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

[Bob kb8tq]

Hi

Some of this gets into just what you are trying to do. Cycle to cycle jitter on the 
PPS output of most GPS modules is in the “many ns” range. If sawtooth correction 
is not being applied, a measurement that is good to an accuracy of 1 ns / 5 = 200 ps
would be plenty good enough. That is a 2x10^-10 (0.2 ppb) sort of accuracy. 

If you are looking at longer term effects, you may need better accuracy / lower drift. 
You can easily get past what a nice new 5071 can do heading down this road … For 
now let’s assume the target above is correct. 

The PRS-10 should be within 0.2 ppb within a few minutes after locking up. It should 
hold this sort of accuracy “on the shelf / power off” pretty much forever and ever. In fact, 
it’s a pretty good bet that it will hold 10X this accuracy for quite a long time. You probably
should warm it up for a day or so if you are after 0.02 ppb. 

Calibration wise, the “easy” way to do it depends a lot on what you have to do the measurement.
If you have a TICC (sounds like you do) - use it. You can also use a scope or a more 
conventional counter. 

If you are going after 0.02 ppb, you would want to set the unit to 0.002 ppb. That gives you
90% of the budget for things like temperature drift and aging. Indeed you might want to go 
to a tighter number …. this is Time Nuts after all. 

0.002 ppb is 2 ns over 1,000 seconds. It’s 200 ns over 100,000 seconds. (Amazing how that 
works :) ). Is your TBolt PPS good to 2 ns? Probably not. Is it good to 200 ns? most certainly. 
Since 100,000 seconds is just over a day, that makes for a fairly easy adjustment process. 
Look at it once a day and tweak it. After a while it will not need tweaks any more. 

The advantage of a “long term” adjustment like this is that it also takes in the temperature 
swings in your lab over a one day period. At the 0.002 ppb level, they will be the dominant 
part of what you see. Will you get to the 0.002 ppb level? It depends a lot on just how stable
your particular unit is and how drafty your lab is. 

Running any sort of electronics gear 24 hours a day is a risk. It may wear out, it may cause
a fire, it may be fine … who knows. It also pulls power and you pay the electric company for that.
It’s generally cheaper / safer to turn stuff off if it is only going to be used rarely. 

Lots of fun !!!

Bob

> On Mar 8, 2019, at 3:18 AM, Forrest Christian (List Account) <lists at packetflux.com> wrote:
> 
> Hopefully you'll all grace me with a few answers to a beginner
> time-nut question or two.
> 
> I have a PRS-10 I've never used other than to power it on with a
> recently-acquired heatsink and verify that it seems to operate
> correctly and that the operational parameters don't seem out of
> tolerance.   I would like to use this in the near future as a 10Mhz
> reference for a TAPR TICC which I'd like to use to measure the jitter
> performance of the PPS output of various consumer GPS receivers, the
> goal being to end up with a jitter histogram.
> 
> So three interrelated questions:
> 
> 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.
> 
> 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.
> 
> 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?
> 
> -- 
> - Forrest
> 
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