[time-nuts] Datum/Symmetricom X72 Rubidium
GandalfG8 at aol.com
GandalfG8 at aol.com
Sat Mar 16 05:41:11 EDT 2013
I've had an "interesting" week or so playing with an X72 and it turned out
not to be quite as straightforward as I first expected.
A google based "X72" search of the list archives seems to throw up more
questions than answers so I thought it might help a bit if I shared what I've
learned, whilst hoping others might be able to fill in some missing gaps
for me too:-)
One of the attractions of the X72 was the option to use a 1PPS input for
frequency conditioning, and one of the first things I learned was that this
depends on the firmware version.
This feature was introduced with firmware version 5.02 in 2003, and this
bright and shiny looking just like new X72 turned out to have 25,000 hours
on the clock and firmware version 4.10 from 2002.
Lesson one, looks ain't everything:-(
Next problem, these are specified at shipment to have an accuracy of
<+/-5E-11 but obviously they age.
Whilst Symmetricom does offer analogue and digital options for adjustment,
more of which in a moment, there's no user option to properly adjust the
startup frequency, as in the FE5680A for example, instead there's a flag that
gets set to conveniently warn the user when it's time to send their X72
back to Symmetricom for a "service".
As received, the locked output frequency of this unit was 9.999,999,986,xx
Mhz, the xx indicating digits still wandering after lock which may reflect
more on the less than ideal antenna placement for the Thunderbolt providing
the counter reference.
When finally in a negotiated position to remove the "do not remove"
warranty stickers both were found to cover access holes, one of which led nowhere
but the other to a trimmer capacitor adjacent to the lamp assembly.
Whether or not it was the intended purpose this did allow adjustment of the
output frequency, unfortunately it ran out of steam at 9.999,999,992,xx
MHz so was reset to where it started.
There are two further options for frequency adjustment, not including the
digitally adjustable CMOS outputs, one is a software command that allows an
offset to be specified, based on the free running frequency and in steps of
2E-12, which does allow for reasonable adjustment relative to the startup
frequency but resets every time power is removed
The other is an anologue control input which can be varied from 0 to 5
volts and allows adjustment to a few parts in 10^11, but which sits at just
over 4 volts to bring this one to 10MHz.
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