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How To guide on Pre-compliance testing

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SiC:
Hi Dave,

I'm a long time viewer of your channel and I love it! One thing I would love to see however, is a how to guide on pre-compliance EMC testing - especially as now you have a low-cost spectrum analyser.

I'm sure there are many hobbyists/entrepreneurs/etc who have designed, built and tested their widget and ready to go to market. But before actually getting to market they have to book expensive lab time to get it EMC certified.

Now if we could test our widgets at our own labs, we could know in advance if its got any chance of passing or if a redesign may be required. So when it comes to the actual test day, we can come much better prepared. These new cheap Spectrum Analysers would certainly be cheaper than booking a testing house lab session and failing! Also it would be great to learn set-up and use (and if possible, even how to build) antenna's for the different frequencies used.

Doable? Or is this cheap equipment really not good enough to do it?

Cheers,
Si.

jahonen:
I think this thing has been discussed in the past and conclusion is that while one certainly can get a general idea how it will look, but it is much more difficult to say whether DUT will pass limits or not with great confidence. It is really not the measurement equipment (spectrum analyzer) price tag, but a calibration of the setup as whole (i.e. how much of RF input to the analyzer corresponds to a certain field strength seen by the antenna) and the way the measurement is done (in an RF-anechoic chamber). This is because one can only measure nearfields with simple means, while actual test measures more or less farfield. Probably one can learn to have some kind of instinct which will be the case but I afraid that comes only via much practical experience.

I think that Dave would have a difficult time to trying to explain all this in reasonable time, although certain quick measurements could be explained with relative ease.

Of course, conducted emissions are much better candidate for homebrew measurement, as there is no requirement for calibrated antenna or anechoic chamber. LISN (Line Impedance Stabilization Network) is not a very exotic device, just few carefully constructed coils and capacitors. But there are some caveats which can destroy the analyzer if one is not careful.

Regards,
Janne

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