TGSL Tuning & troubleshooting

[mikebg]

Unfortunately the only simulator which I'm able to use is CircuitMaker. I'll try with it a.s.a.p., however I'm not saying that the +20° phase is not achievable, but that to get it I'd have to use cap values much too far from the prescribed one and with that the circuit dos not behave well anymore in all its aspects.

Stefano, the CircuitMaker makes it correct:

[dfbowers]

That's very interesting. Maybe worth having two different kinds of coils depending on whether grass or not. Would you prefer original (Ivconic) wiring for beach for example?

I like idea of red and black for TX - also because white and green often twisted pair in good USB cables, good for low noise for RX.

-SB

Excellent point about the white and green being twisted.. I forgot about that. I did notice that some cables are different in terms or wire size. I just picked up two USB-2 cables.. both different.

Yes, I would prefer Ivconics coil wiring for the beach as I feel (subjective) that it's a little more sensitive... Huge difference in wet grass though!

Don

[mikebg]

Phase shift in TX tank circuit

This simulation shows how R1 and C1 cause phase lead in reference voltage. In ideal case (without energy loss in TX tank circuit) the reference phase should be 90 deg, ie the same as the phase of electromotive voltage E induced in the RX coil (see posting #376).

[Qiaozhi]

Hi Qiaozhi:

Yes and no.

Your choice for the null is good because it gives you a reasonable voltage range on the GB JFet due to capacitor C12 while you adjust the GB pot. I would go so far as to recommend a null that keeps the voltage on capacitor C15 fairly non-negative over the Disc pot range.

But you have not addressed the actual ground balancing of the target signal. The null signal phase does not affect the phase range at all (significantly) of the GB or DISC.

I have tested this many times and I just repeated it. I set the null on one side of the minimum, then on the other, so the RX null signal is about 180 deg different in each case. And the ground balance for ferrite is the same in each case. Same for the discrimination of a US Nickel. The null does not affect it. Not for my TGSL, maybe for other detectors.

---------

It's what the math says also. When a target is present, the Rx signal is the sum of the null signal (A) and the target signal (B), two independent signals.

Vrx(t) = Asin(wt + phi1) + Bsin(wt + phi2).

If you put them into a synchronous detector, the phi1 of the first signal does not affect the detection of the second signal. Only the amplitude and phase of the second signal determines its response.

When we ground balance, we adjust the pot to null out the signal of phase phi2 -- it doesn't matter what phi1 is.

The null signal [Asin(wt + phi1)] makes a constant voltage on C12 and C15 which never appears at the output of LM308. It simply does not affect the detection of the ferrite; only the relative phase of the ferrite signal and the ground balance sync pulse make the audio beep that we hear during ground balancing.

For me, theory and tests show the same thing. The null phase does not matter, at least not for the reasons that have been often stated. Instead I think it is useful for keeping the JFets forward biased or for achieving maximum coil overlap, etc. I'm still open to other evidence, but I need to see it.

Regards,

-SB

Unfortunately you are still misunderstanding my point. Of course, the phase-shift caused by the target and the initial phase-shift in the cols are two different things. That much is obvious. Also, as far as the target shift is concerned it doesn't matter how much initial phase-shift there is in the coil. There is no argument there. I think you're convinced I'm saying something that I'm not.

Let's try and put this really simply:
There has been a lot of discussion in this forum by many people about coil balancing. Some members still think that the coils should be nulled to zero. You only have to connect a commercial Tesoro coil to your detector to see that this is incorrect. It doesn't really matter if the initial phase-shift is exactly 20 degrees, or even 200 degrees (depending on the coil), the important thing is that you must not make your coil adjustment too different to the Tesoro coils, otherwise you will not be able to position the GEB sample pulse over the zero-crossing (or close to it for real ground / ferrite) and consequently cannot achieve ground balance. Also, the DISC control relies on the initial phase-shift being reasonably close to the original coils, otherwise the markings on the control panel for the DISC control will be wrong. In other words, with the DISC control set to minimum, Tesoro detectors will reject iron. This may not be the case for a badly adjusted coil.

When the TGSL was initially cloned, a lot of people were simply setting the coil balance to achieve the lowest voltage possible. If you try this experiment you will discover that the detector will not ground balance because the GEB sample pulse cannot be adjusted to the correct position. This is not just theory. I've done the simulations and tested the circuits in practice. From these results I have even designed and built my own detector. I have no problems using many different Tesoro coils, homemade coils, or even some coils from other manufacturers.

Finally, I think you are trying too hard to get the extra few cms that some constructors are managing to achieve. When you make your detector supersensitive on the bench, it will react differently in the field. As an example, I have a Fisher 1266-XB in my collection, which has been calibrated correctly according to the service manual. As most people know, this is a very sensitive detector. But once you take it out in the real world, it's almost impossible to use it at maximum sensitivity. My own homebrew detector is much more stable and achieves the same depth in the field, plus I don't find myself digging an enormous hole and finding a nail!

[dfbowers]

Unfortunately you are still misunderstanding my point. Of course, the phase-shift caused by the target and the initial phase-shift in the cols are two different things. That much is obvious. Also, as far as the target shift is concerned it doesn't matter how much initial phase-shift there is in the coil. There is no argument there. I think you're convinced I'm saying something that I'm not.

Let's try and put this really simply:
There has been a lot of discussion in this forum by many people about coil balancing. Some members still think that the coils should be nulled to zero. You only have to connect a commercial Tesoro coil to your detector to see that this is incorrect. It doesn't really matter if the initial phase-shift is exactly 20 degrees, or even 200 degrees (depending on the coil), the important thing is that you must not make your coil adjustment too different to the Tesoro coils, otherwise you will not be able to position the GEB sample pulse over the zero-crossing (or close to it for real ground / ferrite) and consequently cannot achieve ground balance. Also, the DISC control relies on the initial phase-shift being reasonably close to the original coils, otherwise the markings on the control panel for the DISC control will be wrong. In other words, with the DISC control set to minimum, Tesoro detectors will reject iron. This may not be the case for a badly adjusted coil.

When the TGSL was initially cloned, a lot of people were simply setting the coil balance to achieve the lowest voltage possible. If you try this experiment you will discover that the detector will not ground balance because the GEB sample pulse cannot be adjusted to the correct position. This is not just theory. I've done the simulations and tested the circuits in practice. From these results I have even designed and built my own detector. I have no problems using many different Tesoro coils, homemade coils, or even some coils from other manufacturers.

Finally, I think you are trying too hard to get the extra few cms that some constructors are managing to achieve. When you make your detector supersensitive on the bench, it will react differently in the field. As an example, I have a Fisher 1266-XB in my collection, which has been calibrated correctly according to the service manual. As most people know, this is a very sensitive detector. But once you take it out in the real world, it's almost impossible to use it at maximum sensitivity. My own homebrew detector is much more stable and achieves the same depth in the field, plus I don't find myself digging an enormous hole and finding a nail!

Qiaozhi and Simon,

All very good points! So, maybe we can take this to the next level..
As far as nulling to as close to zero as possible.. Maybe develop a new spec for a null voltage? I think it's time to look at a few commercial Tesoros to see what they are doing. I only have a Cibola and 2 coils, but I can open it and compare two coils at least. Back shortly..

In regards to maximum distance on air tests. This may be a good point of reference when building and testing.. A super sensitive detector is not really of any practical value in the field. The real trick is to have the best sensitivity along with the best stability. As soon as a detector goes out the door, what matters the most is making use of what you have put together. Proper ground balance, a minimum disc level that you can live with, proper coil discipline and in some cases reduced sensitivity. In the long run, I have found out that the ground is the limiting factor and increased levels of sensitivity is of little use.

Case in point.. I have a Nautilus DMC IIb.. adjustable coil null, Tx power, Rx gain, ground balance and the ability to monitor both channels at the same time. It does not go significantly deeper than any other detector that I have where I live. I have found that the best performance is a compromise.. I set the coil balance (null) slightly off to one side, reduced Tx power and Rx gain and a dead on or slightly positive ground balance. It actually detects deeper than when trying to max everything out.

Don

[Qiaozhi]

Qiaozhi and Simon,

All very good points! So, maybe we can take this to the next level..
As far as nulling to as close to zero as possible.. Maybe develop a new spec for a null voltage? I think it's time to look at a few commercial Tesoros to see what they are doing. I only have a Cibola and 2 coils, but I can open it and compare two coils at least. Back shortly..

In regards to maximum distance on air tests. This may be a good point of reference when building and testing.. A super sensitive detector is not really of any practical value in the field. The real trick is to have the best sensitivity along with the best stability. As soon as a detector goes out the door, what matters the most is making use of what you have put together. Proper ground balance, a minimum disc level that you can live with, proper coil discipline and in some cases reduced sensitivity. In the long run, I have found out that the ground is the limiting factor and increased levels of sensitivity is of little use.

Case in point.. I have a Nautilus DMC IIb.. adjustable coil null, Tx power, Rx gain, ground balance and the ability to monitor both channels at the same time. It does not go significantly deeper than any other detector that I have where I live. I have found that the best performance is a compromise.. I set the coil balance (null) slightly off to one side, reduced Tx power and Rx gain and a dead on or slightly positive ground balance. It actually detects deeper than when trying to max everything out.

Don

Hi Don,

I completely agree with everything you said. The ground is the limiting factor, and many workshop wonders fail miserably in the field because of this little problem.
There's nothing worse than an unstable detector. It only leads to frustration.

[greybeard]

Never mind what I tried and did not work.. it doesn't matter. What finally DID work is this:

A USB-2 cable, using all 5 conductors (2 power, 2 signal + ground).

* RED and BLACK wires to run the Tx coil (since they are a heavier gauge).

* GREEN and WHITE for the receive circuit.

* Rx and Tx shields tied together in the coil and wired to cable shield.
Cable shield is then tied to ground on PCB.

* Rx and Tx coils NOT connected to ground inside the coils at all.
In other words, all active parts of the coils do not touch the grounding
system in any way.. Only the cold end of the Tx circuit is connected to
ground on the PCB.

Don

apologies for the slight off topic response.
great tip with the USB cable. i have a HH1 on the bench and have tried RG58 and RG59 coax with varying results. Changed to the USB cable and have great improvement with reduced capacitance.
thanks.
rod

[simonbaker]

Unfortunately you are still misunderstanding my point. Of course, the phase-shift caused by the target and the initial phase-shift in the cols are two different things. That much is obvious. Also, as far as the target shift is concerned it doesn't matter how much initial phase-shift there is in the coil. There is no argument there. I think you're convinced I'm saying something that I'm not.

I think you think I think something I don't think. (BTW, I'm not trying to be argumentative, I really think this null subject is important mainly because there may (or may not) be myths that have people barking up the wrong tree.)

We seem to analyze this differently. You talk about target "shift". There is no target "shift" in my way of thinking -- a target signal simply appears superimposed on the steady null signal. The target signal has its own amplitude and phase. Same frequency as null of course.

Let's try and put this really simply:
There has been a lot of discussion in this forum by many people about coil balancing. Some members still think that the coils should be nulled to zero. You only have to connect a commercial Tesoro coil to your detector to see that this is incorrect. It doesn't really matter if the initial phase-shift is exactly 20 degrees, or even 200 degrees (depending on the coil), the important thing is that you must not make your coil adjustment too different to the Tesoro coils, otherwise you will not be able to position the GEB sample pulse over the zero-crossing (or close to it for real ground / ferrite) and consequently cannot achieve ground balance. Also, the DISC control relies on the initial phase-shift being reasonably close to the original coils, otherwise the markings on the control panel for the DISC control will be wrong. In other words, with the DISC control set to minimum, Tesoro detectors will reject iron. This may not be the case for a badly adjusted coil.

I want to break through our communication barrier, so I'll say:

Why why why?

Why do you feel the "initial phase-shift " (null phase) affects the GEB or DISC pot settings? You state it, but can you give an explanation, equation, anything? I have shown what I think are reasons the GEB and DISC pot settings relate only to the target signal, which is not affected by the "initial phase-shift".

I get it that you don't agree. You think the initial phase shift (null phase) somehow affects where the GEB and DISC pot settings take effect. Can you just give some kind of detail on that?

I'll restate my thinking again:

We set GEB by ear, listening to when the ferrite signal disappears -- it has nothing to do with the null signal phase. When the audio beep disappears, we know we have put the GEB sample pulse over the zero crossing of the ferrite signal. It's the only way we know. You cannot see the ferrite signal by itself on an oscilloscope -- the null signal dominates it.

So you can change the null phase around all you want and you should find that you will set the GEB pot (and disc pot for nickel) virtually the same in all cases. That's what I find.

When the TGSL was initially cloned, a lot of people were simply setting the coil balance to achieve the lowest voltage possible. If you try this experiment you will discover that the detector will not ground balance because the GEB sample pulse cannot be adjusted to the correct position. This is not just theory. I've done the simulations and tested the circuits in practice. From these results I have even designed and built my own detector. I have no problems using many different Tesoro coils, homemade coils, or even some coils from other manufacturers.

I would like to see your simulations and if possible your tests. Maybe I can then understand where we differ in understanding this. Is it possible you are referring to a different kind of MD than the TGSL?

Your testing and mine disagree. I can change the null phase without affecting the DISC and GEB control pots. You say it does affect them. There is room for error in my tests because of noise, etc. and I may be biased. But I'm pretty sure that is what I find.

Finally, I think you are trying too hard to get the extra few cms that some constructors are managing to achieve. When you make your detector supersensitive on the bench, it will react differently in the field. As an example, I have a Fisher 1266-XB in my collection, which has been calibrated correctly according to the service manual. As most people know, this is a very sensitive detector. But once you take it out in the real world, it's almost impossible to use it at maximum sensitivity. My own homebrew detector is much more stable and achieves the same depth in the field, plus I don't find myself digging an enormous hole and finding a nail!

Totally agree! I simply want to build a baseline TGSL, so I want to match what has been done. After that, I hope to try many mods and test against real conditions. Right now I'm trying to develop building techniques that are equivalent (while not identical) to others, and I use the air test just as a way to compare the outcome.

Regards,

-SB

[Aziz]

Hi Vladimir,

Amendment # 312

a small piece of ferrite should do a good job as well. But the ferrite does not generate resistive response like an Al plate. Only reactive response and can be used externally to balance the detector coil.

Aziz

[simonbaker]

Qiaohzi and me

I may have just had a ***flash*** that may help (a little) reconcile Qiaohzi and my ideas on nulling.

There may be more than one use/goal for ground balancing.

The first, which I have been focusing on, is to make the detector immune to targets like ferrite and typical ground material.

The second may be.... to make the detector immune to fluctuations in the null signal. Yeah!

I always assume null signal is constant amplitude and phase. But suppose null signal has some "amplitude noise" (for example oscillator amplitude fluctuations).

If the null phase is set so that a change in amplitude makes the GB and DISC channels both move in the same direction, then it potentially could make a chatter beep (yes?).

However, if we choose a null signal phase that makes the GB and DISC channels move in opposite directions, then fluctuations in the null signal should not make chatter because the LM393 logic (U106) cancels them out.

So if we try to achieve the second goal and the first goal, we probably need to choose a null phase similar to what Qiaohzi says -- otherwise, we can't satisfy both goals simultaneously.

My feeling is that what Qiaohzi has been describing is setting the null phase to satisfy the second goal.

-SB

P.S. There is a subtle part of the second goal: making the MD immune to small phase shifts in the null signal (such as produced by knocking the coils). You want to pick the null phase such that a small change in phase creates small voltage changes in the DISC and GB channels that move in opposite directions. I don't know if this requires a further small adjustment of the null phase, need to think it out.

[Qiaozhi]

Both the GEB and DISC sample pulses are triggered from the TX output. As you adjust these controls they move the position of the sample pulses. Obviously, if your coil is nulled such that the initial phase-shift places the zero-crossings of the RX signal beyond the range of the GEB control, then you will not be able to ground balance. Also. you need to set the coil balance such that the DISC sample pulse will be positioned at the peak of the RX signal when turned fully counterclockwise. Therefore you cannot simply choose any old initial phase-shift you want. You need to use the same coil nulling as Tesoro. With the correct initial phase-shift the GEB control will be somewhere in the middle, and the DISC control will be at minimum.
It's as simple as that.

[simonbaker]

Both the GEB and DISC sample pulses are triggered from the TX output. As you adjust these controls they move the position of the sample pulses. Obviously, if your coil is nulled such that the initial phase-shift places the zero-crossings of the RX signal beyond the range of the GEB control, then you will not be able to ground balance. Also. you need to set the coil balance such that the DISC sample pulse will be positioned at the peak of the RX signal when turned fully counterclockwise. Therefore you cannot simply choose any old initial phase-shift you want. You need to use the same coil nulling as Tesoro. With the correct initial phase-shift the GEB control will be somewhere in the middle, and the DISC control will be at minimum.
It's as simple as that.

We're having fun now...

Not so simple at all.

What do mean by RX signal? Let's start there. I mean the ferrite signal, what do you mean?

Regards,

-SB

[Qiaozhi]

We're having fun now...

Not so simple at all.

What do mean by RX signal? Let's start there. I mean the ferrite signal, what do you mean?

Regards,

-SB

RX is short for the "received" signal. When you are adjusting the coils it is the signal from the output of the preamp with no metallic targets near the search head.

Forget about the ferrite for the moment. It is just a convenient way of setting the ground balance without having to go outside and bob the coil up and down.

[simonbaker]

RX is short for the "received" signal. When you are adjusting the coils it is the signal from the output of the preamp with no metallic targets near the search head.

Forget about the ferrite for the moment. It is just a convenient way of setting the ground balance without having to go outside and bob the coil up and down.

Ok, I understand RX is the received signal. I think we have to be very specific here so I understand what you are saying.

I am assuming the following steps for setting up a coil and ground balancing a TGSL detector.

1. Adjust coils to create some "null" signal. Which exact null signal we will discuss in a moment.

2. Adjust GEB pot to ground-balance the detector using a ferrite target. The pot is turned until the ferrite target no longer makes a clear beep. I would assume the DISC pot is set for minimum discrimination during this procedure.

Now, I think you feel that the choice of the null signal (particularly the phase) in step 1 influences the position of the GEB pot setting to ground-balance the ferrite target in step 2. My feeling is that it does not.

Am I stating that correctly? If so, can you explain in detail why the null signal phase would affect the GEB pot setting? They seem independent to me, especially in terms of how the synchronous detector processes signals.

Regards,

-SB

[Vladimir]

Hi Vladimir,



a small piece of ferrite should do a good job as well. But the ferrite does not generate resistive response like an Al plate. Only reactive response and can be used externally to balance the detector coil.

Aziz

Perhaps there was some confusion in terms, I did not mention the ferrite, but usually iron. The truth is not given a good reactive response but in extreme necessity, and it may help.
Regards