[time-nuts] Time-Nutters-- Adding Rubidium to a Thunderbolt...?

[Skip Withrow]

Hello Mike,
Replacing the OCXO in a Thunderbolt (or one of the many telecom
cousins) is very easy to do.  Just yank out the OCXO, hook the
Thunderbolt EFC to the Rubidium EFC (the Thunderbolt range is 0-6V),
and hook the Rb 10MHz where the OCXO 10MHz went.

That is the easy part.  Understanding how to tune the Thunderbolt, and
what kind of performance to expect is a little harder.  Rubidiums need
a much longer time constant and the Thunderbolt only goes to 1000s.
This can be circumvented by doing some math and 'fudging' the
Thunderbolt gain and damping numbers.  There was a discussion on how
to do this in time-nuts many years ago.  Also, a word of warning, some
of the telecom cousins do strange things when this technique is used
(and some don't).

The other thing to consider is the Thunderbolt DAC and Rb resolution.
A PRS-10 takes the EFC and runs it into an A/D such that the minimum
step in frequency is about 1x10E-12.  You will never get an ADEV below
this value.  Many of the other rubidiums (X72, SA22c, FE5680A,
FE5650A) use DDS in their loops and have similar resolutions.  The
LPRO is one unit that is around has an analog EFC.

The Thunderbolt DAC steps in 10uV steps IIRC.  And it steps its DAC
voltage every second.  With GPS signals jumping around you can still
(will) end up with poorer short term performance than if the Rb was
left to its own devices.  Hint, let the Thunderbolt do its thing with
the Rb, but use Lady Heather and disable disciplining when making
measurements/comparisons.

Regards,
Skip Withrow