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Ultra Low System Clock Speed for FCC Compliance

Ray Russell

Apr 28, 2018
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Hi all,

I have designed a personal device that fits inside a key ring fob.

It does not connect in any way or under any conditions to the AC mains.

The device operates at very low duty cycle and can operate at very low system clock speeds.

The device is aimed at the consumer market.

I am interested in manufacturing this device and realize that the device must pass FCC Part 15b regulations.

As far as I can tell there are two areas within 15b that seem to apply to my situation:

1) This is the regulation:
From §15.103 "(h) Digital devices in which both the highest frequency generated and the highest frequency used (my emphasis in two places) are less than 1.705 MHz and which do not operate from the AC power lines or contain provisions for operation while connected to the AC power lines. Digital devices that include, or make provision for the use of, battery eliminators, AC adaptors or battery chargers which permit operation while charging or that connect to the AC power lines indirectly, obtaining their power through another device which is connected to the AC power lines, do not fall under this exemption."

My comment:
I can certainly reduce my clock frequency down below 1.705 MHz. However, the signal edge speeds will be significantly higher than 1.705 MHz so I imagine that I would run afoul of the "generated" part.


2) And this appears to be the definition of "Digital Device" as used above:
From 47 CFR § 15.3 "(k) Digital device. (Previously defined as a computing device). An unintentional radiator (device or system) that generates and uses timing signals or pulses at a rate in excess of 9,000 pulses (cycles) per second and uses digital techniques; inclusive of telephone equipment..."

My comment:
Notice that in item 2) there is no mention of "generated", it only addresses "timing signals or pulses". So if I reduce my system clock source to less than 9KHz does that knock me out of the "Digital Device" category and thus exempt me from FCC compliance testing?

I'm interested in any and all thoughts on the matter.

Thanks in advance,
Ray Russell
 

Hopup

Jul 5, 2015
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It says quite clearly that if your device generates and/or operates higher than 1.705MHz it needs testing. Also your device should have battery which is replaced or charges outside.

Notice the unintentional radiation part. If your device does generate intentional radiation for medical or other purposes it would probably need testing anyways.
 

Ray Russell

Apr 28, 2018
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Apologies for not being more clear:
1) My device operates on a non-rechargeable coin cell;
2) It is not being used for medical purposes or any of the other listed categories (vehicle, etc).
3) It is NOT an intentional radiator--It is merely a device which senses its environment and outputs an audible alarm. It does not transmit.
4) It is not dependent on any kind of a high speed system clock.

Given the above facts and conditions then my two questions remain:

a) Do you think that the 1.705 MHz requirement in my original post includes system clock edges or is it just the system clock frequency? My guess is "Yes, it does include the clock edges which would almost certainly be faster than that;

b) And do you think that if my system clock is SLOWER THAN 9 KHz (from item 2 of my original post) that my device will be exempt from FCC testing by virtue of the fact that is NOT classified as a digital device?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Regardless of what you think about, or how you interpret the rules, it is the FCC that will make the final determination and interpretation of whether your product is compliant. Disagree and you will have your day (or weeks or months) in court. Penalties the FCC assesses for non-compliance are severe. It is a good idea to seek professional (paid) advice before offering anything for sale as consumer electronics. Any advice you receive here at Electronics Point is worth exactly what you pay for it: nothing.

You can obtain more information by visiting links to pages at this Google result page.

I suggest you read the article titled "Are Your Company's Consumer Electronics Exempt from FCC Marketing Regulations?" authored by Ronald E. Quirk, Jr. and recently published in the April 2017 edition of IEEE Consumer Electronics Magazine. Obtaining the professional services of someone like Mr. Quirk is probably a good idea.
 

Ray Russell

Apr 28, 2018
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Boy, this is a tough crowd. All I'm trying to do is get a feel for how I might design the circuit in order to improve the chances for making my path toward EMC compliance less thorny. I am not ready to manufacture anything at this point.

Thank you for taking the time to reply and thank you for your reference.
 
Last edited:

hevans1944

Hop - AC8NS
Jun 21, 2012
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Well, we don't want you to make a huge financial mistake by getting your "key fob" ready for production and then finding out that the FCC won't approve it. You can build prototypes and rent time in a screen room (Faraday cage) to test for spurious and inadvertent emissions. Or build your own screen room... the purpose is to keep extraneous noise out so what you measure comes from your prototype. A simple wood frame with copper screen tacked on works pretty well. Buy a used wide-band communications receiver to listen for spurious RF emissions, especially "birdies" that will occur at integer multiples of the frequency of some digital waveform. This is a situation where negative results are encouraging but don't mean you are home free. Positive results (spurious emissions heard) mean more work ahead to determine whether they mean non-compliance or can be safely ignored. Calibrated spectrum analyzers come to mind, but those are expensive to purchase or rent, plus you need knowledge of how to use them.

If your prototypes pass muster, then you can consider spending money to have a production version tested and type approved... if that is even necessary. Someone well-versed in the law and interpretation of the Federal Code could save you a lot of money if it turns out testing and type approval is not required.

It doesn't take much to electromagnetically shield something the size of a key-fob running off a coin-cell. Maybe use a die-cast aluminum case, or a metalized plastic case? But first you have to test to see if a problem even exists. For EMC compliance hints, tear open a used cell-phone to see how they did it. Lots of metal inside those cell phones, all around the stuff that makes electrical "noise".
 

Ray Russell

Apr 28, 2018
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I understand completely. In a prior life I designed avionics products for DOD, some of which had to be TEMPEST-qualified, As part of that work I had to design the boards, architect the housings, write the TTPs, etc. So I understand the nuts and bolts of how to minimize EMI. What I don't know is whether there is a lower limit to what the FCC in particular is looking for. From reading the regulations it seemed that there might be. That was the point of my original post.

Since I am doing this key fob thing myself and am on a low budget, I am looking for every LEGAL "angle" that might allow my product to be exempt, if that is possible. The two items that i included in my original post were areas that seemed like they might allow exemption under certain circumstances. I was merely asking if anyone had any experience with them.

I never was, and am not now planning to manufacture the product prior to FCC approval. I was just looking for an avenue around the testing requirement.
 

AnalogKid

Jun 10, 2015
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If your device still is functional with a 9 kHz system clock, then you have a good chance of squeaking by. It still would have to be tested, but (if successful) not have to met any of the radiation limits. IOW, my guess is some testing (or at least a design audit) required to prove it doesn't need more testing.

TEMPEST - Tiny ElectroMagnetic Particles Emitting Secret Things

I've done TEMPEST projects, some of the most professional fun ever.

ak
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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If your device still is functional with a 9 kHz system clock, then you have a good chance of squeaking by.

If you can tolerate slower rising and falling edges you'll reduce possible problems too.
 

AnalogKid

Jun 10, 2015
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If you can tolerate slower rising and falling edges you'll reduce possible problems too.
That's actually the first point I was going to make, but the midnight brain ain't what it used too was. Slowing an edge from 100 ns to 10 us makes a huge difference in radiated spectrum.

ak
 
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