4 ms·
You're overestimating how precise the clocks in normal electronic devices are. A typical PC clock is +/-100ppm. After 1 hour that's 0.36s, which is roughly 100k
by eqvinox 4mo ago
You're overestimating how precise the clocks in normal electronic devices are. A typical PC clock is +/-100ppm. After 1 hour that's 0.36s, which is roughly 100km in distance. A good electronic clock is a tenth that. An OCXO is in the 1ppm range, but that's still a kilometre per hour.
There's a reason GPS satellites are used as reference clock for PPS, PTP and NTP. A naval vessel you could carry a Rubidium clock on, I guess. But on ground vehicles or mobile receivers... nope.
[ed.: OCXOs aren't that large, 1cm^3 box ballpark, too large* for a smartphone or laptop but not a problem on larger quadcopters, cars or military radio equipment. And 1ppm is long term drift, you can try compensating a bit beyond that, so - I guess it's a question of spending the money and energy** on OCXOs.
* thick specifically, can't easily be made thin AFAIK
** the first O there is Oven - roughly 0.5W continuous draw.]
- Const-me 4mo ago> 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 4mo agoYes, 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 4mo 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 4mo agoWe'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 4mo agoadd on top of this that oven controlled crystal oscillators (or any more performant technologies if affordable) would be selected by militaries...
- eqvinox 4mo agoI'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 4mo agoMicrochip 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