[time-nuts] Re: 32.768Khz Crystal Trimming

[Richard (Rick) Karlquist]

No one mentioned tempco, so I will.  Ideally you should do your
calibration at a temperature corresponding to the long term
average in your workshop.  If the crystal is in a piece of
equipment with a temperate rise, it should be accounted for,
and then going forward you have to leave the equipment powered
up 24/7.  The crystal is probably a tuning fork, meaning it
won't be AT cut.  It may have a substantial tempco around
room temp.  In which case that old time-nuts insult may apply:

"congratulations, nice thermometer."

Rick N6RK

BTW, I go back 48 years with crystals.

On 4/1/2022 8:30 AM, Bernd Neubig wrote:
> Hi,
> 
> If you do not want to make it a time-nuts style research project, but just
> look for a quick fix - here is a rule of thumb:
> This kind of crystal usually has a trimming sensitivity of around -10
> ppm/pF. This means, if you increase the value of both capacitors on either
> side by 2 pF will increase the load capacitance by 1 pF and thus lower the
> frequency by  about 10 ppm.
> If you need to vary one cap slightly more than the other, (to get a finer
> resolution), please use the one at the output side of the on-chip oscillator
> stage.
> Take care of the start-up margin (safety to get a reliable start-up after
> power on). The larger the capacitors, the lower becomes the margin,
> sometimes the margin drops rather quickly. Therefore, after having made the
> changes, test the start-up behavior by switching on and off several times,
> preferably with a slow voltage ramp.
> 
> Enjoy the crystal world (as I did for the last 45 years and still doing).
> 
> Bernd
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: Dan Kemppainen [mailto:dan at irtelemetrics.com]
> Gesendet: Freitag, 1. April 2022 14:47
> An: time-nuts at lists.febo.com
> Betreff: [time-nuts] 32.768Khz Crystal Trimming
> 
> Hi,
> 
> I've got a 32.768Khz (USA number format) crystal on a RTCC oscillator of a
> small micro, and it's running fast. Around 10 seconds per day or so.
> This is a bit more than an order of magnitude more than the datasheet
> states.
> 
> The 9 seconds per day error should be a good measurement. The RTCC is
> running a 1 second counter, and that's being compared to a 1 second counter
> derived by clocking the micro from a 10Mhz EXT clock reference.
> This is consistent between multiple copies of the board.
> 
> I'm assuming, the C1/C2 load capacitors to ground needs to be higher in
> value to trim that oscillator closer to the correct frequency. Is this
> correct? Any quick back of the napkin calculations how much additional load
> capacitance would be needed?
> 
> For Ref, this is the crystal:
> https://abracon.com/Resonators/ABS06.pdf
> ABS06-32.768KHZ-1-T
> 
> Thanks,
> Dan
> 
> 
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