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
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