[Freedombox-discuss] Time

Adrian Gropper agropper at healthurl.com
Fri Dec 13 03:02:13 UTC 2013

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?


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?
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Adrian Gropper MD
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