[time-nuts] TIC resolution impact on GPSDO's performance

Ulrich Bangert df6jb at ulrich-bangert.de
Mon Dec 25 08:34:34 EST 2006


Hi Tom,

> 1. Could you clarify for us what points in your plot
> are real measured data and what points, if any, are
> simulated?

the red line is a simulated computed noise due to the 41.6 ns
quantization errors. The HP10811 plot is made up of two parts. One for
taus up to a few 100 s where I measured it against my FTS1200 having
significantly lower ADEV in that range. A second part for longer taus as
measured inside the open loop of my GPSDO. i.e. against sawtooth
corrected pps. The two gps lines are from a closed loop measurement. 

> 2. Have you looked into why your Comparison.pdf
> plot doesn't have the same look as SigmaTauBoth 
> (photo_gallery_44.html)?

I noticed that too and I have no explanation for that. Since I had the
raw data that lead to photo_gallery_44.html no more at hand I had to use
different raw data to construct the pdf in my posting. 

> But the horizon point (where the lines meet) for the 
> SigmaTauBoth plot is around tau 1 day while the horizon point 
> for Comparison.pdf, as best I can tell, is around tau 10^8 
> seconds (about 1000 days).

I would like this problem to get cleared for myself too. I am going to
start some new measurements. While it is a problem worth to discuss: If
it were like you say would that not backup my argumentation because it
would draw the yellow line even a bit lower due to the changed slope and
so increase the distance to the red line? 

> 3. For the sake of contrast, and to make your plot
> even better, would you be able to add another OCXO
> or two?

Yesterday i spend some hours into improving my Plotter utility to make
that possible. Otherwise I would not have been able to generate the plot
of my posting. If you want to send me data then 

a) send me raw phase data so that I compute the statistics on my own

or 

b) if you send me statistical data give them a look similar to

Tau			      Overlapping Allan Deviation
1.00000000000000E+00	1.18508991343051E-09
2.00000000000000E+00	1.00418275497165E-09
4.00000000000000E+00	4.94116108301216E-10
8.00000000000000E+00	2.54210080584921E-10
1.60000000000000E+01	1.32077950356112E-10
3.20000000000000E+01	7.12770070677926E-11
6.40000000000000E+01	4.01265021949822E-11
1.28000000000000E+02	2.40645383337377E-11
2.56000000000000E+02	1.24198310130885E-11
5.12000000000000E+02	6.61924079416740E-12
1.02400000000000E+03	3.39639264963695E-12
2.04800000000000E+03	1.60459982080177E-12
4.09600000000000E+03	8.34317107123469E-13
8.19200000000000E+03	4.23896023761579E-13
1.63840000000000E+04	2.18096321206837E-13
3.27680000000000E+04	1.00382297277920E-13
6.55360000000000E+04	5.27984014158743E-14

The headerline is needed. Any number of columns is allowed. The number
format need not exactly be that way.

> 4. I'm wondering if you could conclude something
> interesting if your plot separated the OCXO issue,
> from the GPS engine issue, from the TIC issue
> rather than lumping the GPS and TIC together into
> one line.

While you are correct that I lump GPS and TIC into one line, we all must
live with that. Because once you make a measurement you have already
lumped the TIC resolution into your data. The only thing that we can do
about is to make the TIC resolution sufficiently small. My GPS data is
measured with 110 ps resolution. The left part of the hp10811 plot has
been measured with 1e-13 @ 1 s resolution.

> For example, HP 53131 (500 ps) vs. 53132 (150 ps)
> vs. 5334 (2 ns) vs. 5370 and SRS 620 (25 ps). You
> can also add Shera (42?ns) and Jackson (8?ns).

As long as the TIC resolution is small against the jitter of the signal
to be measured we will hardly see anything of its influence in an
(lumped) Allan plot. As long as GPS is concerned (even sawtooth
corrected) this is the reason why we live so well with our HP5370s, our
HP53131/2s and our SR620s with the HP53131 being a bit on the boarder.
The 5334's 2 ns reolution and anything that will clearly document itself
in the Allan plots. As for the Shera design: The red line IS an a bit
too optimictic estimation for this case.

I am a bit disappointed that i seem to have failed to make clear the
true tenor of my posting: That the TIC resolution kind of 'modulates'
the measurement results and that this modulation can become the dominant
one depending on the signal's properties and the modulation amplitude.

Best regards
Ulrich Bangert, DF6JB


> -----Urspr√ľngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Van Baak
> Gesendet: Montag, 25. Dezember 2006 00:03
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] TIC resolution impact on GPSDO's performance
> 
> 
> Hi Ulrich,
> 
> Thanks for your note on TIC resolution and the very
> nice plot you attached. Here are some comments:
> 
> 1. Could you clarify for us what points in your plot
> are real measured data and what points, if any, are
> simulated?
> 
> 2. Have you looked into why your Comparison.pdf
> plot doesn't have the same look as SigmaTauBoth 
> (photo_gallery_44.html)?
> 
> Do both represent the same thing (M12/sawtooth
> vs. M12/corrected) or did I misunderstand one of
> them. Because they do seem to have a different
> look.
> 
> The red and black lines are parallel while black
> and yellow appear to merge (like chopsticks).
> 
> But the horizon point (where the lines meet) for the 
> SigmaTauBoth plot is around tau 1 day while the horizon point 
> for Comparison.pdf, as best I can tell, is around tau 10^8 
> seconds (about 1000 days).
> 
> 3. For the sake of contrast, and to make your plot
> even better, would you be able to add another OCXO
> or two?
> 
> For example, perhaps add one that performs closer
> to the 10811 spec (e.g. drift rate of 5e-10/day) and
> one that is a lesser grade, an Ovenaire or CTS class
> of OCXO or TCXO with short-term stability in the -10's
> and drift rate in the -9's. I can provide the data if you
> need it.
> 
> Perhaps other readers can suggest their own OCXO
> or Rb.
> 
> 4. I'm wondering if you could conclude something
> interesting if your plot separated the OCXO issue,
> from the GPS engine issue, from the TIC issue
> rather than lumping the GPS and TIC together into
> one line.
> 
> For example, in addition to different examples of
> OCXO (low, medium, and high-performance) and
> different GPS engines (e.g., Oncore VP, M12 with
> sawtooth, M12/corrected), do you think you could
> you also add different TIC resolutions?
> 
> For example, HP 53131 (500 ps) vs. 53132 (150 ps)
> vs. 5334 (2 ns) vs. 5370 and SRS 620 (25 ps). You
> can also add Shera (42?ns) and Jackson (8?ns).
> 
> Perhaps other readers can suggest different ones.
> 
> I'll have a few more comments later.
> 
> /tvb
> 
> 
> 
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