Hi > On Nov 22, 2020, at 2:40 AM, Tom Van Baak <[email protected]> wrote: > > 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.
If you put a chunk of cable into one leg of the drive circuit, you will get a much more rational measure of the noise floor on a DMTD. You want to “explore” the region from about 15 degrees of phase shift out to 180 degrees. ( If using a time tagger like the TICC ). The reason is fairly simple. The Reference oscillator cancels out very will at “zero degrees”. As you go to longer and longer delays ( this ultimately is a 10 Hz phase shift ) the correlation / dropout degrades. Yes, there are other issues, but this at least gets you into the ballpark Bob > > 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 >> > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
