Originally posted by Tinkerer
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I think we have to keep in mind the di/dt of it all. The current in the target I think should be driven the di/dt of the current in the TX coil. The current in our MD RX coil will be driven by the di/dt of the current in the target. We have double differentiation here the way I see it.
On the sawtooth, when we look at the eddy currents increasing during TX ON, we see that the 15us target eddy currents increase until about 50us, or roughly 3TC, then they start decaying, in spite of the current ramp still raising nearly linear. Does this suggest that the target is "saturated"?
However, the slope of the sawtooth is actually decreasing at a slow rate.
The reason the fast target starts decreasing when it does is because it is tracking the slope of the TX current much faster (than the slow target) and, although it does not completely "saturate" (of course it takes infinite time to completely saturate) to the initial slope of the TX current, it actually grows higher than for the slope of the TX current later on (sorry for that head-scratcher

(Use photoshop and draw a line from the start to the end of the sawtooth ramp and you can see how it is decreasing in slope.)
If the TX current was a perfect linear ramp, even the fast target should be still increasing and approaching a constant level, and naturally way ahead of the slow target.
Pretty sure I got that right...

Could it be that it makes a difference if the eddy currents are still raising at switch OFF, or that they are already decaying?
Looking at the graphs, I think it really is more a matter of how big a jump and how fast the target current changes on its wild ride, which wouldn't seem to depend much on the direction the its eddy currents are flowing, more on the level they are currently at if anything (although it doesn't seem to depend very strongly on that either).
We really need to add the RX signal response to these simulations and look at that, not just the target signal.
-SB
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