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Why You Still Experience EMI If You Already Have a Filter?

There is a reason why you ended up on our web site.  Most likely it is because your equipment that already has at least one or more regular EMI filters is still experiencing EMI problems.  Why would that happen? Bear with us for a few minutes to read the explanation below - it is quite enlightening.  After all, it is just basic physics. For a full article on the gap between EMC regulations and real-life EMI issues subject see this inCompliance Magazine article 

This is at 50/60 Hz, of course – impedance at high frequencies varies wildly. A much more realistic scenario for power line ("line side") output impedance is 0.1 Ohms. As for the "load" side (equipment itself) - anything goes. Industry has settled on 0.1/100 Ohms numbers - 0.1 Ohms for the "line" side and 100 Ohms for the "load" side.  What happens with a regular filter that you would find inside your equipment at 0.1/100 Ohms termination? Filters are built using combination of capacitors and inductors - they form resonances between themselves and the termination impedances.  See the chart showing attenuation of a regular EMI filter (click on it to read a full article on the gap between EMI and EMC regulations published in inCompliance magazine).

Your equipment likely already has some kind of EMC (electromagnetic compliance) certification, such as CE and/or FCC mark - shouldn't it be EMI-free? Not necessarily.  EMC regulations require that electrical noise on power lines (conducted emission) was tested in with typical for laboratories impedance of 50 Ohms.  We couldn't find anywhere in the world a power line with 50 Ohms impedance, nor there are that many loads with precisely 50 Ohms impedance. 

Conventional EMI filter performance - lower frequences are amplified

Attenuation performance of a regular EMI filter

The chart that was done for one of the better regular EMI filters on the market has several traces.  Trace A is how the filter would perform in a 50 Ohms termination - not great at lower end, but not bad at higher frequencies.  Traces B and C are more revealing - they show filter's performance outside of test lab, such as in your setting (0.1/100 or 100/0.1 Ohms termination).   What traces B and C demonstrate is so-called "negative attenuation" which is plain language means "signal amplification."  At lower end of the spectrum the filter which was bought for the purpose of reducing EMI amplifies it instead - up to 10 times at some frequencies.  Is it a bad filter? Not necessarily - it performs its function for the sole purpose for which it was designed, built, and sold - to pass EMC regulations.  Its design was fine-tuned and cost-managed just for that.  To reduce actual EMI in your application you simply need to have a very different filter, and OnFILTER is here to help.

A question may be asked - this EMI amplification happens only at lower frequencies - does it matter?  Yes, it does.  When equipment is tested in the EMC lab for compliance, the cables are relatively short, their own parasitic capacitance and inductance are quite low.  How long do you think are your power cables? Don't just think of that cable with the plug coming from your equipment - think of the network of all power and ground cables in your facility, all with their inductances, capacitances, resonances, resistance, skin effect, and the rest.  When a signal with a broad spectrum such as sharp pulse (shown in the second chart), encounters cable maze, the shape of the waveform changes.  When equipment goes through EMC tests, the low frequency signals are simply not present - nothing to worry about.  But once you install this equipment in your facility, there aren't many broad-spectrum signals anymore but quite a lot of the low-end-of-the-spectrum signals - exactly in the region where regular filters amplify signals rather than suppress them.  This is illustrated in our third figure that perhaps requires not much explanation by now - it all corresponds to the filters' own specifications.  In most facilities there isn't much of energy in signals above 2MHz - for the most effective noise suppression this is the frequency range that is important for actual installations.

Change of the shape of the signal with long cable

Signal shapes with short and long cables

Comparison in performance between regular EMI filter and CleanSweep® AC EMI filter

If most of regular filters amplify noise in real-life situations, why OnFILTER' CleanSweep® filters don't?  Because they are not "regular."  Our filters are designed not with the narrow purpose of just to pass EMC test, but with the wider purpose to help to reduce EMI where it is needed - in your applications.  Our filters are impedance-independent and they provide highly-effective noise suppression  in the entire spectrum, but especially they excel where they are most needed - at lower end of the spectrum.  They work pretty well in an EMC test lab too. 

CleanSweep® AC EMI filters provide effective noise suppression in both differential (Live-Neutral) and common Live/Neutral-Ground) modes and, uniquely, in ground, giving your equipment complete protection from EMI where it needs it most.

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