Alternative clocks for nrf52

[davman]

I'm looking for alternative xtal's (mhz + rtc) for use on boards where space is very limited.

Silego's GreenCLK3 looks like a great option, though that one is right now only available in a 26 MHz + 32.768 kHz combination.

They + SiTime have similar options, but I wonder if there are any RF-aspects that should be considered if using mems oscillators, like phase noise etc.?

[run_ar]

We usually advise customers against using mems oscillators as the phase noise from the mems oscillator could be amplified by the radio pll, and make radio communication unstable. This also means we don't use these ourself so we don't have a lot of experience with them, but we haven't seen any customers succesfully using one either.

So if your space is very limited you might want to consider using only an external 32MHz crystal and using the internal rc for 32.768 kHz if the accuracy is good enough for your application.

[Hao]

Hi run_ar,

Would I be able to use a 32MHz crystal + 32.768KHz SiTime clock? Those SiTime clocks comes at full swing, I think we will need to disable/bypass the amplifier on the clock path.

[run_ar]

Using a mems oscillator for the 32.768 KHz should be fine. The following external clock sources are supported:

• Low swing clock signal applied to the XL1 pin. The XL2 pin shall then be grounded.
• Rail-to-rail clock signal applied to the XL1 pin. The XL2 pin shall then be grounded or left unconnected.

Why do you want to use this oscillator instead of a (cheaper) crystal? are you feeding multiple devices? EDIT: I see they are quite small, I guess that's why?

[Roman]

I'm also would like to use MEMS oscillators. 

Currently, I'm wondering is it possible to use MEMS oscillator at all and which value of the Phase Noise will be applicable?

[Roman]

I mean using MEMS oscillator for High-frequency source.