Optimizing Target Responses

[Carl-NC]

Yes, add a parallel RL circuit and loosely couple the L to the TX coil. Also couple to the RX coil, if you have one. One side of the target RL must be grounded.
This only works for eddy/non-ferrous targets, not magnetic/ferrous targets. Set the RL tau for whatever target tau you want.

[Repwoc]

Great, thanks - I'll give that a go.

[Repwoc]

Actually, in my circuit the TX coil is connected to the +ve rail (N-channel switch). Should the target RL still be connected to ground?

[Carl-NC]

Short answer: yes.
Long answer: Spice requires that every circuit node has resistive path to ground. You could actually connect one node of the RL loop to any other circuit node (including any voltage rail) and that will work. I use ground so that I if I want to plot the RL response it is already ground-referred.

[Repwoc]

Connected to ground is working. Thanks.

paul

[Repwoc]

This only works for eddy/non-ferrous targets, not magnetic/ferrous targets. Set the RL tau for whatever target tau you want.

Is there a way to simulate ferrous targets?

If I set the tau to 1 μS, ie 1 μH inductance + 1 Ω parallel resistor with the coupling set to 0.1, what sort of target would that simulate (size, depth, whatever)?

Is there a list somewhere of tau values for different targets?

paul

[Carl-NC]

A ferrous target can't be modeled with an RL, it requires something more complex. I've worked on it in the past but haven't figured it out.
Here are some tau ranges:

Salt water: 1-3us
Foil: 1-5us
Hammered silver coins: 5-15us (that's a guess)
Cupro-nickel coins: 10-20us
Pull tabs: 20-30us
Copper coins: 40-80us
Small silver coins: 60-100us
Medium silver coins: 100-150us
Large silver coins: 150-500us
Atocha bar: 5000us

The coupling coefficient just determines target strength, correlating that with an absolute depth will depend on the target (tau, diameter, thickness).

[Repwoc]

Thanks.

Atocha bar: 5000us

Not many of those round here :-)