5 ms·

> A typical PC clock is +/-100ppm. After 1 hour that's 0.36s Are you confident in these numbers? They add up to 52 minutes of drift/year. Good modern quartz w

> A typical PC clock is +/-100ppm. After 1 hour that's 0.36s Are you confident in these numbers? They add up to 52 minutes of drift/year. Good modern quartz watches specify 5 seconds/year drift, almost 3 orders of magnitude better.

eqvinox 3mo ago
Yes, albeit 100ppm is bad/cheap crystals. 50-30ppm is normal. The difference with a quartz watch is that it's factory calibrated with the load capacitance on the crystal, and that it's a 32768Hz tuning fork. For a variety of reasons, generating higher frequency clocks off 32768Hz is... "annoying" (huge PLL ratio, very slow feedback loop step), and typical crystals in the 10-100MHz range are just less precise and thermally stable. (Not sure why, I'm not an oscillator manufacturer...) (NB: you can of course correct for initial deviation in software. The actual problem is stability over temperature.) Ed.: https://www.digikey.com/en/products/filter/crystals/171 https://www.digikey.com/en/products/filter/crystals/171 (or, in the hopes the filter on the link works, https://www.digikey.com/en/products/filter/crystals/171?s=N4IgjCBcpgHAzFUBjKAXATgVwKYBoQB7KAbRHgHYBWMAFipAF0CAHNKEAZUwEsA7AOYgAvgTABOWOKQhUkTLgLFIZAAxNhokADYZPACYcAtGAit2kECAJoAnixwcRwoA https://www.digikey.com/en/products/filter/crystals/171?s=N4... ) - look at the options and prevalence for frequency stability & tolerance. Ed.2: a wristwatch also benefits from being kept at constant-ish body temperature.
- Const-me 3mo ago
  > typical crystals in the 10-100MHz range I think most quarts watches oscillate at 32 kHz = 2^15 Hz, high precision quartz watches at 8.4 MHz = 2^23 Hz. > The actual problem is stability over temperature Apparently, designers of these watches compensating for that somehow: https://en.wikipedia.org/wiki/Quartz_clock#Thermal_compensation https://en.wikipedia.org/wiki/Quartz_clock#Thermal_compensat... > benefits from being kept at constant-ish body temperature Some people take off their watches every day before going to sleep. These high-end quartz oscillators are probably too expensive to use in commodity computers. Still, the cost shouldn’t look too bad when compared to a price or an airplane, marine vessel, or most military equipment.
  - eqvinox 3mo ago
    We're in agreement; 1ppm is 31.5s/yr so this lines up with OCXO performance / keeping the crystal at constant temperature. It's still 1km zone of spoofability per hour without resync. (GPS sync is a question of nanoseconds.)
DoctorOetker 3mo ago
add on top of this that oven controlled crystal oscillators (or any more performant technologies if affordable) would be selected by militaries...
- eqvinox 3mo ago
  I'm unaware of any technology between OCXOs and Rb standards. The latter have gotten smaller but not tiny and also need quite a bit of ongoing maintenance and calibration.
  - raron 3mo ago
    Microchip has some "chip-scale atomic clock"s, not much bigger than an OCXO, but a lot more expensive. https://www.microchip.com/en-us/product/csac-sa65 https://www.microchip.com/en-us/product/csac-sa65

eqvinox 3mo ago

Yes, albeit 100ppm is bad/cheap crystals. 50-30ppm is normal. The difference with a quartz watch is that it's factory calibrated with the load capacitance on the crystal, and that it's a 32768Hz tuning fork. For a variety of reasons, generating higher frequency clocks off 32768Hz is... "annoying" (huge PLL ratio, very slow feedback loop step), and typical crystals in the 10-100MHz range are just less precise and thermally stable. (Not sure why, I'm not an oscillator manufacturer...) (NB: you can of course correct for initial deviation in software. The actual problem is stability over temperature.) Ed.: https://www.digikey.com/en/products/filter/crystals/171 https://www.digikey.com/en/products/filter/crystals/171 (or, in the hopes the filter on the link works, https://www.digikey.com/en/products/filter/crystals/171?s=N4IgjCBcpgHAzFUBjKAXATgVwKYBoQB7KAbRHgHYBWMAFipAF0CAHNKEAZUwEsA7AOYgAvgTABOWOKQhUkTLgLFIZAAxNhokADYZPACYcAtGAit2kECAJoAnixwcRwoA https://www.digikey.com/en/products/filter/crystals/171?s=N4... ) - look at the options and prevalence for frequency stability & tolerance. Ed.2: a wristwatch also benefits from being kept at constant-ish body temperature.

Const-me 3mo ago
> typical crystals in the 10-100MHz range I think most quarts watches oscillate at 32 kHz = 2^15 Hz, high precision quartz watches at 8.4 MHz = 2^23 Hz. > The actual problem is stability over temperature Apparently, designers of these watches compensating for that somehow: https://en.wikipedia.org/wiki/Quartz_clock#Thermal_compensation https://en.wikipedia.org/wiki/Quartz_clock#Thermal_compensat... > benefits from being kept at constant-ish body temperature Some people take off their watches every day before going to sleep. These high-end quartz oscillators are probably too expensive to use in commodity computers. Still, the cost shouldn’t look too bad when compared to a price or an airplane, marine vessel, or most military equipment.
- eqvinox 3mo ago
  We're in agreement; 1ppm is 31.5s/yr so this lines up with OCXO performance / keeping the crystal at constant temperature. It's still 1km zone of spoofability per hour without resync. (GPS sync is a question of nanoseconds.)

DoctorOetker 3mo ago

add on top of this that oven controlled crystal oscillators (or any more performant technologies if affordable) would be selected by militaries...

eqvinox 3mo ago
I'm unaware of any technology between OCXOs and Rb standards. The latter have gotten smaller but not tiny and also need quite a bit of ongoing maintenance and calibration.
- raron 3mo ago
  Microchip has some "chip-scale atomic clock"s, not much bigger than an OCXO, but a lot more expensive. https://www.microchip.com/en-us/product/csac-sa65 https://www.microchip.com/en-us/product/csac-sa65