[time-nuts] Low-long-term-drift clock for board level integration?
albertson.chris at gmail.com
Mon Feb 20 03:48:33 UTC 2012
On Sun, Feb 19, 2012 at 3:56 PM, Bill Woodcock <woody at pch.net> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
> Hi. This is my first posting to this list, and I'm not a timekeeping engineer, so my apologies in advance for my ignorance in this area.
> I'm building a small device to do one-way delay measurements through network. Once I'm done with prototyping, I'm planning a production run of several hundred of the devices. They'll have a GPS receiver, probably a Trimble Resolution SMT, and they have a bit of battery so they can initially go outdoors for ~30 minutes to get a good fix, but then they get taken indoors and plugged into the network, and probably never get a clear view of a GPS or GLONASS satellite again.
> - From that point forward (and we hope the devices will have an operational life of at least ten years) they'll be dependent on their internal clock and NTP, but we really need them to stay synchronized to within 100 microseconds. 10 microseconds would be ideal, but 100 would be acceptable. And in order to be useful, they need to stay synchronized at that level of precision essentially forever.
So you can live with a 100 uSec drift over ten years or you say 10
uSec per year is OK.
How many uSec are there in one year? I get 3.1E+13. So you can
tolerate 10 parts in 3E13 or 1 part in 3E12 drift per year. And you
have a $300 budget. Somehow I think either the spec of the budget
will have to move by orders of magnitude.
Of you can have both with margin to spare if you can keep a GPS
antenna in view of the sky continuously
Your plan to sync the system to GPS be exposing it briefly to the GPS
signal will not work
The reason is that, let's say you wanted to adjust your wrist watch by
adjusting the fast/slow lever. Assume you have a perfect clock in
your house. You adjust the time just fine. But now if you only wait
5 minutes to see if the watch is moving fast or slow you will not get
good result. but if you wait a week then maybe you can measure a
difference in the two rates. Same for NTP. It needs a bit of
time, maybe hours or days to measure the relative rates. The math is
not hard. GPS, after it has "settled" for about an hour or so can get
the time to about 50 nano seconds. So you capture the time, Now you
wait an hour and capture it again. You could easy have 0.1 uSecond
per hour error in the rate. You say you's like 10 uSecond per year.
So you need either a better GPS or wait longer than one hour.
So it's not like you can sync time to GPS in an instant. it takes
at least a few hours if you care about microseconds.
Again this becomes easy and within $300 if you can have an outdoor antenna.
Redondo Beach, California
More information about the time-nuts