[time-nuts] Time interval counters and low S/N signals.
phk at phk.freebsd.dk
Fri May 20 11:30:08 EDT 2005
In message <428DDE92.2010407 at medphys.ucl.ac.uk>, "Dr. David Kirkby" writes:
>Anyone ever tried this sort of thing? Comments?
Amplitude noise translates directly to time jitter in time-interval
There are two sources of jitter: zero-crossing slope and false
zero-crossings. As your S/N decreases, the risk of false zero-crossings
zero-crossing slope results in gaussian time jitter and averages
out well if you can average enough samples.
false zero-crossings are usually stratified on the noise-spectrum
and may or may not average out at all. In this case you need to
do a histogram and apply relevant statistical tools depending on
If you run the counter in 50Ohm mode and with decent voltages, you
don't have to worry about charge-threshold issues, which is a very
hairy ball of vax to get into. (At low signal levels or high-impedance
mode the trigger point is not only a voltage but also a certain
amount of charge to fill the input capacitance).
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