TGSL Tuning & troubleshooting

[Vladimir]

The shield of the cable should only be connected to the GND on the PCB. One wire from the RX pre-amp (the one that goes to the inverting input of the opamp) should be connected to the shield in the search head, not GND on the PCB. Obviously other configurations do work, but the way Tesoro does it results in less noise pickup. Some of the Chinese detectors provide the GND connection in the socket on the control box, but the reason has more to do with flexibility, allowing the coil to be used on many different designs, rather than best practice. However, be aware that each configuration changes the balance of the coil. You can try wiring the RX coil without connecting one side to the shield, or even using a center-tapped RX coil with the tap connected to the shield. It may be purely subjective, but it appears to me that Tesoro's approach is more stable, less noisy, and easier to coil balance.

Does such a connection at your Tesoro working on wet grass correctly?
Regards

[dfbowers]

Hi Dfbowers.
Modest attention.
In radioamateur and measurement systems, the general principle has always been to connect the screen to the ground only on the PCB. Reason - the countervailing currents.
The combination of ground to the coil and the PCB exits the screen frequency, then the earth loses its meaning.
Do you have a good electrical connection to the Al foil in the head coil?
It is always problematic.
Best regards Chris.

Chris,

So in theory this would suggest that Tx and Rx lead would not be tied to the shields + ground inside the coil, correct?

I have been using copper foil with shields soldered directly to the copper foil.

Don

[Krzysztof]

Hi Dfbowers.
Copper Tape - super!
Combination of Cu in the head shielded cable wire screen in only one place, then the screen cable directly to GND on PCB.
No additional connection to another cable shield Tx / Rx in the head.
Then the current compensation loop is created, results in additional parasitic vibrations leading to higher noise and lack of enclosure of the screen.
Best regards Chris.

[simonbaker]

Hi Dfbowers.
Copper Tape - super!
Combination of Cu in the head shielded cable wire screen in only one place, then the screen cable directly to GND on PCB.
No additional connection to another cable shield Tx / Rx in the head.
Then the current compensation loop is created, results in additional parasitic vibrations leading to higher noise and lack of enclosure of the screen.
Best regards Chris.

Did not Qiaohzi say that some commercial MD's have only 3 wire cables, with the shield being a common ground for the TX and RX? That would be similar to how we wire the TGSL.

-SB

[simonbaker]

One difficulty I am having with trying different grounding configurations is that the coils seem to null at different places depending upon where the shields are grounded, so I don't think that I can cast a set of coils in epoxy with wires exposed. I'm just going to have to make several complete coils and evaluate from there. I may be able to make some sort of quick disconnect to try different cables though. Waiting for epoxy to harden..

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

-SB

[simonbaker]

Hi Dfbowers.
Modest attention.
In radioamateur and measurement systems, the general principle has always been to connect the screen to the ground only on the PCB. Reason - the countervailing currents.
The combination of ground to the coil and the PCB exits the screen frequency, then the earth loses its meaning.
Do you have a good electrical connection to the Al foil in the head coil?
It is always problematic.
Best regards Chris.

Is anyone sure really how/why the coil shield works? Is it for electrostatic buildup on grass? RF interference?

I'm guessing not RF interference, because blocking RF in the detection band maybe would also block the target signal??? Also, I think many MD coils are now using graphite type shields which are high resistance (relatively) and not RF shields.

For electrostatic shielding, a lot of effects could be going on. Moving the coil through an electrostatic field gradient -- is that a problem -- and would a shield block it? Dissipating electrostatic buildup from touching grass -- how would that work when the coil is inside a plastic (insulating) head?

Is there well understood science to this, or just works in practice???

-SB

[Vladimir]

Does anyone here own home-made detector test in real operating conditions or be reduced to laboratory testing and theory?
So many illogical conclusions and misunderstanding of the functionality of MD. Some ideas should be tested but most of the questions to discuss here, are the basis of detection IB metal detectors. I suggest to start school on the basis of metal detection and not wasting time on discussion of basic issues.
Many times has been written on the forum of static Shild the coil, its single purpose is to static voltage (which can be several thousand volts) brought to GND, nothing else. With such high-voltage plastic body of the coil is not isolation.
Regards

[Krzysztof]

Hi Simonbaker.
Then the good the old school and the practician of line the feeding RF.
The point of earth only one in GND on PCB.
Further to shield only one separate isolated line, the most often shield.
Test in practice resistor 100kom and more between end the shield and the foil the Cu of shield.
The effect, it acts how the graphite shield and the smaller hums from smaller currents of static electricity.
Plastic near movement causes creating on its surface static loads. Grass' touch will unload to the ground this.
These short of discharge give the effect of bangs and hum in MD.
Idea on today: Leading the plastic or the foil the stuck on bottom of shield Cu; but from slit - the lack of compact roll.
Best regards Chris.

[Qiaozhi]

Does such a connection at your Tesoro working on wet grass correctly?
Regards

The method I described is the one used by Tesoro. There are only 4 wires plus the shield in the cable, but only the shield and 3 of the wires are used. Sometimes Tesoro coils will start to react to wet grass when they get old. This is because the connection to the electrostatic shield is broken.

The purpose of the electrostatic shield is to reduce capacitive effects between the coil(s) and the ground ... or your hand / forearm.

[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

[Vladimir]

The purpose of the electrostatic shield is to reduce capacitive effects between the coil(s) and the ground ... or your hand / forearm.

???
Why is called the "electrostatic"? it is true that reducing capacitive effects but it is not primary purpose.
Regards

[Qiaozhi]

???
Why is called the "electrostatic"? it is true that reducing capacitive effects but it is not primary purpose.
Regards

Maybe there is some confusion here.

When two objects are brought in close proximity to each other there is an electrostatic field potential between them. In other words, you are creating a capacitor, with one plate being the detector coils and the second plate being the object. When the metal detector is being used in the field, the search coil can brush against wet grass. or other objects. The electrostatic field produced can cause unwanted signals in the detector, and that is why a Faraday shield is used in the search head to shield the coils from nearby objects. For obvious reasons the Faraday shield is also known as an electrostatic shield. In many cheap coil arrangements, like the Micronta detectors, there is no shield. Consequently this is one of the reasons these detectors do not have a good reputation.

[Vladimir]

Maybe there is some confusion here.

When two objects are brought in close proximity to each other there is an electrostatic field potential between them. In other words, you are creating a capacitor, with one plate being the detector coils and the second plate being the object. When the metal detector is being used in the field, the search coil can brush against wet grass. or other objects. The electrostatic field produced can cause unwanted signals in the detector, and that is why a Faraday shield is used in the search head to shield the coils from nearby objects. For obvious reasons the Faraday shield is also known as an electrostatic shield. In many cheap coil arrangements, like the Micronta detectors, there is no shield. Consequently this is one of the reasons these detectors do not have a good reputation.

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).

[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.

[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