4 ms·

It's surprisingly hard to keep a clock accurate over time. Typical crystal oscillators can gain or lose seconds per day. If you want to do better than that, you

It's surprisingly hard to keep a clock accurate over time. Typical crystal oscillators can gain or lose seconds per day. If you want to do better than that, you either need internet access (for NTP), a radio receiver of some kind (for atomic clock broadcasts, GPS, etc.), or a significantly more expensive oscillator. Having a frequency reference with perfect long-term accuracy that any electrically-powered clock can use for free is pretty convenient! I've often wondered how hard it would be to make portable battery-powered clocks use 60 Hz noise as a timekeeping source. Maybe I should make a project out of it.

deleted 2y ago
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rrr_oh_man 2y ago
> Typical crystal oscillators can gain or lose seconds per day. Are you sure? I only owned a quartz watch when I was a kid, but all sources I find on the web say even the lowest quality quartz watch movements are accurate to +- 20-30 seconds a month. For comparison: my several decades old manual Poljot loses some seconds per day.
- AdamH12113 2y ago
  The Casio G-Shock I got in the 90s was ~66 ppm fast, which worked out to ~5 seconds/day. There are cheap crystals on Digi-Key with ~10 ppm accuracy, which is around 0.86 seconds/day. But crystals drift with age (and a tiny bit with temperature). Perhaps watches have improved since I last looked, but I would want to measure the actual real-world accuracy before believing anything better than 1 second/day.
- crote 2y ago
  Yeah, that sounds about right. Crystal oscillators typically have an accuracy of about 20ppm, which is 1.73 seconds per day. Keep in mind that those number are a guaranteed worst-case performance - real-world performance is often much better.
agos 2y ago
how more expensive is a "more expensive" oscillator?
- lxgr 2y ago
  A few cents, as far as I understand, but that's not the only problem: You can get significantly more precision using an oven-controlled oscillator (frequency is temperature-dependent, and it's easier and more precise to control temperature to a known high setpoint than to compensate for the temperature-frequency curve using a thermometer), but these are, as the name implies, small ovens that consume additional electricity (on the order of 1-2 W, as far as I know). An additional base load of 1-2 W per oven and microwave would be pretty bad for the energy budget on a global scale, so just providing a time source from the power line they're already connected to is more efficient :) Other than oven-based oscillators, you can also use temperature-corrected oscillators; the Apple Watch does that, for example, and is supposedly about four times more accurate than a non-temperature-corrected MEMS, which in turn is already better than a quartz oscillator [1]. Not that it matters much, given that all models have at least a GPS receiver these days, as well as a battery life of about a day without charging, but I suppose it's a real benefit to all Apple Watch users working in faraday cage isolated bunkers without Wi-Fi :) [1] https://www.watchuseek.com/threads/apple-watch-oscillator-and-synchronization.2728930/ https://www.watchuseek.com/threads/apple-watch-oscillator-an...
  - AdamH12113 2y ago
    > A few cents, as far as I understand, but that's not the only problem Nitpick: looking on Digi-Key, in quantity, a sub-ppm oscillator is ~5x more expensive than a 10 ppm crystal -- ~85 cents for the cheapest TCXO I can find vs. ~18 cents for a crystal. The frequencies are also much higher (tens of megahertz), which won't help with the power consumption. You're right about the power consumption, though.
- generuso 2y ago
  It depends. On the lower end of the spectrum, there are high volume products that offer somewhat better performance for just a tiny increase in cost, as mentioned in other comments. On the higher end of the spectrum there are laboratory-grade oscillators, for example the legendary Hewlett-Packard 10811 [1] and similar, which cost hundreds to thousands of dollars. These overlap in price with the lower end atomic clocks. [1] http://etoysbox.jp/Memo/3_Test_Equipments/HP_10811_OCXO/HP_10811_OCXO_Spec.html http://etoysbox.jp/Memo/3_Test_Equipments/HP_10811_OCXO/HP_1...
crote 2y ago
The problem is that grid frequency has terrible mid-term performance. The frequency itself will be reasonably close to 50/60Hz, but it's guaranteed to be off by a little bit due to the way it operates - and that small offset accumulates over time. For example, the European supergrid only makes time corrections if it has drifted by more than 20 seconds. That's good enough for the clock on your microwave, but it's pretty bad compared to literally any other time source.
- AdamH12113 2y ago
  Wow, that's pretty bad! In Texas it's +/- 3 seconds[1]. Which I wouldn't want to use for precise event logging, but for human use is fine. [1] http://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf http://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf
eternityforest 2y ago
I wouldn't be surprised if you could do it in software alone, using a wire on an ADC as the receiver

deleted 2y ago

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rrr_oh_man 2y ago

> Typical crystal oscillators can gain or lose seconds per day. Are you sure? I only owned a quartz watch when I was a kid, but all sources I find on the web say even the lowest quality quartz watch movements are accurate to +- 20-30 seconds a month. For comparison: my several decades old manual Poljot loses some seconds per day.

AdamH12113 2y ago
The Casio G-Shock I got in the 90s was ~66 ppm fast, which worked out to ~5 seconds/day. There are cheap crystals on Digi-Key with ~10 ppm accuracy, which is around 0.86 seconds/day. But crystals drift with age (and a tiny bit with temperature). Perhaps watches have improved since I last looked, but I would want to measure the actual real-world accuracy before believing anything better than 1 second/day.
crote 2y ago
Yeah, that sounds about right. Crystal oscillators typically have an accuracy of about 20ppm, which is 1.73 seconds per day. Keep in mind that those number are a guaranteed worst-case performance - real-world performance is often much better.

agos 2y ago

how more expensive is a "more expensive" oscillator?

lxgr 2y ago
A few cents, as far as I understand, but that's not the only problem: You can get significantly more precision using an oven-controlled oscillator (frequency is temperature-dependent, and it's easier and more precise to control temperature to a known high setpoint than to compensate for the temperature-frequency curve using a thermometer), but these are, as the name implies, small ovens that consume additional electricity (on the order of 1-2 W, as far as I know). An additional base load of 1-2 W per oven and microwave would be pretty bad for the energy budget on a global scale, so just providing a time source from the power line they're already connected to is more efficient :) Other than oven-based oscillators, you can also use temperature-corrected oscillators; the Apple Watch does that, for example, and is supposedly about four times more accurate than a non-temperature-corrected MEMS, which in turn is already better than a quartz oscillator [1]. Not that it matters much, given that all models have at least a GPS receiver these days, as well as a battery life of about a day without charging, but I suppose it's a real benefit to all Apple Watch users working in faraday cage isolated bunkers without Wi-Fi :) [1] https://www.watchuseek.com/threads/apple-watch-oscillator-and-synchronization.2728930/ https://www.watchuseek.com/threads/apple-watch-oscillator-an...
- AdamH12113 2y ago
  > A few cents, as far as I understand, but that's not the only problem Nitpick: looking on Digi-Key, in quantity, a sub-ppm oscillator is ~5x more expensive than a 10 ppm crystal -- ~85 cents for the cheapest TCXO I can find vs. ~18 cents for a crystal. The frequencies are also much higher (tens of megahertz), which won't help with the power consumption. You're right about the power consumption, though.
generuso 2y ago
It depends. On the lower end of the spectrum, there are high volume products that offer somewhat better performance for just a tiny increase in cost, as mentioned in other comments. On the higher end of the spectrum there are laboratory-grade oscillators, for example the legendary Hewlett-Packard 10811 [1] and similar, which cost hundreds to thousands of dollars. These overlap in price with the lower end atomic clocks. [1] http://etoysbox.jp/Memo/3_Test_Equipments/HP_10811_OCXO/HP_10811_OCXO_Spec.html http://etoysbox.jp/Memo/3_Test_Equipments/HP_10811_OCXO/HP_1...

crote 2y ago

The problem is that grid frequency has terrible mid-term performance. The frequency itself will be reasonably close to 50/60Hz, but it's guaranteed to be off by a little bit due to the way it operates - and that small offset accumulates over time. For example, the European supergrid only makes time corrections if it has drifted by more than 20 seconds. That's good enough for the clock on your microwave, but it's pretty bad compared to literally any other time source.

AdamH12113 2y ago
Wow, that's pretty bad! In Texas it's +/- 3 seconds[1]. Which I wouldn't want to use for precise event logging, but for human use is fine. [1] http://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf http://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf

eternityforest 2y ago

I wouldn't be surprised if you could do it in software alone, using a wire on an ADC as the receiver