Hi As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something.
Bob On Nov 5, 2012, at 5:42 AM, Bill Dailey <[email protected]> wrote: > Thanks guys, > > Like usual more complicated than I thought. I was hoping that this would > cancel any stability issues common to both the reference and the signal > thus giving me best case ability. I seem to be getting numbers too good to > be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly > linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there > was some kind of gotcha. > > Doc > > > On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson <[email protected] >> wrote: > >> On 11/05/2012 06:30 AM, Tom Van Baak wrote: >> >>> Ah, I see what you mean now. Yes, that setup can give you a rough >>> estimate of the counter's noise floor. >>> >>> I can't give you specific numbers but one danger with this sort of test >>> is that the input and the timebase are artificially locked together (i.e. >>> fixed phase relationship) through the common reference. Your measurements >>> may thus show artificially less noise than a real-life case of independent >>> input(s) and reference. >>> >>> This can happen if your sub-ns counter is based on interpolators. Because >>> the input and the timebase are locked in phase, the counter lands near the >>> same point of the interpolator scale on every single measurement, rather >>> than experiencing the noise (and non-linearity) of the entire scale. >>> >> >> It's a little more complex than interpolator non-linearities alone. You >> also need to include cross-talk between the signals. This cross-talk is >> usually higher between A and B inputs than from reference, but never the >> less. >> >> You would need to sweep the trigger input delays to illustrate these >> non-linearities. From a single measurement you can get both a better or >> worse number compared to the average which is what you would expect to see >> for free-running signals. >> >> So, you can get a rough idea about the baseline, but it is not a >> sufficient method. >> >> See the SR620 manual for a plot of non-linearities. >> >> Cheers, >> Magnus >> >> >> ______________________________**_________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/** >> mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> >> and follow the instructions there. >> > > > > -- > Doc > > Bill Dailey > KXØO > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
