[time-nuts] Help with my ADEV measurement setup

[Dan Watson]

Oddly enough I do have a PPS output from the TCXO I was measuring. It's on
a little board I made and there is a PicDiv right on it. I'll have to play
around with that.

I did notice the aliasing issue trying to measure a 12MHz crystal. It
appeared to have incredible stability and accuracy for a plain old XO...
Also it has a frequency offset of over 1kHz, and I noticed that I had to
manually type in the correct initial frequency during setup to have
meaningful data in the frequency difference view. i.e, 12001053 instead of
12E6. But of course with a marginally stably oscillator, that poses a
problem. How long do I wait to find a mean frequency to type in...? It
makes total sense why this is so in TI mode, but still it's one more thing
to deal with.

I think I'll stick with frequency mode for most things. Many of the
oscillators I want to measure are right around 10^-8 or 10^-9, and I'd hate
to constantly be fighting the noise floor of the instrument. I'll treat the
data from frequency mode as relative and that should get me what I need. At
least until I own a better instrument.


Thanks

Dan

On Tue, Jun 30, 2015 at 10:49 PM, John Miles <john at miles.io> wrote:

> Yep, it will ignore any non-numeric data like "us" suffixes.  It will
> always interpret incoming data as seconds, so the 1E-6 scale factor is
> appropriate if the counter is returning microseconds.  I'll tweak the
> mouseover help text for the scale factor field to clarify that.
>
> I think you're basically getting valid data.  The 53131A's one-shot
> resolution is 0.5 ns, and you're seeing about 2E-9 residual ADEV at t=1s.
> It's in the right ballpark, anyway... e.g., on a 20-ps HP 5370, the
> residual ADEV at t=1s will usually be in the neighborhood of 30-60 ps.
>
> I would, however, be worried about aliasing with a TCXO.  If its frequency
> is more than 5E-8 off -- meaning it drifts more than 50 nanoseconds per
> second with 10 MHz at the STOP jack --  its error will end up
> underrepresented in the measurement.  In this case your oscillator is
> drifting quite a bit (as expected) -- look at the 'w' view of the original
> phase compared to the unwrapped 'p' phase.  You could try putting 1pps on
> both the START and STOP jacks but that'll require more futzing with scale
> factors, 1pps dividers and the like, and may leave you more vulnerable to
> trigger uncertainty from various causes.  For measurement of a TCXO, I'd
> stick with frequency mode.  The ADEV plots won't be 100% kosher but they'll
> be fine for relative comparisons with other plots from the same measurement
> setup.
>
> -- john, KE5FX
> Miles Design LLC
>
>
> > -----Original Message-----
> > From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Dan
> > Watson
> > Sent: Tuesday, June 30, 2015 5:05 PM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] Help with my ADEV measurement setup
> >
> > I tried both the PPS and the 10MHz signal on channel 1, with the 10MHz
> DUT
> > on channel 2. Tom emailed me and it turns out the software was not
> > detecting the units correctly from the serial string. (What are 6 powers
> of
> > ten between friends, right? :) ) Likely a settings mistake on my part, I
> > sent him a screen cap to see what's up.
> >
> > None the less, I manually enter the time units and was able to plot some
> > data. I attached a new screen cap. How does this look?
> >
> > Thanks
> >
> > Dan
> >
> > On Tue, Jun 30, 2015 at 5:36 PM, Bob Camp <kb8tq at n1k.org> wrote:
> >
> > > Hi
> > >
> > > Are you using a PPS as the “start” and the 10 MHz as the “stop” or
> > > comparing two PPS signals?
> > >
> > > Bob
> > >
> > > > On Jun 30, 2015, at 2:47 PM, Dan Watson <watsondaniel3 at gmail.com>
> > wrote:
> > > >
> > > > Hi,
> > > >
> > > > I'm trying to take some ADEV measurements with my 53131A, and I'm
> > having
> > > > some issues. This is my setup:
> > > >
> > > > - 53131A with the OCXO option. Calibrated against a T-bolt. I also
> did
> > > the
> > > > TI Quik cal and it passed
> > > > - I'm using RS-232 out with a null modem cable into a Serial-USB
> > > converter
> > > > - Software is TimeLab in talk-only mode. 53131A check box is checked.
> > > > - The counter is in TI mode, with a T-bolt on Channel 1 and DUT on
> > > Channel 2
> > > > - A delay of 1 second is set on the counter. TimeLab seems to
> accurately
> > > > detect this interval
> > > >
> > > > I started measuring various devices, and could never seem to get
> better
> > > > than around 1x10^-6. Even my Rb was showing a 1 second ADEV of 10^-6.
> > > > Finally I put the T-bolt on channel 1 with common mode on to both
> > > channels,
> > > > and it still measures around 10^-6. A picture of that is attached.
> Surely
> > > > this can't be right.
> > > >
> > > > I tried frequency mode and it gives ADEVs of 10^-12 on the Rb and
> T-bolt,
> > > > as expected. I understand the issues with filtering that the 53131A
> does
> > > > internally on this mode, but at least it shows my setup is working to
> > > some
> > > > degree. It's TI mode that seems to be wonky.
> > > >
> > > > I'm probably doing something really stupid.
> > > >
> > > > Thanks for any help you all can suggest.
> > > >
> > > > Regards,
> > > >
> > > > Dan W
> > > > <adevtest.png>_______________________________________________
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