[time-nuts] Low-long-term-drift clock for board level integration?

[Bill Hawkins]

What you are looking for is the Caesium standard on a chip that
is presently only available for mostly military projects. This
will become available as war surplus after WW III.

But if you are going to correct it with NTP, a simple crystal
oscillator will do. If you're using NTP, why do you need to
initially set it to GPS accuracy?

Your best solution is to maintain the GPS antenna and only use
GPS to discipline a good crystal oscillator.

Do you plan to regulate the ambient and power environment to
some degree of accuracy?

Note that 10 microseconds is 1 part in 10E5. The folks on this
list deal in parts per 10E12. $300 will buy you a standalone
GPS receiver that does parts in 10E9, but it is bigger than a
circuit board.

Can you use a standalone receiver to always generate a 10 MHz
signal or pulse per second signal that is distributed to all
of the measurement devices in a facility?

Bill Hawkins, who has ideas but is not a professional


-----Original Message-----
From: Bill Woodcock
Sent: Sunday, February 19, 2012 5:57 PM

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. 

My plan, such as it is, was just to get the best clock I could find within
budget, integrate it onto the motherboard we're laying out as the system
clock, and depend on NTPd to do the right thing with it.  

Anyone have any thoughts or advice on clocks I could use that would be, say,
under USD 300 in quantity 500, and would be optimized for minimal long-term
drift?  Power-use is not particularly constrained.  It needs to be
integrated onto our board, but space isn't too constrained either. 

I'm also happy to pay for a few consulting hours if people want to give me
detailed advice on a professional basis. 

Thanks,

               -Bill