Hi, I need to select the values for the crystal capacitors (C1 & C2) for a nRF51422 pcb.
From page 9 of "Crystal Oscillator Design Considerations" I see:
C1' = C1 + C_xc1 + C_pcb
C1' = 2CL
therefore:
C1 = 2CL - (C_xc1 + C_pcb)
Where:
CL is the load capacitance from the xtal datasheet, C_xc1 is an unknown with a typical value of 4pF from Table 19 of the 51422 Product Spec v2.0 C_pcb is an unknown (the reference design uses .5pF and my design is reasonably close to it)
So it seems like my initial guess for C1 & C2 is = 2CL - (4pF + .5pF)
Here are my questions:
- Is C1 = 2CL - (4pF + .5pF) for an initial guess reasonable? I would like to get as close as possible for my first guess and don't know if there is a better approach.
I have seen three values of C_xc1 to choose from: 4, 1, and 3.5 pF.
The Product Spec states 4pF as typical and 1pF as a test value.
I got 3.5pF from calculating its value from reference design as follows: C_pcb = .5pF (pg 61 of Prd Spec 2.0), CL = 8pF and C1 = 12pf (pg 63). C1 = 2CL - (C_xc1 + C_pcb) 12 = 2*8 - (C_xc1 + .5) C_xc1 = 3.5pF
- Given that C_pcb and even C_xc1 are unknown I will likely have to try many load capacitors until I find the right values. I will have to add and remove these surface mounted caps by hand.
In the "Crystal Oscillator Design Considerations" this is written: "Too low capacitive load will result in a crystal oscillator frequency higher than specified, or too high a load results in a lower oscillation frequency."
Knowing this I will be able to tell if the capacitance is too high or too low. Any suggestions on how I can quickly narrow down to the correct value?
Thanks!