TESORO GOLDEN SABRE

[simonbaker]

I'm still puzzling the question of null phase shift.

Here are observations I am starting with:

If you think of the RX and TX coils as an ideal transformer and the RX coil not connected to anything, then there should be two phases possible for the RX coil as you slide it across the TX coil. One is where the net magnetic flux balances one way, and one where the net flux balances the other way. The two phases should be virtually 180 degrees apart. As you slide the coils toward the null point, the RX signal should diminish in amplitude, but the phase doesn't change. It goes to zero at the null point, then suddenly shifts phase 180 deg and begins to increase again as it passes the null point.

Now connect the RX coil to a capacitor, throw in some non-ideal resistance, and you have a resonant circuit that will have a different phase, but again just two distinct phases. Are they 180 degrees apart???? Agh, need to do the math. But the phase will be determined by the LCR parameters.

With our actual TGSL, near the null point we observe that the phase shifts smoothly from one point to another. I suspect the reason is that we have some capacitive coupling from the cable or shield. The shield also magnetically couples the coils probably, and perhaps adds a signal that has intermediate phase.

Now, what is important and what can we control? Qiaohzi has explained the reason we want to choose a null phase that allows us to set the ground balance pot near its middle position to allow best practical adjustments if the ground changes. This is based on the fact that ferrite-like material modulates the amplitude of the null signal but does not change its phase.

I agree with that in theory, but I'm not sure there is much practical choice to be made with the TGSL. I think when we try to fine-tune the null signal phase by shifting the coils, we are really just varying the relative size of the capacitive component of the coupling and not changing the magnetically coupled phase. Not sure about that, but I suspect it. Anyway, my thought is that the capacitive component is not modulated by ferrite, so it really should be ignored. If you could see only the magnetically coupled null signal, it may be true that it is mainly one of two phases, like the ideal case; in which case, you are stuck with choosing one or the other by choosing one side of the null point or the other.

If you really want to change the phase of the null signal, you need to change the RX resonant frequency relative to the TX frequency. But the TGSL design pretty much fixes it in a small range if you follow the design.

In other words, my feeling is that you can't accomplish much by tweaking the RX null signal phase during nulling the coils. Picking the best side of the null is the real mission, and just enjoy whatever phase you end up with. Hopefully, the important magnetic component of the null signal will be fine and dandy.

... to be continued no doubt...

-SB

[Qiaozhi]

Thanks Qiaozhi..

I'm wondering if that is just to get the phasing for DISC an GB to work correctly, and if there is some mathamatical explanation that's easy to understand.

Although the TGSL electronics is the reason why the coil must be nulled that way, there is nothing to stop you having a different phase-shift (as long as the residual amplitude is fairly low) and you redesign the electronics to sample at the correct position. Also, if you study the output of the pre-amp for different coil overlaps, you will see that only one position satisfies all the criteria for the TGSL to operate correctly. I don't know if there is an mathematical explanation, and it may just be down to good old trial and error.

[dfbowers]

yes - GB pot mounted on that panel.

Hmm... Try this. Unmount it and see if that fixes your problem. Or, ground the case of the pot with an extra wire right to ground.. I will bet that your problem goes away!

I think that adding a small bit of capacitance to the pot with your hand is just enough to cause a small phase shift in the GB circuit.

[dfbowers]

The 20 degree phase shift is a requirement of the electronics in the TGSL. When the GEB pot is set to its middle position, the sample pulse is approximately 20 degrees offset from the TX signal.

Thanks Qiaozhi..

I'm wondering if that is just to get the phasing for DISC an GB to work correctly, and if there is some mathamatical explanation that's easy to understand.

I'm in the middle of writing a TGSL "101" manual, sort of like the "TGSL for Dummies" idea.
I would like to get that detail correct as that question comes up frequently.

When I complete it, maybe add it to the "TGSL compete details".. It may curb a lot of the repetitive questions from new builders that don't read or cannot find a lot of the important points buried in the huge thread! But.. reading all the threads give great insight on how things are arrived at!!

Of course, one of the best ways to learn something is to write about it..

[habitbraker]

yes - GB pot mounted on that panel.

[dfbowers]

thanks ! I read that thread - lots of good info. About null phase - its clear.
About my touching enclosure problem, thats how its looking:



I discovered that problem occurs only when I touch front (showed) panel. Back panel is spayed too, but no toucing problems. And in disc mode I actually can discriminate out that problem. About 60% of disc and its silent. But I disriminate useful coins as well....

Do you have the GB pot mounted on the panel?

[Qiaozhi]

I'm trying to find a link where the 20 degree phase shift theory comes from, so maybe someone else can answer.

The 20 degree phase shift is a requirement of the electronics in the TGSL. When the GEB pot is set to its middle position, the sample pulse is approximately 20 degrees offset from the TX signal.

[habitbraker]

thanks ! I read that thread - lots of good info. About null phase - its clear.
About my touching enclosure problem, thats how its looking:



I discovered that problem occurs only when I touch front (showed) panel. Back panel is spayed too, but no toucing problems. And in disc mode I actually can discriminate out that problem. About 60% of disc and its silent. But I disriminate useful coins as well....

[dfbowers]

thanks for response!
I guess you are right about lm308. But I will try it anyway when I receive it.
If you say that your tgsl also dont detect coin with ferrite then I guess its ok too.

TRIED that coil with thichness of 2.5mil and didnt saw differences. Inductances/dimensions was the same so I guess I need to chang those parameters to see differences.


Eduardos PCB not finished yet so cant tell my exp.

I put everything in the box today and attached actual cable (1m instead of 20cm).
All is ok but I got one problem: when i touch the boxes conductive areas I got falses. I got plastic box which have inside (and few areas of outside) sprayred with Kontakt EMI http:http://www.gemalta.com/aerosol-emi-35-200ml.html
Is this normal? touching non-conductive areas and cable, even a coil (not overlap area) is ok with no beeps...

i got few questions:
I have read about null phase and if I understand correctly - if I can ground balance, i got phase right.
But also heard that better is phase of 20 deg. So question is : 20deg leading (+) TX or lagging(-) tx?

Is it ok that my response of silver ring depend on how I have ground balanced? i discovered that I got about 30 deg turning angle on my gnd bal. pot. where I can balance out ferrite. When I gnd balance on one end of those 30deg I got no response for silver. All other metals ok. But when I gnd balance on other end of those 30deg i got silver response..
I hope you understand.

I can create false signals by touching any area of my cable that does not have a shield (like the 4 pin connector on my coil cable) but I generally do not handle that area anyway. I think that if I had a metal enclosure and it were grounded to the connector and circuit ground, I could eliminate that problem. I had problems with an external GB pot until I added a ground to that as well.

There has been a lot of discussion here in the past on null phase:

http://www.geotech1.com/forums/showthread.php?p=114987#post114987

If I understand this right, it provides for maximum range for the GB adjustment pot. Plus, ideal ferrite would be rejected when the Rx signal zero crossing occurs at the center of the sample pulse.

I'm trying to find a link where the 20 degree phase shift theory comes from, so maybe someone else can answer.

The GB setting is critical in that advancing the GB pot too far, you will suddenly reject thin silver. So.. you may want to set your GB pot to where a ferrite slug is just breaking up. With a PCB mounted GB pot, I start fully CCW and advance CW until ferrite is rejected. Actual ground conditions were I live allow me to set the GB pot several degrees CCW from where ferrite is rejected.

Don

[habitbraker]

thanks for response!
I guess you are right about lm308. But I will try it anyway when I receive it.
If you say that your tgsl also dont detect coin with ferrite then I guess its ok too.

TRIED that coil with thichness of 2.5mil and didnt saw differences. Inductances/dimensions was the same so I guess I need to chang those parameters to see differences.


Eduardos PCB not finished yet so cant tell my exp.

I put everything in the box today and attached actual cable (1m instead of 20cm).
All is ok but I got one problem: when i touch the boxes conductive areas I got falses. I got plastic box which have inside (and few areas of outside) sprayred with Kontakt EMI http:http://www.gemalta.com/aerosol-emi-35-200ml.html
Is this normal? touching non-conductive areas and cable, even a coil (not overlap area) is ok with no beeps...

i got few questions:
I have read about null phase and if I understand correctly - if I can ground balance, i got phase right.
But also heard that better is phase of 20 deg. So question is : 20deg leading (+) TX or lagging(-) tx?

Is it ok that my response of silver ring depend on how I have ground balanced? i discovered that I got about 30 deg turning angle on my gnd bal. pot. where I can balance out ferrite. When I gnd balance on one end of those 30deg I got no response for silver. All other metals ok. But when I gnd balance on other end of those 30deg i got silver response..
I hope you understand.

[dfbowers]

I tried the ferrite plus coin test. It appears as though the ferrite masks the coin somewhat, but that was with a U.S. nickel and quarter. Maybe it also depends on where you have everything set. I don't believe that's a condition that you would encounter in the field though.. Maybe a coin next to a hot rock or nail that you have discriminated out.

I have tried a similar experiment in the past where I cover a coin with a clay brick. A detector cannot typically detect the coin under a single brick but if you pulled a brick out of an entire sidewalk and put the coin under a brick in the sidewalk and replaced the brick it would detect it OK..

Also, I have found little difference between the LM308 and TL071's. By picking through a handful of LM308s I see some suttle differences but that's about it.

As far as Eduardo's version goes, audio is improved with external speaker for sure but the advantage is not that great if headphones are used. I have also been contemplating replacing the 1m Ohm resistors in the DC stages with the original 460k Ohm resistors. Too noisy in actual use and sensitivity cannot be reduced enough! The doubled gain increased sensitivity but also increases noise as well. I like running a quiet detector just under where the sensitivity control causes instability but am having trouble with this version.

[simonbaker]

hello!
After reading hundreds of posts here, I finally lighted up my first TGSL and its working pretty good.
Disc is perfect and I can totally reject ferrite stick.
Its all on the bench for now and i dont have coil enclosure. I am planing to use vacuumforming method to form the enclosures. I already purchashed 1.5mil plastic sheets.

My coils have classic dimensions - d=25cm, described in "ivconic coil making PDF"
Tx=6mH, Rx=6.5mH. Wounded with 0.28mm wire. Spacers = 5mm

My air distances for now are about 25cm with Latvian coin and thick silver ring in disc mode. I am using dfbowers threshold mod, because with ordinary threshold control I could put it on max sensivity and still dot get chatters or false signals. I dont have eiro, so I cant compare with others here, but I will get one. I can put sensivity so high that coin is detected 30 cm far but then I can detect my hand - no good

I am using tl071 instead of LM308. TX Frequency is 14.1khz voltage is about 17Vpp. I am using external pot for GND bal.
Actually disc is more sensitive than all metal mode. (about 3cm more) I dont know why

First I tried nulling with unshielded coils and I could get about 20mVpp meassuring at U101 pin 7. Then I shielded it with adhesive Al tape and after that, my min null became about 1.5Vpp.
Sensivity isnt much reduced, but I think that large null is because Al tape is pretty thick and very conductive. I tried several methods of nulling, but finally I just adjusted coil so that it almost mach waveforms shown in pdf called TGS_Compare1. After that Its working pretty good.

It just my first one and my future plans are:
1. Building another TGSL or more. I have one Silverdog PCB half assembled...
2. Try LM308. I have ordered a few..
3. Wind different coils. Now I have one coil with spacers of 2.5mm drying. Will post results.
4. Try to find different shielding method.
5. Build a coil enclosure using vacuum forming. Never done it before.
6. Test it on real conditions in real soil
7. Add volume control
8. Read another 1000 posts posted in geotech Now I started to read TGSL tining&troubleshooting - I simply love that tread - so much info!!

All I want is to get famous 30-35cm.

oK . Pretty long text I have made
One question: Is this normal that waving ferrite rod together with coin I dont get response? Isnt that mean that in soil I will not get response for that coin? Single coin is detected....

Later I will post some pics

I say: good work! All your specs sound normal for a TGSL, including the large null with a shield -- very similar to TGSL from dfbowers that I use for comparison.

My coils are usually 28 to 30 cm diameter, but now I am beginning to think that 25 cm is maybe a better choice for some reason. I'll have to drill some new holes and try one that size.

Not sure about the ferrite and coin together, someone else can test sooner than me. It may depend on size of ferrite too. Also, remember the ferrite in ground is spread wider than the coin and that probably makes a big difference.

Regards,

-SB

[habitbraker]

My coil winder. counter was specially made for this purpose - with -1 option and sound indication. Using PIC. It counts only up to 255 - enough for TGSL. if enyone interested I coud give shematic and source code


Shield spacers and one shielded coil. In my oppinion too thick so its no good for nulling


My setup One of plastic sheets I mentioned

[habitbraker]

hello!
After reading hundreds of posts here, I finally lighted up my first TGSL and its working pretty good.
Disc is perfect and I can totally reject ferrite stick.
Its all on the bench for now and i dont have coil enclosure. I am planing to use vacuumforming method to form the enclosures. I already purchashed 1.5mil plastic sheets.

My coils have classic dimensions - d=25cm, described in "ivconic coil making PDF"
Tx=6mH, Rx=6.5mH. Wounded with 0.28mm wire. Spacers = 5mm

My air distances for now are about 25cm with Latvian coin and thick silver ring in disc mode. I am using dfbowers threshold mod, because with ordinary threshold control I could put it on max sensivity and still dot get chatters or false signals. I dont have eiro, so I cant compare with others here, but I will get one. I can put sensivity so high that coin is detected 30 cm far but then I can detect my hand - no good

I am using tl071 instead of LM308. TX Frequency is 14.1khz voltage is about 17Vpp. I am using external pot for GND bal.
Actually disc is more sensitive than all metal mode. (about 3cm more) I dont know why

First I tried nulling with unshielded coils and I could get about 20mVpp meassuring at U101 pin 7. Then I shielded it with adhesive Al tape and after that, my min null became about 1.5Vpp.
Sensivity isnt much reduced, but I think that large null is because Al tape is pretty thick and very conductive. I tried several methods of nulling, but finally I just adjusted coil so that it almost mach waveforms shown in pdf called TGS_Compare1. After that Its working pretty good.

It just my first one and my future plans are:
1. Building another TGSL or more. I have one Silverdog PCB half assembled...
2. Try LM308. I have ordered a few..
3. Wind different coils. Now I have one coil with spacers of 2.5mm drying. Will post results.
4. Try to find different shielding method.
5. Build a coil enclosure using vacuum forming. Never done it before.
6. Test it on real conditions in real soil
7. Add volume control
8. Read another 1000 posts posted in geotech Now I started to read TGSL tining&troubleshooting - I simply love that tread - so much info!!

All I want is to get famous 30-35cm.

oK . Pretty long text I have made
One question: Is this normal that waving ferrite rod together with coin I dont get response? Isnt that mean that in soil I will not get response for that coin? Single coin is detected....

Later I will post some pics

[simonbaker]

i don't know.. i am dumb or what i can't find any cu enamelled wire within 0.27-0.28 on ebay only one i found is 0.3 is it possible that there is nothing such like 0.27 - 0.28?

I think .3 mm should work fine. You might look for 29 gauge wire too, maybe not common though. 28 or 30 gauge might work too, that's what I often use.

-SB