Jason I believe has a GPS4. I have not seen any on eBay in a while.
http://www.brandywinecomm.com/literature/bwc_lc_GPS4.pdf

There is also the HP 58540A that looks a lot like the Thunderbolt, but
the specs are not nearly as good. I have not seen any of those on eBay
in a while either.
http://www.realhamradio.com/58540A.htm

Then there is the Fury, by Jackson Labs. This one is very new, so I
would not hope to find one on eBay, but it is much less expensive than
the Thunderbolt as new, and the specs are very good.
http://www.jackson-labs.com/docs/Fury_Flyer.pdf

Didier KO4BB

Richard W. Solomon wrote:

The last time I saw a Thunderbolt on eBay was waaaaay back in June/July I
think.

I did get one of those Brandwine GPS4's that one person listed a bunch of
(one of the last).

I just took a picture of the inside of my GPS4, and I also uploaded the
manual if you would like to read over it.

http://www.rabel.org/archives/Images/Brandywine_GPS4/

http://www.rabel.org/archives/Brandywine_GPS/

My unit has the "Medium Performance" Brandywine labeled OCXO. (Page 35 & 36
of the manual explain the different oscillator options). From the manual it
looks like they use (or used at one time) Furuno, Motorola, and some Sirf
(unknown model) receiver.

I've tried contacting the Brandywine people a few times in the past, and
they answered only one of my emails (incompletely at that). I don't know if
I just caught them at a bad time or what, but I wouldn't hold my breath if I
had to email them again.

FYI, I'm running firmware 17, I'm pretty sure there is at least a firmware
19. If anyone else out there has one, what firmware is yours running and
what is its configuration?

Jason

On Sat, Jan 27, 2007 at 09:57:30PM -0600, Didier Juges wrote:

I'm curious as to whether anyone has actually MEASURED 58540A

performance and compared it with a Thunderbolt.  Clearly the standard
58540A has MUCH worse specified holdover performance, but unless that is
important to you the actual 1 PPS jitter statistics (ADEV) and accuracy
relative to UTC and the related 10 mhz stability in various taus would
be what would count for most of us as actual holdover operation happens
less often, at least for significant intervals.  And then of course
phase noise, close and far... counts for many applications too.

The Thunderbolt has lots of tweaks and the 58540A few, but for

many uses this is not a big issue.  Just curious as to how good or bad
the relative performance is WHEN LOCKED.

--
Dave Emery N1PRE,  die@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."

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.

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.

Didier KO4BB

David I. Emery wrote:

On Sun, Jan 28, 2007 at 12:33:00AM -0600, Didier Juges wrote:

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 ?

Both units were spec'd in the SA on days...

--
Dave Emery N1PRE,  die@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."