On 2023-12-13 23:23, Paul Koning wrote:
On Dec 13, 2023, at 5:10 PM, Johnny Billquist <bqt%softjar.se@localhost> wrote: On 2023-12-13 22:52, Mouse wrote:But a drift of a couple of % would not be unexpected.Um, yes, it would be unexpected. The crystal spec in that era was 0.01%, [...] If you see a couple % drift, that's definitely not the crystal.I'm not so sure. My experience is that older machines keep worse time than newer ones, even high-quality older machines and cheapo newer ones. I suspect that crystal frequencies drift with time (or perhaps with something that tends to correlate positively with time, such as wide temperature ranges in storage).Chrystals age, and over time get farther from the nominal frequency. They also deviate based on temperature. And a bit on load. So I would say more than 100ppm is definitely more or less to be expected. But I might have exaggerated a little when I said "several percent". :-) But just pull up the data sheet for any modern crystal, and you'll find what the expected deviation will be based on both time and age. (Well, actually, for age they usually just say what to expect the first year, after that there isn't much...)I pulled up a random modern manufacturer, Crystek. Their HC49 (through hole, metal can) crystals are specified at 50 ppm calibration tolerance, 100 ppm frequency stability, and aging less than 3 ppm for the first year. That's the basic spec; they offer better specs as an option.
Well. Here is the one I'm currently dealing with at work: https://eu.mouser.com/datasheet/2/530/CM8V_T1A-1510858.pdfAnd it's basically a choice of +-20 ppm or +-100 ppm at manufacture. In addition aging the first year is +-3 ppm. But the most interesting thing is the affect of temperature. It's basically an exponential curve on the deviation from frequency related to temperature, centered at +25 degrees celsius. Go up or down, and frequency start dropping.
And, as you yourself noted, these 32768 Hz chrystals are usually aimed at better precision than some random 1MHz clock.
It's not that uncommon these days to have temperature compensation for the crystal, because otherwise, this is how all crystals behave.
And for grins I found a 1972 catalog from International Crystal Mfg., perhaps the best known supplier for many decades. Their "general purpose" crystals in the MHz range have a calibration spec of 100 ppm, frequency within 50 ppm from -30 to +60 C, and no aging spec that I can see.
So that would be better than what you usually find today. You commonly want clocks with temperature compensation built in if you want good stability, because temperature is by far the worst thing affecting crystals.
Anyway, I'm not sure why are having an argument about this. No matter what the actual deviation is, it's a simple fact that no crystal is absolutely perfect. And if you can observe a clock drift that depends on load, then the error comes from somewhere else. If you are just seeing something of a constant drift, then it's a fair chance it's because of the crystal.
Johnny
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