11 Myths of EMI/EMC: Myth #11

Myth #11: Adding filters to PCB I/O signal lines will reduce radiated emissions.

Adding a low-pass filter will attenuate differential mode (DM) noise appearing between a signal and signal ground. To reduce common mode (CM) noise the capacitors must be connected to a clean, typically chassis, ground. CM noise appears on every conductor on a PCB including the ground (signal return). When the capacitor is connected to a noisy digital ground, then the currents are conducted via the capacitor to the signal lines. Removing the capacitor may actually decrease the common mode currents on the signal lines.

11 Myths of EMI/EMC: Myth #10

Myth #10: Adding a ferrite core or balun on a cable will always reduce radiated emissions.

If no current probe is available, adding a ferrite on a cable is often used as a diagnostic tool to determine if an RF current is flowing on the cable. If a cable is electrically long (greater than 1/2 wavelength), then the current on the cable is maximum at some location and zero at another. If a ferrite balun is placed on the low-impedance section, then any RF current within the frequency range of the ferrite will be reduced due to the impedance of the ferrite.

11 Myths of EMI/EMC: Myth #9

Myth #9: A PCB should only be grounded in one place.

Single-point grounding is used to prevent common ground impedance circuit coupling. That is to say, the entire circuit system is regarded as a structural point of ground reference point, and all of the ground connections are connected to this point. Single-point grounding is divided into two types: series and parallel. Because the series type generates common ground coupling, it is better for the low-frequency circuit to use parallel single-point grounding. To prevent power frequency and other interference from stray current in the signal ground line, signal ground and power ground paths should be isolated from the ground and the chassis, and only connected on the safety ground bolt to earth ground wire by power ground, chassis ground (except floating).

11 Myths of EMI/EMC: Myth #8

Myth #8: Going to a four-layer printed circuit board will always reduce EMI.

Measurements made on differential signal traces and single signal traces above a ground plane in both a microstrip and stripline configuration show the following: the differentially driven stripline PCB has a lower level of emissions only at certain frequencies when compared to two differential traces close to and over an image plane (a ground plane underneath the traces). At high frequencies the level of emissions is similar for both configurations. Other comparisons between differential stripline and differential traces with an image plane also show that the stripline radiates at a lower level.

11 Myths of EMI/EMC: Myth #7

Myth #7: Along the same line, a power line filter tested alone with a power supply will be just as effective when mounted in the final system enclosure.

A power line filter that is unshielded with unshielded input power lines can have system noise coupled to both the filter components and the input connections to the filter. This is most likely when the filter components are mounted on a PCB containing noise sources such as switching power supplies or digital logic. The problem may be exacerbated when the input power is routed some distance away from the intended load. If the noisy output side of the power supply is run close to the filtered AC input power line side, then the resultant cross talk can either reduce the filter attenuation or even negate it completely.

11 Myths of EMI/EMC: Myth #6

MYTH #6: A commercially purchased power line filter will perform in the equipment as specified by its data sheet.

Still another common mystery of EMC is why a power line filter with an impressive level of specified attenuation does not perform as specified when wired into the real system. When a manufacturer of an inexpensive filter specifies a level of attenuation, the attenuation must be determined if it is specified for common mode (CM) noise or for differential (DM) noise. The majority of commercial-type power line filters only include a CM inductor and a capacitor across the line for the DM noise. Thus, for DM noise, only the capacitor is effective while for CM noise only the inductor is effective. Even the most inexpensive filter is tested to an MIL-STD-220 test method.

11 Myths of EMI/EMC: Myth #5

Myth #5: The impedance of a capacitor will keep decreasing with increasing frequency OR the impedance of an inductor will keep increasing with increasing frequency.

These statements are only true for ideal components. A real capacitor will have a parasitic equivalent series inductance (ESL) and inductance from the connecting wires or PCB traces and parasitic resistances as well (ESR). A series resonance circuit is formed by the LCR series components.

11 Myths of EMI/EMC: Myth #4

Myth #4: All conductive coatings or plastics are the same.

Many manufacturers quote shielding effectiveness of conductive coatings only for plane wave fields, which are relatively easy to shield against. The engineer must determine what type of field is dominant in their product whether it is plane wave, electric field, or magnetic field. A low-conductivity (or high-resistance) coating such as carbon may be effective against an E field, but not very effective for a magnetic field or plane wave. In reality it is often required to achieve plane wave and magnetic field attenuation, and here a high-conductivity (or low-resistance) coating is required.

11 Myths of EMI/EMC: Myth #3

Myth #3: To meet EMI requirements, we only have to put the equipment in an all-metal enclosure.

This particular EMI myth ignores the radiation from cables entering and exiting the enclosure and the weak link in an enclosure, namely seams and apertures. When PCBs are contained in an enclosure, the fields generated by the PCBs set up noise currents on the inside walls of the enclosure. This noise current flowing on inside surfaces can then flow out onto the external surface of the enclosure through any seams or apertures causing radiation. It is this “inattention” paid to seams and apertures that can result in radiated emissions and immunity/susceptibility failures. So, limit the size of seams and openings in the enclosure.

11 Myths of EMI/EMC: Myth #2

Myth #2: Cable shields are only connected to ground at one end.

When there is a shielded cable connecting two pieces of equipment, engineers were always told that the shield should be connected to chassis or ground at only one end. The most common reason given to do this is to avoid ground loops. Cables routed over long distances will cross different ground potentials that may exist. This difference in ground potentials will create a noise current flow in the cable shield. Audio engineers especially do not like this situation. This results only in electric field shielding with limited plane wave shielding and very little magnetic field shielding. For high-frequency shielding, the shield must be grounded at both ends.