How close could we get with just software on an Android phone? The phone would be disabled to whatever point it took to maintain a counter, charge the battery, occasionally listen for GPS or wifi. How stable could that be?
Adrian On Thursday, December 12, 2013, Doug wrote: > It seems that time is important. So how do we know what time it really > is, without leaving a trace? > > Many years ago I knew someone who did some experiments with time. He > wanted a really accurate timekeeper, but did not have a lot of money to > spend. This was long before GPS existed, and before NTP servers became > easily accessible to nearly everyone. You could tune a radio to WWV, or > call up the time lady at the phone company, but that was about it. > > So he built his own OCXO. He built a little circuit board, with a crystal > and an oscillator circuit. It also had another circuit, electrically > isolated from the first: a heater circuit, consisting of a transistor, and > a temperature-sensing element, in a feedback arrangement. (I think this > was right around the time when fairly precise and inexpensive > temperature-sensing ICs first became available.) He set this circuit to > maintain the temperature on the board at a temperature higher than any > likely to be encountered in his room, I think about 60C. > > He had a small Thermos brand vacuum jug, the kind used to keep a bowl's > worth of soup warm. Of course this has a double-wall glass envelope, > aluminized on the interior walls of its vacuum chamber, and is an excellent > thermal insulator. He cut a small hole through the screw-on cap and ran > wires through it, and suspended his circuit board in the middle of the jug. > > Outside the jug he set up a power supply for the crystal circuit, designed > to be as stable as he could afford, and another power supply for the heater > circuit. He powered it all up and waited for hours for the temperature to > equilibrate. He then used this module as a clock source for his other > projects that needed really stable time. > > (He had to wait for hours because he designed the heater circuit to max > out at a fairly low power level, knowing that very little power would be > needed to maintain 60C in that insulated jug. He wanted to keep overall > power consumption as low as possible, because he wanted to operate time > bases over long periods of time and therefore needed to run his clock > source, and his counters, on trickle-charged battery power, and didn't want > to invest a lot in batteries.) > > There were a number of problems he had to solve. The insulating jug was > so good that he had to worry about the temperature overshooting the set > point, as the temp feedback circuit was not the only thing in there that > was dissipating power, and it itself dissipated power even when the heater > transistor had no current. He had to choose the right kind of crystal, as > different kinds have different degrees of stability. He had to choose the > right frequency range. He baked the crystal for a long time to age it. He > had to put small faraday cages around both the crystal circuit and the > heater circuit to keep noise out of the oscillator. > > IIRC he estimated a stability of something like one second per year, at a > very low cost (most of the parts coming out of his spare parts box, or the > kitchen). > > Of course these techniques have been in use in commercially available > clock sources for decades, and I see you can now buy OCXO modules for as > low as $50 at places like Digikey. But I am not aware of any cheap > mass-produced OCXO-based appliances suitable for use in a personal NTP > server you can keep in your home. > > Imagine such a module, with a small low-power computer attached (open > hardware, perhaps an Arduino would be enough, or perhaps this one? > http://www.kickstarter.com/projects/214379695/micro-python-python-for- > microcontrollers ), with a lithium battery so you can carry it outside, > and with a USB and/or ethernet connector. You have to connect it to an > accurate clock occasionally, so it knows what time it is. Then after that, > it just sits there and counts. Connect it to your local network, or plug > it directly into your Freedombox, and there you are. > > With this kind of frequency stability, you don't need to calibrate it > against external time bases very often. Perhaps carry it outside > occasionally so it can see GPS signals, or carry it somewhere where it can > see someone else's wifi. Perhaps just design it so that you can sync it up > by plugging it into a smartphone. Or all of the above. > > With the growing importance of accurate timekeeping, and of leaving > minimal traces behind while doing it, perhaps the time has come for an open > hardware project for such a timekeeper. (Kickstarter, anyone?) > > Seems to me such an appliance could be produced for less than what a lot > of us spend on a good gateway router. > > > Would something like that be accurate enough for our purposes? > > > _______________________________________________ > Freedombox-discuss mailing list > Freedombox-discuss@lists.alioth.debian.org > http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/freedombox-discuss > -- Adrian Gropper MD
_______________________________________________ Freedombox-discuss mailing list Freedombox-discuss@lists.alioth.debian.org http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/freedombox-discuss