How to fix every EMC problem...
Where EMC interference occurs (for whatever reason) or where a product fails to meet an EMC standard (any standard, emissions or immunity test), there are actually only ever three ways to fix it.
It sounds so simple. But it doesn’t necessarily follow that you can pick one of the above to resolve any and every EMC problem, or that the actual solution itself is a straight forward one. If we are lucky, EMC solutions can be clear-cut, but, in our experience, they can also be extremely complicated, requiring a complex combination of these three techniques. The point here though, is that these are the only three methods which are available to resolving an EMC problem.
The 3 options
Typically, the “kill it at source” option is applied up front at the design stage. However, a good designer, one who considers EMC at the outset, will also think of incorporating diverting and blocking techniques too. Once a product is out there (whether in the field experiencing an interference problem or being tested in a laboratory), we are normally limited to only the “divert it” or “block it” solutions. If they don’t work, then it can be an expensive trip back to the design board.
Common mitigating techniques
Clearly, the designer has a big responsibility for the EMC performance of a product and they should consider reducing clock speeds and fast edges, keeping fast track lengths as short as possible, confining fast digital circuitry in an enclosed or protected area of the PCB and using a brushless motor when considering their “kill it at source” approach. However, they may also apply effective diverting techniques, such as ground planes and decoupling capacitors, and blocking techniques, such as inline ferrites or resistors on PCB tracks that feed onto long tracks or external cables, or perhaps a screened enclosure around the electronics.
A filter utilises both diverting and blocking techniques. For instance, take a mains filter. It has capacitors that divert high frequency currents to earth (Y caps) or back to the source (X2 caps), and it has inductors or common mode chokes that block high frequency currents. If either of these techniques were used alone, they would not be nearly as effective as they are when working together. Multiple stage filters take advantage of mixing it up with more diverting and blocking techniques.
How do you solve an ESD problem?
Diverting it with conducting paths taking the discharge currents away from the sensitive circuitry, or blocking it with insulation, such that the discharge does not occur, are both effective solutions.
As frequency increases, a complex set of components start to exist, meaning that quite often the circuit impedances start to become unknown. A diverting technique works well on high source and load impedances (taking currents away), whereas a blocking technique works well on low source and load impedances (holding back voltages).
Have you ever used a ferrite, which gives you a good 6dB of attenuation at a given radiated emissions frequency in one situation, but then does nothing for you the next time you use it? If a blocking ferrite doesn’t work, change to a diverting technique, or a mix and match of both.
It's as 'simple' as 1, 2, 3
When dealing with EMC failure, you can’t see the impedances, the currents or the voltages that are causing the problem. However, if you can accept that there are only three possible routes to fixing the problem, then you are already half-way there to finding the solution.
Tags: EMC Testing