TGSL Tuning & troubleshooting

[pebe]

UHF and MD have only a theoretical resemblance. If you to be used by your recommendations in practice, you will see that it will not want to do. The basic problem is that you do not understand the essence of metal detectors. The picture I posted is a proven method that works for a long time in the real field. I found a lot of money, the detector is very stable and I am proud to make changes and updates that I did. If you did something you can practically give the presentation here on the forum. Theory, unless supported by practical testing, it makes no sense.

P.S. This picture does not show coaxial cable !.... You'll carefully read the previous posts

Well, your 304 posting certainly looks like coax cable - I don't know of a flat braided cable. In any event, the wire that feeds current to the Tx coil is also part of the RX circuit and will cause a voltage drop in both circuits due to the resistance of the screening wire. The Rx current introduced in this way will be in phase with the Tx current so overall the circuit will act like the coils are overcoupled. Obviously, you take care of that when you null the coils.

I wouldn't mind betting that if you separated the screens along their length and only connected them at the PCB, you would need to null the coils again - but the screening would be just as effective.

[Vladimir]

Hi Vladimir -- those voltages were at output of LF353 pin 7, not directly at the RX coil leads. Maybe still high, I don't know. It's just the way my coils are. I'll check dfbowers' coil that he sent me...

I just checked dfbowers coil, it is about 1V pp at same point. It seems a little high, but again it is amplified by LF353. I wonder if it has shifted a little during shipping or my handling. I tried deforming the coil head with pressure of my hands, but it is very solid and the null does not change at all -- nice solid coil head I must say!

Regards,

-SB

Perhaps the balance is disturbed during the closure of the coil, the attempt to use small plates such as aluminum 3x3 mm or less and move them to the top surface of the coil. At some point you'll notice a sudden drop in voltage on the pin 7th Thus it is possible to subsequently adjust the balance of the coil. When you get 0.0V then glue strengthen Al plate. If you need to get the ideal voltage, aluminum plate may be other dimensions. If you do not succeed with aluminum, attempt to use the tiles from the iron. It depends in which direction is phase shifted.
This procedure does not affect adversely the work of the detectors on the ground.

It is best to balance the coil measured by the pin 7 .... it is wrong to measure directly on the coil, is actually the result of more accurate to pin 7.
Regards

[simonbaker]

0.2V pp is a very large voltage......for obtain maximum sensitivity is required voltage 0.00V pp. for this type of detector, of course, but this is not the rule in some other concept.

Hi Vladimir -- those voltages were at output of LF353 pin 7, not directly at the RX coil leads. Maybe still high, I don't know. It's just the way my coils are. I'll check dfbowers' coil that he sent me...

I just checked dfbowers coil, it is about 1V pp at same point. It seems a little high, but again it is amplified by LF353. I wonder if it has shifted a little during shipping or my handling. I tried deforming the coil head with pressure of my hands, but it is very solid and the null does not change at all -- nice solid coil head I must say!

Regards,

-SB

[Vladimir]

Vladimir,

Can you give details about your coil? Particularly, the mechanical details?
Thanks
Don

DD coil, diameter 27cm. 0.27mm wire
RX = 6.5mH, 21-23oma (107 wires)
TX = 6.0mH, 19-22oma (98 wires)
"D" shape size 255 * 137mm

Both Rx and Tx coils were pinched synthetic thread and then taped with super glue. Over this is a layer of insulator strips, a layer of Al foil - self-adhesive - and the final one more layer of insulator strips. Both coils are adjusted and tightened the plastic body with epoxy resin and then pour the Polyurethane-insulation , the body of the coils final closed with reinforced fiberglass.
Way of connecting with a cable I gave in previous post # 304

In this way, I got a coil with good electrical, mechanical and thermal stability. Coil is lightweight and does not change the characteristics of different weather conditions.

[dfbowers]

UHF and MD have only a theoretical resemblance. If you to be used by your recommendations in practice, you will see that it will not want to do. The basic problem is that you do not understand the essence of metal detectors. The picture I posted is a proven method that works for a long time in the real field. I found a lot of money, the detector is very stable and I am proud to make changes and updates that I did. If you did something you can practically give the presentation here on the forum. Theory, unless supported by practical testing, it makes no sense.

P.S. This picture does not show coaxial cable !.... You'll carefully read the previous posts

Vladimir,

Can you give details about your coil? Particularly, the mechanical details?
Thanks
Don

[Vladimir]

Hi Vladimir.
The use of coaxial cable does not make sense here. It does not matter that the hot wires are in the middle, so the circuit and closes the top, although the balance.
The measuring technique used two separate wires from each coil above the common screen or two coaxial cables for each coil, it is equivalent.
Coils connected together in one point, and nothing more.
At the top of braided coaxial cables together at one point to GND on PCB. Braided coaxial cable is not connected to anything at the bottom.
A separate simple (in isolation) data cable between the point GND / PCB at the top and bottom shield.
Then there is no possibility of countervailing currents.
It works well in the UHV technique, and MD / PI also have high di/dt.
Best regards Chris.

UHF and MD have only a theoretical resemblance. If you to be used by your recommendations in practice, you will see that it will not want to do. The basic problem is that you do not understand the essence of metal detectors. The picture I posted is a proven method that works for a long time in the real field. I found a lot of money, the detector is very stable and I am proud to make changes and updates that I did. If you did something you can practically give the presentation here on the forum. Theory, unless supported by practical testing, it makes no sense.

P.S. This picture does not show coaxial cable !.... You'll carefully read the previous posts

[Krzysztof]

Hi Vladimir.
The use of coaxial cable does not make sense here. It does not matter that the hot wires are in the middle, so the circuit and closes the top, although the balance.
The measuring technique used two separate wires from each coil above the common screen or two coaxial cables for each coil, it is equivalent.
Coils connected together in one point, and nothing more.
At the top of braided coaxial cables together at one point to GND on PCB. Braided coaxial cable is not connected to anything at the bottom.
A separate simple (in isolation) data cable between the point GND / PCB at the top and bottom shield.
Then there is no possibility of countervailing currents.
It works well in the UHV technique, and MD / PI also have high di/dt.
Best regards Chris.

[Vladimir]

Playing around with two shielded coils and null at LF353 output pin 7...

6 foot USB cable... only Tx J2-2 wire and cable shield grounded at PCB.

With Rx J2-2 wire, Tx J1-2 wire, cable shield connected at coil end of cable:
-------------------------------------------------------------
1. No coil shields connected -- null = .3V pp
2. Tx shield connected to grounded wires -- null = .4V pp
3. Rx shield connected to grounded wires -- null = .25V pp
4. Coil shields connected to each other and to cable wires -- null = .2V pp
5. Coil shields connected to each other, but not to cable wires -- null = 6V pp!!!!!!!!!!!!!!!!!!!!!!!!!

Coils were re-nulled for each test.

Air depth hard to test due to noise and loose coils, but seemed fairly same regardless of shield connections.

Test 5 very hard to understand, will check some more.

-SB

0.2V pp is a very large voltage......for obtain maximum sensitivity is required voltage 0.00V pp. for this type of detector, of course, but this is not the rule in some other concept.

[Vladimir]

If you touch the unshielded coil first to discharge field, then wave your hand, do you still get response? It's hard to say what is due to electrostatics and what is true detection of a conductive medium (like salty liquid). Can someone do that test?

-SB

Effect of capacitance reduction is secondary purpose.
If the detector responds to a handful or uneven ground, it can be adjustable ground balance that no false signals.

[Vladimir]

And Graphic...

[dfbowers]

Playing around with two shielded coils and null at LF353 output pin 7...

6 foot USB cable... only Tx J2-2 wire and cable shield grounded at PCB.

With Rx J2-2 wire, Tx J1-2 wire, cable shield connected at coil end of cable:
-------------------------------------------------------------
1. No coil shields connected -- null = .3V pp
2. Tx shield connected to grounded wires -- null = .4V pp
3. Rx shield connected to grounded wires -- null = .25V pp
4. Coil shields connected to each other and to cable wires -- null = .2V pp
5. Coil shields connected to each other, but not to cable wires -- null = 6V pp!!!!!!!!!!!!!!!!!!!!!!!!!

Coils were re-nulled for each test.

Air depth hard to test due to noise and loose coils, but seemed fairly same regardless of shield connections.

Test 5 very hard to understand, will check some more.

-SB

Yes, I can confirm your observations. Deepest null achieved with configuration #4

If I read everything right, this is the choice that Tesoro uses as well.
Don

[dfbowers]

Hi dfbowers - when you measure null, do you measure RX coil leads, or output of LF353 pin 7?

-SB

LF353 pin 7.

Don

[simonbaker]

The problem is not so much that an electrostatic field potential exists across the coil-ground capacitance, but the fact that it changes when the coil is in motion. These changes can be sensed by the detector circuitry and give a false indication. Even moving your hand across the face of an unshielded coil and can create an audio response.

If you touch the unshielded coil first to discharge field, then wave your hand, do you still get response? It's hard to say what is due to electrostatics and what is true detection of a conductive medium (like salty liquid). Can someone do that test?

-SB

[simonbaker]

Playing around with two shielded coils and null at LF353 output pin 7...

6 foot USB cable... only Tx J2-2 wire and cable shield grounded at PCB.

With Rx J2-2 wire, Tx J1-2 wire, cable shield connected at coil end of cable:
-------------------------------------------------------------
1. No coil shields connected -- null = .3V pp
2. Tx shield connected to grounded wires -- null = .4V pp
3. Rx shield connected to grounded wires -- null = .25V pp
4. Coil shields connected to each other and to cable wires -- null = .2V pp
5. Coil shields connected to each other, but not to cable wires -- null = 6V pp!!!!!!!!!!!!!!!!!!!!!!!!!

Coils were re-nulled for each test.

Air depth hard to test due to noise and loose coils, but seemed fairly same regardless of shield connections.

Test 5 very hard to understand, will check some more.

-SB

Still don't understand test 5 yet, but I found if RX coil is not grounded at the coil head to the TX ground, and coil shields also not grounded but connected together (as in test 5), that a small null can be found -- but then there is no detection!

Looking at the target signals at output of LM308, it seems phase of target is different if RX lead not grounded -- there are signals, but the GB and DISC channels go in opposite directions, essentially discriminating out the signal -- just because RX lead not grounded.

Probably not important, since current wisdom says to ground shields and TX and RX lead at coil head - just do it and you should be fine. Still, it makes me want to learn more about how target signal is received and how phase is affected by these grounding choices.

-SB

[Qiaozhi]

Exactly so, this is the essence of the whole problem .... all other matters related to this issue, is reduced to how best to take Unwanted voltage to GND. It is undesirable if any part of the electrostatic Shild found in the "air", it must be connected to GND or virtual GND (eg, SMW).

The problem is not so much that an electrostatic field potential exists across the coil-ground capacitance, but the fact that it changes when the coil is in motion. These changes can be sensed by the detector circuitry and give a false indication. Even moving your hand across the face of an unshielded coil and can create an audio response.