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I have a small electronic device that I'm considering bringing to market. It doesn't have any features like wifi or bluetooth, so one of the prerequisite steps before I can sell it is to ensure it doesn't exceed FCC emissions limits for an unintentional emitter.

However, emissions testing can be expensive. I've gotten quotes from 3 different labs, and the cheapest one was $1100, and that if you fail the test, you'll need to fix your device and then pay for an additional test. So obviously, I want to do everything I can to ensure I pass the test on the first try.

I've read online that a good way to get a general sense of your device's emissions before testing is to buy an AM radio and see what effect the device has on it. So I bought a generic $8 AM/FM radio and I found that my device does indeed effect the AM channels. From 10 feet away, I tuned into the clearest channel I could find, and then I monitored the reception as I got closer. From 1 foot away, the reception is still perfectly clear. From 6" away, there's a noticeable whistle and extra static in the reception. From 3" away, the channel is completely static.

I did a similar test with the FM reception, and there was no interference at any distance.

So my question is, how do I determine if this means my device wouldn't pass verification testing? I know my description isn't terribly quantifiable, but is there a general rule-of-thumb for this kind of thing? Is complete disruption of AM reception from 3-6" away considered unacceptable by the FCC? I've been personally using my device, and have had it running continuously for over a month, and I haven't noticed any negative effects.

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  • \$\begingroup\$ Unfortunately an AM/FM radio only covers quite a narrow spectrum of radio frequencies. Even to get that to work over those ranges you would need to sweep the tuner to listen to all of them. A single channel will only tell you about a very tight frequency range. "I haven't noticed any negative effects." means nothing either... the guy across the street with his short wave radio may be cursing you, or the ILS system at the local airport may be acting screwy... \$\endgroup\$ – Trevor_G Oct 16 '17 at 15:49
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You are likely to be dealing with FCC part 15 rules. https://en.wikipedia.org/wiki/Title_47_CFR_Part_15 is a good primer. The idea behind the rules in to ensure that devices you buy don't prevent other devices you own from working with regard to radio. You will probably be interested in 15.1XX part of the rules found here. You will want to see if you are exempt for a particular reason, or you will have to try to do a rough calculation of the radio power coming from your device. The AM radio is a good start, but I am not sure if the distances are good enough.

I would try to find other products that are in a similar realm to yours and see how they measure up. If they have similar characteristics, you are probably in good shape ( I am no means an expert). Something else you could do potentially is get a spectrum analyzer and put a simple antenna on it for an input and see at what frequencies you are offending and maybe have a better idea of from where. And as always, shielding or a grounded metal case can solve most problems.

As a note, FM will 99.999999% of the time not be effected by this sort of thing. Both the frequency and modulation type make it fairly immune to the sort of interference caused by things like switching power supplies

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You should know what class your device falls into and which FCC regulations you need to pass. That part requires research and is beyond the scope of anyone here to answer for you.

Unfortunately an AM/FM radio only covers quite a narrow spectrum of radio frequencies. AM will be affected much more then FM. But even to get that to work over those ranges you would need to sweep the tuner to listen to all of them. A single channel will only tell you about a very tight frequency range.

"I haven't noticed any negative effects." means nothing either... the guy across the street with his short wave radio may be cursing you, or the ILS system at the local airport may be acting screwy...

FCC certification testing is expensive because of the paper work involved.

However, if you are in the right neighborhood it is sometimes possible to buy some time for a quick test just to see if there are frequencies you need to work on. If you have larger electronics firms around they may also have in-house testing capabilities that a few phone calls and maybe a case of beer or two will get you some access to.

Do some networking, you are likely not the only one around that has faced this issue.

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