As mentioned in (EMI Noise, Part 1: Don’t Be the Problem), there is always a source, a path, and a victim when it comes to electromagnetic interference (EMI) and other types of electrical noise. This post focuses on the victim and what you can do to avoid becoming one. To recap, a variety of passive components can be used to suppress electrical noise, including: multilayer ceramic capacitors (MLCCs), feedthrough capacitors, specialized EMI filters, ferrite beads, and common mode chokes. These same devices are also used to reduce EMI radiation. Assuming smart circuit board layout has been implemented, each of these devices offers unique advantages depending on the nature of the electrical noise coupled to a system.
To solve electrical noise problems effectively, the source of the EMI, its coupling path to the victim, and the victim must be identified. So, before proceeding, let’s review some common sources of EMI:
- Electric Motors
- Electric Power Lines
- Mobile Electronics
- Loose Wiring Harnesses
- Garage Door Openers
There are four possible mechanisms for EMI to be coupled into a system:
- Conducted coupling occurs when source and victim circuits share current paths.
- Electric field coupling occurs when there is a difference in charge between two lines placed in the near field, a distance of less than half the wavelength of the radiated noise. This difference in charge induces a voltage between the two lines.
- Magnetic field coupling occurs when current flowing through a wire induces a magnetic field that couples to a nearby wire, which in turns induces a current in that wire.
- Radiated coupling occurs when far field — a distance of several wavelengths between source and victim — radio transmissions couple into a system.
- Using a MultiLayer PCB with uncut Vcc and Ground planes
- Using proven decoupling methods
Identifying the source
Identifying the source of EMI noise is not always intuitive. For example, if lightning strikes near your home during a storm and the lights go out, you can deduce that the lightning was the source of the problem. However, if it’s a clear day outside and you notice the television flickering, you may be tempted to think that the television is defective, warranting repair or return, but the problem could likely be due to a nearby EMI source, such as power lines, a new RF tower, or even construction equipment, all of which can produce disruptive noise.
Identifying the current path
When trying to identify the EMI coupling path, it is important to remember that current flows through the path with the least amount of impedance, which is the sum of the resistance and reactance of the current loop.
The impedance equation above uses the inductive reactance, since the impedance of the current loop largely depends on the inductance of the path that the current takes. At low frequencies (typically below 1MHz), the path that the current takes is largely dependent on “R.” Since R>>2ΠfL, the current path will be through the path of lowest resistance.
At higher frequencies (typically 1MHz and above), the path that the current takes is largely dependent on its inductive reactance. Since 2ΠfL>>R when “f” is high, the current will tend to return through the path of lowest inductance. At low frequencies, signals will travel through the low resistance path. At high frequencies, signals will travel through the low inductance path. As such, to determine the current path, just compare the impedance of the possible current loops at the operating frequency.
Available EMI filtering solutions
Capacitors, inductors, and shields are some of the many available tools that can help suppress EMI in a system. With regard to capacitors, standard MLCCs and feedthrough capacitors can both be used to decouple noise from non-critical and automotive systems. MLCCs are relatively low cost components, which can be an advantage in many applications, but feedthrough capacitors are typically preferred for EMI filtering due to their broad, high-frequency filtering range and higher-level filtering capabilities. Often more critical applications — including medical, aerospace, and space applications — require more specialized filters, which are available from several manufacturers.
Inductive devices are also effective at suppressing noise. For example, ferrite beads exhibit low impedance at low frequencies and very high impedance at high frequencies to suppress high frequency noise. Common-mode chokes are also frequently used to filter common-mode noise out of parallel wires in applications ranging from high data rate USB and HDMI to power systems.
Implementing EMI filtering
(See Demcko, R.; Mello, C.; and Ward, B., “AVX EMI Solutions.”)
In addition to the use of filters, effective EMI suppression also requires smart PCB layout techniques. Depending on the source, the coupling mechanism, and the victim, there are several layout solutions that can be implemented to reduce the amount of noise being coupled to the system. For example, EMI can be significantly reduced through PCB layout techniques including:
- Using a high frequency decoupling capacitor at each IC
- Using a high frequency decoupling capacitor at the regulator
- Connecting decoupling capacitors in the lowest inductance method possible
- Grounding both ends of cables
- Considering multiple grounds on a ribbon cable
- Multilayer varistors (MLVs) clamp bi-directionally in the on-state and like EMI filters in the off-state
- Placing the MLV as close to the transient source as possible
- Considering a dedicated Vcc line to clocks
- Considering limiting the number of 90° trace features, since two 45° trace features typically radiate less
- Using balanced trace design if possible
- Filtering at connector pins if necessary
This list is by no means exhaustive and doesn’t apply to every application, but it is a solid collection of good guidelines to take into consideration when laying out PCBs to minimize EMI noise.
In summary, to avoid becoming a victim of EMI: Identify its source, coupling path, and victim; utilize filtering solutions well suited to your application; and implement smart PCB layout techniques tailored to your system.