[time-nuts] Housing LPRO and Thunderbolt together

WB6BNQ wb6bnq at cox.net
Fri Apr 22 21:37:05 UTC 2011


Hi Pete,

As Bob, K6RTM, pointed out the Thunderbolt and the Rubidium are two different
animals all together.

True, you can treat a Rubidium like it was a normal crystal oscillator, but it is
not the same.  The Rubidium has a definite life span, the more you run it the less
the life.  A high quality crystal oscillator, on the other hand, just gets better
the longer you leave it on.  Aside from nominal electrical component failures, the
crystal blank in a properly designed circuit has no short term failure mechanism and
will last for decades with constant applied power.  The Rubidium’s life span is, at
best, 10 years.  The question is how long was it running before you got it ?

A high quality crystal oscillator has excellent short term specs but does have drift
and aging functions that severely limit its use for long term purposes.  Long term
meaning more than a few hours for the best.  That is where the Rubidium oscillator
takes over as its drift function is measured in days to a month or more.

For high quality measurements, the crystal excels for measurement times of less than
10 seconds, as the Rubidium is noisier in that time frame.  That is, for taking
readings on a one second to second basis, such as with a high resolution time
interval counter, the crystal excels.  However, if the period of the measurement is
longer, then the Rubidium would be a better choice.  For portable purposes the
Rubidium also excels as its retrace is much better than a crystal oscillator.  You
also do not need to wait the thirty to sixty days for the crystal to stabilize.  The
Rubidium will be very close to its original set point in about 20 minutes.

Adding GPS to mix has its own issues.  First, you need to know the coordinates
precisely or spend a couple of days getting a damn good fix.  The GPS is quite noisy
in the short term and the oscillator that is steered by the GPS has that noise show
up in its output.  That is mitigated by having a high quality crystal oscillator
where the GPS control loop seldom makes corrections; perhaps once an hour or more.
That is how the Thunderbolt works and depending upon its internal crystal
oscillator, it may possibly be tweaked to perform better then the standard factory
settings.

As for use, it all depends upon what and how you’re making measurements.  With a
nominal 8 or 9 digit counter, for example, you may not notice all of the above
issues because they are typically beyond the resolution of the equipment in most
cases.  In other measurement processes it may be of major concern.

As for your project boxes, I would use the rack mounted box to house the
Thunderbolt, distribution amps and perhaps a couple of other oscillators (like the
hp 10811) along with quality power sources.  Because crystal oscillators like a
constant operating condition, do consider battery power for the lab to handle those
occasional mains power drops.

I would use the portable box for the Rubidium oscillator and include a battery
option depending upon your intent.  The emphasis should be to have very quiet and
stable power supplies for both projects.  Even batteries have a fair amount of noise
so make the mains power (and battery) voltage high enough to allow for running a
quality regulation circuit.

My two cents !

73....Bill....WB6BNQ


g4gjl at btopenworld.com wrote:

> I have a dilemma and wish to access the collective wisdom of the group to advise
> a solution.
>
> I am building a clock generator based on a Thunderbolt. I have an LPRO and would
> also locate this in the same enclosure. I will also add a distribution amp and a
> divide chain in due course.
>
> The ultimate purpose of the set up is to provide a self contained clock
> generator set for my other test equipment, and also an experimental workstation
> for Rubidium and GPS disciplined experiments.
>
> Most of my other equipment is for 19-inch rack mounting.
>
> I have two potential solutions for housing the timing kit:
>
> 1. An old dismantled HP 4U scope chassis which will fit in with my other
> equipment physically, and can be racked if necessary. The PSU would have to be
> built into the same enclosure.
>
> 2. A pair of Anritsu instrument cases which once house a bit error test set. The
> two units clip together beautifully, and are free standing. As there are two
> units, this solution would allow me to build the PSUs in one case and the more
> sensitive timing electronics in the other. These units cannot be racked on
> account of their form factors.
>
> Both solutions will require me to do some bespoke metalwork, but that is no
> problem for me and amounts to about the same amount of work for either solution.
>
> So what does the group advise? Is it vitally important to keep PSU components
> isolated from the timing electronics? I want to create the least noisy clock
> source given the components I have.
>
> Looking forward to hearing some opinions...
>
> Pete
> G4GJL
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