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

[Bob kb8tq]

Hi

> On Mar 8, 2019, at 7:47 AM, Attila Kinali <attila at kinali.ch> wrote:
> 
> 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. 
> 

Putting some dimensions on this:

A “telecom” Rb should be in the 0.01 ppb at one second (ADEV) range. It should 
improve by sqrt(tau) so at 100 seconds it will be at 0.001 ppb.  It should improve 
some past that. Counting on 0.0001 ppb at 10,000 seconds …. not so much. 

A cheap / small OCXO may be at the same 0.01 ppb at 1 second level. An OCXO
does not have a “predictable” improvement rate. You may have the same at 100 
seconds as at 1 second. 

A very good (but maybe cheap … who knows …) OCXO could get you to 0.001 ppb
at 1 second. You also could buy a few dozen to get one that actually does that. Testing
could be involved to sort them out. 

Both the Rb and the OCXO have temperature stability specs on them. What those 
numbers are may be unknown on a surplus item. The “best case” claimed temperature
stability on a telecom Rb is often better than the “best case” stability on your cheap
OCXO.

Indeed there are (rare) OCXO’s out there that get down to 0.0001 ppb  at 1 second
and hang in there out to 1,000 seconds. Finding one at all can be tough. Finding one
cheap … you would have to be very lucky. Working out that you *do* have one will involve
testing. 

Bob



> 
> 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
> 
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