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)
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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
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