[time-nuts] PRS-10 Warm-up Time, Calibrating/Adjusting, and long-term poweron

[Attila Kinali]

On Fri, 8 Mar 2019 01:18:45 -0700
"Forrest Christian (List Account)" <lists at packetflux.com> wrote:

> 1) Assuming the PRS-10 has been off for a long time, how long should I
> plan on leaving this on for the 10Mhz to stabilize?   I see the
> longest warmup time on the spec-sheet is 7 minutes -  although this
> seems a lot shorter than I'd likely use in real life,  I'm also not
> sure if there's much benefit to an excessively longer warmup time
> (like days), would like opinions on this.

The warm-up time is from "I switched it on" to "there is a 10MHz signal
and it's 'stable'". I.e. at latest after 7 minutes you should be able
to use the ouptut of the PRS10. But keep in mind that the stable-after-warm-up
is not tha same as stable-for-a-time-nut. A Rb vapor cell standard has
a retrace (ie change of frequency after being switched on, for details see
John Vig's tutorial) of several weeks to a few months, depending on the
exact construction of the Rb cell, how long it has been in use and how
long it has been off.

 
> 2)  Longer-term I'd like to use the 1PPS output from a Trimble
> Thunderbolt to calibrate the PRS10 and adjust if necessary just to
> trim out any aging drift on the PRS10.  Initially I thought I was
> going to discipline the PRS10 on a continual basis with the
> Thunderbolt using the PPS input on the PRS10, but I've recently
> realized that leaving the PRS10 on permanently might not be the best
> option (see Question 3).   So I'm looking for opinions on how to keep
> the PRS10 calibrated/adjusted.  I.E. trim with the trimmer, adjust
> using digital commands, etc.

Switching it on and off several times a month would be worse in
terms of longevity  than having it running continuously. The vapor
cell itself is usually run between 60°C and 100°C and the Rb lamp
usually between 120°C and 170°C. As you can imagine, power cycling
gives a huge thermal strain on all the components. The inner construction
of the PRS10 is not optimized for power cycling but thermal insulation
of the sensitive components (see e.g. http://time.kinali.ch/Rb/PRS10/ )

So I would rather feed the PPS to the PRS10 directly and let itself
do the work. This will also give you an optimized control loop for
the performance of the PRS10 without you having to go through the
process of desiging and optimizing the loop.


> 3) As implied in #2, I was originally planning on leaving the PRS10 on
> a continuous basis.   I've read a couple of things which imply that
> there is little benefit to doing so, and that every hour it's on
> consumes the lamp life.   Assuming I only need the highly stable PRS10
> source every few months for things like jitter measurements on 1PPS
> sources, is there any benefit to leaving the PRS10 on?

If you want to do just jitter measurement, then I wouldn't use a Rb standard
but a stable OCXO instead. At averaging times below 1-10s an OCXO will beat
any Rb standard, even one that has an OCXO like the PRS10 (admittedly, it's
not a high quality OCXO but it's one none-the-less). An OCXO has a shorter
warm-up time (usually 2-3 minutes until stable, usually less than a day
until time-nuts-stable, less than a month until nutty-time-nut-stable).

I personally, would use an OCXO as reference to the TICC (or TICCs?)
and use the PRS10 as an additional input to the TICC. This way you
can do a multi-way comparison between the GPSDOs and the Rb, removing
out the (in-)stability of the reference OCXO. 


Beside those comments, you have not told us at what kind of performance
you are aiming at. There is a huge difference whether you need 1e-10 or
1e-13 and whether you need it at 1s, 1ks or even 100ks averaging times.


				Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neal Stephenson