It’s the holiday season again, when we celebrate the serious things in life and have some fun too.
When EMC engineers come bearing gifts…tis NOT the season to be jolly. Fa-la-la, le-la-la-laa. In fact, it’s time to be worried. For to the EMC engineer, GIFTS is an acronym for: Grounding, Isolation, Filtering, Transient suppression, and Shielding. Sometimes it takes a lot of GIFTS before a system achieves electronic happiness.
In keeping with the spirit of things, this blog write-up is going to take a fast look at the GIFTS. We can’t do much else with a limited number of words, so don’t expect this to be a detailed how-to-do-it piece.
GROUNDING
There are several different grounding schemes (i.e. chassis, signal and power to mention three). The choice is based on the purpose of the ground. In general, a hybrid single-point/multi-point grounding system should do the trick. This will provide a short multi-point ground reference for all equipment within a system while simultaneously reducing common mode currents.
Chassis ground serves as the reference for filters and transient suppressors, and is directly connected to the equipment/device frame. Signal and power grounds can be routed through the cable back to the single point ground.
Component bonding should be bare metal to metal with some form of conductive corrosion protection, and have less than 2.5 mohms resistance with an impedance of no more than 20 ohms at 30 MHz (≈100 nH inductance). The impedance requirement is to keep people from using long wires for grounding.
ISOLATION
Whenever possible, wiring should be categorized and segregated to isolate noisy circuits from sensitive circuits. Place like-function wires together and isolate from others (i.e., analog, digital, or power). This also holds true for PCBs. In that case, analog PCB sections should be physically separated from digital sections to prevent image-current from digital signals from wandering into the analog areas of the PCB.
If the physical layout allows, wires, cables, and traces should be routed over a continuous ground or power plane along their full length. This keeps the common mode loop area to a minimum. Long parallel runs should be avoided and incompatible categories should be run perpendicular . . . yeah right!
FILTERING
Filters are used to provide bandwidth limiting of interference. If needed, the filters should be placed as close as practical to the source of interference or to the susceptible component. If an enclosure is being used, filters should at least be used at the shield entry points to protect the integrity of the shield.
TRANSIENT SUPPRESSION
Transient suppression is used to provide amplitude limiting of interference. The transient should be suppressed at the source with voltage limiters to prevent spreading of the generated RF noise. Lightning, ESD, and EMP are all exceptions to this rule. For these we have to protect the sensitive circuits. Suppression can be done with silicon diodes, RC snubbers, or Pi filters. The choice is determined by the transient’s frequency and current.
SHIELDING
Shielding works! It can be used as a standalone solution BUT to be cost effective, good EMC design practices should be done first. The goal should be a minimum of 40 dB at 1 GHz unless it’s a military device. In this case, 80 dB would be better. The limiting factor on shield integrity is the number and size of required penetrations, including cables. Using shielded twisted wire pairs (STP) will significantly reduce coupling onto harnesses.
Apertures, including seams, should be made as small as possible, and all mating surfaces should be conductive. RF gaskets can help in this regard but they are often unnecessary.
Back in one of the early 1970s IEEE EMC Symposiums, EMC Engineer Bob Cowdell presented a paper that demonstrated that a conductive plated paint can lid embedded in the side of an enclosure was capable of providing >120dB SE with no fasteners and no RF gasket material! And, it’s watertight! An analysis of this configuration shows its action to be similar to that of a knife edge arrangement with a single row of fingers.
Connector shells are considered to be a part of the cable shielding and are electrically connected to the equipment/device enclosure. In general, cable shields are grounded at both ends with 360 degree shield terminations. In cases where a lot of high frequency noise exists on the cable shield, first try to eliminate the source, then use ferrites on the cable. If it’s a low frequency problem, an additional shield may have to be used or one end of the cable may have to be electrically floated using a capacitor. In all cases, avoid Pigtails.
Well, it was a lot longer than I planned but hopefully, this piece will help you in the new year. Have a Merry Christmas and may all the other holidays that you celebrate at this time of the year be great too.
– Ron Brewer
