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

Sat Mar 9 07:04:15 UTC 2019

This seems like as good of a message to reply to as any.   Thanks to
everyone who has replied so far -  lots of good information, which is
right in the category of what I was looking for.

Let me clarify what I'm hoping to measure.   I apologize if some of
the terminology below is wrong, I'm still learning correct use of some
of the terms - I've tried to explain in a way where it is obvious what
I'm talking about even if the terms are slightly wrong.

The GPS receivers I'm hoping to measure are used in an application
where the 1PPS is used to synchronize radio transmissions.  Generally
one wants the pulse to be within a couple uS or so of the top of the
second.    Older radios cared less, newer ones want it to be more
precise.   Sorry about the soft spec, it unfortunately is what it is.
  One note is that there is some expectation that there will be be
some fixed offset from the UTC second due to cable delays and the
like, so I'm less worried about a reasonably low fixed offset  and
more about the variable (jitter) offset.    I.E. 1uS +-100nS is
perfectly fine, 1uS +-1uS isn't so hot.

In evaluating a new set of GPS receivers for this application, I'm
hoping to gather Pulse-to-Pulse delay figures for every pulse the GPS
receiver outputs over a week or so.   I might end up repeating it in
various forms, including moving the GPS receiver into an
"environmental chamber" (aka modified chest freezer) to verify that
they still behave in low or high temperature - or if I can figure out
a reliable way to do so, to simulate various signal impairments such
as partial sky view.

My intent is to then process the raw data to get a feeling for the
statistical nature of the 1PPS out of that particular GPS receiver.
For instance, how many pulses were outside the +-ns spec for the
receiver.  What did the histogram of the in-spec pulses look like?
And so on. For some of the processing it's pretty obvious that I need
a clock source for the TICC which is going to be stable over the
entire period or else I'll end up measuring errors in the clock source
instead of the GPS receiver.    It seems to me that if I'm looking for
a histogram supposedly confined to +-100nS, I'm probably going to want
to be able to measure around +-1nS with confidence.   Much more than
that (0.1nS) seems a bit excessive, less than that (10nS) is too
coarse.

Intuition and past experience implies that if I want to measure +-1nS
with confidence, I really need a slightly more accurate clock source
than the measurement itself.  0.1ppb over the measurement period seems
reasonable, 0.2ppb is probably good enough.  I don't think I need
0.02ppb.

I replied in particular to Bob's message below since it seems to
confirm what I was thinking, even without me being verbose enough
about my application.  It also sounds like powering this on for a day
or so before I start (or getting started and just throwing the first
day's data away) might be reasonable to do and I don't need to let
this thing run for months ahead of time to get to the accuracy I need.
   Of course, I'll do a bit of analysis on the data to try to verify
that the unit isn't drifting.

Out of curiosity, I'm also going to do some measurements on the couple
of standalone OCXO's in my collection to see if either would be
accurate enough for this particular measurement.  The best one in my
(admittedly limited) collection seems to be spec'd worse than I'd
need, although I'm not sure how much aging it's done.  None of them
have seen any meaningful poweron time just because I've been too busy
to play with them.  I guess that at some point I should also up my
game as far as good quality OCXO's since I do see how the right
well-aged low-drift OCXO would probably be fine for this measurement.

After reading everyone's response I'm still considering what to do as
far as long-term runtime on the PRS10.   I do know that I'm going to
need this off and on for the next few months so once I get it in a
permanent enclosure and get it powered up it will probably stay on
continuously at least until I am done with this round of experiments,
at which point it might get turned off with the hopes of extending
it's service life.

On Fri, Mar 8, 2019 at 8:06 AM Bob kb8tq <kb8tq at n1k.org> wrote:
>
> 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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>
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-- 
- Forrest