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    How to Perform In-House Pre-Compliance Testing for Radiated Emissions

    June 26, 2017

    girl blocking guy from kissing her


    Have you ever had a wonderful moment ruined by a smell? Once I went out on a date with a girl I’d been interested in for some time. Everything was going great, we got to the end of the date and I walked her to the door. I leaned in for a kiss offered a breath mint and a goodnight hug. Sometimes you can feel like you’re putting out good vibes, but you’re really emitting something else. The same goes for wireless circuits. You go through design and testing, then send your board off to an EMC lab only to find out your board is radiating the electronic equivalent of garlic breath. Performing in-house pre-compliance testing can help you avoid an embarrassing rejection. You’ll have to spend some money on equipment, but it will save you repeated attempts. Once you have the required contraptions you’ll need to know how to use them and read their outputs. Luckily spectrum data is easier to understand than women.


    Benefits of Pre-Compliance Testing

    Sometimes I forget to put on deodorant when I go out. Then a mid-day sniff test reveals my error, and I have to go back and home and put some on. Now I do a sniff test before I leave the house. Pre-compliance is a morning sniff test that will keep you from having to go back to the beginning. It can save you lots of time and money as you take your product to market.


    Things tend to get a bit hectic towards the end of the production cycle. The last thing you need is to find out that your device failed EMI compliance and needs some major fixes. Not only that but sometimes the test lab won’t give you many indications of what went wrong, leaving you to guess and check with another cycle. If you have your own equipment, you can do in-house pre-compliance checks during the design phase. This can greatly increase your chances of passing the final examination on the first try.


    Putting your board through compliance testing again takes time as well as money. Final evaluations can cost upwards of $5,000 a pop. Problems can also cost less to fix during the design phase. It’s cheaper to change your PCB to fix a problem rather than finding a workaround after the board has been finalized. Pre-compliance equipment will cost you a pretty penny, but it can be less expensive than going through EMC checks 2 or 3 times. If you’re planning on developing more than one product, in-house pre-compliance capabilities are definitely a good investment.


    man with futuristic glasses that radiate EMI
    I don’t think this would make it past the FCC.


    Equipment Required

    There are a lot of different ways to fix bad breath. For radiated emissions testing there are fewer options. There are some things you will absolutely need, and there are a few others you can throw in if you’re feeling rich. You should expect to spend at least $3,000 on tools.


    • Testing Area (Required) - The first thing you need is a space to test in. Professionals perform assessments in expensive anechoic chambers. You’re not a professional, so a rural outdoor area, conference room, or basement will do. These places will help mitigate outside signals that can interfere with your experiments.

    • Antenna (Required) - Performing a radiated EMI test without a proper antenna is like getting a goodnight kiss from your dog, it doesn’t count. Which antenna you use will depend on which frequency range you’re operating in. You’ll want to be able to measure frequencies up to the third harmonic of your operating frequency, so choose your antenna accordingly.

    • Spectrum Analyzer (Required) - This is what analyzes the signals coming in from your antenna. It can also be fairly expensive, the Tektronix RSA306B will cost you close to $4,000. This machine is one of the key components for pre-compliance though, so it’s worth dropping a few dollars on.

    • Software (Required) - In order to interpret the data coming out of the spectrum analyzer you’ll need a computer program. I would recommend buying a spectrum analyzer that either comes with software or is compatible with free software.

    • Pre-Amplifier (Optional) - Some people buy medical strength deodorant because they’re paranoid. I say if you don’t need it, don’t buy it. A pre-amp will boost the signals coming into your antenna, making it easier to detect EMI from your gadget. I would only get one if your signal strength is so low that you really need one.

    • Semi-Anechoic Chamber (Optional) - You can also buy other things like a small semi-anechoic chamber. If you need something like this, you’re going to need a bigger budget. You also might think about farming out your pre-compliance testing to a lab or renting equipment.


    overweight man on couch
    Tell your son it’s time to move out of the basement, you need it for radiated emissions testing.



    After that fateful night I started carrying around Listerine strips. Unfortunately, I haven’t had the chance to test them, maybe I do need to upgrade my deodorant. Luckily, pre-compliance testing only takes one person. Now that you have your equipment it’s time to try it out.


    I would love to go through all the ins and outs of the actual testing and readings, but that is beyond the scope of this article. Fortunately, several more qualified companies have done just that. Check out EMCfastpass’s EMC beginners guide, or Tektronix’s pre-compliance guide for a deeper look into EMC testing.


    You might think pre-compliance testing is a joke, but hell hath no fury like the FCC. Avoid the pain of rejection and extra expenses by checking what kind of vibes you’re putting out before the final test. In order to assess your radiated emissions you’ll need some specialized equipment. That paraphernalia will eventually pay itself off if you use it correctly. You’ll then need to do a bit of research on the art of EMC testing if you’re going to woo those regulatory agencies.


    Brushing your teeth will help with any stench, but it’s easier to just not eat smelly foods in the first place. Following EMI design guidelines at the beginning can help more than pre-compliance testing. Things like dedicated grounds, high-speed routing, separating AC/DC, differential pair routing, blind and buried vias, and microvias can all help you nip EMI in the bud. Great design software like CircuitStudio will also help. It has a wide range of features that will help you build the perfect board.


    Have more questions about EMI design? Call an expert at Altium.

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