EMI is any interference in the form of either radiation through the air or conduction over wiring, shields, capacitive or inductive (electromagnetic) coupling.
EMI can make other devices in the neighborhood or powered via the same power source behave erratically, unexpected, or can degrade their performance.
EMI can also cause problems within a device or even within a module/PCB of a device.
Electronic devices not only should be designed to have maximum immunity to radiated (via air, magnetic fields, inductive or capacitive coupling, radio frequency waves …) and conducted interference (via wires, shields, DC power supply cables, AC mains input) caused by other devices. They should also be designed to minimize emitted radiation and conducted interference towards other devices in the neighborhood or that are using the same power source.
EMI exists over a broad frequency spectrum, also covering RFI (Radio Frequency Interference). RFI can be seen as a subset of EMI, only covering the radio frequency part of the frequency spectrum.
EMI has 2 components : Electric fields (E-fields) and Magnetic fields (H-fields), which can exist as an individual static field. When the electric field changes, the magnetic field also changes. And when the magnetic field changes, the electric field changes. So dynamically both are interrelated and can be presented as 2 perpendicular vectors that are perpendicular to each other (90 degrees angle).
Electric fields are caused by accumulated static charge that is not moving, like on a plate of a capacitor or like with static electricity.
This charge causes an electric force that will attract opposite charges and repel charges with an equal sign. Electric fields are associated with potential differences of charged objects or, in other words, voltage. Capacitance refers to the amount of energy that can be stored in an electric field. The reluctance of voltage to change quickly is called capacitance.
Magnetic fields are either caused by permanent magnets or by a charge that is moving, whereby the magnetic field is proportional to the speed of the charge. So magnetic fields are associated with moving charges or, in other words, current. Inductance refers to the amount of energy that can be stored in a magnetic field. The reluctance of current to change quickly is called inductance.
When an electric charge is being accelerated back and forth fast enough, this will cause the static electromagnetic field, that is associated with this moving charge, to become airborne, meaning it is being emitted as radiation. How much or how easy it is radiated depends on the combination of the frequency, thus wavelength (m)= speed of light (m/s) / frequency (waves/s), and the distance over which the wave can travel. If the travel distance of the wave is a multiple of the wavelength associated with the frequency of the wave, a standing wave can form with maximum amplitude, which makes it easier for the electric field to become radiated.
Noise in the frequency spectrum below 30 MHz is indicated as conducted EMI, and above 30 MHz it is indicated as radiated EMI. There is no magic switchover point between conducted and radiated EMI, but 30MHz is commonly used in the EMC Directives (standards) because above this frequency, the associated wavelength starts corresponding with the trace- and cable-lengths found in devices.
The high frequency content of conducted EMI turns into radiated EMI when the conductor length comes close to 1/4 of the wavelength of the interference frequency.
1/4 wavelength for 1 MHz is about 7.5 meters.
1/4 wavelength for 100 MHz is about 75 cm.
1/4 wavelength for 1 GHz is about 7.5 cm
1/4 wavelength for 10 GHz is about 7.5 mm
By limiting the high frequency content of conducted EMI using filters (f.e. single-ended or common mode filter, bypass filter, noise filters, special attention to PCB layout), we also limit radiated EMI.
EMC (Electromagnetic Compatibility) is both the capability of a device to cope with external EMI (immunity) and the level of EMI that the device itself generates (emission). EMC does not only cover immunity to and emission of conducted and radiated EMI, but also immunity to ESD (electrostatic discharges) and fast transients, surge pulses injected on the power supply input.
Below is a list of the EMC tests:
Emission of EMI from the device to the outside world
Radiated emission of EMI
Conducted emission of EMI
Immunity of the device for EMI from the outside word
Immunity for radiated EMI
Immunity for conducted EMI
Immunity for fast transients on the power supply
Immunity for surge pulses on the power supply
Immunity for ESD (electrostatic discharge)
Immunity for voltage drops of the power supply
Immunity for magnetic fields
Sparks in Brushed DC motors, brushed alternators, cause high frequency EMI because of the very high dI/dt (change of current over a period of time).
Rotating fields in brushless and 3 phase AC motor causing changing high voltage fields.
Frequency converters for AC motors
Fast PWM signals.
Capacitive coupling over parasitic capacitance, between surfaces carrying a changing voltage (AC), PCB layers ...
Inductive/magnetic coupling into wiring, PCB tracks, between inductors, coils, between a coil and a wire, between 2 wires ...
Switched-mode power supplies. Especially isolated DC-DC converters cause high frequency common-mode currents to flow and return via parasitic capacitance over a larger loop than differential mode currents that return via their ground reference and form a small loop. The larger the loop that
the current flow makes, the larger the inductance. This makes it easier for higher frequencies and noise to couple into other parts of the circuit or adjacent modules.
High frequency common-mode currents will find a path through cables adjacent to the source, ground planes or any other possible return path.
The larger the inductance in this path, the larger the voltage noise caused by these high frequency switching currents (high dI/dt) of the DC-DC converter.
Common mode currents are caused when the return current of high frequency signals is obstructed or is mixed with other currents, causing unbalance between the signal and return path. These high frequency currents find their way via larger loops and thereby contribute to EMI.
Ground current loops caused by multiple protective earth grounding points or multiple ground points at the shielding. Ground loops allow low frequency noise to be coupled into this loop. The bigger the loop, the easier noise can get coupled into the loop. When there is a potential difference between grounding points, caused by either power supply currents or induced currents flowing through the shield or ground, this will cause problems when the signals are processed. When the signal is referenced to a certain ground potential at the transmitter, while the receiver of the signal references the signal to a different ground potential, the signal can be seen as high, while it was actually low or vise versa because it gets lifted above or pushed below the threshold. This difference in ground potential can be quite high for short periods of time due to f.e. inductive surge currents flowing through the grounding system. Ground loops can be broken using transformers or opto-coupler or by using only one grounding point.
PCB layout which is not optimized for EMI. In every situation where high frequency AC signals don't have a proper low inductance return path, these signals will become air-borne (radiated) via the electromagnetic fields that they create. These electromagnetic fields couple into other conductors and circuits, causing common mode currents that will return to the source via parasitic capacitance into ground connections.
Spikes created by the recovery time of rectifier diodes in switched-mode power supplies.
Light dimmers or motor controllers using phase control (conduction for a part of the AC mains supply sine wave using a Triac or Thyristor)
Crosstalk caused by capacitive or inductive coupling between signal wires.
PCB layout :
Use of decoupling and filter capacitors. Use multiple capacitors in parallel f.e. 10uF in parallel with 100nF in parallel with 10nF to decouple over a broad frequency range, since each capacitor covers only a part of the frequency spectrum.
Place the high frequency power supply decoupling capacitors as close as possible to the power supply pins of the individual components to minimize series inductance. Position of DC buffer capacitors close to the circuits that are drawing currents with high rise and fall times.
Check that you have solid grounding with the lowest possible impedance
Check the current paths and make sure the current can return to the source using a path with the lowest possible impedance. This means that the return path should not be cut up, not be pierced with a lot of via's, not via a long detour.
Bear in mind that DC currents take different routes than high frequency AC currents.
DC currents flow from and back to the source via a path of the least resistance, which is a straight line in the best case.
High frequency AC currents, though, flow from and back to the source via a path of the least inductance. This means that the return path will not be a straight line back to the source, but as close as it can get to the forward path, preferable in the copper layer directly under the forward path.
Why ? Because current like to take the path of least resistance (impedance) and when talking about AC currents, the path of least impedance is a loop with the smallest possible area, thus lowest inductance.
Proper use of solid ground planes/layers and layer stacks that are optimized for EMI. Sandwich the high frequency signals layer between layers that are use as ground planes. This will reduce the radiation of these signals to a minimum. The ground planes should have the lowest possible impedance by using enough via's connecting the plane to the ground reference.
Snubber networks to decrease high frequency content.
Ferrite beads to minimize very high frequencies.
Running cables through ferrite cores to filter out the high frequency noise.
Current limiting resistors in digital signals to lower the rise time of the signals and thus reducing high frequency harmonics.
The lower dI/dt will lower voltage drops over the trace and component inductances.
Decrease inductance of tracks for high frequency signals by providing a return path directly under the track. This minimizes the loop area between signal and return path, thereby minimizing inductance.
Provide low resistance return paths for DC currents
Prevent excessive puncturing of the ground plane with via's
Isolate analog grounds from digital grounds using cut-outs or taking care that the analog and digital return currents do not use the same area in the ground plane.
Decrease the common mode currents generated by magnetic fields (inductive coupling) by using common mode chokes at the input and/or output, especially when using isolated switched-mode DC/DC converters. The inter-winding capacitance in the transformer of an isolated switched-mode DC/DC converter, between the primary and secondary winding, can cause common mode currents.
Decrease the rise time of signals will both lower the mutual inductance and mutual capacitance between signals. The
Common mode currents can also be decreased by putting the power cord as a whole through a toroidal ferrite or a by using a clip-on ferrite on the power chord, because the power cord is the most common EMI radiator. Even in a linear power supply, the rectifying diodes and filter capacitors cause pulsed currents with high harmonics content, thus noise over a broad frequency spectrum. With isolated switched-mode power supplies, there is even more broad spectrum noise on the power chord.
The influence of electric fields can be decreased by using shielded cables or by shielding sensitive electronics using a grounded metal enclosure. The shield prevents that a changing electric field will couple capacitive into signal-wires or traces connected to high impedance inputs, thereby causing extra noise currents. A further advantage of a shield around signal wires is that it provides a fixed parasitic capacitance between the signals and the shield. Without a shield, the parasitic capacitance between the signals and the environment would change when relocating the signal wire (cable) and the parasitic capacitance would be determined by the charge of nearby objects. So a shield generates a very reproducible and fixed parasitic capacitance. If a proper enclosing shield is not possible, then moving the wires close to a ground plane will lower the effect of electric/electrostatic fields.
The influence of magnetic fields is more difficult to decrease, because nothing can actually stop a magnetic field. The magnetic field lines will always close and return to either of the 2 poles of the magnetic field. The only thing you can do is relocating the magnetic field lines using mu-metal, ferrite or iron that is thick enough to absorb the magnetic field. Using twisted pair cables, balanced signals will minimize the influence of magnetic fields. When the wires are not twisted, the current induced in the wire closest to the magnetic field will be higher than the current induced in the wire further away from the magnetic field. By twisting the wires, the effect of distance to the magnetic field is averaged out, and the cumulative effect will be minimal. Also, good to know is that a magnetic field drops with the distance squared. That means that when doubling the distance, the magnetic field will drop to a quarter of the original value. So when f.e. power cables crosstalk into cables with sensitive signals by means of inductive coupling, moving the cables away from each other will decrease the coupling effect drastically.
When implementing a power supply filter with an inductor/ferrite bead, make sure that the input and output of the filter are not situated on overlapping layers. When the in- and output of the filter is overlapping each other, this will create a parasitic capacitance of maybe 100's of picofarad, causing the high frequency noise to be able to bridge the filter. Additionally, the parasitic capacitance will form a resonance circuit with the filter inductor/ferrite bead.
Especially for switched mode power supplies, identify the switching nodes in the circuit that have a high dV/dt (rate of change of the voltage over a period of time). Keep these nodes small and away from other parts of the circuit, so the electric field can not couple into these other parts.
Identify the current loops in the circuit that have a high dI/dt (rate of change of the current over a period of time). Keep this loop as small as possible and away from the other parts of the circuit, so the magnetic field can not couple into these other parts.
For regulators or filters : keep the ground return of the input capacitor separate from the ground return of the output capacitor. It is bad practice to connect the ground return of the input and the ground return of the output together to one point and connect this point f.e. with via(s) to the ground plane layer. This will couple noise between input and output.
Conducted EMI in the frequency spectrum between 150 KHz and 30 MHz is measured by connecting the device power supply to the AC mains supply or DC power supply via a LISN (Line Impedance Stabilization Network). The LISN forms a high impedance (using LC filters) for the conducted noise that is caused by the device, and it blocks all noise and transients that are present on the AC mains or DC power supply from entering the device. The LISN provides a 50E measuring output, so the conducted noise can be measured with a spectrum analyzer that has a 50E input impedance. This way, the repeatability of the measurement is guaranteed.
Conducted EMI can also be measured using an HF current probe. The power cable of the device is routed through the HF current probe and the probe measures the common mode current. Common mode current is the symptom that is caused by the EMI disease in the device as a result of bad grounding, unbalance in differential pairs, high frequency signals that don't have a return path direct under the signal. All energy that is radiated within the device will be translated into common mode current. So the common mode current is a good indicator for EMI problems.
Conducted EMI is measured in dBuA (decibel microampere) over 50E.
dBmW (decibel milliwatt) = dBuA - 73.
dBuA = dbuV - 34
Radiated EMI (including RFI) in the frequency spectrum between 30 MHz and 10 GHz is measured using antenna's that are placed at fixed distances from the device under test.
Radiated EMI is measured in dBuV (decibel microvolt) over 50E.
dBmW (decibel milliwatt) = dBuV - 107
0 dBmW = 107 dBuV = 0.2236 Vrms
-60 dBmW = 47 dBuV = 223.61 uVrms
-80 dBmW = 27 dBuV = 22.361 uVrms
dBuV = 20*log (Volts) + 120
When a device is malfunctioning or behaves unexpectedly due to EMI and power supply noise is being suspected, the power supply can be replaced temporarily by a battery to check if this changes the behavior of the device.