Nulling IB Coils

[Anteros]

Hello again,

I've been working a lot on my walkthrough metal detector (Induction balance type) and I've already detecting with model antennas (15cm x 30cm) but when I try to null them by overlapping one with the other, I can't remove all the signal.

The least I can get is about 5mV peak to peak, is this normal or should I get less than that?

What considerations should I have with coils when nulling?

Thanks

[Carl-NC]

When overlapping 2 coils, you almost never get complete balance because of the thicknesses of the coil bundles. You might be able to add a "tweaking" loop -- a single loose loop -- that you can move around to fine-tune the balance. This is a common method used in hobby coils.

- Carl

[Anteros]

Thanks Carl.

I also though of adding an op-amp in substractor mode after the initial preamp stage so I can remove the part of the signal that doesn't get nulled, also giving more freedom over the calibration and the possibility of future microcontroller-based auto calibration.

[KingJL]

Thanks Carl.

I also though of adding an op-amp in substractor mode after the initial preamp stage so I can remove the part of the signal that doesn't get nulled,

In the ML Relic Hawk, they feed a portion of the Tx signal (~10mv) to the preamp to cancel out the slightly non-nulled Rx coil. I believe it also allows them to go slightly past null on the overlap giving them an ever so slightly larger active signal area. The correct way to null those coils is by looking at the output from the pre-amp and cancel the Tx sample.

Regards,
J. L. King

[Anteros]

Thanks KingJL, that's what I was thinking about. I hope it works.