[time-nuts] Different Thunderbolt versions
mkern at fastmail.fm
Tue Feb 24 19:29:48 UTC 2009
On 22.02.2009, 21:12 Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> Even with sawtooth correction the performance of the M12+T was found
> inadequate for the LOFAR <http://www.lofar.org/p/systems.htm> array.
> They use SRS FS725 rubidium sources disciplined by M12+T GPS timing
I didn't mean using the M12 by itself, obviously a clock stable enough
over the time the M12 pps must be integrated has to be used.
If we are using the ADEV limits you proposed then at 50 MHz (= 3ns
acceptable error) the timing requirement is an ADEV of 3*1E-(8+x) at
tau = x seconds. From the measurements at
http://www.leapsecond.com/pages/gpsdo/ it seems the Thunderbolt gets
pretty close to that.
The LOFAR clock system is described at . In section 18.104.22.168 they
"Some Crystal Oscillators have the advantage that they have a better
Allan variance for periods of up to 10s and therefore it can be claimed
that they have a better performance than the SRS-FS725 Rb-reference
standard. The performance for time periods above 10s, the SRS-FS725
performs better. Therefore choosing an OCXO would require a maximum
calibration interval of 10s and it would require a significantly better
GPS (or GALILEO) receiver because de Rb-reference is used to average
the PPS signal from the GPS receiver thereby making it possible to
identify the time difference between stations at receive frequencies
I think this means that they are using pps integration times above 10
seconds. I couldn't find any reference to the actual value though.
LOFAR is also working at frequencies up to 240MHz so the timing
requirements are definitely higher. They say that a station time
offset of 200ps does not affect performance as long as it remains
stable over time.
> They also state that the ionosphere contribution to ADEV is about 8E-12
Yes, from which they infer that "the reference clock shall have an
Allan variance of 1e-11 or less over 10s." I am not sure if this has
to do with the propagation of the GPS signal or if they mean that they
need a clock stable enough to later compensate for the different
delays of the observed signal through the ionosphere.
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