[time-nuts] FMT on October 13

[Tom Van Baak]

>> While were at it, in the case mentioned above I'm a curious
>> about their FMT frequency standard -- if it's really accurate
>> to parts in 10^12, as they imply, over 10 minutes. I could
>> believe this if it were an Rb or Cs-based GPSDO.
> 
> We're using an Austron 1250A OXCO that's been measured as better than
> 9x10e-13 for averaging times of 1 second out to 1000 seconds; over a
> broader range, it's better than 3x10e-12 from 0.1 seconds to 40,000 seconds.

Ah, if "they" is you, then I have no more worries. Yes, using
that free-running 1250A is the perfect solution; much better
than using the output of a GPSDO.


> Now, an important point -- we're not trying to trim the Austron to be
> precisely on frequency.  We're going to let it run at whatever offset it
> happens to be.  That will help make sure that the signal doesn't have
> lots of zero's at the end, even though the resolution of the
> synthesizers driving the transmitters is limited to 0.1 Hz.

Clever.


> We'll be comparing the Austron against a Z3801a (via my TSC-5120A
> analyzer) and logging the frequency difference for at least several
> hours prior to the test until several hours following.  The TSC gives 16
> digits over 1000 seconds; depending on how much jitter we see, we'll
> probably throw away the last two or three.  Even though the Z3801A may
> be wandering around a bit, with successive 1000 second measurements we
> should have confidence in the actual frequency over 1000 second periods
> to at least parts in the 12s, ultimately limited by the Austron's
> stability.  But since that's known to be in the 13s over the averaging
> period of interest, we think we're safe in claiming accuracy and
> stability of parts in the 12s.

Yes, running the measurement for hours before and after is
the right thing to do. All sounds good.

Make sure not to get near any of the equipment. Free-running
oscillators are sensitive to vibration or shock. You've probably
heard the story of my best Sulzer oscillator making small phase
or jumps which I eventually correlated to when the kids flushed
the toilet down the hall.


> Tom, if I'm missing something in this analysis, I'm seriously open to
> education...

Nothing missing; you nailed it.

/tvb