Geometrics MetalMapper 2x2 Operation Manual page 63

We acquire data during the positive decay and again during the negative decay. The entire
measurement spans two cycles of the primary background frequency of concern (the one that
needs to be canceled).
By inspection, one can see that the background contribution during the negative decay will
exactly cancel that during the positive decay, thus we see just the decay curve we're interested
in and not the background.
The data we report is simply the difference between that measured during the positive decay
and the negative decay. Thus the amplitude of the reported decay will be 2x the decay in a
single cycle.
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