How to Perform In-House Pre-Compliance Testing for Radiated Emissions
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 immunity 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 a costly 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, we have all the info you'll need.
Benefits of Pre-Compliance Testing
Sometimes I forget to put on deodorant when I go out. Then a mid-day whiff 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 with design engineers at the end of the product development 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 any indication of where or 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 testing 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 and money. Final evaluations can cost upwards of $5,000 each. 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 testing 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 more than one round of the product development cycle, in-house pre-compliance capabilities are a good investment.
I don’t think this would make it past the FCC.
There are a lot of different ways to fix PCB garlic breath. For radiated emission testing, there are even fewer options. Some pieces of equipment for testing are critical, and others are useful but not mandatory. A minimum expenditure is around $3,000.
Testing Area (Required) - The first thing you need is a space to test in. Professionals perform assessments in expensive anechoic chambers. Since that would be overkill for what we're disucssing, 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 limits or voids the rest of this process. Which antenna you use will depend on which frequency range you’re operating in and the spectrum analyzer it connects to. You’ll want 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. A spectrum analyzer is one of the key components for pre-compliance, so it’s worth dropping a few dollars on.
Software (Required) - To interpret the data coming from the spectrum analyzer, you’ll need the right software. I would recommend buying a spectrum analyzer that either comes with software or is compatible with free software.
Pre-Amplifier (Optional) - A pre-amp will boost the signals coming into your antenna, making it easier to detect weak electromagnetic energy developing from your gadget. It's usually best to wait on purchasing this equipment until you know you need it.
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.
There's probably no testing interference out here, though you may need a generator
Now that you have your equipment it’s time to try pre-compliance testing out.
I would love to go through all the ins and outs of the actual emission testing and readings, but that is beyond the scope of this article. Fortunately, several 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 the FCC disagrees. Avoid the pain of rejection and extra expenses by EMC testing the vibes you’re putting out before the final test. The equipment needed will pay for itself if used correctly.
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