Electronics > Metrology

doing in house EMC pre-compliance testing - where do I start?

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Simon:
We had a product at work fail EMC in CE marking. So we have bought a spectrum analyser and will be getting the required licence on it for the required filters etc. We have an aerial, and that is all I know.

How do we go about setting this up for the pre-compliance the spectrum analyser claims it can do? The analyser itself has some sort of output as well as input. Is this so that it can self characterise the setup and make the required adjustments to get a roughly good metric against the standard?

I am sorry to say that Telonic have been pretty useless in advising on what we need to the point that I have an aerial but no cable for it, they must be doing so well that dealing with customers in order to make a few more quid on some cable is totally not worth their while.

fourfathom:
What kind of antenna?  What kind of product?  What was the failure (frequencies, amplitudes)?  I assume this is a free-space test.  And what spectrum analyzer?  What antenna?

I used to do FCC and European (pre-EU) pre-compliance testing for our telecomms gear, and before that for wireless microphones and receivers.  At the simplest, I used a spectrum analyzer, several antennas (a telescoping dipole for the lower frequencies, and a biconical for the higher ones, and sometimes a log-periodic for still-higher frequencies).  Each antenna has a calibration chart.  You may need a preamp -- that and the coax cable also need to be calibrated, or at least have known gain/loss at the frequencies of interest.  My test range was sometimes a grassy field, or at a different company where we did a lot of this we laid down a wire "hardware cloth" screen under a layer of asphalt.

So that's the setup.  This was before the era of modern spectrum analyzers with pre-loaded compliance limits and calibration factors, so I had to do that part by hand: identify the emission peaks apply the correction factors, and compare that to the compliance limits.

I highly doubt that your analyzer can self-calibrate the measurement system using just a measuring antenna.  You're probably going to have to enter the calibration factors yourself.  Again, you need antenna factor calibration data.

If you have the test results for the failed run, you may be able to useful mitigation without a calibrated system.  Just find the specific emissions (frequency, dB over limit) and measure the unmodified system with whatever you have.  Start doing your EMI fixes, and see how much you can drop those specific emissions -- this is a relative measurement, so calibration isn't critical.  If you failed by 10dB and can reduce the emissions by 20dB then you are probably ready to re-test.

The test results can be greatly affected by external cabling to/from your DUT.  Be realistic, don't cheat, and try to maintain the same configuration between tests.

Simon:
Yes my first aproach would be to look at the system as it failed and use that as a metric against the failure test. then change stuff and see how much better I can make it. First order of business is sniffing around with a near field probe to locate the cause.

I have a tekbox biconical antenna, yes it has a chart of gains at different frequencies so that the analyser can take that into account. What I am confused about is how do I actually set up to do a test with the analyser that will apparently give me the ability to compare to standards. If I am not set up right then what I am doing is meaningless.

fourfathom:

--- Quote from: Simon on August 21, 2022, 03:16:47 pm ---Yes my first aproach would be to look at the system as it failed and use that as a metric against the failure test. then change stuff and see how much better I can make it. First order of business is sniffing around with a near field probe to locate the cause.

I have a tekbox biconical antenna, yes it has a chart of gains at different frequencies so that the analyser can take that into account. What I am confused about is how do I actually set up to do a test with the analyser that will apparently give me the ability to compare to standards. If I am not set up right then what I am doing is meaningless.

--- End quote ---

Quick response!  See my additions to my previous post.

There has to be a "How to do an open-field compliance test" tutorial out there somewhere.  It's really pretty simple:  your correction factor is (Antenna factor @ Freq Of Interest + preamp gain @FOI + cable attenuation @FOI).  See what the test range distance is for your particular requirement, and put your antenna there.  Make measurements, probably with vertical and horizontal polarizations (antenna rotation) at the higher frequencies.  Usually your DUT is put on a carousel, but one way or another you need to test at different orientations.  Compare measurements to the standard. 

Your spectrum analyzer probably has compliance limit templates and the ability to enter correction factors, but if that's your question -- sorry, I've never used that stuff.  The last time I did these measurements was perhaps 35 years ago.  But I guarantee that antennas, coax, amplifiers, and radio propagation haven't changed all that much since then.

Simon:
So I replicate the distances of the standard and use the correction factors of the antenna plus losses/gains of stuff in between and I'm sort of doing the same as the test house.

Interestingly I actually have a 9kHz to 3GHz signal generator so is there a way of testing my setup by generating a known signal and see what I get.

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