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Boost success rate with EMC pre-compliance test (Part 3)

23 Dec 2015  | Andy Eadie

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Part 1 can be read here and Part 2 here.

4. GTEM Cell
Cost: Medium ($5k-$50k)

Learning curve: High

Usefulness: Medium

Main equipment required: GTEM, Spectrum analyser/receiver

These things are pretty cool. They're like semi-anechoic chambers but much smaller. They're shaped like a wedge and integrate a stripline antenna that can act as a receiving or transmitting antenna. The EUT is placed inside the GTEM (Gigahertz Transverse Electromagnetic), between the antenna and a resistive load. The size of the EUT is restricted to a cubic area within the GTEM, but GTEMs do vary in size from small (50 cm) to a few meters. GTEMs are useful for both emissions testing and radiated immunity testing. Given the small dimensions, it's much easier to generate a large electric field of 30 V/m or higher using much smaller power-amps than you would need in a SAC.

The FCC have issued a couple of KDBs (Knowledge database articles) about the usage and acceptability of measurement results obtained from GTEMs. Under certain circumstances, the FCC will accept measurement data from GTEMs as proof of compliance.

I chatted with a technical assessor at an accreditation body about GTEMs and they strongly advise against their usage as they can yield much different results both for emissions testing and radiated immunity testing. They warned that even for pre-compliance testing their results should be taken with care because they can vary so much from an OATS or chamber. However, many reputable companies still choose to use them and if you're interested in exploring their usefulness further, Nokia released a good technical note on the correlation between their GTEM, OATS, and chamber, which makes for some interesting reading.

The ease of use, cost and time saving (you can do 8 hours work of emissions testing in less than 1 hour in an automated GTEM) draw many manufacturers in. If you include a healthy margin (say 10dB) and make sure the GTEM has an automated turntable so that the automation software can resolve the emissions profile from 3 dimensions, you can still get meaningful data.

I know several companies that legally use a GTEM to show compliance to radiated emissions and immunity standards in Europe (for CE Mark compliance) as well as use it for pre-compliance testing.

5. Cable current clamps
Cost: Low ($1k-$10k)

Learning curve: Medium

Usefulness: Medium

Main equipment required: Spectrum analyser/receiver, current probes

Kenneth Wyatt describes a measurement technique that can be very useful in determining the radiated noise contribution of external cabling. By measuring the RF common mode currents present on each of the cables connected to your product, you can easily extrapolate an approximation to the far field electric field strength that would be measured at an EMC test lab due to these cables.

This can be a really good and inexpensive method to sanity check whether your cabling is likely to meet the radiated emissions requirements at a test lab.

Conducted emissions pre-compliance
For most electronic equipment that connects to a public AC power source, either directly or via a power adapter, regulatory bodies prescribe limits to the amount of noise that the equipment can inject back onto the power grid.

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