Even defining when the sand timer is "done" is not a real
simple thing. Waiting for that very last particle to drop may
not be the best approach.
Bob
Correct. Marking time with an hour glass is not that different
from marking time with a 1PPS. Each signal has a rise time;
one picks the appropriate live trigger level or sampled slope
waveform model to minimize jitter.
Waiting for the last grain is like waiting for the last millivolt of
a TTL 1PPS pulse; no one does that.
Note that [this] hourglass interval has a standard deviation on
the order of 10 seconds. So my initial goal is 1 second timing
resolution, which turns out to be pretty easy to do optically.
Whether sand or cesium, phase comparators have a minimum
resolution. While lower resolution is better, if the ADEV of the
DUT is too far above the ADEV of the comparator then that
resolution is wasted. So detection to a granularity of 1 second
is sufficient for this application.
/tvb
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