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.
Best Answer
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.