MPP PI Pinpointer probes - one of them has peculiar behavior - hyper sensitive to EF - ideas?

[green]

Have you tried increasing main sample delay suggested reply #8? The decay signal looks overdamped. Maybe sampling some X signal from the ferrite since it detects swinging ever so slowly.

[Polymer]

Have you tried increasing main sample delay suggested reply #8? The decay signal looks overdamped. Maybe sampling some X signal from the ferrite since it detects swinging ever so slowly.

Not really. I can only adjust from 9 to 16us at the moment. What ballpark figure would be interesting to test for (20,40,80,160us) ?

[dbanner]

I am having intuition that the ferrite coil is saturating (or near to saturation), causing a decrease in inductance and invoking a transient overdamped condition.
Remove some turns if you can, and see what happens.

[dbanner]

I am thinking that the ferrite you used is from some VHF type apllication and isn't suitable for lower freq. The pulse width and current is high enough to invoke some strange reaction of this ferrite.
Increasing the main sample delay will defeat the original purpose of your pinpointer probe.

[Carl-NC]

I am having intuition that the ferrite coil is saturating (or near to saturation), causing a decrease in inductance and invoking a transient overdamped condition.
Remove some turns if you can, and see what happens.

I agree, sounds like core saturation. You could also reduce the TX pulse width or add some series R to the coil.

When I was at White's, the first pinpointer I built used a wrong ferrite and it turned out to be a very effective compass.

[green]

I agree, sounds like core saturation. You could also reduce the TX pulse width or add some series R to the coil.

When I was at White's, the first pinpointer I built used a wrong ferrite and it turned out to be a very effective compass.

Is there a way to measure core saturation? Maybe connect a 1 ohm resistor from +battery to coil and scope Tx on current(voltage across coil).

[Qiaozhi]

Is there a way to measure core saturation?

It's tricky, as you would really need to plot the B-H curve.
Here's some food for thought:
https://www.nutsvolts.com/magazine/a.../probing_cores

[dbanner]

The inductance(ie number of turns) is too high for the Al value of this ferrite core PP2, causing it to saturate.
Reducing the number of turns would likely cure this. Or limiting the level of the pulse.

[dbanner]

On second thought, perhaps am radio ferrite rod is best.

[dbanner]

It's tricky, as you would really need to plot the B-H curve.
Here's some food for thought:
https://www.nutsvolts.com/magazine/a.../probing_cores

Taken from the above article:
"typical core materials show a straight line which curves towards the horizontal at both ends (see Figure 1). The curved tips show the core's permeability falling at high currents." End quote

Green, you are welcome to build the B-H test circuit, as I know you are a TESTING freak-a-zoid.

[green]

Inductance: 170uH
Rprobe: 0.8 Ohms
Rseries to probe 56 Ohms
Windings: 64
Shielded: yes
probe specs from reply #1

charge TC=3us, peak current=.2A, coil current should be about .125A@3us, .173A@6us, .19A@9us. Was wondering if Polymer compared the current(voltage across a 1R resistor in series with the coil)with predicted if he couldn't tell if coil was saturating. If saturating, should get to .19A in less than 9us? Maybe charge wouldn't look exponential. If could paste recording in Excel it could be charted linear log to see if it charts a straight line.

[Polymer]

Gosh, so many replies - Thank You!

One correction: Rseries to PP-Probe is 68 Ohms (not 56 Ohms) - sorry

[Polymer]

Inductance: 170uH
Rprobe: 0.8 Ohms
Rseries to probe 56 Ohms
Windings: 64
Shielded: yes
probe specs from reply #1

charge TC=3us, peak current=.2A, coil current should be about .125A@3us, .173A@6us, .19A@9us. Was wondering if Polymer compared the current(voltage across a 1R resistor in series with the coil)with predicted if he couldn't tell if coil was saturating. If saturating, should get to .19A in less than 9us? Maybe charge wouldn't look exponential. If could paste recording in Excel it could be charted linear log to see if it charts a straight line.

Heres a scope shot of the shunt voltage measured across the 68 Ohm series resistor of PP2:

[Polymer]

The inductance(ie number of turns) is too high for the Al value of this ferrite core PP2, causing it to saturate.
Reducing the number of turns would likely cure this. Or limiting the level of the pulse.

It looks like you are right. I reduced the windings in steps of 5 and testing this "compass" (Thanks Carl) until I got to 35 windings less.
The problem is practically not noticable anymore. I also played with the sample delay - it did not make much difference.

The pre-amp output looks much different now, yet it still detects a nickel at around 3 inches.

Here's a scope shot:

[dbanner]

It looks like you are right. I reduced the windings in steps of 5 and testing this "compass" (Thanks Carl) until I got to 35 windings less.
The problem is practically not noticable anymore. I also played with the sample delay - it did not make much difference.

The pre-amp output looks much different now, yet it still detects a nickel at around 3 inches.

Here's a scope shot: [ATTACH]48215[/ATTACH]