TGSL Tuning & troubleshooting

[Krzysztof]

I greet.
In every the technique the RF the screen enfeebles about individual percentage the working and the tenderness of system but he was done for stability - therefore the decrease the slow EMI and it is the quick variables more valuable.
The already simplest systems BFO without screen are more tender (slightly) but difficult in repeatability and stability.
Additional nonconnected nonferrous metal in neighbourhood of coil behaves in dependence from kind variously and distance from the ground - variable unstable capacity.
This never nothing good it does not foretell it - if would better was not at all!
Christopher

[simonbaker]

Very interesting.
I'll wait to see possibly for confirmation...

Yes, needs confirmation. Some people find nulling critical. But in all my tests, slight changes in null did not make sudden improvement or reduction in detection. But I never made a coil that achieved 30 cm, so perhaps it works differently when everything is perfect. But for now, that is my theory.

-SB

[Krzysztof]

Dennis the Mennis
"Can you tell me what the difference is , in a detection percentage % rate , between a shielded coil and an unshielded coil ?
For example : with a shielded coil the improvement in detection is 15 % ... "

Until now Coil to screen the EMI reduction, should be formally unchanged. Only the additional capacity of the coil is a free screen can be unpredictable. Better link.
Krzysztof

[Stefano]

Null phase not as important as most think.

I believe that the phase relationship of the RX null signal (compared to the TX signal) is completely independent of the phase of the received target signal.

The most important phase relationship is for the received target signal -- which is too small to see. The only way to estimate it is using the discrimination control, and see where it cuts in and out.

The phase of the nulled signal is easily shifted by moving the TX and RX coils as we all know. That's what we do when we null a coil. But it doesn't change the phase of the target signal.

The importance of the nulled signal phase for the TGS design (I believe), is mostly on the bias voltage created by the detector output. We want a positive voltage so the JFet won't become forward biased. But otherwise, I don't think the phase of the null signal matters that much to the detector, because it doesn't affect the phase of the target signal, which rides on top of it.

-SB

Very interesting.
I'll wait to see possibly for confirmation...

[Qiaozhi]

Can you tell me what the difference is , in a detection percentage % rate , between a shielded coil and an unshielded coil ?
For example : with a shielded coil the improvement in detection is 15 % ...

regards ,

Dennis the Mennis

I am unable to give you an objective answer to your question, but subjectively I don't think there is any noticeable difference.

[ivconic]

Thanks Ivconic / Qiaozhi ,

I guess I've spend hours for nothing removing the adhesive Al foil .
A simple wire should have solved that.

Can you tell me what the difference is , in a detection percentage % rate , between a shielded coil and an unshielded coil ?
For example : with a shielded coil the improvement in detection is 15 % ...

regards ,

Dennis the Mennis

I never tried unshielded coil so far..

[Dennis the Mennis]

No problem as Qiaozhi said.
But there is another thing; there is good connection from the inner side of tape too!
You can not realize that right now, but once Al tape applied over coil and if during wrapping it was constarined good, it makes good contacts too.
I discovered that by coincidence. Once i rend the tape during wrapping over coil. It was almost at the end until gap! Ohh i was angry! Than i started to wrap the rest 2-3cm over existed Al layer and finished it until gap. I was thinking: " The hell...i will scrap it anyway...." . But than checked with ohmmeter at both sides of gap and it was continous!? I moved it , swinged it a bit...no problem! So i used that coil also, not scraped it.
That coil is in function until today.
Later in many occasions i rechecked that and it is true. Somehow adhesive is "spacing" when tape wraped and on wider areas there is pretty good conntact from the inner side also. Besides..if you wrap it spiral as Qiaozhi said, than you have "upper" side of tape pressed from "bottom" adhesive side of tape with much chances to make good contact.
However...this is not important at all. Al tape wraped in sprial is forming continous shield over coil , so really doesn't matter if there is no contact from adhesive side..

Thanks Ivconic / Qiaozhi ,

I guess I've spend hours for nothing removing the adhesive Al foil .
A simple wire should have solved that.

Can you tell me what the difference is , in a detection percentage % rate , between a shielded coil and an unshielded coil ?
For example : with a shielded coil the improvement in detection is 15 % ...

regards ,

Dennis the Mennis

[simonbaker]

I've checked components around pre-amp and they're ok.
So I've tried to re-adjust coil nulling: I've made a 7 turns-5cm loop and connected in series with rx coil.
Then moving it I' ve verified that I'm able to change phase shift and tension, but noticed that the minimum voltage doesn't correspond to the correct phase shift.
I've found a position where I get 4us delay in the rx signal respect to tx which should be 20 degrees (70/360*20)=3.8 but neither in this position nor in any other, I'm able to get a full good working. I mean I have always not a single but a double or even triple sound if target is swept slow and near the coil, and big iron is not completely rejected in discrimination mode.
Noticed also that ground connection of Rx coil shifts a bit the phase:
increments if connected to purple wire and decrements if connected to grey wire.
Detection depth is bad as before too.

So:
Is the method I used with the coil correct to fix the phase and offset of possibly not good original coil?

Is it correct a time difference of 4us for a DD coil (measured at output of pre-amp) for a required phase diff of 20°?

Which side of Rx coil should I connect to ground?

Should a DD coil give just a single tone for each target sweep or is it normal have a double tone?

Just a quick comment that I believe is true (and I've said before):

Null phase not as important as most think.

I believe that the phase relationship of the RX null signal (compared to the TX signal) is completely independent of the phase of the received target signal.

The most important phase relationship is for the received target signal -- which is too small to see. The only way to estimate it is using the discrimination control, and see where it cuts in and out.

The phase of the nulled signal is easily shifted by moving the TX and RX coils as we all know. That's what we do when we null a coil. But it doesn't change the phase of the target signal.

The importance of the nulled signal phase for the TGS design (I believe), is mostly on the bias voltage created by the detector output. We want a positive voltage so the JFet won't become forward biased. But otherwise, I don't think the phase of the null signal matters that much to the detector, because it doesn't affect the phase of the target signal, which rides on top of it.

-SB

[ivconic]

I've tried to shield a coil with self-adhesive aluminium tape but due to the glue / adhesive layer there was no conductance between the pieces of tape .
With kitchenfoil you can measure a couple of ohms on both sides of the airgap.

Is it a problem when the alu tape without 'conductance' is used ?


regards

Dennis the Mennis

No problem as Qiaozhi said.
But there is another thing; there is good connection from the inner side of tape too!
You can not realize that right now, but once Al tape applied over coil and if during wrapping it was constarined good, it makes good contacts too.
I discovered that by coincidence. Once i rend the tape during wrapping over coil. It was almost at the end until gap! Ohh i was angry! Than i started to wrap the rest 2-3cm over existed Al layer and finished it until gap. I was thinking: " The hell...i will scrap it anyway...." . But than checked with ohmmeter at both sides of gap and it was continous!? I moved it , swinged it a bit...no problem! So i used that coil also, not scraped it.
That coil is in function until today.
Later in many occasions i rechecked that and it is true. Somehow adhesive is "spacing" when tape wraped and on wider areas there is pretty good conntact from the inner side also. Besides..if you wrap it spiral as Qiaozhi said, than you have "upper" side of tape pressed from "bottom" adhesive side of tape with much chances to make good contact.
However...this is not important at all. Al tape wraped in sprial is forming continous shield over coil , so really doesn't matter if there is no contact from adhesive side..

[Qiaozhi]

I've tried to shield a coil with self-adhesive aluminium tape but due to the glue / adhesive layer there was no conductance between the pieces of tape .
With kitchenfoil you can measure a couple of ohms on both sides of the airgap.

Is it a problem when the alu tape without 'conductance' is used ?


regards

Dennis the Mennis

That's not a problem as long as you loosely spiral wrap a drain wire around the whole coil. In fact, that's the method I use.

[Dennis the Mennis]

The aluminum foil method is a tried and tested method that works. It does not adversely affect the sensitivity. as long as you leave a gap in the foil and insulate the two coils from each other where they overlap.

I've tried to shield a coil with self-adhesive aluminium tape but due to the glue / adhesive layer there was no conductance between the pieces of tape .
With kitchenfoil you can measure a couple of ohms on both sides of the airgap.

Is it a problem when the alu tape without 'conductance' is used ?


regards

Dennis the Mennis

[Qiaozhi]

I'm still not ready for it, anyway I was wondering about the best shieldind solution in the coil making process:

I have 10" Hays shell which is the one I'm planning to use for tgsl.
I was thinking of 2 solutions: one is the sistem explained by ivconic: alluminium kitchen foil, bare wire around it for the contact to the screen, and, once nulled, everything sinked in a epoxy cast. The advantage is that one can lock the windings in the right nulling position just putting hot glue spots directly on the shell.
The other solution is to glue a bare wire on the 2 shells with cyanoachrilate, spray the inside with spray graphite (I have DueCI N77) as Tesoro does, but then how to lock the coil onto the shell for the nulling? I suppose that the glue can't attach well on the graphite sprayed...
Isn't the first method worse in therms on sensibility? I've seen that for PIs the second is preferable...
I want to be sure because that's the only shell I have and don't want to waste it...

The aluminum foil method is a tried and tested method that works. It does not adversely affect the sensitivity. as long as you leave a gap in the foil and insulate the two coils from each other where they overlap.

[Stefano]

I'm still not ready for it, anyway I was wondering about the best shieldind solution in the coil making process:

I have 10" Hays shell which is the one I'm planning to use for tgsl.
I was thinking of 2 solutions: one is the sistem explained by ivconic: alluminium kitchen foil, bare wire around it for the contact to the screen, and, once nulled, everything sinked in a epoxy cast. The advantage is that one can lock the windings in the right nulling position just putting hot glue spots directly on the shell.
The other solution is to glue a bare wire on the 2 shells with cyanoachrilate, spray the inside with spray graphite (I have DueCI N77) as Tesoro does, but then how to lock the coil onto the shell for the nulling? I suppose that the glue can't attach well on the graphite sprayed...
Isn't the first method worse in therms on sensibility? I've seen that for PIs the second is preferable...
I want to be sure because that's the only shell I have and don't want to waste it...

[ivconic]

If you read most people's posts, after constructing a TGSL or similar design, they usually end up with around 20cm detection distance for a 1 euro coin when using an 8" coil. In general, the detection distance for a 1 euro coin is approximately equal to the coil diameter.

However, by using different coil inductances, so that the TX and RX frequencies are closer together, the sensitivity can be increased. But there are a number of problems with doing this. Firstly, it becomes more difficult to balance the coils. Secondly, you may not be able to achieve the correct initial phase-shift. Thirdly, the residual RX amplitude could overload the RX pre-amp.

It's a fine balancing act. That's why the manufacturers (such as Tesoro) go for a more conservative approach to make the coils easier to manufacture.

The bottom line is that there is unlikely to be anything wrong with the electronics. Try testing the circuit with a commercial coil to see what I mean.

Exactly.
Proper coil nulling is not an easy job. I think most people understood nulling as just simple task. Pitty, it is not like that.
At some coils i spend 5-6 hours to null them properly. Amateur conditions do not offering me comfort, so nulling is time&nerves consuming tough task.
Than again...at some coils i achieve good balance in just 20 minutes.
It is all about quality made and details.
Usually i am using hot melted plastic glue gun and put "punkts" (points) over the coil to attach it to holder good. Than i must wait hot glue to dry and get cold. Than again to correct balance by slitest (parts on millimeter) movements of some coil segments. Than again to wait to see influence of temperature changes in workshop....and again..and again. All the time RX coil is attached to millivoltmeter and observed. TX coil is attached to TGSL TX out.
Sometimes process is going pretty smooth and everything ends in 20 minutes....but sometimes it took time to be done. It can took hours sometimes.
Once coils are balanced well i use joint filler (2 components epoxy) to finish coils. It all must be done in one move, fast, not to allow temperature changes to affect balance. If you balance coils and let them overnight....those will move for sure and spoil the balance much.
Tough job to make good coil in amateur conditions. Not easy.


....

That's why i am thinking to give up of making DIY coils finally, lately. It is more cheaper to buy original coil and adapt detector to work good with it.

[Qiaozhi]

If you read most people's posts, after constructing a TGSL or similar design, they usually end up with around 20cm detection distance for a 1 euro coin when using an 8" coil. In general, the detection distance for a 1 euro coin is approximately equal to the coil diameter.

However, by using different coil inductances, so that the TX and RX frequencies are closer together, the sensitivity can be increased. But there are a number of problems with doing this. Firstly, it becomes more difficult to balance the coils. Secondly, you may not be able to achieve the correct initial phase-shift. Thirdly, the residual RX amplitude could overload the RX pre-amp.

It's a fine balancing act. That's why the manufacturers (such as Tesoro) go for a more conservative approach to make the coils easier to manufacture.

The bottom line is that there is unlikely to be anything wrong with the electronics. Try testing the circuit with a commercial coil to see what I mean.