MPP with DD coil 580 uH and short delay

[Teleno]

I'm using a MineLab compatble DD coil by DETECH with the following characteristics:

Tx coil: 580 uH, 1 Ohm. Damping: 470 Ohm.

Rx coil: 500 uH, 5.4 Ohm. Damping: 1KOhm

Modifications.

The induction balanced DD coil has a very short transient and I can sample at 6 us. This increases sensitivity to gold.
Some modifications are necessary for making the best of with this coil:

- R3 (3.3 Ohm) shorted to get maximum Tx current.
- Tx pulse increased to 100us: R7 (1K2) increased to 2K2.

These changes allow the coil to peak at 1.1 A.

- Minimum sampling delay set to 6us via trim pot R6.

The coil is 18 cm x 15 cm, so the expected detection depth is about 16 cm, which matches the tests.


1. Completed PCB on the test bench.





2. PCB and power pack crammed in a general purpose housing. Power pack hash 8 x AA Ni-MH, Panasonic BK-3HCCE, 2450 mAh.




3. The MineLab-compatible DD coil





4. Everything mounted on a universal telescopic shaft and ready to go:





4. Some indoor air tests.

Cheers!

[Teleno]

Hi guys! I'm back from my vacation in NW Spain where I had lots of detecting and gold prospecting fun. I must say the MPP passed the beach tests with fying colors, it's able to find tiny coins like "perra chica" (1870) at great depths in a mixture of wet sand and rubble.



Met people who are detecting nuggets in old riverbeds but I hadn't the time to gove it a try. Panning did show up some flakes, though:



I'll be back next summer and would be glad to meet you guys over there to share the fun. Detecting is legal, no permits required. The same for gold prospecting and there's plenty of it. Who would like to join?

[Teleno]

I have shorted C12 in my MPP Rev-D following the advice of Eric Foster in this thread:

http://www.geotech1.com/forums/showt...381#post219381

Being absent from the Forum for a while, I have not followed the Minipulse Plus thread and what changes/improvements have been made. I have only just looked at the schematic and done some quick comparisons with the Minipulse B schematic of January 1989. I will say, however, that there was never a problem with EF with the integrate and hold circuit that I started to use around that time. The same arrangement was also used in the Superscan and the waterproof Aquastar when they were in production.

Where a problem can arise is if the coupling capacitor (C12 in the MPP circuit) is too low a value. In the original MP it was 22uF, while in the MPP it is 0.47uF. The Garrett XL500, for which I designed the circuit, had EF problems for the same reason; they substituted a low value capacitor for the larger one I originally used. Luckily I checked an early production one and wrote a report for Charles Garrett.

If you scope across R19, with the 0.47uF, you will likely see that the pulsed waveform no longer has a flat d.c baseline but is a ramp. You need a flat, or substantially flat baseline to get good EF cancellation, so it is either a big capacitor or direct coupling. In the MP I used a tantalum capacitor and offset the preamp output to -0.5V to give the capacitor a bit of dc bias.

Today, with better I.C.s and tighter component tolerances, I generally couple the preamp to the integrator input without a blocking capacitor. With good balance in the differential integrator any small offset changes in the preamp cancel out anyway.

Eric.

The detector seems to work just the same (indoor tests). Recovery after a strong target signal seems faster.

I'll check whether the EF signals I had been noticing will go away.

[green]

Hi Teleno

You might want to try a pot(maybe 100 ohms) with the ends going to the 2k2 resistors and the wiper to the preamp. I haven't been with the 1C integrator but needed to add the pot with the last circuit I made.

[Ferric Toes]

Hi Teleno

You might want to try a pot(maybe 100 ohms) with the ends going to the 2k2 resistors and the wiper to the preamp. I haven't been with the 1C integrator but needed to add the pot with the last circuit I made.

Yes, that is a good thing to do in addition to have Jfets matched for ON resistance; all resistors 1%, and 470nf capacitors matched. It is surprising how narrow the notch is on the 100R preset for maximum CMR.

Eric.

[6666]

I have shorted C12 in my MPP Rev-D following the advice of Eric Foster in this thread:

Now that you have eliminated the AC coupling, wont you now have to look at the DC offset from the preamp ?

[Teleno]

Now that you have eliminated the AC coupling, wont you now have to look at the DC offset from the preamp ?

I might, but it all seems OK because I can adjust the sound threshold as usual and the frequency swing caused by targets is just like before.

[6666]

I might, but it all seems OK because I can adjust the sound threshold as usual and the frequency swing caused by targets is just like before.

Thanks for reply, I would be very interested in , if you still have your pcb open on the bench what the DC off set from the output of the preamp is ?

With the surf pi for instance , it needs a zero off set for best target signal, thanks.

[Teleno]

Yes, that is a good thing to do in addition to have Jfets matched for ON resistance; all resistors 1%, and 470nf capacitors matched. It is surprising how narrow the notch is on the 100R preset for maximum CMR.

Eric.

Don't forget matching the sampling periods! (2x caps & 2x resistors)

I would suggest a different approach. Since the EF remains practically constant in one Rx period, tuning the duration of the late sampling is equivalent to tuning the gain of the EF branch of the integrator. A constant voltage bias can be introduced to simulate the EF during the adjustment.

[green]

Don't forget matching the sampling periods! (2x caps & 2x resistors)

I would suggest a different approach. Since the EF remains practically constant in one Rx period, tuning the duration of the late sampling is equivalent to tuning the gain of the EF branch of the integrator. A constant voltage bias can be introduced to simulate the EF during the adjustment.

The detector I'm using uses the same timer for the target sample and EF sample. I've been holding a ceramic magnet on the coil and quickly lifting away while monitoring integrator out with an oscilloscope when adjusting the trim pot. Maybe there is a better way.

[Teleno]

I've been holding a ceramic magnet on the coil and quickly lifting away while monitoring integrator out with an oscilloscope when adjusting the trim pot.

The magnet is being stimulated by the Tx pulse and will respond the way magnetic ground does.

in contrast EF is not being affected by the Tx, so your simulation equivalent shouldn't be either.

[green]

The magnet is being stimulated by the Tx pulse and will respond the way magnetic ground does.

in contrast EF is not being affected by the Tx, so your simulation equivalent shouldn't be either.

I think the ceramic magnet isn't effected by the Tx pulse. There integrator output is the same with no magnet or with the magnet held constant on the coil.

[green]

I think the ceramic magnet isn't effected by the Tx pulse. There integrator output is the same with no magnet or with the magnet held constant on the coil.

Checked the magnet response to see if I remembered correctly(craft ceramic magnet made of ferrite(1/2 inch diameter, 3/16 inches thick). With the magnet held stationary above the coil center, integrator out maybe .1mv change at any height from no magnet. If held on coil near edge around 3mv about the same as a US nickel at 9 inches. Moving magnet away from coil center shouldn't be a problem, moving it close to edge could be. I'm using an IB coil and there is a small change in amplifier out during Tx. Checked with GEB off.

[Teleno]

Now that you have eliminated the AC coupling, wont you now have to look at the DC offset from the preamp ?

The offset is 1V. The offset of the differential integrator is 25mV with no target.

[6666]

The offset is 1V. The offset of the differential integrator is 25mV with no target.

Thanks.