Editorial Note:Today’s design article comes from the book “Signal Integrity Issues and Printed Circuit Board Design” by Douglas Brooks. More information about the book can be found in this posting. This is the first half of chapter 9 - Electromagnetic Interference (EMI). The second half of this chapter is now available here along with the first half of chapter 10 - Reflections and Transmission Lines. The second half will be made available in the future.
Chapter 9 Electromagnetic Interference (EMI)
Current is the flow of electrons. When electrons move down a trace or a wire, current flows. Electrons are negatively charged particles. Therefore, we can envision that there is an electrical (negative) charge around a wire as current flows, and the magnitude of this charge depends on the number of electrons (magnitude of the current). We call this charge an electric field. Furthermore, as the current flow changes in magnitude, this electric field changes in intensity.
Also, as electrons flow, a magnetic field is generated around the wire or trace. Andre Marie Ampere, for whom the measure of current is named, is credited as the first person to state this law. This is the basic relationship behind an electro-magnet. The strength of the magnetic field is related to the magnitude of the current flow, so the magnitude of the magnetic field will change with changing current levels.
Therefore, around any wire or trace carrying a current, there will be an electrical field (often designated by the letter E) and a magnetic field (often designated by the letter H). Together, these two fields form an electromagnetic field. The two components of the field (electrical and magnetic) must flow together at the same speed. The speed at which they can travel is determined by the medium they travel through, the medium surrounding the current flow, and specifically the relative dielectric constant of that medium. Thus, the propagation speed of a signal is determined by how fast the electromagnetic field can travel through the surrounding medium.
In 1831 Michael Faraday published Faraday’s Law of Magnetic Induction. This law states that a changing magnetic field (itself caused by a changing current flow) can induce an electrical current in an adjacent wire. So if we send a changing (AC) current down a wire or trace, it can induce a similar current in an adjacent wire or trace. This can be a good thing: It is the basic principle behind radio and television transmission and reception. But it can also be a bad thing: It is the cause of crosstalk and FCC compliance testing problems. As board designers, our challenge is to design boards that maximize this effect when we want to transmit a signal and minimize this effect when we don’t. Transmissions we don’t want are called EMI, or crosstalk.
In one sense, all of our traces are antennas. A good transmitting antenna is also a good receiving antenna. Therefore, any trace that is a good “radiator” is also a good receiver. Designs that emit EMI are also more susceptible to EMI. In this chapter we will look at design techniques that help reduce EMI. We will see later that the very same techniques also help minimize crosstalk.
Fields and Cancellations
Figure 9-1 illustrates the magnetic field lines around a wire when the current is flowing out of the page. The direction of the magnetic flux lines can be determined by the right-hand rule. Point the thumb of your right hand in the direction of the current flow. The magnetic flux lines curl in the direction of your fingers.
Figure 9-1 Magnetic flux lines curl around a wire following the right-hand rule.
Now consider Figure 9-2. Here there are two conductors, one with current coming out of the page and one with current going into the page. If these two wires are carrying a signal and its return current, then the currents will be equal and opposite. Therefore the magnetic fields will also be equal and opposite. Think of yourself as positioned off to the right-hand side of this page. One of these wires will be closer to you than the other (the one with current going into the page). Thus the EMI radiation from that wire will be greater than that from the other. You will be able to measure this EMI radiation, and under the right conditions this radiation might be significant.
Figure 9-2 A signal and its return have magnetic fields that approximately cancel.
But if we move the two wires closer together, then you, as an observer off to the right, would begin to see radiations from each wire that are more equal. And since they are opposite in phase, or polarity, they tend to cancel. In the limit, if the wires were very close together, their EMI radiations would completely cancel and would not be measurable. Herein is one of our most basic principles: If you want to minimize EMI radiation, keep the signals and their returns close together. That, of course, may be easier to say than to do.