Sierra Circuits’ Webinar on PCB Design for Low EMI

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Primarily, the root cause of many design issues revolves around the circuit board design. Poor board designs can result in radiated emissions, radiated immunity, and electrostatic discharge (ESD) compliance failures, among others. Bad board designs often result in endless cycles of trial and error mitigation – compliance testing – board spins – and more troubleshooting/mitigation/spins. This drags out the schedule and is very costly.

Sierra Circuits recently hosted a webinar presented by Ken Wyatt, Principal Consultant at Wyatt Technical Services LLC (Colorado). As an EMC consultant for over 10 years, Mr. Ken Wyatt, had chances to review or help troubleshoot hundreds of electronic products and he has found that most of those products that fail compliance testing are due to poorly designed PC boards. In the webinar, he thus has explained why reducing circuit loop areas is only part of the solution and to understand proper board design for lowest EMI requires understanding how signals propagate in boards.

According to him, once we understand how signals move via electromagnetic fields, then board stack-up and trace routing should become very clear and one should be able to design a low-EMI board the first time.

WEBINAR: PCB Design for Low EMI

According to Ken Wyatt, there are only three things that can be done with electromagnetic fields. They can be stored, moved, or converted to kinetic energy. To propagate an electromagnetic wave requires a transmission line comprised of two pieces of metal, usually separated by a dielectric. In the case of a standard PC board, this would be the microstrip or stripline and a return plane.

Click here to view the cross-section of a standard microstrip. One can see the H field flux wraps around the trace while the E field is mainly concentrated between the trace and return plane. The actual signal in the form of an electromagnetic wave travels through the dielectric space between the trace and return plane. The trace merely serves as a way of guide and guides the path of the signal energy.

One important concept to understand is that for low-frequency signals, the return conduction current path is relatively spread out along the return plane and mostly takes a direct path from load back to the source. It is called the path of least resistance. So what constitutes low frequency? Something less than 50 to 100 kilohertz. So this concept is most important for audio or other low-frequency technologies.

For high-frequency signals, return conduction current path is relatively confined along the return plane and directly underneath the signal trace. This is the path of least impedance and is due to mutual inductance between the trace and plane. So what constitutes high frequency? Well, something greater than 50 to 100 kilohertz. So this concept is most important for most other digital and RF technologies.

As seen in the simulation of this concept, with the return conduction current in green, for the 1-kilohertz example on the left, one can see the return current is spread out and basically travels from the load directly back to the source. In the 1-megahertz example on the right, one can see the return current located directly underneath the circuit trace. This will be very important when partition circuit board between analog and digital circuitry.

So just how signals move in circuit boards? Most of the designers have been generally misled in circuits’ class. It was at least implied that the current was electrons flowing through copper wires from source to load. And for DC circuits and ignoring the initial turn-on transient, this is accurate. However, the movement of electrons does not occur at near light speed. They are too tightly bound to the copper molecules and only move at about one centimeter per second.

For AC circuits, the standard circuit theory model cannot be modeled as simple wires but as transmission lines. Understanding how digital signals propagate in PC boards gives a competitive advantage over other companies who may not understand this to how signals move in PC boards, the circuit’s point of view and the field’s point of view. In reality, they are related. That is we can’t have one without the other. Now the circuit theory point of view considers only the signals and power return back to their source.