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

SAIDJACK at aol.com SAIDJACK at aol.com
Mon Feb 20 05:02:15 UTC 2012

Hello Bill,
this is potentially possible with the small M9108 or the Jackson Labs  
Technologies GPSTCXO.
Some caveats:
1) The Trimble Resolution-T May work, but the above stated units have a 50  
channel WAAS/EGNOS/MSAS GPS receiver and are also GPS Disciplined 
Oscillators  not just timing GPS receivers. The trimble unit may only be a 12 channel 
 receiver like the Resolution-T and doesn't seem to support SBAS?
2) The two mentioned units above may work with indoor GPS reception. This  
would then be able to get to your 100us goal no problem. Indoor GPS 
reception  depends on your setup, it works better when there are windows that allow 
signal  propagation and multipath to reach the indoors antenna. The antenna 
won't have  to sit next to the window. It will depend on a case-by-case 
basis if  these units can get GPS reception indoors, but we even had units 
receiving GPS  signals inside a metal thermal chamber without an antenna 
connected(!)... so it  may be possible
3) The M9108 has an external 1PPS input you could use to feed a 1PPS signal 
 from a 1588 or NTP system into it as an alternative to the GPS. That 1PPS 
should  be fairly accurate though (within +/-200ns to UTC) to get your 
<100us per 24  hours holdover accuracy
4) The above units will give you position, velocity, and time as NMEA  
strings as requested, with WAAS accuracy (typically better than 0.8 meters  
horizontal rms) when they are used with an outdoor antenna.
5) The above units are priced in the ballpark of your goal in quanity,  and 
have very highly stable oscillators (OCXO and TCXO) that should help with  
your stability requirements. They are rated at 25ppb and 75ppb over 
temperature  for example, and that would mean you could reach ~100us drift without 
any  external reference (units in holdover) over 24 hours with a +/-5 Degree 
C  temperature variation.
In a message dated 2/19/2012 19:49:21 Pacific Standard Time,  
albertson.chris at gmail.com writes:

On Sun,  Feb 19, 2012 at 3:56 PM, Bill Woodcock <woody at pch.net> wrote:
> 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  

Chris Albertson
Redondo  Beach,  California

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