Practical Shielding, EMC/EMI, Noise Reduction, Earthing and Circuit Board Layout

Editor's Notes

• #2: Electromagnetic Interference requires two parties: a source, and a victim. Any electronic device has the potential to radiate or conduct emissions, and at the same time be susceptible to them itself. Some problems may exist where the victim is also the source. Understanding of the mechanisms by which this takes place is essential to solving EMC problems.
• #3: To ensure compatibility between one item of equipment and any other in a given electromagnetic environment requires a comprehensive understanding of the ways in which this source to victim coupling is possible. The slide above outlines the ways in which this can happen. Keep in mind that some of the paths only exist because of the parameters which are not taken into account - the inductance and capacitance of cables, for example.
• #4: EMI works like this. Always a noise source, a coupling path and a receptor.
• #5: The solution for reducing interference can be at the source, path or receptor.
• #6: Noise sources are everywhere.
• #7: Above is a list of considerations when changing the coupling between source and victim.
• #8: Receptors everywhere!
• #9: Any EMI problem will involve any or a combination of the above coupling mechanisms.
• #10: A minor re-routing can often eliminate a problem by removing a common conductor, which we are inclined to forget has resistance, inductance and capacitance whether we like it or not.
• #11: AC current in a conductor creates a magnetic field. This can couple with a nearby conductor and induce a current in that conductor. This usually happens where there are large and/or fast variations in current (i.e. high di/dt). Note that there is no need for a common ground for this to take place - it can happen between isolated circuits.
• #12: One conductor can be affected by a changing voltage on another. The extent of this depends on the ratio of impedances between the load and the source. The source impedance will be determined by the distance between conductors, their effective areas and the composition of whatever is between them (dielectric). Electrostatic (capacitive) coupling usually happens where there are large and/or fast variations in voltage (i.e. high dv/dt).
• #13: A combination of coupling mechanisms exist in the real world. One type of coupling can dominate, but sometimes a combination is present.
• #14: This graph shows the effects of the spacing between conductors on their ability to couple from one circuit to another.
• #15: Although the picture above of the supply system impedance, this is in an ideal world. In reality, the impedance is determined very largely by whatever is connected to the supply. This is shown by the graph above, and makes predictions of coupling difficult.
• #16: Any conductor with an applied voltage and current will generate an electric and a magnetic field as shown. The way in which these fields develop will be determined by the physical layout. Whether the electric or magnetic field will dominate depends on whether the current or voltage is predominant.
• #17: Radiated emissions can be divided into ‘near field’ and ‘far field’. In the near field, separate electric and magnetic fields exist. Which one will predominate depends on the source impedance as shown above. It is important to understand this, because they are measured differently and different measures are used to counteract a magnetic or electric field. In the far field, the two merge into a composite electromagnetic plane wave. This is often considered to take place at about one sixth of a wavelength (/2), mainly because of Maxwell. This is true of point sources, but is substantially affected by the source dimensions as shown by Rayleigh, and illustrated in the chart above.
• #18: In most cases, the wanted signal is produced in differential mode. It is also possible to have interfering signals which are induced into a conductor in differential mode, and can in turn be radiated in differential mode. Ground plays no part in this case. The cable can also carry signals in common mode, usually interfering signals they could also be internally generated). These are referenced to ground, very often by stray capacitances and inductances and can of course be radiated as common mode signals. Common mode signals become a problem when they are converted into differential mode, and get confused with the wanted signal. In antenna mode, currents are equally induced in the signal conductors, and in the reference plane (often happens in aircraft travelling through magnetic fields). Becomes a problem if converted to differential mode. Although depicted as happening between two separate modules, it is very possible for this to take place on a PCB.
• #19: The circuit drawn above shows a ‘small loop antenna’ smaller than  /4 at the frequency of interest. (All PCB tracks are ‘small loop antennae’ up to a few hundred MHz). The field varies with the square of the frequency, and is directly proportional to the signal current and the loop area, and can be substantial. Keep the area small.
• #20: Cable radiation is generally common mode, which is generally far worse than differential mode. These common mode voltages can easily arise due to poor termination, as we will see in the module on cables and connectors. The loop area is uncontrolled, hence unpredictable. Common mode current generally needs to be under 5 µA to meet cable emission standards!
• #21: As in the drawing above, where there are high switch-mode frequencies, capacitance to ground plays a major part. This allows large common-mode voltages to develop relative to ground. In addition, differential mode signals can appear on the supply line or the signal cable as a result of SMPS noise being getting through to the signal cable from the supply lines, or directly onto the Live and Neutral from the switching oscillator.
• #22: Coupling can take place either directly to the circuitry, or via I/O or mains lines. Common mode interference eventually translates into differential mode, and resonance within an enclosure must be regarded as a possibility.
• #23: Transients and spikes are different from continuously generated EMI. Above is a list of likely sources.
• #24: Virtually all transient wave-forms are classified in this way. There is some variation in where the second time period, T2, starts from. In this slide it is shown as being from almost the start of the rise of the wave. However, as both of these times are specified as being + or - 30% in most cases, it is difficult to see why anyone bothers!
• #25: The graph above shows the results of a study carried out on mains supply and telecom lines to record the number and amplitude of transient voltages. These figures obviously depend on the lightning strike density in various parts of the world, and the degree of heavy load switching in the vicinity of a particular site. Particularly with lightning (but also with switching surges) the mains connection density plays a part.
• #27: This slide shows different transients that can exist in a 12V automotive system. If not designed for, 12V components will fail at these high voltages.
• #28: The supply voltage can exhibit a variety of disturbances.
• #29: The ITIC (Information Technology Industries Council) curve shown above, demonstrates the fluctuation levels and time periods which are likely to upset a PC. From this it is obvious that short-duration voltage variations can definitely affect electronic equipment. It is just as necessary to ensure that mains-powered equipment doesn’t introduce any of these phenomena which are voltage dips (short-duration reductions in voltage) interruptions (complete absence of power for longer than ~ 3 s) harmonics unbalance (voltage differences between phases) flicker (rapid voltage variations which are annoying as they affect incandescent lighting) transients