[time-nuts] Re: 32.768Khz Crystal Trimming

[Mike Millen]

Remember that the load cap value in the data sheet isn't what you fit in 
the circuit.
First deduct the total stray capacitance on the pcb and the IC 
connections, then double the remainder... that's the value you place 
either side of the crystal.

E.g. if the data sheet says 12.5pF and you estimate the strays to be 
2.5pF then you should place a 20pF at each side of the crystal.
(Because the two caps are effectively in series)

Mike - M0MLM

On 01/04/2022 14:01, Bob kb8tq wrote:
> Hi
>
> Given the (very normal) lack of information on the crystal,
> it becomes a “try it and see” sort of thing. They seem to
> want 12.5 pf as the load cap for the first one listed on the
> data sheet. How far off of that is your circuit as it sits?
>
> First step would be to take the “output” cap up one value
> and see what happens.
>
> Bob
>