[time-nuts] Datum/Symmetricom X72 Rubidium

Herbert Poetzl herbert at 13thfloor.at
Sat Mar 16 14:01:55 EDT 2013

On Sat, Mar 16, 2013 at 05:41:11AM -0400, GandalfG8 at aol.com wrote:
> 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 :-(

Did you open up the unit completely?
If so, did/could you take/make some photos/scans of the


> 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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