[time-nuts] Help with my ADEV measurement setup

John Miles john at miles.io
Tue Jun 30 22:49:06 EDT 2015


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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