TGSL Tuning & troubleshooting

[Qiaozhi]

Hi Don,

What method do you use to shield your coils?

[dfbowers]

Sounds excellent!

Have to take the bait... going by the ferrite model as Qiaozhi explained, I assume you mean that your null phase zero crossing centers under the sync pulse nicely near the middle of the GB pot range. The ferrite model says that the slug should only make a very small additional pot adjustment no matter where you put the zero crossing, as long as you center the sync pulse over it. If otherwise, the slug is not acting like ideal ferrite. So are you saying with other coils, the zero crossing was not near the middle of the GB pot range?

Back to your coil -- usually there are two stable null points, one on each side of the minimum. Which one did you pick?

Also, would you measure the DC voltage on capacitor C15 (JFet source of DISC channel) while turning the DISC pot through its full range? Does it change sign, or stay positive or negative the whole time? Where does it peak in magnitude?

What do you think is the secret to coil 7? Did you use exact same shielding (I forget exactly what that is)?

-SB

Yes, things are behaving exactly as you described above. I never had an issue of trying to get the GB to be in the center position for ferrite, but centering the sync pulse would always require me to way overcouple or undercouple the coils, so not close to a null voltage.

In this case, it just happens to occur very close to a null voltage.. just slightly overcoupled. When I get home tonight , I will measure what's going around C15.

I did a few things differently with this one.. mostly getting the frequencies just right. And, I shielded a little differently. I ran a very thin strip of Cu tape around the outside of the coil and left a tab to solder to. Then, wrapped the the coil with conductive mylar so that every lap around the coil would touch the copper foil on the mylars conductive side. Very little copper though.


Don

[simonbaker]

Yes, things are behaving exactly as you described above. I never had an issue of trying to get the GB to be in the center position for ferrite, but centering the sync pulse would always require me to way overcouple or undercouple the coils, so not close to a null voltage.

In this case, it just happens to occur very close to a null voltage.. just slightly overcoupled. When I get home tonight , I will measure what's going around C15.

I did a few things differently with this one.. mostly getting the frequencies just right. And, I shielded a little differently. I ran a very thin strip of Cu tape around the outside of the coil and left a tab to solder to. Then, wrapped the the coil with conductive mylar so that every lap around the coil would touch the copper foil on the mylars conductive side. Very little copper though.


Don

Thanks, there are still design issues with GB I'm mulling over.

I assume you first position the zero crossing, then balance with ferrite? About how much adjustment required when using the slug?

-SB

[dfbowers]

Sounds excellent!

Have to take the bait... going by the ferrite model as Qiaozhi explained, I assume you mean that your null phase zero crossing centers under the sync pulse nicely near the middle of the GB pot range. The ferrite model says that the slug should only make a very small additional pot adjustment no matter where you put the zero crossing, as long as you center the sync pulse over it. If otherwise, the slug is not acting like ideal ferrite. So are you saying with other coils, the zero crossing was not near the middle of the GB pot range?

Back to your coil -- usually there are two stable null points, one on each side of the minimum. Which one did you pick?

Also, would you measure the DC voltage on capacitor C15 (JFet source of DISC channel) while turning the DISC pot through its full range? Does it change sign, or stay positive or negative the whole time? Where does it peak in magnitude?

What do you think is the secret to coil 7? Did you use exact same shielding (I forget exactly what that is)?

-SB

Simon.

I took a few measurements of the DC voltage on C15. On mine, while adjusting the disc pot through it's full range, it's near zero except for around the lowest and highest disc settings. At both ends it rises to around .05V.

Also, regardless of where I center the sync pulse, little if any adjustment is needed on the GB pot.

Don

[simonbaker]

Simon.

I took a few measurements of the DC voltage on C15. On mine, while adjusting the disc pot through it's full range, it's near zero except for around the lowest and highest disc settings. At both ends it rises to around .05V.

Also, regardless of where I center the sync pulse, little if any adjustment is needed on the GB pot.

Don

Thanks for info.

Your DC voltage on C15 seems beautiful -- like you are balanced for ground on DISC channel as well as GB channel; interesting. I'll have to doublecheck what I get, but I think usually with null on one side of minimum I see a change of sign over range, with null on other side I see max somewhere in middle. I'll check again.

It could be that the oscillator frequency has an important impact on how well the ground balancing operates, and also on how the discrimination control works. I wonder what the best frequency is? And why/how do other MD designs use different frequencies?

-SB

[simonbaker]

Thanks for info.

Your DC voltage on C15 seems beautiful -- like you are balanced for ground on DISC channel as well as GB channel; interesting. I'll have to doublecheck what I get, but I think usually with null on one side of minimum I see a change of sign over range, with null on other side I see max somewhere in middle. I'll check again.

It could be that the oscillator frequency has an important impact on how well the ground balancing operates, and also on how the discrimination control works. I wonder what the best frequency is? And why/how do other MD designs use different frequencies?

-SB

On a related note, I'm considering a funky single-channel TGSgold design optimized for gold prospecting. The idea is to center the sync pulse over the phase-center for gold, and choose an oscillator frequency so the phase-center for iron naturally falls at the sync pulse zero crossing. Then choose a null phase that also falls at the sync pulse zero crossing for ground balancing.

Because it is single channel, it would be advisable to build a coil with an external loop that can be used to adjust the null phase, since ground balancing is done by adjusting the null phase to the sinc pulse. The DISC control would be an internal pot for fine adjustment to gold, or not needed at all.

This is really topic for TGSL Experiments thread; I'll continue there if worth pursuing.

-SB

[dfbowers]

On a related note, I'm considering a funky single-channel TGSgold design optimized for gold prospecting. The idea is to center the sync pulse over the phase-center for gold, and choose an oscillator frequency so the phase-center for iron naturally falls at the sync pulse zero crossing. Then choose a null phase that also falls at the sync pulse zero crossing for ground balancing.

Because it is single channel, it would be advisable to build a coil with an external loop that can be used to adjust the null phase, since ground balancing is done by adjusting the null phase to the sinc pulse. The DISC control would be an internal pot for fine adjustment to gold, or not needed at all.

This is really topic for TGSL Experiments thread; I'll continue there if worth pursuing.

-SB

Interesting idea. Just a couple of thoughts:
The phase for gold can occur on about half the discrimination scale depending upon how big. So that might be a design consideration. ( I have never prospected).

The null phase might only be useful for a purely reactive ground.

Iron is unpredictable, especially if it's a closed circle like a washer, nut or ring.. or is large like a horseshoe. ( I have a few 1 gal jugs filled with crap that I thought were good targets.

Also, I may already have such a circuit if you want to experiment with it. It's a Nautilus DMC IIb!.. It has an adjustable coil balance (Signal loop balance) that feeds back both phase and amplitude to the Rx circuit... I am thinking that the DMCIIb still uses feedback in some way like the older patents


I did build the attached circuit a while back. It may accomplish what you are describing. Might be an easy way to implement a external null phase.

The feedback network is at the bottom left of the schematic.

[simonbaker]

Interesting idea. Just a couple of thoughts:
The phase for gold can occur on about half the discrimination scale depending upon how big. So that might be a design consideration. ( I have never prospected).

The null phase might only be useful for a purely reactive ground.

Iron is unpredictable, especially if it's a closed circle like a washer, nut or ring.. or is large like a horseshoe. ( I have a few 1 gal jugs filled with crap that I thought were good targets.

Also, I may already have such a circuit if you want to experiment with it. It's a Nautilus DMC IIb!.. It has an adjustable coil balance (Signal loop balance) that feeds back both phase and amplitude to the Rx circuit... I am thinking that the DMCIIb still uses feedback in some way like the older patents


I did build the attached circuit a while back. It may accomplish what you are describing. Might be an easy way to implement a external null phase.

The feedback network is at the bottom left of the schematic.

Good points. Yes, I can image gold could have a range of phase, wish there was some good data out there. It takes big nuggets to design a gold machine...

As for iron, I agree, especially it seems with rusty stuff, it fools me frequently -- and in those cases a discriminator doesn't help either I've found, so probably just have to dig. Of course part of the nice thing about designing gold hunting machines is that you hope for minimum trash in the areas you hunt -- not a good assumption around old mines however.

If the ground has a perfectly constant conductivity, you could set the null phase for that and it should still work for ferrite I think. But if the ground conductivity fluctuates, probably only a discriminator would take it out. So that could be a problem around rivers. If the ground conductivity is far enough from gold and you choose the frequency right, maybe you could attenuate typical conductive ground just by zero crossing placement.

That Nautilus circuit looks like a venerable old MD. I don't know if I'll be able to make sense of it without a lot of work -- I was thinking along the lines of a 1/2 TGSL -- but I'll give it a look. Maybe it has already been done right there. I like the simplicity of the parts used.

Looking forward to hearing how your coil works in the field.

Sometime try a Euro on top of the ground -- I'd like to know what air depth you get with your MDs in that case.

-SB

[dfbowers]

Good points. Yes, I can image gold could have a range of phase, wish there was some good data out there. It takes big nuggets to design a gold machine...

As for iron, I agree, especially it seems with rusty stuff, it fools me frequently -- and in those cases a discriminator doesn't help either I've found, so probably just have to dig. Of course part of the nice thing about designing gold hunting machines is that you hope for minimum trash in the areas you hunt -- not a good assumption around old mines however.

If the ground has a perfectly constant conductivity, you could set the null phase for that and it should still work for ferrite I think. But if the ground conductivity fluctuates, probably only a discriminator would take it out. So that could be a problem around rivers. If the ground conductivity is far enough from gold and you choose the frequency right, maybe you could attenuate typical conductive ground just by zero crossing placement.

That Nautilus circuit looks like a venerable old MD. I don't know if I'll be able to make sense of it without a lot of work -- I was thinking along the lines of a 1/2 TGSL -- but I'll give it a look. Maybe it has already been done right there. I like the simplicity of the parts used.

Looking forward to hearing how your coil works in the field.

Sometime try a Euro on top of the ground -- I'd like to know what air depth you get with your MDs in that case.

-SB

Simon,

I will do a short test of air depth for a Euro on the ground, probably late today or tomorrow and let you. I wouldn't expect much different from what you have because I know that the two TGSLs perform very similar. But.. maybe different ground or background noise would be the two variables. I know that if I bury stuff in my backyard, it's radically different than at the beach.. It's just that hard packed clay knocks off about 1/2 my depth.

Also, for a neat little circuit to play with (single channel) I might recommend KT315's Volksturm.. I built an 3 of these last winter but added a phase shifter right after the Rx op amp.. Might fit the bill if you just want to play with phasing and nulling.

The real advantage to single channel is that it goes deeper in the ground.. at the expense of digging everything. Odd thing is that this circuit seems to work best with an untuned Tx (Just a square wave).

Don

[simonbaker]

Simon,

I will do a short test of air depth for a Euro on the ground, probably late today or tomorrow and let you. I wouldn't expect much different from what you have because I know that the two TGSLs perform very similar. But.. maybe different ground or background noise would be the two variables. I know that if I bury stuff in my backyard, it's radically different than at the beach.. It's just that hard packed clay knocks off about 1/2 my depth.

Also, for a neat little circuit to play with (single channel) I might recommend KT315's Volksturm.. I built an 3 of these last winter but added a phase shifter right after the Rx op amp.. Might fit the bill if you just want to play with phasing and nulling.

The real advantage to single channel is that it goes deeper in the ground.. at the expense of digging everything. Odd thing is that this circuit seems to work best with an untuned Tx (Just a square wave).

Don

Very interesting -- yes, I'm trying to confront the problem of the GB channel limiting the sensitivity so much -- single channel one way around that, at a price.

Also interesting about the square wave driver -- wonder what that means? I'll have to take a look at it.

BTW: those MDs are so good looking if I weren't married I'd ask them out...

-SB

P.S. What's your favorite MD you've made so far and why?

[dfbowers]

Very interesting -- yes, I'm trying to confront the problem of the GB channel limiting the sensitivity so much -- single channel one way around that, at a price.

Also interesting about the square wave driver -- wonder what that means? I'll have to take a look at it.

BTW: those MDs are so good looking if I weren't married I'd ask them out...

-SB

P.S. What's your favorite MD you've made so far and why?

Well, I have only made 3 IB detectors to completion and used them so far.

For straight relic hunting, I might pick the Volksturm since it might go deeper, (in the ground anyway) but the TGSL is more practical for hunting. I have also built both Stuart PIs and 2 HHs. I did some relic hunting with the HH and it worked OK. I did have some great success with the TGSL earlier in the season so I have some confidence in it.

I did notice that in my test garden, the Volksturm would hit some deeper coins that the TGSL could not find, but it would pick up every little nail and small pieces of iron as well. The Volksturm can discriminate or ground balance, but not both at the same time.

Don

[dfbowers]

Well, I have only made 3 IB detectors to completion and used them so far.

For straight relic hunting, I might pick the Volksturm since it might go deeper, (in the ground anyway) but the TGSL is more practical for hunting. I have also built both Stuart PIs and 2 HHs. I did some relic hunting with the HH and it worked OK. I did have some great success with the TGSL earlier in the season so I have some confidence in it.

I did notice that in my test garden, the Volksturm would hit some deeper coins that the TGSL could not find, but it would pick up every little nail and small pieces of iron as well. The Volksturm can discriminate or ground balance, but not both at the same time.

Don

Oh yea.. almost forgot, I built several of the Tarsos circuits too. Just to learn about IB.

[maikl]

Oh yea.. almost forgot, I built several of the Tarsos circuits too. Just to learn about IB.

Hi dfbowers. How to TARSOS showed on the field (except the humid grass)? Tests in the air is much better and simpler to create than TGSL.

[dfbowers]

Hi dfbowers. How to TARSOS showed on the field (except the humid grass)? Tests in the air is much better and simpler to create than TGSL.

The Tarsos shows very good distance in the air (I cannot remember just how far) but more than most VLF designs. if I remember, it has a high power Tx circuit though (100V?). I did not test it outside extensively but it definitely works.. It needs the autotune option to work outside though.

Don

[maikl]

The Tarsos shows very good distance in the air (I cannot remember just how far) but more than most VLF designs. if I remember, it has a high power Tx circuit though (100V?). I did not test it outside extensively but it definitely works.. It needs the autotune option to work outside though.

Don

Thanks for the reply dfbowers.