[time-nuts] Cheap jitter measurements

Tom Van Baak tvb at LeapSecond.com
Wed Apr 25 07:01:10 EDT 2018


Hi Gary,

> A little coding later and there are nice plots.  They were compared to
> the output of tvb's adev.c program.  Results are similar.

Whoa there cowboy. That doesn't mean it's right. Comments:

> gps.png looks as expected.

1. No, it would appear something is wrong with your data. You show the ADEV for tau 1 s to be about 1e-2. That seems off by a factor of a million. It's probably not your reference, or your TICC, or the NEO-M8M, but something in how you are scaling the data? or glitches in your data? Hard to say without more info.

2. The artifacts at the end of the traces are weird. That's either too small a statistics sample or you have one or more bad data points that are ruining your data set.

3. It's best not to show ADEV or MDEV on the same plot as TDEV. They are different concepts and have different units. Also use ADEV as a label not OADEV.

4. At this stage of being a time-nut, it's best not make canned ADEV plots without first making phase and frequency plots. ADEV is just a statistic and you can feed it garbage and it will still happily compute numbers. By plotting phase (or phase residuals) and frequency (or relative frequency error) your eye can catch most of the bugs that occur to first-time ADEV'ers. Creating phase and frequency plots also makes you aware of the units, scale, and magnitude of your data, something you can use as a sanity check.

5. Since you are using batch tools instead of interactive tools, I suggest showing the first few and last few raw data lines from the TICC. Also the first and last few lines of the data that you input to your ADEV calculation. Or just post the raw data. This is helpful to debug bad plots like this.

> GPSTLXO.png shows the quality of the JL part,

1. Hmm, that just looks like some self-test data. I'm suspicious when I see 1e-17 MDEV numbers. Something's not right. The TICC is nice, but not that good.

2. Wait, you're using the JL 10 MHz as the TICC reference and then you're measuring the JL 1PPS with the TICC? So that's not really a measurement of the "quality of the JL part"; it's more just a self-test of a TICC channel.

3. Once you get correct-looking ADEV plots for NEO-M8M and JL then we can talk about what effect (if any) using Rb as a TICC reference will have. Remember that for time interval measurements the quality of the reference is not that important.


I strongly, pretty please, strongly advise you to use TimeLab for a while before you roll your own tools and plots. That is, "learn to drive before you design your own car". I know you have a grumpy old man aversion to using Windows, but lots of people on the list seem to run TimeLab easily on their non-Windows systems.

If nothing else, at least look at what TimeLab does. The PDF user manual is a superb tutorial on working with time & frequency data.

Anyway, a good start to your NTP measurement project. Thanks for posting.

/tvb


----- Original Message ----- 
From: "Gary E. Miller" <gem at rellim.com>
To: "time-nuts" <time-nuts at febo.com>
Sent: Tuesday, April 24, 2018 8:45 PM
Subject: Re: [time-nuts] Cheap jitter measurements


Time-nuts!

I went ahead and bought the TAPR-TICC, it is a very impressive
instrument.  For this setup it is combined with a Jackson Labs
GPSTLXO as the 10MHz reference.  The JL is a GPS disciplined temperature
compensated crystal oscillator.

The first setup uses the TAPR-TICC in Period mode, outputting the PPS
period individually for channel A and channel B.

Channel A is the PPS of a plain u-blox NEO-M8N.  Channel B is the
PPS of the JL GPSTLXO.

Simple to get the cycle times from the USB serial port.

Then I grabbed a copy of the easy to use Python Allantools.
    https://github.com/aewallin/allantools

A little coding later and there are nice plots.  They were compared to
the output of tvb's adev.c program.  Results are similar.

Results are attached.  gps.png is the plain NEO-M8N.  GPSTLXO is the
JL part.

gps.png looks as expected.  GPSTLXO.png shows the quality of the JL part,
but does have some odd divots in the plot.  Maybe artifacts of using the
PPS derived from the reference 10MHz?  Or an artifact of the 10e6 divider?

There are adev's of Rb standards here: http://www.ke5fx.com/rb.htm

My guess is that the oadev at 1s would be about 50x better with
a Rubidium?  But similar at 10k seconds?

Comments?

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com  Tel:+1 541 382 8588

    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin



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