EMC shielding

Thread Starter


Joined Jan 4, 2007
I am having trouble with noise from mains ac lines affecting my instrumentation.

What earth systems or procedures should I follow to stop any noise interference?


Joined Apr 20, 2004

Do you have any means to characterize the noise? Fluorescent lamps and SCR controlled loads are good sources of noise.

Is there a quality line to earth in the building? If so, something like an Isobar power strip and good case connections to earth will help. Electrical/electronic noise elimination is more of an art than a science, especially if you can't find the source - taking spikes off the mains is diffeent from having RF noise come in from a nearby transmitter.


Joined Feb 24, 2006
Conducted noise from the mains can be attenuated by lowpass LC filters, configured as series L and shunt C. Many 3-prong instrument power receptacles have this circuitry built in, and laptop power adapters have a molded ferrite barrel around the connection to the computer.

For radiated emissions there is capacitative coupling of electric fields and inductive coupling of magnetic fields. A 360 degree conductive surface will eliminate electric fields inside the box so to speak. The inductive coupling of the magnetic field requires that you reduce the area of all loops. That's why twisted pairs are used. Identifying the noise source goes a long way toward solving the problem.


Joined Nov 1, 2006
You didn't say what type of instrumentation you're using, but a general rule to avoid ground loops is to ground every enclosure & chassis & use signal isolators on interconnect cables & possibly an isolation or [Sola CVS type] constant voltage transformer to power this equipment. The last two steps are a bit pricey so try all the other suggestions first.