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

[WB6BNQ]

Hello Bill Woodcock,

Many, many questions come to mind.  Is this a fixed network that never changes its character ?  Or by network do you mean via the internet where you have no control over path variations ?  I guess the latter based upon your comments thus far.

What is driving the requirements of your delay measurements ?  Are they realistic ?

It seems to me that to do a one-way delay measurement, the precise absolute time of transmission would be quite important.  Otherwise how would you know the start of the timing pulse ?  How would you otherwise account for variables in the path ?

Doing a few fixes for 30 minutes will, under best conditions, get you somewhere on a circumference around your location with a radius of 15 meters (50 feet).  For GPS to get a useful coordinate result with meaningful data will take longer than 30 minutes or so.  Typically, you would want to do a 48 hour “survey” of your position to try to achieve a 3 to 5 meter resolution.  However, that only gives you an idea of where the GPS antenna was
located and specifically not where the start of the path is located.  Your intended use of GPS will not help you with the time at all because once you lose the GPS signals (i.e., going back inside the building) the reported time is meaningless because the GPS internal oscillator is no where near stable enough to maintain that time properly.  This is the case for all but a few special GPS units.

Trying to study SC verses AT cut crystals and other minutiae is a complete waste of your time.  Either one in its proper circuit will do the same job.  No matter which, for any decently designed ovenized oscillator, it takes 30 days to truly achieve stable thermal equalibrium and reach the best specifications, as to drift, for that particular unit.  In the mean time transporting, jarring around and warmup retrace factors will guarantee the
oscillator will not be where it was at its last long term runup.

Not knowing the requirements for your measurements makes it tough to give any meaningful answers.  Clearly, it seems that your needs are to correlate data from many nodes at indeterminate positions.  If this is truly the case, then “time” would be the critical factor that needs to be focused on, more so then location in my opinion.  If so, then your budget may need to increase more than your planning for.

Bill....WB6BNQ


Bill Woodcock wrote:

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> On Feb 19, 2012, at 5:07 PM, Bill Hawkins wrote:
> > If you are going to correct it with NTP, a simple crystal
> > oscillator will do.
>
> Yeah, my assumption was that something like a DOCXO or a VCTCXO would be about the best I'd get within budget.  But I'm very new to this, and just beginning to dip into all the articles about SC cut versus AT cut, etc.
>
> > If you're using NTP, why do you need to
> > initially set it to GPS accuracy?
>
> We need the GPS fix for location, and get the time for free, so figured we'd use it.
>
> > Your best solution is to maintain the GPS antenna and only use
> > GPS to discipline a good crystal oscillator.
>
> That would be nice, of course, and we'll get the best GPS antenna we can afford and fit, but we can't have an externally-cabled antenna, or require that people put these on windowsills, or anything like that…  There will be too many of them, and the level of clue of the people plugging them in out in the field is likely to be too low.
>
> > Do you plan to regulate the ambient and power environment to
> > some degree of accuracy?
>
> Yes…  They'll all be indoors, which helps, and temperature-regulated crystals seem to be relatively widely available, if in a dizzying number of styles.  Regulated power is something that we need to just have someone spec out, or use a reference design, if one exists.  Anyone have pointers?
>
> >
> > Note that 10 microseconds is 1 part in 10E5.
> > The folks on this list deal in parts per 10E12.
>
> So that's something I've been having a hard time understanding…  If that's the amount of inaccuracy _per oscillation_, then at the time-scales I'm dealing with, it would quickly accumulate and become unuseful…  that is, 10 microseconds of drift per second is almost a second of drift in a day, whereas I need, ideally, something I can discipline to within 100 microseconds _total_, using just a single GPS fix, plus NTP over the long haul.
>
> Now, I don't know whether what I want is possible or not, but that's why I'm asking these questions.
>
> Or am I misunderstanding the parts-per-foo notation?
>
> > 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?
>
> There will only be one device per location, and we can't have any external stuff plugged into them.  Else yes, I'd just use GPS and be done with it.
>
>                                 -Bill
>

Bill Woodcock wrote:

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> 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