[time-nuts] LPRO-GPSDO
Magnus Danielson
magnus at rubidium.dyndns.org
Sat Jan 9 00:18:03 UTC 2010
Robert,
Robert Atkinson wrote:
> Hi Bob,
> Indeed! I must admit I thought at first that the LPRO was one of unit with built-in discipling to an external 1PPS. If this guy was selling in the UK he would be breaking advertising rules with his claims. I can see no validity for his claim of NIST traceability. NIST specifically say that the GPS signal (let alone the output of a GPSDO) is only traceable after the fact by analysing the monitoring data.
Which is the correct answer within the context of traceability, such as
ISO 17025 (previously ISO 25). Many confuse the issue of adjustment with
that of calibration and traceability. Adjustment may be used in a
calibration procedure, but is not necessary for traceability. The
calibration procedure however involves measuring and recording the
deviations in accordance to some strict procedure, and this is done with
reference to some reference standard, calculate the tolerances and
uncertainties of the measure and also include the tolerance and
uncertainty of the reference standard. Traceability is achieved by the
combined records of all the transfer standards to the national standard
which in itself should be traceable to the international standard.
You can have a clock being all wrong, but with propper traceability and
no adjustment, you could make readings within certain limits traceable
to the national standard. You can adjust all you want on a clock, but
have no evidence of its current or previous reading.
Disciplining does not give traceability, but you can get adjustments and
for some applications sufficient precission. Traceability is thus a
heavy word to swing.
Disciplining an LRPO to GPS seems like a good idea thought. Can get you
very nice hold-over properties.
Cheers,
Magnus
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