Getting ready to build the MPP

[KRinAZ]

You may consider the Infineon CoolMOS series Mosfets. Watch the Coss when selecting. Even with a diode in series you still want to keep the capacitance as low as possible to get the most out of your coil. Higher voltage seems to give output capacitance, which makes sense. For the diode I found the Vishay VS-5EWH06FNHM3 had good specs. Would you mind publishing a photo of your coil. Interested to see how you went about it

Hmmm, thx Achillion, I'll check the mosfet & diode out, & I added a coil pic earlier today at the bottom of the post about the coil build, let me know if you have trouble accessing it - look for coil.jpg

[Achillion]

You may consider the Infineon CoolMOS series Mosfets. Watch the Coss when selecting. Even with a diode in series you still want to keep the capacitance as low as possible to get the most out of your coil. Higher voltage seems to give output capacitance, which makes sense. For the diode I found the Vishay VS-5EWH06FNHM3 had good specs. Would you mind publishing a photo of your coil. Interested to see how you went about it

[KRinAZ]

OK, I've build yet another 8" round SSBW (self-shielding basketweave) coil and I'm going to stick with it as I'm finally happy with the coil - I finally got it as tight as I wanted - to improve the magnetic field's strength, quality, and consistency. It uses exactly the same length of wire as the last one so has the same resistance, just wound tighter, and gave me a wonderful 20% increase in inductance (& presumably magnetic field strength) - without sacrificing even 1uS in speed, very nice. Previous coil was 300uH.
Here are it's spec's:
7 basketweave turns
360 uH
3.0 ohms (due to the small gauge 26AWG wire)
2455 ohms critical damping
Flyback spike recovered at 4uS after end of TX pulse - by monitoring the coil, and also at TP3.
Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet, anyone have suggestions on a good higher voltage replacement? 800 volts would be nice...

Also since this coil has has relatively high resistance & about the same ohms as current limiting R3 (3.3 ohms) I removed & jumpered R3.

This coil should be very good for trying out the timings I'm going to experiment with. I am almost done putting the MPP on a pole setup, so more soon...

[greylourie]

Hi 6666,

I will see if I can pm you the link.( As its about DD coils. Don't want to sully KR's good thread.)

[6666]

He also has one or two posts that might be of help to anyone looking at building DD coils for coping with more difficult ground.

Hi GL where have you been looking at for that info ?
thanks

[KRinAZ]

Ok, I spent some quality time with my MPP today & tweaked & experimented with the TX pulse width & rate. I've been wanting to experiment with these so I finally replaced the resistors with pots (more below). Since I have wound yet another experimental 8" round BW coil, I also wanted to do some measuring. This coil is 7 BW turns, 301uH, critical damping at 1620 ohms, passive resistance is 3.0 ohms (I want to get the ohms lower, using 26AWG now, will have to get larger wire). I got some great results by changing both the TX pulse rate & width. But first I had to fix some instability issues with fast SAT & high sensitivity so I determined by swapping & rechecking that the minimum series resistor for the sensitivity pot is 1K, & the minimum series resistor for the SAT pot is 37k. Well now my MPP threshold is nice & stable even with delay at minimum, fastest SAT, & highest sensitivity. Now to tweak the TX pulse. I swapped R7 for a 4K7 trimmer pot (it adjusts the TX pulse width) & R8 with a 22K trimmer pot (adjusts the TX pulse rate, a 30K would be better but this is what I had on hand), with the 22K pot I can't go down to the default 1K TX pulse rate, but can go much higher - which is what I wanted to do. I left the sample pulse widths at 15uS & the main & secondary delays unchanged. I tried many combinations of TX pulse widths & rates & expected to increase sensitivity & depth with longer pulses - but - I got much better sensitivity & depth [on small low conductive targets] when I ran the combination of 2500 TX pulses per second, & 40uS TX pulse width! That's a 33% drop in TX pulse width/power & I got no benefit when increasing the TX pulse width all the way up to 85uS. This was quite unexpected & there's more - I verified the EFE is still nulled out so that's good, I found there's no target response at all to a large ferrite core (very nice), so I experimented with how it handles AZ's high iron mineralized ground - & with this tweak it is MUCH smoother on bad ground - so much better I think I could use it without ground balance on a lot of the ground here! Wow! I now need to get this thing on a pole setup & get out in the goldfields with it. When I get a chance I'll do some reasonably scientific sensitivity & depth comparisons between the two settings. I wouldn't be surprised if this is part of the timings Minelab runs for their "Fine Gold" setting...

By the way, by "[on small low conductive targets]" what I mean is very small gold and lead bits (since lead and gold behave so similar to a detector) - this added for the sake of clarity...

[KRinAZ]

Wow KRinAZ !

1620 ohms damping must make for super fast switch off. Well done. You could experiment with shorter widths for TX and samples. And possibly get more benefit from lower battery consumption whilst still getting/or improving detection of the targets you desire. Check out the link for possible inspiration.

http://www.findmall.com/read.php?34,171965,171978#msg-171978

Its the hard work of Qiaozhi and many other posters (such as yourself), with their experiments that make this a great forum for learning.

Thanks greylourie, good links & info you've pointed out there!

[KRinAZ]

Well done KRinAZ.

I was going to explain why your new settings gave improved depth and sensitivity on smaller targets, but I see that greylourie's link to an Eric Foster post has already supplied the answer.
Please keep us informed of the test results. When you've decided on the final settings, I think this information should be added to the MPP Project Archive, and the MPP Completed Projects.

Many thanks Qiaozhi! I will keep at the tweaks & post when I have something significant. I don't know yet if I will hit this goal but I am attempting to get the sensitivity to quite small gold nuggets (.1 gram) that a Whites GM V/SAT has. This MPP platform is great to experiment with as it's easy to understand and tweak. Thanks also for creating & putting this project out & also for your excellent documentation & technical discussions.

[greylourie]

I have another link. It highlights some of the tweaks and advantages done by people like baum (with his preamp mods), and KRinAZ with his work on his minipulse. I think the original author of the linked post might be a member here, and I would like to thank him too. He also has one or two posts that might be of help to anyone looking at building DD coils for coping with more difficult ground. Anyway, hope his post provides inspiration and encouragement.

http://www.findmall.com/read.php?34,460404,460404#msg-460404

[Qiaozhi]

Well done KRinAZ.

I was going to explain why your new settings gave improved depth and sensitivity on smaller targets, but I see that greylourie's link to an Eric Foster post has already supplied the answer.
Please keep us informed of the test results. When you've decided on the final settings, I think this information should be added to the MPP Project Archive, and the MPP Completed Projects.

[greylourie]

Wow KRinAZ !

1620 ohms damping must make for super fast switch off. Well done. You could experiment with shorter widths for TX and samples. And possibly get more benefit from lower battery consumption whilst still getting/or improving detection of the targets you desire. Check out the link for possible inspiration.

http://www.findmall.com/read.php?34,171965,171978#msg-171978

Its the hard work of Qiaozhi and many other posters (such as yourself), with their experiments that make this a great forum for learning.

[KRinAZ]

Further interesting tweaks on my MPP

Ok, I spent some quality time with my MPP today & tweaked & experimented with the TX pulse width & rate. I've been wanting to experiment with these so I finally replaced the resistors with pots (more below). Since I have wound yet another experimental 8" round BW coil, I also wanted to do some measuring. This coil is 7 BW turns, 301uH, critical damping at 1620 ohms, passive resistance is 3.0 ohms (I want to get the ohms lower, using 26AWG now, will have to get larger wire). I got some great results by changing both the TX pulse rate & width. But first I had to fix some instability issues with fast SAT & high sensitivity so I determined by swapping & rechecking that the minimum series resistor for the sensitivity pot is 1K, & the minimum series resistor for the SAT pot is 37k. Well now my MPP threshold is nice & stable even with delay at minimum, fastest SAT, & highest sensitivity. Now to tweak the TX pulse. I swapped R7 for a 4K7 trimmer pot (it adjusts the TX pulse width) & R8 with a 22K trimmer pot (adjusts the TX pulse rate, a 30K would be better but this is what I had on hand), with the 22K pot I can't go down to the default 1K TX pulse rate, but can go much higher - which is what I wanted to do. I left the sample pulse widths at 15uS & the main & secondary delays unchanged. I tried many combinations of TX pulse widths & rates & expected to increase sensitivity & depth with longer pulses - but - I got much better sensitivity & depth [on small low conductive targets] when I ran the combination of 2500 TX pulses per second, & 40uS TX pulse width! That's a 33% drop in TX pulse width/power & I got no benefit when increasing the TX pulse width all the way up to 85uS. This was quite unexpected & there's more - I verified the EFE is still nulled out so that's good, I found there's no target response at all to a large ferrite core (very nice), so I experimented with how it handles AZ's high iron mineralized ground - & with this tweak it is MUCH smoother on bad ground - so much better I think I could use it without ground balance on a lot of the ground here! Wow! I now need to get this thing on a pole setup & get out in the goldfields with it. When I get a chance I'll do some reasonably scientific sensitivity & depth comparisons between the two settings. I wouldn't be surprised if this is part of the timings Minelab runs for their "Fine Gold" setting...

[KRinAZ]

Yes, that's a side effect. Not if it interferes with Rx chain.Thanks, It is a big fun. There are a very few countries as poor with minerals as Croatia, so knowing there is gold is just precious.

Well I hope you find a nice bit of AU there in Croatia & also get a chance to enjoy the beaches some - I once new an attractive woman - Dubrovka - who told me of Croatia's beautiful beaches...but I digress haha...

[greylourie]

I really like the idea of a variable pitch and level. Thanks for sharing your example, mickstv. That CD4046 is very interesting. Lot of info on google and and some fun examples on youtube.

[Davor]

if I understand correctly the absolute value circuit would correct this, the same way it corrects the response hole created by the GEB circuit & sampling. Yes?

Yes, that's a side effect.

I believe the tone is originated by a 400Hz square wave oscillator. In my opinion 440Hz would be a better choice as it's an in tune musical note (it's an A4) and will sound more musical.

Not if it interferes with Rx chain.

On the gold in your back yard, how cool! - hope you make a good find - maybe there's a nugget with your name on it back there...

Thanks, It is a big fun. There are a very few countries as poor with minerals as Croatia, so knowing there is gold is just precious.