagnetic compatibility (EMC) testing requirements within that particular jurisdiction. These tests are used to measure how much radiation a device emits, conducts, or receives from an external source. EMI comes in two forms: radiated and conducted.

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