[Freedombox-discuss] Time

Doug dj0fb at frombob.to
Thu Dec 12 23:56:20 UTC 2013

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? 
), 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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