32 MHz external clock source

[JGusler]

I know part 15 talks about bluetooth requirements, could someone tell me precisely where the FCC says bluetooth devices need a 32 MHz external clock source? Thank you

[ketiljo]

There's no such requirements from the FCC. Part 15 isn't specifically for Bluetooth, it's the other way around, a Bluetooth radio has to satisfy the requirement under part 15. 

[JGusler]

I saw a reply to a post from Sigurd 2 years ago saying "You need external 32 MHz crystal in order to use the Radio and Bluetooth." and then on this post  General PCB design guidelines for nRF52 series under Crystal and Clock it says a 32MHz crystal is mandatory. Could you clarify these please? 

[ketiljo]

A 32 MHz _crystal_ is mandatory to be able to use the radio. The 32 MHz RC oscillator is way to inaccurate to be uses as a reference for the radio PLL. This is a requirement for the chip itself and has nothing to do with the FCC. The radio simple doesn't work without a crystal. 

[JGusler]

And pardon this naivete, by radio you mean the bluetooth transmitter?

[ketiljo]

Yes, it's a transceiver (transmitter and receiver).

These are the minimum external components you need for the nRF52840:

https://infocenter.nordicsemi.com/topic/ps_nrf52840/ref_circuitry.html#unique_329143743

[JGusler]

Okay, thank you so much ketiljo!