[time-nuts] Re: 32.768Khz Crystal Trimming

[Dan Kemppainen]

After playing with cap values the crystal is running about 6.5ppm fast. 
Plenty good for what it is. Of course this was with a sample size of 1.

Being a time-nut, I did try a few different cap values just to see if it 
could be made any better. The Next closest cap combo gives 8.3ppm slow.

Putting a little heat on the crystal showed it tended to drop in 
frequency, so 6.5ppm fast is probably better than 8.3ppm slow.

Looking at the slope/intercept of the values tried, the needed value 
isn't easily achievable. Again, n=1, so sample to sample variation is 
untested.

It looks like the 'ideal' cap value might be achievable, but that would 
require a pretty big mismatch between C1/C2. More than 2:1, probably 
closer to 3:1. I'm guessing that would be a 'no-no'?

Anyway, Thanks to all who replied!

Dan






On 4/2/2022 3:27 AM, time-nuts-request at lists.febo.com wrote:
> 
> All,
> 
> Ahhh, Yes. Checking notes, during the initial build I slapped a pair of 
> 10pF caps down (all I had on hand) just to get the RTCC oscillators 
> somewhat working. I can see now that is WAY under needed capacitance.
> 
> Measurements are taken right in the micro, so no external leads to 
> influence the board. A serial debug line spits out the difference in 
> elapsed time for the RTCC and time derived from the 10Mhz EXT REF 
> (GPSDO). (No extra wires/leads etc.)
> 
> Honestly this RTCC doesn't need to be stellar. It's more of a holdover 
> oscillator when the EXT CLK is lost. But, this is time-nuts and all, so 
> might as well squeeze everything out of it! :)
> 
> 
> 
> Bernd,
> 
> Your post, and rule of thumb are what I was looking for. I will 
> certainly archive this in my notes. This should be good enough to save 
> destroying the board by soldering/de-soldering many 0603 SMT caps.
> 
> 
> Thanks to all who replied. This should be enough to keep me busy over 
> the weekend!
> 
> Dan