[time-nuts] Different Thunderbolt versions

[Markus Kern]

On 22.02.2009, 21:12 Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> Markus

> 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
> receivers.

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 [1]. In section 3.1.3.3 they
say:

"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
above 10MHz."

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
> @10s.

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.

Markus

[1] http://www.lofar.org/operations/lib/exe/fetch.php?id=public%3Adocuments%3Alofar_documents&cache=cache&media=public:documents:19_detailed_description_of_clock_sync.pdf