What is the 20ns internal offset and how do I use it?
There is a 20ns internal offset. The front ends are offset by
this amount for improved Linearity and will allow use of the full dynamic
range of the TDC.
Most, if not all TDC's including ours, have start-up non-linearities.
If the model 7186 & 7187 were not to insert a pedestal, you would
find the first 20ns to be highly non-linear. Thus, in the fastest range
(100ns Full Scale) you would only be able to use the remaining 80ns.
We adjust the module to subtract 20ns from each START/STOP interval,
so that the full 100ns of dynamic range is usable. The same 20ns is
subtracted from all Full Scale Ranges (100ns, 200ns, 400ns, or 800ns). Thus
in the 100ns range, 20ns digitizes to 0 counts and 120ns digitizes to 4000
counts. Likewise, in the 200ns range, 20ns digitizes to 0 counts and 220ns
digitizes to 4000 counts (same for the 400ns & 800ns ranges).
If you want to see times down to 0, add a 20ns delay to the STOP line.
By using a 20ns cable, both the start and stop will be offset by
20 ns so your range will be 0ns to 100ns. This still compensates for
the first 20ns of non-linearity since the start is still offset by this amount.
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