I am assuming that you plan on sharing the external oscillator signal with other ICs in your design. Is this the case? Clock sharing is not recommended for the nRF52 series. The receiver is very sensitive to noise on the reference clock and any noise here will affect the sensitivity of the receiver. Hence, we strongly recommend using a dedicated 32MHz crystal to ensure the performance stated in our product specifications.
I don't want to share the external oscillator. I just want to have a more accurate clock for the nRF52840, this is why i want to put a TCXO instead of a crystal.
The HFXO on the nRF52 is designed for crystal operation only due to phase noise requirements, hence using an external clock signal or a TCXO is not advise by us as we cannot guarantee the performance. You may try it out on your own risk, for all I know it might work, but this has not been characterized or verified by us.
as far as I know there are no plans for developing a reference design with the nRF52 family that uses a TCXO. The nRF52 family has been designed with an XO as the clock source for the radio and crystals with a 10ppm frequency tolerance and 10ppm frequency stability over the nRF52 familiy's recommended operating temperature range (-40 to +85 C) are available, e.g. this and this. This should give a lot of headroom for the +/- 40ppm requirement for BLE.
