[time-nuts] Thunderbolt GPS Receivers
David I. Emery
die at dieconsulting.com
Sun Jan 28 01:52:33 EST 2007
On Sun, Jan 28, 2007 at 12:33:00AM -0600, Didier Juges wrote:
> Yes, the disparity in holdover specifications is striking, even more so
> considering the HP 53540A was obviously designed as a form, fit,
> function replacement (except for the software interface) for the
> Thunderbolt, based on the mechanical dimensions.
The interesting thing about the holdover spec of any similar
unit is that unless something bizarre was done in the design of the DAC
circuitry to cut corners, it almost entirely depends on the stability of
the OCXO chosen. Aside from clever games with sensing internal
temperature and the like (which might make SOME difference I admit), for
the most part the EFC DAC is frozen while in holdover or perhaps
periodically loaded with a value that tracks the with first moment of
the correction and as such the design of the rest of the unit should
make rather little difference to holdover performance. It is all
determined by the drift with time and temperature of the OCXO with a
more or less fixed EFC input.
So I'd at least naively think that if there was a huge
difference in holdover performance that was real (not just an artifact
of specsmanship or marketing games somewhere) it would have to reflect a
much worse OCXO in the 58540A. I do know they supplied various OCXO
options - perhaps the standard Thunderbolt had a more expensive one than
the standard 58540A ?
> Also consider that the Thunderbolt spec was written when SA was in
> effect. We should expect better performance today with SA turned off.
> Trimble never revised the spec. I asked the question (along with a bunch
> of other questions) from the sales guy a month ago but he did not answer
> that one. I guess they probably have not done comprehensive testing
> since SA was turned off.
Both units were spec'd in the SA on days...
Dave Emery N1PRE, die at dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."
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