3 ms·

Quartz clocks can be inaccurate and can vary greatly with temperature. Beyond temperature-compensated crystal oscillators (TCXO) which are designed to be reason

Quartz clocks can be inaccurate and can vary greatly with temperature. Beyond temperature-compensated crystal oscillators (TCXO) which are designed to be reasonably stable over a prescribed temperature range, there are oven-controlled crystal oscillators (OCXO)[1], that are held at a stable temperature in a small can with a heating element controlled by something like PID control. For anything requiring affordable stable timekeeping (this includes high data rate/quantization multimeters like the HP 3458A), an OCXO is a pretty good way to go. GPS is a fine reference to start one from and with specialized hardware GPS itself can be used as a MHz clock. For everything else there are TCXOs, and if you really don't care on-die RC oscillators can be used (this is the internal oscillator on an Atmega, for example, the default until you add an external crystal). If money is no object, atomic clocks are of course also an option. Past press mentioned that Google uses GPS-referenced atomic clocks to keep their data enter clocks in sync [2]. As an amusing aside, my kitchen oven clock has a poorly-chosen TCXO, such that it is not stable over the temperature range normally experienced by an over control panel. It's a great example of not designing to the application. Whenever I bake something, time accelerates on the oven clock. It's like a cookie-making time machine. If you are interested in learning more about the hardware of crystal oscillators, this application note from HP is terrific: http://www.leapsecond.com/pdf/an200-2.pdf http://www.leapsecond.com/pdf/an200-2.pdf 1. http://www.vectron.com/products/literature_library/ocxo.pdf http://www.vectron.com/products/literature_library/ocxo.pdf 2. http://www.wired.co.uk/news/archive/2012-09/20/google-gps-powered-database http://www.wired.co.uk/news/archive/2012-09/20/google-gps-po...

comboy 11y ago
> First of all, to be clear, I'm not talking about the hardware clock. That really only matters when you read from it, and you generally do that once: at boot.
- tomkinstinch 11y ago
  That's absolute time that's read at boot, from a battery-backed real-time clock (RTC). Relative time can still drift. Crystal resonators with multipliers and PLL are used to supply the CPU with the signal distributed by its clock network. The crystal is a voltage-controlled oscillator (VCO), so it is tightly controlled (could be by voltage controlled by propagation delay within the chip, for example), but it can still drift over time and with temperature. As the article mentions, this can be responsible for system time error, "Your clock's pace will always vary a tiny little bit for different reasons". Operation of the computer shouldn't be impacted since things sync to the clock, but absolute timekeeping can be.
  - comboy 11y ago
    My bad, I was before my coffee. One additional thing that wasn't yet mentioned is that freqency depends also on gravitation, so orientation of your motherboard could matter (for the curious - https://www.youtube.com/watch?v=zILwgQhjC_Q https://www.youtube.com/watch?v=zILwgQhjC_Q ). I'm not really sure it's big enough to matter though.
a3n 11y ago
> Whenever I bake something, time accelerates on the oven clock. Bonus! You can make thirty minute brownies in twenty minutes. http://www.uffaz.com/entertainment/i-cook-thirty-minute-brownies-in-twenty-minutes/ http://www.uffaz.com/entertainment/i-cook-thirty-minute-brow...
Matumio 11y ago
The article mentions corrections beyond 500ppm. I worked on time synchronization (using PTP instead of NTP), and when we are talking about "cheap oscillators" we mean those specified at 50ppm (50us per second) which you find in every Ethernet card. In reality we measured +/- 8ppm. Yes, the drift changes over time, and you can see in the log when a door is opened in the morning. But NTP can only reach milliseconds precision anyway, so there is plenty of time to react. Whatever the problem is with those PCs mentioned in the article, using a temperature-controlled oscillator would be overkill.

comboy 11y ago

> First of all, to be clear, I'm not talking about the hardware clock. That really only matters when you read from it, and you generally do that once: at boot.

tomkinstinch 11y ago
That's absolute time that's read at boot, from a battery-backed real-time clock (RTC). Relative time can still drift. Crystal resonators with multipliers and PLL are used to supply the CPU with the signal distributed by its clock network. The crystal is a voltage-controlled oscillator (VCO), so it is tightly controlled (could be by voltage controlled by propagation delay within the chip, for example), but it can still drift over time and with temperature. As the article mentions, this can be responsible for system time error, "Your clock's pace will always vary a tiny little bit for different reasons". Operation of the computer shouldn't be impacted since things sync to the clock, but absolute timekeeping can be.
- comboy 11y ago
  My bad, I was before my coffee. One additional thing that wasn't yet mentioned is that freqency depends also on gravitation, so orientation of your motherboard could matter (for the curious - https://www.youtube.com/watch?v=zILwgQhjC_Q https://www.youtube.com/watch?v=zILwgQhjC_Q ). I'm not really sure it's big enough to matter though.

a3n 11y ago

> Whenever I bake something, time accelerates on the oven clock. Bonus! You can make thirty minute brownies in twenty minutes. http://www.uffaz.com/entertainment/i-cook-thirty-minute-brownies-in-twenty-minutes/ http://www.uffaz.com/entertainment/i-cook-thirty-minute-brow...

Matumio 11y ago

The article mentions corrections beyond 500ppm. I worked on time synchronization (using PTP instead of NTP), and when we are talking about "cheap oscillators" we mean those specified at 50ppm (50us per second) which you find in every Ethernet card. In reality we measured +/- 8ppm. Yes, the drift changes over time, and you can see in the log when a door is opened in the morning. But NTP can only reach milliseconds precision anyway, so there is plenty of time to react. Whatever the problem is with those PCs mentioned in the article, using a temperature-controlled oscillator would be overkill.