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In this article, we’ll explore the coupling methods of electromagnetic interference (EMI) and delve into the types of currents that EMI generates in printed circuit boards, specifically focusing on differential mode and common mode currents. Understanding these currents is fundamental to designing circuits that can effectively manage interference, maintaining performance across diverse environments.
In one of the previous lessons, we reviewed how electromagnetic compatibility (EMC) breaks down into two main branches: electromagnetic interference, often shortened to EMI, and electromagnetic susceptibility, also known as electromagnetic immunity. These two branches help us categorize and address the different aspects of how electronics interact with electromagnetic energy, whether it’s to prevent unintentional emissions or protect devices from interference. Within these branches, each is further divided for more targeted analysis and control.
For electromagnetic interference, or emissions, we typically separate these into conducted emissions and radiated emissions. For susceptibility or immunity, we similarly split this into conducted susceptibility and radiated susceptibility.
To better understand how these types of emissions and susceptibilities arise and how they travel, it’s important to clarify two main distinctions regarding the origin and distance of a signal. The space around an EMI source divides into two primary regions:
The Near Field region
The Far Field region.
Close to the source, in the near field, we analyze electric and magnetic fields separately. Their properties are influenced strongly by the source itself. As we move farther away, in what’s known as the far field region, these fields merge into what we call an electromagnetic plane wave.
Here, the field characteristics are shaped less by the source and more by the properties of the medium through which they propagate. The boundaries between these regions are determined by the signal’s wavelength.
This separation between near and far fields doesn’t have an exact cutoff point but instead passes through what we call the transition region. Typically, this transitional zone is located at a distance of about lambda over two pi (where lambda represents the signal wavelength), although this can vary.
In the far field, the ratio between the electric field and magnetic field stabilizes, reaching a consistent value referred to as the wave impedance, equal to approximately 377 ohms in free space.
In the near field, however, the ratio between electric and magnetic fields depends on the source and varies with the observation distance. This variability occurs because electric and magnetic fields attenuate differently depending on the type of source.
For example, in a dipole antenna (a source tipically characterized by high voltage and low current), the wave impedance is greater than 377 ohms, making the electric field dominant. Here, the electric field strength decreases inversely with the cube of the distance, while the magnetic field diminishes inversely with the square of the distance. As the distance increases, this ratio lowers until it reaches the wave impedance of 377 ohms.