Getting ready to build the MPP

[Davor]

Regarding the absolute value circuit, it is good to have it with GB because beyond the hole response is inverted, and in case you have indication only for positive going responses, you can't have any response of a long TC target directly underneath the coil. You'll find that all PI machines with GB have absolute value circuitry for that reason. Minipulse is not originally designed as a GB machine, and therefore no absolute value circuit. There are a few ways to add it in.
The way I introduce GB to minipulse is by introducing a GB sample that is in ratiometric relationship to the main sample, so that ground responses are eliminated. It most certainly introduces a hole, and it most certainly reverses phase for TCs larger than a hole.

If I understand correctly the threshold you speak about, you have a quiet(ish) constant tone which increases when targets are nearby, and it is a nice mellow sinus. I also played with such indication, only I call these modulators. Any balanced modulator will perform this way, and by means of introducing a slight mismatch to the balance (e.g. an offset) you get a quiet background tone. Most balanced modulators have very limited balance, and they attenuate a LO by only about ~60dB, which is quiet but audible. I believe this is the best possible indication method, as there is no real threshold that obliterates tiny targets, and it must work preferably well for seeking gold.
Since I found a few creeks in my backyard with gold, and a friend of mine found a real nugget in one of them, building a rig that can find them started making sense. I myself panned a small amount of gold dust, and it was a great joy. I'd starve if I had to earn money by digging gold, but it is a joy.

[mickstv]

This is the output section that I'm using with my GB detector, it could be modified to use with a GB version of the MPP.

GND would become -5v and +16v would become +5v. The ground symbol is TX coil common.

The output is variable pitch and level.

[Qiaozhi]

Since I'm primarily a prospector searching for gold, I personally usually don't use disc modes if present on a detector - only all metal mode - so I'm actually referring to the Whites threshold sound in all metal mode (Classic series) or just the Goldmaster ("black box" & GMT/GMZ models) threshold (no disc mode so all metal all the time, where the threshold tone is always present (unless threshold control is turned down too low) as a faint hum & gets progressively louder with RX signal strength, but the freq is constant at around 1khz to 1.5khz (I believe, will try to verify) & sounds like a nice sine wave signal, hope that helps & let me know if you'd like more info on it.

OK - now I understand.
The MPP has a VCO-type audio output, where the amplitude of the target signal controls the audio frequency. This arguably provides the most sensitive method of detection. However, if you would prefer a non-VCO audio, where there is a change in loudness instead of frequency, then I suggest you take a look at Fig. 16 in the Hammerhead Article.

[KRinAZ]

OK - now I understand.
The MPP has a VCO-type audio output, where the amplitude of the target signal controls the audio frequency. This arguably provides the most sensitive method of detection. However, if you would prefer a non-VCO audio, where there is a change in loudness instead of frequency, then I suggest you take a look at Fig. 16 in the Hammerhead Article.

Many thx for this article! - I'll take a look at the Hammerhead, as I personally very much prefer the amplitude modulation (Whites style) threshold tone.
For what it's worth this (non-VCO AM) is the threshold tone method used on the most sensitive gold specific detectors - the Whites GM series and the Fisher Gold Bug II (I think most would agree for either IB VLF or PI technology no other detectors match the sensitivity to small gold of these two).

[KRinAZ]

Regarding the absolute value circuit, it is good to have it with GB because beyond the hole response is inverted, and in case you have indication only for positive going responses, you can't have any response of a long TC target directly underneath the coil. You'll find that all PI machines with GB have absolute value circuitry for that reason. Minipulse is not originally designed as a GB machine, and therefore no absolute value circuit. There are a few ways to add it in.
The way I introduce GB to minipulse is by introducing a GB sample that is in ratiometric relationship to the main sample, so that ground responses are eliminated. It most certainly introduces a hole, and it most certainly reverses phase for TCs larger than a hole.

If I understand correctly the threshold you speak about, you have a quiet(ish) constant tone which increases when targets are nearby, and it is a nice mellow sinus. I also played with such indication, only I call these modulators. Any balanced modulator will perform this way, and by means of introducing a slight mismatch to the balance (e.g. an offset) you get a quiet background tone. Most balanced modulators have very limited balance, and they attenuate a LO by only about ~60dB, which is quiet but audible. I believe this is the best possible indication method, as there is no real threshold that obliterates tiny targets, and it must work preferably well for seeking gold.
Since I found a few creeks in my backyard with gold, and a friend of mine found a real nugget in one of them, building a rig that can find them started making sense. I myself panned a small amount of gold dust, and it was a great joy. I'd starve if I had to earn money by digging gold, but it is a joy.

OK, on the absolute value circuit - an observation - in my testing of the 8 turn 8x10 BW coil, one experiment I tried was 2 coils (since I now have two 8x10 BW's) - the 7 turn as the TX and the 8 turn as the RX - and I ran into the phenomenon where - while air testing so no ground effect - there was no target response as a target approached the coil, but a response when the target retreated from the coil - was quite strange & 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?

On the threshold tone, yes you've got it right, this would be a form of amplitude modulation, perhaps more simple than the balanced modulation you outlined though - I believe the implementation is the target response produces a varying voltage which biases the transistor that drives the audio output - much like the output of a standard audio amplifier (that's my understanding anyway). The Geotech IDX project is based on the Whites Classic IDX (it's in the Whites Classic series I referred to) & I believe uses this approach for threshold, there's a schematic in that forum (don't remember the post number) but some of the components listed in the schematic are incorrect (the RX preamp for instance has the wrong component) & 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.

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...

[KRinAZ]

This is the output section that I'm using with my GB detector, it could be modified to use with a GB version of the MPP.

GND would become -5v and +16v would become +5v. The ground symbol is TX coil common.

The output is variable pitch and level.

Hmmm, many thanks for this, I'll take some time & look over...

[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.

[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.

[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...

[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...

[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,...978#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.

[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]

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,...404#msg-460404

[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.

[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,...978#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!