[time-nuts] Disceplining a Rubidium with a Thunderbolt
EWKehren at aol.com
EWKehren at aol.com
Wed Jul 13 13:19:05 UTC 2011
Bill
I agree with what you say. I am a fan of Brook Shera's loop and have
used it with a Rb and between a Rb and a D 1000. I did modify the input to
"fool" it to get smaller steps and the only problem is the AD 1861.
Availability is not the problem, they are readily available at a price much less than
the original catalog price.
The problem is its performance. It was never intended for what we use it,
but it was perfect for us at that time and it has no DNL specification.
Richard Mc Corkle has run some tests and DNL is 8 bits which is not good
for any digital loop let alone a loop with very long time constant. A 16 bit
LTC 1655 would be perfect for Rb's with resolution, tuning range and aging
over 10 years. I like to use my D 1000 as clean up for particular tests and
since those tests are time limited again the 1655 would be perfect. I
whish Brooke would consider a software change or authorize some one to do so.
I am now a TB fan and use it as a frequency source for all my equipment and
even as a 1 PPS GPS receiver. It will take some time but I plan to
compare TB with the best affordable GPS receiver controlling a Rb when I find a
way to get around the AD 1861 or some one else's loop.
In the mean time I am looking forward to some one publishing test results
for a Rb TB combination. But without a clean up OCXO I for one do not
consider it an improvement as you pointed out.
Bert Kehren
Hi David,
Just to clear the AIR, all Rubidium frequency standards have a crystal
oscillator as
the primary signal source within the Rubidium device. The Rubidium
portion of the
standard is just a very high Q filter whose properties can be controlled
such that
it's filter's center frequency has extremely small drift. That small
drift factor is,
typically, way less then the resulting factors that control drift in a
Quartz
resonator.
In order to gain the properties of the Rubidium's longer term stability
and the short
term noise properties of a very good Quartz oscillator you would need both
items. You
select a very good Quartz device and phase lock it to a really good
Rubidium (with its
own Quartz oscillator). You would adjust the loop constants to correct at
a very slow
pace consistent with the quality of the very good Quartz oscillator.
To get to the next level (connection to the Nation's reference), you would
discipline
the Rubidium against a GPS device with an even slower loop. So, in the
end you have
two separate loops with three separate devices. This is not your "Nickel
& Dime
store" plug-and-play set up. It would have to be set up with care and
some
experimentation to get it right.
For a reference on the basic process, you should read the QST article on
Brooke
Shera's GPS disciplined oscillator system. Contained within it is a
description of
the loop process I referred to above. To utilize his method would require
upgrading
his circuit design (some parts not available any longer) and some software
upgrading
as well to account for those changes.
To obtain the QST article go to Shera's web site at
http://www.rt66.com/~shera/
Also click on the "more information line" for further reading.
Bill....WB6BNQ
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