Radiated emissions testing is a key part of electromagnetic compatibility (EMC) compliance, and following a holistic EMC design process can give you the best chance of meeting the relevant requirements and regulations. A holistic EMC approach considers end market specifications, PCB layout, and signal integrity. But there’s one key principle we haven’t covered yet: error margin.
Being an engineer, I decided to conduct an experiment to illustrate this point. I shipped an Analog Devices, Inc. (ADI) evaluation board to three different EMC test houses, one in Ireland, one in China, and one in the United States. Before we get to the results, let’s talk about why margins matter.
In radiated emissions testing, margin is a safety cushion added to the measured emission levels to account for various uncertainties and potential sources of variation. This margin ensures that even if some result variations occur during the testing process, the device can still comply with the required standards. Typically, a margin of 4 to 6 dB is considered sufficient for most industrial applications.
While EMC testing follows standardized procedures, variations between test houses can produce discrepancies in test results. Differences in equipment, test environments, and product layout are variables that can tip the scale between your product passing or failing. The value of an error margin becomes clear once these differences are understood.
In his book “Electromagnetic Compatibility,” Henny Ott has an excellent section diving into some detail on why a margin is necessary and how it accounts for test result variations.[i] Ott describes five categories of uncertainty in radiated measurement. These can be summarized as two main areas: First, anechoic chamber and equipment, and second, test set-up and procedure.
Root mean square addition of the above figures shows that we should allow an uncertainty of +/-3.6 dB for anechoic chamber and equipment differences.
Significant result variations can originate from small differences in how the device under test is positioned and oriented during testing. For example…
We can give an allowance of +/- 2 dB for test set-up and procedure. This gives a total of 4-6 dB margin.
Remember the ADI evaluation board I shipped to three different EMC test houses?
The device under test was a ADM3053 evaluation board, selected because it has easily identified peaks in its emissions profile. It contains isoPower and produces an emission at 360 MHz. Each lab tested the 360 MHz emission in its own 10 m semi-anechoic chamber.
Respectively, they found a quasi peak max emission of 5.0 dB, 7.2 dB and 8.4 dB below Class B, giving a max delta of 3.38 dB—well under the recommended margin of 4-6 dB.
In one of my first blogs, we spoke about the secret to good EMC design: Building EMC compliance into your product from the beginning rather than at the end. There, I said that choosing components and reference designs that have already been tested and proven in the EMC lab can give you the best chance of creating an EMC compliant design—and this is true!
Yet even when we follow this best practice, as in the case of my ADM3053 evaluation board, errors can arise due to variations between test houses. In my experiment, differences in equipment calibration, anechoic chamber performance, product variation, and/or test procedure resulted in a delta of 3.38, demonstrating that a 4-6 dB margin is essential for components or assemblies you are using in your final product. Variations between test specimens can introduce further discrepancies.
There can be a cost to developing extra margin, like selecting higher tolerance or more robust components, but it’s worth considering these tradeoffs from your design get-go. Leaving margin for error can make the difference between passing or failing EMC testing, and nobody wants a redesign!
[i] Ott, Henry W. Electromagnetic Compatibility Engineering, First Edition. Wiley, 2009.
