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In this lesson, we are going to talk about radiated emissions, how they are created, and how they depend on the layout of the printed circuit boards that we design.
We will also discuss how to design our printed circuit boards to minimize the issues these radiations can cause, allowing us to pass EMC tests and obtain also certifications like FCC and CE marks, enabling us to sell our product in the marketplace.
And so this is going to be a long article, but it's definitely one of the most important lessons that I can share if your goal is to design PCB with low EMI.
Controlling Radiated Emissions
The control of radiated emissions should always be approached as a design issue from the very beginning of the project. It is essential to allocate both financial and engineering resources early on, particularly when the goal is to minimize emissions and create a product that adheres to the EMC (Electromagnetic Compatibility) directive.
The primary point to understand is that radiated emissions are caused by time-varying currents, not by voltage. Currents flowing through wires or printed circuit boards (PCBs) inherently radiate electromagnetic energy. Therefore, the question is not whether they will radiate but rather how effectively they will radiate and how much energy will be emitted. This leads to the next consideration: the differentiation between radiated emissions stemming from differential mode currents and those from common mode currents.
Differential mode currents arise from the normal operation of electronic devices and follow the current loop formed by the conductors within the circuit. In other words, differential mode currents represent the expected, desired flow of current for proper circuit operation. As current travels through the system, it forms a loop starting at the current source, reaching the load, and then returning to the source through the reference plane, thus completing the loop.
This current loop, in reality, forms instantaneously, or more accurately, at the speed of light in the dielectric medium between conductors. This is due to the displacement current, which follows the signal's wavefront at this speed. However, for the sake of simplicity, we consider the current loop only after the current flows through the load and returns to the source.
These current loops essentially act like antenna structures, which predominantly contribute to the radiation of magnetic fields. This inherent behavior makes it critical to design with emission control in mind from the outset, ensuring the product is compliant with the EMC directive while minimizing the potential for unwanted radiated emissions.
Another key concept related to radiated emissions is described by Faraday’s law of electromagnetic induction, which states that a changing magnetic field induces an electromotive force (EMF) in a conductor, which in turn generates an electric current if there is a closed loop. This principle also operates in reverse: an alternating current flowing through a conductor creates a magnetic field according to the same law.
Returning to the idea of current loops, while these loops are fundamental to circuit function, managing their size is critical to minimizing radiation. The larger the loop, the more significant the potential for radiation, particularly in high-frequency circuits. Controlling the size of these loops is a crucial aspect of reducing emissions.
On the other hand, radiation caused by common mode currents results from parasitic effects within circuits and their conductors. These effects create unwanted voltage drops in the system, leading to current flowing through the return reference plane. The differential mode current passing through the return reference plane impedance creates these voltage drops, leading to common mode currents.
The impedance responsible for these issues stems from the actual physical characteristics of conductors, which include not only resistance and capacitance but also inductance. The inductance of the return path plays a particularly significant role as the frequency increases. Digital signals, which contain high harmonic content, can exacerbate these effects.
When cables are connected to the return reference plane, they are subjected to the common mode potential, effectively forming an antenna that primarily radiates electric fields. This common mode voltage drop is caused by the differential mode current flowing through the return path. The parasitics in the conductors—resistance, capacitance, and impedance—produce this voltage drop, which in turn generates common mode currents that radiate.
The challenge with common mode currents is that they are not explicitly part of the circuit design. These parasitic effects are not accounted for in schematics or layouts, making them difficult to detect and control. Despite often being smaller in magnitude than differential mode currents, common mode currents can be much more disruptive, as they produce greater radiation. This is because common mode currents flow in the same direction, causing the electromagnetic fields they generate to add together, unlike differential mode currents whose fields cancel each other out due to opposite flow directions.
This behavior highlights two important points when debugging radiated emissions: with differential mode currents, the opposing fields tend to cancel out, particularly if the current-carrying cables are equidistant from the measurement point. The closer the cables are to each other, the more effectively their radiated fields cancel. However, this is not the case for common mode currents, as the currents—and consequently, their fields—flow in the same direction and add together. Even if the cables are close to each other, there is no significant reduction in radiated fields.