Target delay time?

[green]

Some scope pictures. First column, adjusted damping resistor, top picture under damped, bottom picture over damped. Second column, placed a ferrite bead on coil with best damping. Used an IB coil, Rx(two 8in. round figure eight) Tx(oval surrounding Rx)constant rate 6250 amps/sec. Scope external triggers when coil driver commands off((T), top screen second division from left). Bead causes a signal change when coil is on. Bead causes a signal change out to 5usec after current is turned off even though the coil volts looks flat 1.5usec after turnoff. Probe was placed near fet because connecting it to the coil caused a change in preamp out. Should target delay time be set past where a ferrite bead causes minimal change or doesn't it matter?

[Carl-NC]

Ferrite represents non-viscous ground, which PI's normally ignore because no one runs a sample delay much below 10us. Once you start using an IB coil, it becomes easy to sample all the way down to zero, and then you can see ferrite. If you can see ferrite, then you will see mundane ground as well. So, yes, you want a sample delay that minimizes ferrite response.

[Teleno]

Ferrite represents non-viscous ground, which PI's normally ignore because no one runs a sample delay much below 10us. Once you start using an IB coil, it becomes easy to sample all the way down to zero, and then you can see ferrite. If you can see ferrite, then you will see mundane ground as well. So, yes, you want a sample delay that minimizes ferrite response.

My MiniPulsePlus with IB coil is sampling at 6us and indeed sees mineralized ground. An alternative to increasing the delay is to increase the sound threshod.

[Ferric Toes]

My MiniPulsePlus with IB coil is sampling at 6us and indeed sees mineralized ground. An alternative to increasing the delay is to increase the sound threshod.

Have you tried it on a wet saltwater beach? I have found that anything less than 10uS becomes difficult to use.

What sort of IB coil do you have? Is it DD or concentric? I favour concentric as you can get good cancellation of the ground signal if you get the ratio of the diameters correct. You never get rid of it completely with this method, but it is effective in reducing the ground signal when the coil is within a couple of inches of the ground surface. This is because the ground between the two coils gives an opposite polarity signal which subtracts from the signal picked up within the area of the inner receive coil.

Eric.

[Teleno]

Have you tried it on a wet saltwater beach? I have found that anything less than 10uS becomes difficult to use.

What sort of IB coil do you have? Is it DD or concentric? I favour concentric as you can get good cancellation of the ground signal if you get the ratio of the diameters correct. You never get rid of it completely with this method, but it is effective in reducing the ground signal when the coil is within a couple of inches of the ground surface. This is because the ground between the two coils gives an opposite polarity signal which subtracts from the signal picked up within the area of the inner receive coil.

Eric.

My MPP is set to 6us minimum sampling delay and it performs well on the salty beach with excellent depth (given the small size of the coil).
The IB coil is a 2x 500uH DD (Minelab).
The Tx pulse is 100us which ramps up to 1A at 12V.

[green]

I've been using a LFB250150-000 ferrite bead core Digikey #240-2285-ND. Nine LFB090050-000, Digikey #240-2135-ND give similar results. Did some experimenting on the bench. Set target delay at 3,4,5,6,7 and 8usec, adjusted target sample time to cancel ground with a fixed 100usec GEB sample time at each target delay setting. It worked at the lower settings, was expecting to see a lower target delay time where the Alabama ground, brick and hot rock didn't cancel at the California GEB setting. Maybe a small difference The integrator out signal increased at the lower settings for a 4 grain nugget even though the target sample time decreased to cancel the ground signal. I think it still might be better if the ferrite core isn't detected but need to check lower target delay times on the ground not the bench. I'm finally building a detector for ground testing. Scope pictures from thread #1.