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I'm working on a BLE capable device using the Nordic Semiconductor nRF52 SoC, and we're going to have to go through FCC testing. But one thing that I am not clear on, at all, is the tolerance in output power from device to device.

For example, let's say my target output power is 0dBm on BLE channels 37, 38, and 39 (2404MHz, 2426MHz, and 2480MHz), and the sample we send to FCC testing transmits at exactly 0dBm on all 3 frequencies. How much variation is allowed from device to device? Is okay to be at 0.5dBm? 1dBm? -0.5dBm? Or is it more of a "thou shalt not transmit more than sample + xdBm"?

The nRF52 chip only gives 4dBm steps in RF output power, and I'm worried that might not be enough control to get every device close enough in terms of RF output power, but I have not found anything authoritative on what the FCC requires for device to device difference in RF output power.

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As far I know, FCC only limits the maximal EIRP power (mean value in all directions, measured @3meters). So no problem to be +/-1dBm under condition that the most powerfull device stays under limit (And it should not be a problem if you follow the reference design).

This Nordic White Paper gives some information about regulation.

However to have precise information about the requirements you should contact Nordic or you local certification lab. They are most used to this.

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The FCC sets the upper limit. None of your devices may exceed it, at all.

The easy way to ensure you don't exceed the power limit is to tune a representative piece of hardware to meet the limit, dial it back a bit, and then use the same setting for all of your devices. This gives some wiggle room for any transmitters that are a bit "hot" compared to the rest. One issue with this technique is that none (few) of your devices will be transmitting at their allowed maximums.

A more time-intensive method is to test and tune each individual unit. This requires extra effort when commissioning the hardware, and implies some sort of non-volatile storage to retain the individual power settings. The benefits include each of your devices transmitting as loudly as they are allowed to, and that you can keep a paper trail in case you run afoul of the FCC in the future.

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