FCC certification for Metro Mini

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curtisb
 
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FCC certification for Metro Mini

Post by curtisb »

There have been a few other threads on the forum regarding FCC Part 15 and RF communications, and it seems like many people (including me) are confused by the regulations. I'm interested in building a saleable product that contains a Metro Mini to PWM control a case fan, which I believe would be considered an unintended radiator.

Generation 1
- read voltage off of a potentiometer to determine setpoint
- read analog temperature sensor to measure actual
- send PWM signal to 10W case fan (fan has signal wire and internal electronics, so there will be no additional FET, etc. to modulate the power)

My understanding is that the Metro Mini will be an unintended radiator, since it operates at a clock frequency > 9kHz. I noticed that the FCC logo is printed onto the Metro Mini PCB, so it would appear that this is already tested / certified. Do you have a report / datasheet for this board that explains test conditions and conditions of use to maintain compliance? As long as the PWM signal stays below 9kHz, my assumption is that this will not trigger any need for re-testing, as the only component operating at radio frequency has already been tested, and I will not be transmitting any RF signals.

Generation 2
- same as above, but including additional I2C sensors (CCS811 gas sensor breakout board)

In this case, there will be an I2C clock line running > 9kHz from the board to the sensor. Does this change the assessment? Does the testing of the Metro Mini include any off-board components, that would allow for this type of use to also be pre-approved?

Thank you!

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adafruit_support_mike
 
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Re: FCC certification for Metro Mini

Post by adafruit_support_mike »

I'll check to see if we have an FCC cert for the Metro Mini, but the information in it will be largely irrelevant to your own design.

The compliance conditions for connected circuits break down into two basic chunks:

1) The connected circuit doesn't generate signals in the regulated frequency bands on its own. If it does, it requires its own certification.

2) The connected circuit doesn't amplify the power of signals generated by the Metro Mini, in the regulated frequency bands, above the regulated transmission levels. The FCC cert basically says the Metro Mini's GPIO pins don't generate enough EMF under their own power to require a license (or a redesign). Things like the SPI signals to a TFT don't have enough energy to infringe.


Point 2 sounds like it could be relevant to your product. The main question is who controls the power to the fan's motor.

If you're using a plain motor with a fan stuck on its spindle, controlling the power through it with mosfets, and driving those mosfets with signals from the Metro Mini, it would be a good idea for you to get in touch with an FCC rep and get an official opinion on what compliance measures you'll need to take. Even if your PWM frequency is below the regulated bands, square waves have a lot of high-order harmonics which could infringe.

If you're using a fan with its own built-in control module, and that control module accepts low-current PWM signals to control the motor speed, you should be in the clear. The fan's control system will have to be certified on its own, and the signals from the Metro Mini shouldn't ever take it outside its certified operating range.

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