Calculating frequency offset for FSK

[joeyjoejoe]

I've got a nice test circuit with a MICRF112 ( http://ww1.microchip.com/downloads/en/DeviceDoc/MICRF112.pdf ), that I am using in frequency-shift mode. It is working nicely

The specs have a bit of info on how it operates.



I'm wondering how I can calculate XTAL_MOD, so that the FSK offset is closer to 35kHZ. Right now it is closer to 6khZ.

This is my current schematic, with XTAL_MOD using a 3.3pF capacitor.



I'm hoping the math isn't too far above my head.

[joeyjoejoe]

Might be on to something...

http://ww1.microchip.com/downloads/en/appnotes/00826a.pdf

Crystal Pulling
Series or parallel resonance crystals can be pulled
from their specified operating frequency by adjusting
the load capacitance (CL) the crystal sees in the circuit.
An approximate equation for crystal pulling limits is:
Where ∆f is the pulled crystal frequency (also known as
the load frequency) minus fs

[joeyjoejoe]

So I'm not quite sure what fs and C0 are.

This is the data sheet for the crystal, which tells me Cl and C1.
https://www.mouser.ca/datasheet/2/741/LFXTAL036679Bulk-998814.pdf