[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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