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:d...@irtelemetrics.com] Gesendet: Freitag, 1. April 2022 14:47 An: time-nuts@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 _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
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