On 22/03/2021 21:57, alan_m wrote:
On 22/03/2021 20:03, Chris Hogg wrote:


I have always thought that some quartz crystal oscillator circuits can
be fine-tuned a few cps either side of the principal frequency, by way
of a simple variable capacitor, and observing the result using an
accurate oscilloscope. I may be wrong, and even if I'm not, I wouldn't
recommend trying it unless you're going to scrap the oscillator anyway
and making a complete balls-up won't matter.

They drift by x parts per million per degree celcius hence why with a
crystal if you wanted repeatable accuracy you put it in temperature
controlled environment.

Clock crystals are generally cut so that their linear temperature
coefficient is almost zero and there is a tiny quadratic variation with
ambient temperature typically -10ppm at about 10 and 40C spot on at 25C.

Figure 6 in the TI datasheet:
https://www.ti.com/lit/an/slaa322d/slaa322d.pdf

It is pretty easy to get within 5ppm on average over that range.
Cars in winter can get both hotter and colder though.

--
Regards,
Martin Brown