[time-nuts] Datum/Symmetricom X72 Rubidium

GandalfG8 at aol.com GandalfG8 at aol.com
Sat Mar 16 05:45:46 EDT 2013

Hi All
This messsage was a work in progress and was sent in error, in  particular 
it was due to be converted to something more akin to a series of  bullet 
points as it was becoming much too verbose, will edit later and add the  rest 
of the information.
Apologies for this, please ignore for now.
In a message dated 16/03/2013 09:41:31 GMT Standard Time, GandalfG8 at aol.com 

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 
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 
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 
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 
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  
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 
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  
offset to be specified, based on the free running frequency and  in steps 
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  
over 4 volts to bring this one to  10MHz.

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