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Boost success rate with EMC pre-compliance test (Part 1)

21 Dec 2015  | Andy Eadie

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One of the surefire ways to improve your chances of passing EMC testing and lower the risk of failure is through pre-compliance testing. There are a ton of options, and with a focus on emissions testing, this overview aims to comprehensively break them down according to several factors such as cost, usefulness, and skill level.

Why do pre-compliance testing?
Many companies don't do any pre-compliance testing before they send their product off to a test lab. And honestly, some get away with it – they pass first time and they're off to the races. But for most companies, the prospect of failing EMC testing poses a substantial enough risk to project timelines and budgets that pre-compliance testing is a necessity.

Hitting EMC problems at an EMC lab can literally cost your company tens of thousands of dollars in debugging time, re-design costs, re-testing costs, re-manufacturing costs, and delays to your critical time to market schedule.

Pre-compliance testing is essentially an exercise in de-risking the certification process. It attempts to transform a relatively unknown risk into a known risk and seeks to give a higher level of confidence that a product will sail through finals testing first time around.

One of the most important factors in pre-compliance testing is the concept that EMC problems are much much cheaper and easier to fix the earlier they are caught in the design cycle. A few reasons for this are:

 • Any problems you find can be fixed in a PCB or product revision that you were going to do anyway for other reasons. For example, if you do pre-compliance testing on a pre-production batch run and find an EMC problem. Any changes and fixes can be rolled into the final production run.
 • In-house pre-compliance testing is generally much cheaper to do than full compliance testing at an accredited EMC test lab. Typical lab rates are anywhere between $180/hr and $250/hr. If you catch any potential issues before you get to a test lab, then you're going to save a lot of expensive lab time.
 • If you've gone ahead and assumed that your product was going to pass EMC testing and already pressed the 'Go' button on a production run of product which you've sold to your customers and you subsequently fail testing, then you've just made some expensive beer coasters. It's sometimes possible to re-work existing product to implement EMC fixes, but this is usually labour intensive and high cost in terms of EMC suppression products. This happens way more often than you might hope.
 • Related to the last point, it's much cheaper to implement an EMC fix on a circuit board than it is to add a remedial fix once the design is finalized. For example, an R-C low pass filter on data signals just before they traverse a radiating cable may cost you $0.002 per PCB, but if the PCB is finalized and you need to find a solution to radiating cables without altering the PCB, a cable choke may cost you $1+ per cable plus debug time. Another example is solving power supply noise on a PCB rather than having to buy a much more expensive low noise power supply adapter or brick. Careful design of the PDN (power distribution network) could save you several dollars per unit if you're relying on a low noise power supply to suppress noise.
There are a ton of reasons to do pre-compliance testing. If you're not doing it, you're leaving the door open to an unknown cost and project delay liability.

What is pre-compliance testing?
Pre-compliance testing can take many forms. In most cases, pre-compliance testing involves an attempt to simulate the methods (or a close approximation to it) that a full compliance lab would undertake on your product. For EMC, that means quantifying the emissions coming from the product – both radiated and conducted, and for the products that will be subjected to immunity tests (for example CE testing and some product standards), some other electromagnetic phenomena will be applied to the product as well. For wireless products, you can also do RF pre-compliance tests, but that's outside the scope of this blog post.

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