Getting ready to build the MPP

[Qiaozhi]

And when I first posted I had the names of the photos swapped, I immediately edited the post to fix that but it seems you saw the post with wrong pic names haha - solves that mystery...

So I'm not completely losing it after all.

[Achillion]

Hi Jacob, my damping resistor is actually a resistor network at 2455 ohms. Keep in mind - as Dan pointed out in post #295: "...did you keep the original 1K input resistor on the input to the preamp? If so the actual 'in circuit' critical damping resistance with the 2455 ohm coil resistor in parallel with that 1k input resistor is 710.5 ohms..." I used an o'scope and Qiaozhi's damping tool to find the highest ohms value possible while just barely (but completely) eliminating the ringing, are you doing something similar to this? - Kyle

Yes mate, I have the 1k still in place, added the diode and have the basket weave coil (350uH, 26AWG, 3.4ohm). Coil resister is only about 350 ohm though. I will check tonight and upload a scope image. There is a little overshoot on the pre-amp output but a nice response to small gold. Theoretically this would mean that I have a lot of capacitance sitting somewhere in the coil circuit right? Well, I sorted out quite a bit of issues with my modded MPP but this puzzled me. Cheers,
Jacob

[KRinAZ]

KRinAZ, you have made some mods to the standard MPP. I guess you have tuned the damping resistor. Do you know what value it is.I'm testing at the moment and although I'm able to sample at 7-8us, I find the value quite low (between 350 -370 ohm).Jacob

Hi Jacob, my damping resistor is actually a resistor network at 2455 ohms. Keep in mind - as Dan pointed out in post #295: "...did you keep the original 1K input resistor on the input to the preamp? If so the actual 'in circuit' critical damping resistance with the 2455 ohm coil resistor in parallel with that 1k input resistor is 710.5 ohms..." I used an o'scope and Qiaozhi's damping tool to find the highest ohms value possible while just barely (but completely) eliminating the ringing, are you doing something similar to this? - Kyle

[KRinAZ]

Hi KRinAZ That is good results for a such a small target. Qiaozhi my view shows gold color for the nugget. The lead has a dark blue color with the nickel a blue hue. Must be different browsers. I am on Goggle Chrome.Have a good day,Chet

Hi Chet, many thanks!

[KRinAZ]

If you imagine looking at a cross-section of the coil windings, with the magnetic field lines circling around the cross-section, you can see why the parallel target is strongest at the coil center, and weakest at the edge. And vice-versa when the target is perpendicular to the coil.What are the diameters of the gold and lead test targets?By the way, do you have the names of the Gold and Lead images swapped over? The lead target looks gold in color, and the gold looks lead colored.

OK I guess that makes sense, I never noticed the orientation act that way before with coin targets & such - I'm going to do some more tests with different metals... The test gold and lead pieces are both 1/4" / .635cm wide, the gold thickness if 1/32" / .90mm and the lead is 1/32" / .95mm thick, the gold weighs .22 grams, the lead is .23 grams. And when I first posted I had the names of the photos swapped, I immediately edited the post to fix that but it seems you saw the post with wrong pic names haha - solves that mystery...

[Qiaozhi]

KRinAZ, you have made some mods to the standard MPP. I guess you have tuned the damping resistor. Do you know what value it is.
I'm testing at the moment and although I'm able to sample at 7-8us, I find the value quite low (between 350 -370 ohm).

Jacob

The mods made by KRinAZ are here -> http://www.geotech1.com/forums/showt...586#post206586

[Qiaozhi]

Qiaozhi my view shows gold color for the nugget. The lead has a dark blue color with the nickel a blue hue. Must be different browsers. I am on Goggle Chrome.

Have a good day,
Chet

What?
I'm sure they were the other way round, and I checked it more than once. Must have had too much wine.

[Achillion]

KRinAZ, you have made some mods to the standard MPP. I guess you have tuned the damping resistor. Do you know what value it is.
I'm testing at the moment and although I'm able to sample at 7-8us, I find the value quite low (between 350 -370 ohm).

Jacob

[6666]

gold color for the nugget. The lead has a dark blue color with the nickel a blue hue.

Me to.

[Chet]

Hi KRinAZ

That is good results for a such a small target.

Qiaozhi my view shows gold color for the nugget. The lead has a dark blue color with the nickel a blue hue. Must be different browsers. I am on Goggle Chrome.

Have a good day,
Chet

[Qiaozhi]

If you imagine looking at a cross-section of the coil windings, with the magnetic field lines circling around the cross-section, you can see why the parallel target is strongest at the coil center, and weakest at the edge. And vice-versa when the target is perpendicular to the coil.

What are the diameters of the gold and lead test targets?

By the way, do you have the names of the Gold and Lead images swapped over? The lead target looks gold in color, and the gold looks lead colored.

[KRinAZ]

I recently received a .22 gram flat gold nugget as a thank you, & tried it out as a test target on my MPP - got some interesting and different results than expected:

Air tests with delay at shortest, sensitivity at most sensitive, SAT about at project spec -

1) target oriented parallel to coil, repeatable target signal in center of coil - distance of about 2.5" 6.35cm
2) target oriented parallel to coil, repeatable target signal when right over coil winding at edge of coil - distance of about .75" 1.14cm

1) target oriented perpendicular to coil, repeatable target signal in center of coil - distance of about .25" .635cm
2) target oriented perpendicular to coil, repeatable target signal when right over coil winding at edge of coil - distance of about 1" 2.54cm

I repeated the above with a test lead target I made to be reasonably close to the gold target in physical dimensions and got about the same results.

The good thing is that - this is the smallest gold & shortest TC target I have had a target response from.

It's interesting how the target response is reversed based on the orientation - and that the response is stronger right over the coil windings for the perpendicular target. When scanning the ground this would have the effect of two target signals.

The down side is that when I adjusted my MPP to be reasonably stable while scanning over mineralized ground (much lowered sensitivity & much faster SAT) I could not detect the little nugget.

So I guess I'm back to wanting GEB on my MPP, I'll reread Davor's & other's posts on GEB & think on it for a bit & see what it will take to try to add GEB to my MPP...

Each target is next to a US nickel (coin)

[KRinAZ]

Hi KR if you are going to just build the PS and TX and maybe preamp, BBS made a post about uping the plus and minus 5 volt supply on the preamp to knock off a uS or 2 from the preamp recovery time, I simed it with about 8-10 volts on the preamp and on the sim the higher voltage did knock off 1-2 uS, just might be worth looking at.

Many thanks 6666, very interesting, I'll look into that.

[6666]

Thinking of building & measuring fast coils - I think I'm going to get another MPP board & just build the power sources [needed] and just the TX circuit - to purely observe the coil with no RX circuit influence - I'll do that before the next coil build. Also thinking of fast coil sampling - anyone read the KB article about the MPF technology on Minelab's new SDC 2300? It's linked to on the product page and quite an interesting read!

Hi KR if you are going to just build the PS and TX and maybe preamp, BBS made a post about uping the plus and minus 5 volt supply on the preamp to knock off a uS or 2 from the preamp recovery time, I simed it with about 8-10 volts on the preamp and on the sim the higher voltage did knock off 1-2 uS, just might be worth looking at.

[KRinAZ]

-------------------------------------------------------------------------I've been using 24 AWG. Tried 26 AWG but did not like the result. Since each jump in wire gauge doubles the cross sectional area and lowers resistance perhaps you should give the 24 AWG a try. There is a trade off with increasing wire sizes in terms of greater eddy currents, inter-winding capacitance, weight, and physical wire bulk in the 3DSS coil form slots.Regards,Dan

Hmmm, ok thanks Dan. I'm curious if you tried other larger sizes & found a size that was as large as practical? That would establish a ball park range of sizes - 24AWG being the minimum to ???AWG being the maximum - that would be very good to know! It would be quite interesting to wind identical coils with each AWG size in the range & document all the performance specs - SRF, inductance, ohms, damping resistance, spike recovery time, etc.Thanks - Kyle