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

[Polymer]

Hi everyone,

Have built the MPP E and have built one coil and two pinpointer probes.

The coil and my first probe both work just fine.

The second probe ist a bit thinner and detects just as good as the first when stationary.
But when I start moving it, it detects the earth's magnetic field.
When testing with a weak ferrite magnet, I can detect it up to 40cm away!

When waving the magnet pretty close to the properly working pinpointer prope I can see
the output of the preamp bobbing up and down in sync with the waving and have no audio output.
I assume the EFE circuitry is working properly here.

I don't have a clue as to why my second pinpointer is behaving so differently when connected to the same PCB.

Inductance: 170uH
Rprobe: 0.8 Ohms
Rseries to probe 56 Ohms
Windings: 64
Shielded: yes

Any ideas where I could start looking to find out why this one is misbehaving?

Thanks.

[green]

Hi everyone,

Have built the MPP E and have built one coil and two pinpointer probes.

The coil and my first probe both work just fine.

The second probe ist a bit thinner and detects just as good as the first when stationary.
But when I start moving it, it detects the earth's magnetic field.
When testing with a weak ferrite magnet, I can detect it up to 40cm away!

When waving the magnet pretty close to the properly working pinpointer prope I can see
the output of the preamp bobbing up and down in sync with the waving and have no audio output.
I assume the EFE circuitry is working properly here.

I don't have a clue as to why my second pinpointer is behaving so differently when connected to the same PCB.

Inductance: 170uH
Rprobe: 0.8 Ohms
Rseries to probe 56 Ohms
Windings: 64
Shielded: yes

Any ideas where I could start looking to find out why this one is misbehaving?

Thanks.

Some thoughts, maybe correct maybe not. The ferrite has a X component. If coil current hasn't decayed completely the target sample and EFE sample won't be the same and cancel. Could disconnect coil from Tx, Rx only and see if EFE works. If it does maybe damping adjustment would help. Mostly guessing.

[Qiaozhi]

Green makes a good point regarding damping. Did you adjust the damping resistor for each case?
Check the preamp output with a scope to see if it's ringing.

[Polymer]

Thanks for the suggestions. It's not the problem though.
No ringing at all on the pre-amp output, it's an exemplary curve actually.

That's what has me so stumped.

Will do photos of the PP-Probes and Osci-Curves tomorrow. Look and see is more fun ... I believe ...

[Polymer]

Here are the oscilloscope pics of the decay curve of my MPP coil, pinpointer probe 1 and pinpointer probe 2.

The pinpointer probe 2 is the one that is so sensitive to magnetic fields.

Varying the damping resistance from 200 Ohms thru to 1200 Ohms didn't change anything respective to magnetic sensitivity of probe 2.

I did notice that probe number one got slightly sensitive to the EF simulation magnet when strongly over-damped (200 Ohms-ish).

Still wondering and will be poking around more tomorrow.

[waltr]

Here are the oscilloscope pics of the decay curve of my MPP coil, pinpointer probe 1 and pinpointer probe 2.

The pinpointer probe 2 is the one that is so sensitive to magnetic fields.

Varying the damping resistance from 200 Ohms thru to 1200 Ohms didn't change anything respective to magnetic sensitivity of probe 2.

I did notice that probe number one got slightly sensitive to the EF simulation magnet when strongly over-damped (200 Ohms-ish).

Still wondering and will be poking around more tomorrow.

[ATTACH]48141[/ATTACH]

What is the yellow trace on the scope?

Only difference I see is that the coil and PP1 has a rise above zero after decay whereas PP2 does not. Seems maybe PP2 is damped more than coil or PP1.

What is the timing on the EFE sampling?

Interesting problem that I have no idea why.

[Qiaozhi]

Here are the oscilloscope pics of the decay curve of my MPP coil, pinpointer probe 1 and pinpointer probe 2.

The pinpointer probe 2 is the one that is so sensitive to magnetic fields.

Varying the damping resistance from 200 Ohms thru to 1200 Ohms didn't change anything respective to magnetic sensitivity of probe 2.

I did notice that probe number one got slightly sensitive to the EF simulation magnet when strongly over-damped (200 Ohms-ish).

Still wondering and will be poking around more tomorrow.

[ATTACH]48141[/ATTACH]

Maybe the problem is associated with the second sample (EFE) pulse rather than the main sample.
Can you post scope shots of the whole cycle?

[Qiaozhi]

When a metal target gets near the coil, eddy currents are generated in the object, which in turns generates its own magnetic field. This affects the decay curve of the received signal, and also decays away quickly over time. If the coil is moved within the Earth's magnetic field, or a magnet is moved near the coil, the signal generated in the receive coil is substantially the same throughout the whole TX period. In this case the main sample may contain a target signal plus a contribution from the external [unwanted] magnetic field, and the second [EFE] sample will contain only the unwanted signal. Hence subtracting the second sample from the main sample should yield only the target signal.

With above description in mind, I'm wondering if increasing the main sample delay improves the rejection of the magnet. Also, what sample width are you using? Since the first sample is being taken on the decay curve, and the second sample is on the flat portion, there may be a significant difference in the sampled signals if the pulse width is very narrow. In this case you could try increasing the sample pulse width. The presence of a ferrite core (as opposed to air core coil) in the two pinpointers may also be a contributing factor.

[dbanner]

Just out of curiosity, are your PP cores air or ferrite?
Maybe the PP2 ferrite core is too "hard" for pinpointer application.

[Polymer]

Just answered all questions and wanted to post pictures, everything froze, text gone ... sigh ... more time tomorrow

[Polymer]

Thank you for all your replies!

@dbanner
The PP coils are made with ferrites.
The well working ferrite is from induction welding.
That's "soft" in my opinion and has a surface resistance of 10-15kOhm.

The second PP is also made with a "soft" ferrite. 15-20kOhm.
I do not know what it was used for originally.

I stay away from the "hard" am-radio type ferrites for PI after
reading about the differences here on the forum.

What advantage(s) would an air wound PP coil have vs. one with ferrite?

I can only think of it as having more windings and parasitic capacitance.
I have not experimented towards the differences though, so have an unclear picture.

@waltr
The yellow trace is the main sample delay.

Timings:

Pulse Width: 31us
Main Sample Delay: 15us
Sample width 1: 16.5us
Pause Width: 111us (this one starts at same position as sample width1 - not consecutive)
Sample width2: 16.5us

@Qiaozhi
I can't take scope shots of all signals at once.
Could make separate shots and make a composite overlay if this is what it takes.

Ohh f**k, changing the sample pulse width would mean soldering through the conformal coating I have
already applied. I hope to get around that.

I am still puzzled and have dissassembled pinpointer 2 step by step, (taken out of pipe, taking away shielding etc...)
The "raw" ferrite with only coil windings still behaves the same.

I have pre-written this and saved it as not to lose it again.
Hope the formatting isn't all bonkers.

Cheers, polymer

[Qiaozhi]

... changing the sample pulse width would mean soldering through the conformal coating I have
already applied. I hope to get around that

You should only have to adjust the external Gain pot to change the sample pulse width.

[dbanner]

[QUOTE=Polymer;262676

I do not know what it was used for originally. [/QUOTE]


AH HA! It has lost its softness due to effects from the previous application. Or maybe it is saturating.
Just kidding, I don't really know. You have a little mystery to solve.
Can a ferrite core degrade over time, I never heard such thing, but thermal effects can cause changes, maybe?????Thermal stress failure.

[Polymer]

You should only have to adjust the external Gain pot to change the sample pulse width.

Thanks for the reminder of how to change SPW! I had a high wire bridge in there I could cut and attach a pot to.

I tested at 16.5, 33, 50 and 65us respectively. Sadly no change in the magnetic sensitivity.
Even when the coil is dangling and swinging ever so slowly it can be heard .... weird.
I optimised the damping for this PP coil and managed to get to 9us and detect a nickel at 4+". Hope the next one will be as good without the hyper sensitivity to magnetic fields.

Ferrites seem to be somewhat like cats ... dunno why I'm saying this ...

I will put this down to "weird ferrite" and build a new one. The rod I used here is the only one of this type that I have.

[Polymer]

AH HA! It has lost its softness due to effects from the previous application. Or maybe it is saturating.
Just kidding, I don't really know. You have a little mystery to solve.
Can a ferrite core degrade over time, I never heard such thing, but thermal effects can cause changes, maybe?????Thermal stress failure.

Yes, there is little mystery here ... ferrites do seem to be sensitive critters.

I have a faint idea what might have gone wrong from pp experiments I did a few years ago, yep, your writings triggered that memory from the back of my mind.
Not sure at all, must verify ... only have an inkling, will follow wherever it leads.