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Boost success rate with EMC pre-compliance test (Part 2)

22 Dec 2015  | Andy Eadie

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Part 1 can be read here.

Top radiated emissions pre-compliance options
Let's start with the most common EMC tests for radiated emissions. These tests are required for most electronic and electrical equipment that will be sold into any major market in the world. FCC testing for instance mandates radiated and conducted emissions testing, as does CE Mark (Europe), VCCI (Japan), RCM (Australia), CCC (China), etc.

1. Design review
Cost: Low ($1200 or cheaper if you do it yourself)

Learning curve: Low for outsourcing, high for DIY

Usefulness: Very High

Equipment Required: None

One of the most overlooked pre-compliance methods is a very robust and systematic schematic and layout design review. It isn't really pre-compliance 'testing', but it does exactly the same thing. i.e., it's designed to substantially reduce the risk of a product failing at an EMC test lab. At my old test lab, we offered a proprietary EMC design review to our clients only – it was our 'secret sauce' that our clients couldn't get enough of. As design engineers, I think we're allergic to the idea that some problems don't have definitive solutions. We refused to believe EMC was 'black magic' and instead we distilled several textbooks worth of guidance from some of the world's best EMC 'gurus' into a set of design rules that we could manually check on every design.

This is really my favourite method to increase chances of passing testing first time around because you can catch so many potential emissions and immunity problems in one design review. Rather than doing pre-compliance testing for radiated emissions, ESD, surge, etc., a solid design review will catch many of the root causes of these issues even before your first PCB is manufactured.

2. In-house semi-anechoic chamber (SAC)
Cost: High ($100k +)

Learning curve: High

Usefulness: High

Main equipment required: Chamber, spectrum analyser/receiver, antennas, turntable

This option is for the high rollers. Typically reserved for companies with deep pockets and those that design several products per year. A second hand 3m chamber may cost $100k+. You can typically get results quite close to those that a test lab would measure for final testing. Factor in a few dB margin of error and you basically have yourself a fully functioning emissions measurement facility.

In this case, you'll also need a decent spectrum analyser or EMI receiver, and antennas covering the frequency range of interest for your product.

It is possible to find used chambers, but beware that the effectiveness of the material lining the chamber, used for absorbing the RF energy and prevent it from bouncing off the metal walls, degrades over time. As the material degrades, reflections increase and measurements become less reliable.

When you have a chamber installed, you need to do an NSA (normalized site attenuation) survey which will tell you how close your setup is to an idealized site.

3. In-house OATS (Open area test site)
Cost: Medium ($5k-$50k)

Learning curve: High

Usefulness: Medium

Main equipment required: Spectrum analyser/receiver, antenna(s), automated mast, software

An open area test site (OATS) is what many accredited test labs use to perform the final measurements when testing products. An ideal OATS is constructed according to the standards ANSI C63.7 and C63.4 and/or CISPR 16-1-4 for Europe. As there are no walls, the signal received at the antenna is just a combination of the signal received directly from the equipment under test (EUT) and the signal that has bounced off the metallic ground plane.

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