[time-nuts] Low cost GPS module for < 100ns timestamping error

nuts nuts at lazygranch.com
Mon May 5 18:49:43 UTC 2014

On Mon, 05 May 2014 14:55:20 +0100
Tony <tnuts at toneh.demon.co.uk> wrote:

> On 03/05/2014 18:41, Tom Van Baak (lab) wrote:
> > Tony, Chris, Bert,
> >
> > Since all you want is a 10 ns time stamp / data logger you do not
> > need a GPSDO, or OCXO, or VCXO.
> >
> > The solution is cheap and very simple.
> >
> > Your GPS receiver provides a 1PPS to the microprocessor. Use a
> > plain XO or TCXO; the frequency does not need to be accurate, just
> > stable to about 1e-9 (many $1 xtals do this). Each second your code
> > [re]computes the drift between the clock and GPS. You may average
> > over 10 to 100 seconds if you wish.
> >
> > Even though your clock is off-time and off-frequency your software
> > knows what the offset is. Therefore, you can simply adjust the time
> > stamp reading by the current clock error.
> >
> > This "software GPSDO" gives equal or actually slightly better
> > performance than a real GPDSO but it is much simpler: no DAC, no
> > EFC, no OCXO, no VCXO, no PLL.
> >
> > /tvb (i5s)
> > _______________________________________________
> Tom,
> Yes - that is exactly what I intended. The problem though is
> maintaining sufficient accuracy during periods when the GPS clock is
> unavailable or unreliable (perhaps due to local interference), but I
> don't have any handle on how long that may be or how often it occurs.
> Clearly there are no absolute guarantees - eg. the GPS selective
> availability could be turned on again in exceptional circumstances,
> so I accept that 100ns accuracy can't be absolutely guaranteed.
> The question then is, in the experience of users of GPS timing 
> references, for a decent but low cost receiver with a reasonably well 
> sited antenna and assuming there is no significant interference, how 
> long and how frequently is time synchronisation lost? If for example 
> it's only 2 or 3 seconds every few weeks, then there isn't much of a 
> problem. If 5 minute outages occur every few days then the holdover 
> performance of the local oscillator is much more critical.
> What about in more difficult circumstances - eg. in urban environment 
> with an antenna that has a restricted view of the sky? Not that I
> expect to operate in such circumstances but it would be interesting
> to get a feel for how good or bad timing is maintained in less
> favourable situations.
> Thanks, Tony H

The maintenance of time between loss of GPS is why so many time
references hit the surplus market. The hold over specs got to the point
where they had to go to a rubidium reference. 

We should be up to our arm pits in Symetricoms, Trimble, etc, but I
presume the stuff got crushed. I snagged two "new old stock" GPSDO
(crystal based) from a cellular tech. At least the Chinese sold their
references on ebay.

Having second sourced products over the years, it is often easier just
to take a published spec for an item and start from there. 

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