[time-nuts] DMTD and data display

[Tom Van Baak]

Hi Skip,

The input menu for both TimeLab and Stable32 allow you to set tau0 and 
also the phase scaling factor. Given your choice of frequencies your 
sample rate will be 10 Hz, so set tau to 0.1 s. And, yes, the scaling 
factor will be 5 MHz / 10 Hz or 5e5. Both tools expect the scaling to be 
a multiplier (not a divisor) and so use 1/5e5 which is 2e-6.

Now, a word of caution. Do not make a plot that blindly goes down to 
1e-17 or 1e-19 or 1e-20. That's likely wrong.

There are several ways to test your DMTD. One of best ways is to measure 
two oscillators for which you already know the ADEV. For example, if you 
happen to have two fine OCXO with ADEV (tau 1 s) of 3.0e-13 and 4.0e-13 
then the expected stability measurement will be 5e-13. [1]

If your DMTD reports 5e-13 then well done! On the other hand, if you get 
the "wrong answer" then there's a problem with your actual sample rate, 
or your scaling factor, or too much noise in your offset oscillator, or 
problems with your ZCD or TIC, or you have internal noise in your DMTD. 
If the latter then re-try the test using two oscillators with poorer 
stability. At some point you will find a pair of oscillators for which 
your DMTD results agree perfectly with known values, or values measured 
using other comparators or counters that you have lying around.

A temptation is to put the "same signal" into both inputs and see  how 
low the "noise floor" is. For several reasons this can lead to bogus 
conclusions. It would be like building a DIY voltmeter and testing its 
accuracy, stability, linearity, and resolution by how well it reports 
"0.0 volts" when you short the inputs.

So get some 1e-9, 1e-11, 1e-13 sources and see how well your DMTD 
reports their actual stability. Do some short-term runs (say, tau 0.1 s 
to 100 s) and mid-term runs (say, tau 100 s to tau 1 d). You may run 
into other interesting problems along the way so stay in touch.

In the end you should have a reliable instrument that's trusted at e-13 
levels. FYI: the timing of your posting is serendipitous since I'm going 
through the same process this month with a DMTD from Corby.

/tvb

[1] In other words the RMS sum, which in this example is the Pythagorean 
triple 3:4:5 so rms(3,4) = sqrt(3^2 + 4^2) = 5.


On 11/21/2020 1:44 PM, Skip Withrow wrote:
> Hello Time-Nuts,
>
> I have made significant progress on a DMTD instrument, and hope to be
> making measurements relatively soon.  However, I have a question on
> getting the correct results.
>
> I have a box with DUT(5MHz), OFFSET (4.999990 MHz), and REF (5MHz)
> inputs, with DUT and REF outputs that will feed my 5370B TIC.  My
> understanding is that I gain an increase in resolution of 5x10E6 Hz/10
> Hz = 5x10E5.  My question is how do I make this display correctly in
> TimeLab (or Stable32)?
>
> When I use just the 5370B and plot the noise floor (feeding a delayed
> version of the A input into the B input) I get a nice straight line
> with -1 slope starting at 1.15x10E-10 at 1sec and going down.With the
> DMTD connected does this mean that I just put in a scale factor of
> 5x10E5 in TimeLab (and also change the sample time to 0.1Hz)?
>
> Or, is there a better way to set things up?  Any insight appreciated.
>
> Regards,
> Skip Withrow
>