TGSL troubleshooting questions

[ashefr10]

Hello, since a few days my crackling same detector sensitivity minimum. the batteries are new. it occurs in all metal and discrimination. I changed all integrated circuits anything done. I check if the coil is not cut no problem either. thank you if you can help me.

[ashefr10]

OK, thanks for the information. I will try to put the 10 pf capacitor and I'll see what happens later

[Davor]

In case your Rx coil has no leads grounded in a connector, you could un-ground your preamp inputs to promote a differential mode of operation. It is also good to use the sama capacitor (10pF?) in parallel to the 220k in the non-inverting path to make it symmetrical with the inverting path.

[Qiaozhi]

HELLO, my TGSL is sensitive to wet grass, in fact when I prospects in a field where the grass is wet the detector emits false signals. What art can I do for this problem? thank you

Reacting to wet grass is usually caused by poor coil shielding.

[ashefr10]

HELLO, my TGSL is sensitive to wet grass, in fact when I prospects in a field where the grass is wet the detector emits false signals. What art can I do for this problem? thank you

[der_fisherman]

Good one......thanks SB.

[simonbaker]

I agree totally in principle. I'm guessing we don't have to worry about ground loops in the cable itself though and it would be nice to not over-engineer the cable if we don't have to.

I'd argue it this way:

There is a matter of degree difference between the skinny ground loop in the cable and the kind of wide ground loops that thread through circuits and between components. The cable loop has awfully little area to pick up any stray EMI with -- and even if it did, it totally pales in comparison to the honking EMI wad that the RX coil picks up!

Could the other TX (red) lead induce an eddy current in this shield-to-black lead loop? I'd argue that it has the wrong orientation because the lead lies parallel to both sides of the loop, so any EMF would cancel. Likewise, any current that did manage to arise in the ground loop would have cancelling directions relative to any of the RX leads that lie parallel to it, so it wouldn't transmit to the RX leads.

Even if it did couple to an RX lead, I wouldn't worry about it coupling any significant TX power because the hot TX lead probably does that many orders of magnitude more -- at least in a USB cable.

And even then, any such cross-talk of the TX signal often is handled in the motion detection circuit.

But that's just intuition, and so often wrong in my case, so tests are always the way to go.

And would I eliminate that ground loop if easy to do so? Yes. It is a good habit, may be more important in a different MD design.

Ground loops are a good subject to chew on because I think they are not trivial or simple to understand and sometimes have a critical effect, sometimes not.

Regards,

-SB

Anything worth doing is worth overdoing...

P.S. Just read Don's post, he said it simpler - but to be sure, it is a very worthy point der_fisherman brings up.

[simonbaker]

Hi Don,

maybe my terminology, although correct according to Wiki here (you need to read well down the page by the way):-

http://en.wikipedia.org/wiki/Ground_loop_%28electricity%29

Where you will see:-

A star topology should be used for ground distribution, avoiding loops.

May not be quite clear enough for everyone.

As I mentioned before, look at any loop in any cable as a shorted single turn transformer. You will induce voltages from AC sources into that loop.
The loop being shorted will run a (relatively) high current with a very low voltage.
Unless you break the loop and insert an ammeter, you will never know what currents are flowing in the loop.
Such loops also re- "transmit" low levels of AC further on.

You can also read at that Wiki link the following:-

Ground loop issues with television coaxial cable can also affect any connected audio devices such as a receiver. Even if all of the audio and video equipment in, for example, a home theater system is plugged into the same power outlet, and thus all share the same ground, the coaxial cable entering the TV is sometimes grounded to a different point than that of the house's electrical ground by the cable company. The potential of this ground is likely to differ slightly from the potential of the house's ground, so a ground loop occurs, causing undesirable mains hum in the system's speakers.

You don't have to do anything new if you don't want to, but ground loops and similar may effect stability and sensitivity of MDs, who are as easily as sensitive as any radio or audio system.

Generally, in an audio system, unless the designer / builder took steps to prevent it, going from one unit to another with grounded stereo Cinch cables/connectors may cause problems due to having two ground paths, which is why in many pieces of equipment one ground is not connected at the socket......or special electronics are used (opto for example) to obviate the ground loop effects.

You should at least know about them, that was my aim. You still don't have to agree though!! Have a great day.

regards

Andy

I agree totally in principle. I'm guessing we don't have to worry about ground loops in the cable itself though and it would be nice to not over-engineer the cable if we don't have to.

I'd argue it this way:

There is a matter of degree difference between the skinny ground loop in the cable and the kind of wide ground loops that thread through circuits and between components. The cable loop has awfully little area to pick up any stray EMI with -- and even if it did, it totally pales in comparison to the honking EMI wad that the RX coil picks up!

Could the other TX (red) lead induce an eddy current in this shield-to-black lead loop? I'd argue that it has the wrong orientation because the lead lies parallel to both sides of the loop, so any EMF would cancel. Likewise, any current that did manage to arise in the ground loop would have cancelling directions relative to any of the RX leads that lie parallel to it, so it wouldn't transmit to the RX leads.

Even if it did couple to an RX lead, I wouldn't worry about it coupling any significant TX power because the hot TX lead probably does that many orders of magnitude more -- at least in a USB cable.

And even then, any such cross-talk of the TX signal often is handled in the motion detection circuit.

But that's just intuition, and so often wrong in my case, so tests are always the way to go.

And would I eliminate that ground loop if easy to do so? Yes. It is a good habit, may be more important in a different MD design.

Ground loops are a good subject to chew on because I think they are not trivial or simple to understand and sometimes have a critical effect, sometimes not.

Regards,

-SB

Anything worth doing is worth overdoing...

[dfbowers]

Hi Don,

maybe my terminology, although correct according to Wiki here (you need to read well down the page by the way):-

http://en.wikipedia.org/wiki/Ground_loop_%28electricity%29

Where you will see:-

A star topology should be used for ground distribution, avoiding loops.

May not be quite clear enough for everyone.

As I mentioned before, look at any loop in any cable as a shorted single turn transformer. You will induce voltages from AC sources into that loop.
The loop being shorted will run a (relatively) high current with a very low voltage.
Unless you break the loop and insert an ammeter, you will never know what currents are flowing in the loop.
Such loops also re- "transmit" low levels of AC further on.

You can also read at that Wiki link the following:-

Ground loop issues with television coaxial cable can also affect any connected audio devices such as a receiver. Even if all of the audio and video equipment in, for example, a home theater system is plugged into the same power outlet, and thus all share the same ground, the coaxial cable entering the TV is sometimes grounded to a different point than that of the house's electrical ground by the cable company. The potential of this ground is likely to differ slightly from the potential of the house's ground, so a ground loop occurs, causing undesirable mains hum in the system's speakers.

You don't have to do anything new if you don't want to, but ground loops and similar may effect stability and sensitivity of MDs, who are as easily as sensitive as any radio or audio system.

Generally, in an audio system, unless the designer / builder took steps to prevent it, going from one unit to another with grounded stereo Cinch cables/connectors may cause problems due to having two ground paths, which is why in many pieces of equipment one ground is not connected at the socket......or special electronics are used (opto for example) to obviate the ground loop effects.

You should at least know about them, that was my aim. You still don't have to agree though!! Have a great day.

regards

Andy

I think that we essentually said the same thing. your example above just refers to a TV cable being grounded at a different place than power. Same as with computer equipment.

I think that we also agree on what cable scheme is better as well.

But, if you run two conductors close together and tie the ends together, any current must run the same way from anything induced. Any induced current from noise will be the same direction in each conductor . Just like litz wire. You have to separate the conductors to have a loop like you describe- a single turn inductor.

[der_fisherman]

Ground loops

Hi Don,

maybe my terminology, although correct according to Wiki here (you need to read well down the page by the way):-

http://en.wikipedia.org/wiki/Ground_loop_%28electricity%29

Where you will see:-

A star topology should be used for ground distribution, avoiding loops.

May not be quite clear enough for everyone.

As I mentioned before, look at any loop in any cable as a shorted single turn transformer. You will induce voltages from AC sources into that loop.
The loop being shorted will run a (relatively) high current with a very low voltage.
Unless you break the loop and insert an ammeter, you will never know what currents are flowing in the loop.
Such loops also re- "transmit" low levels of AC further on.

You can also read at that Wiki link the following:-

Ground loop issues with television coaxial cable can also affect any connected audio devices such as a receiver. Even if all of the audio and video equipment in, for example, a home theater system is plugged into the same power outlet, and thus all share the same ground, the coaxial cable entering the TV is sometimes grounded to a different point than that of the house's electrical ground by the cable company. The potential of this ground is likely to differ slightly from the potential of the house's ground, so a ground loop occurs, causing undesirable mains hum in the system's speakers.

You don't have to do anything new if you don't want to, but ground loops and similar may effect stability and sensitivity of MDs, who are as easily as sensitive as any radio or audio system.

Generally, in an audio system, unless the designer / builder took steps to prevent it, going from one unit to another with grounded stereo Cinch cables/connectors may cause problems due to having two ground paths, which is why in many pieces of equipment one ground is not connected at the socket......or special electronics are used (opto for example) to obviate the ground loop effects.

You should at least know about them, that was my aim. You still don't have to agree though!! Have a great day.

regards

Andy

[dfbowers]

Ground loops.. Hmmm.. We might be getting lost in definitions here. I have been working on main frame size computers for almost 30 years now. We have always referred to "ground loops" as a difference in ground potential and electrical current would flow from one piece of equipment to another through a ground wire. This would be from an improperly grounded peripheral or computer, or a "bad" ground on one end.

I was looking for the NEC definition and it seems to describe the same thing. Or,
"A ground loop in the power or video signal occurs when some components in the same system are receiving its power from a different ground than other components, or the ground potential between two pieces of equipment is not identical. "

In the TGSL application, I think that you can have all the ground wires that you want, as long as they all go the same place!! So.. IMHO, I don't think that a "loop" means that the ground wires form a complete loop.

Also, I don't think that two Rx wires twisted together in a USB-2 will be a problem run together with the TX pair.. UNLESS one end of the Rx cable is grounded!! The differential mode would be lost. We get around the problem by using the two individually shielded pairs with Belden-M (8723) and running a single ended Rx signal.

Don

[der_fisherman]

Good points all, thanks; and worth testing in practice since the TGSL is surprisingly tolerant of many things (such as rail voltage spikes that are synchronized to the oscillator frequency and the size of the null signal).



I'm not sure that qualifies as a ground loop in the classic sense -- just two wires acting like one, like a mini Litz wire. I'm no ground loop expert, but I think the problem is when they are embedded in or span the circuit causing voltage drops or currents here and there.

But your advice sounds like good practice in general; however I always like to do real tests to see "what we can get away with" in the interest of cheap production techniques. And theorize ad nauseum when I'm too dilatory to get out of my chair ...

Cheers,

-SB

Any cables running in a loop are suspect, whether they are grounds, +12 volts or whatever......mostly the talk is of ground loops......the signal in the loop (even if DC is there too), is always an AC signal, be it slow or fast!!

Loops are bad as you get, due to the transformer effect, a shorted loop with circulating currents.

Testing yourself is always the best way to learn.

Best regards

Andy

[simonbaker]

Nice to hear from you Simon. I will try and answer your questions to the best of my knowledge:-



1) Microphone cable here is basically two tiny coax cables, with the outer insulation "glued" to each down one side, making a "flat" cable, the shields do not touch. For cables where the shielded parts are in full contact, that works as a single shield....(Belkin cable?)



2) The original TGSL USB2 Cable connection diagram, has a very obvious loop in it if you look, its not the cable, its the connection method, the effects of which may be a lot worse than just "wet grass" sensitivity, if the truth were known.....(I have added that drawing below for quick reference.) The loop is via the shield and the black connection of the Tx lead, connected at both ends = a loop. Not forgetting what has been done with the coild shields if they touch anywhere where they cross over.......



3) I personally don't like the idea of USB cables because I know that there is a lot of substandard ones with very thin conductors around. Chinese?

4) Also in a USB cable, the Tx and Rx coil wires are not separately shielded from each other, just all wires in together.....So you have a relatively high level Tx signal, traveling perhaps more than a meter right next to the low level Rx wiring where micro-volts are significant......it's just not my way to do that. Now if "TWO" USB cables were used.........



5) I would bet that coils that can only be nulled down to 4mVolts are suffering from pickup over such cables.....theoretically, well nulled means ZERO pickup.....(I will check the validity of this statement sometime in the next couple of weeks..... on my IGSL)



6) The "weight" of the cable does not seem to make any difference on any previous detectors I have made, future IGSL testing is planned.....cables must always be well fastened down....

7) As to why your Belkin cable didn't work, you would need to examine how it was connected in fine detail.....I cannot answer you there. That is the type of cable that really needs to be used to my mind. I believe Ivconic uses it too if I remember correctly on his machines....my mike cables are electrically similar......



That simply depends upon the way the electronics are designed, it would appear that for TGSL, the Tx coil connects to ground on one side anyway.....then it can be done with (probably!!) no problems.

Regards

Andy

PS Some of my comments are based on previous experience/knowledge, but are not based on knowledge with regard to either TGSL or IGSL metal detectors, yet anyway.......I will drop you Guys a line when I find out for sure (or not!!)

Good points all, thanks; and worth testing in practice since the TGSL is surprisingly tolerant of many things (such as rail voltage spikes that are synchronized to the oscillator frequency and the size of the null signal).

2) The original TGSL USB2 Cable connection diagram, has a very obvious loop in it if you look, its not the cable, its the connection method, the effects of which may be a lot worse than just "wet grass" sensitivity, if the truth were known.....(I have added that drawing below for quick reference.) The loop is via the shield and the black connection of the Tx lead, connected at both ends = a loop. Not forgetting what has been done with the coild shields if they touch anywhere where they cross over.......

I'm not sure that qualifies as a ground loop in the classic sense -- just two wires acting like one, like a mini Litz wire. I'm no ground loop expert, but I think the problem is when they are embedded in or span the circuit causing voltage drops or currents here and there.

But your advice sounds like good practice in general; however I always like to do real tests to see "what we can get away with" in the interest of cheap production techniques. And theorize ad nauseum when I'm too dilatory to get out of my chair ...

Cheers,

-SB

[der_fisherman]

Loops in Shielding

Nice to hear from you Simon. I will try and answer your questions to the best of my knowledge:-

I have not seen a cable where the two inner shields are electrically insulated from each other -- but I assume they are insulated in your mic cable to satisfy your design.-SB

1) Microphone cable here is basically two tiny coax cables, with the outer insulation "glued" to each down one side, making a "flat" cable, the shields do not touch. For cables where the shielded parts are in full contact, that works as a single shield....(Belkin cable?)

I think the "USB" style cable does not really make a ground loop, does it? Unless you consider the TX grounded lead and the shield to make a loop because they are connected at both ends. But they have so little separation, could it matter? And the loop is outside the circuit.-SB

2) The original TGSL USB2 Cable connection diagram, has a very obvious loop in it if you look, its not the cable, its the connection method, the effects of which may be a lot worse than just "wet grass" sensitivity, if the truth were known.....(I have added that drawing below for quick reference.) The loop is via the shield and the black connection of the Tx lead, connected at both ends = a loop. Not forgetting what has been done with the coild shields if they touch anywhere where they cross over.......

However I like the idea of the TX pair and the RX pair being shielded from each other, although haven't seen it prove superior in practice (judging from dfbowers MDs). The USB cable does have a twisted pair for the non-power pair, which helps isolate.-SB

3) I personally don't like the idea of USB cables because I know that there is a lot of substandard ones with very thin conductors around. Chinese?

4) Also in a USB cable, the Tx and Rx coil wires are not separately shielded from each other, just all wires in together.....So you have a relatively high level Tx signal, traveling perhaps more than a meter right next to the low level Rx wiring where micro-volts are significant......it's just not my way to do that. Now if "TWO" USB cables were used.........

One problem with heavy-duty shielded cables is they perhaps can look like a lot of metal to the search coils, and so I'd think it even more important to fasten them down securely to prevent any jiggling.-SB

5) I would bet that coils that can only be nulled down to 4mVolts are suffering from pickup over such cables.....theoretically, well nulled means ZERO pickup.....(I will check the validity of this statement sometime in the next couple of weeks..... on my IGSL)

I have definitely gotten different behavior when I switched between USB and Belkin cable (couldn't get the Belkin to quiet down), but maybe some additional tweaking of the circuit would make them more similar.-SB

6) The "weight" of the cable does not seem to make any difference on any previous detectors I have made, future IGSL testing is planned.....cables must always be well fastened down....

7) As to why your Belkin cable didn't work, you would need to examine how it was connected in fine detail.....I cannot answer you there. That is the type of cable that really needs to be used to my mind. I believe Ivconic uses it too if I remember correctly on his machines....my mike cables are electrically similar......

It would be interesting to study the difference between grounding the TX shield to the grounded TX lead in the head or not.-SB

That simply depends upon the way the electronics are designed, it would appear that for TGSL, the Tx coil connects to ground on one side anyway.....then it can be done with (probably!!) no problems.

Regards

Andy

PS Some of my comments are based on previous experience/knowledge, but are not based on knowledge with regard to either TGSL or IGSL metal detectors, yet anyway.......I will drop you Guys a line when I find out for sure (or not!!)

[simonbaker]

It depends upon the cable type used to a great degree, but a shield may only have one connection to the ground plane.......many talk about "star" connecting.
Although I have never used USB cables personally, I have nothing against them, except that until one cuts it open, you don't know exactly how much was saved in very thin wiring.....I personally hate thin wiring as it presents an extra resistance.

I am sure that a good quality USB cable would not suffer from that......

I have added a couple of simple drawings showing the connection with twinCore cable and with Dual Microphone cable where each core is shielded, my preference....no loops....

I tend to bring all connections separately into the electronics box as I can both best measure and see that all is ok.

A typical mistake is to cause a short between the Rx and Tx Faraday's shield where they cross one another, make sure a good piece of insulation is placed in these 2 areas......(in most VLF designs that I have seen....). To stop a loop, you need to not use one of the shields, us just one (assuming its a low Ohm short....)

Regards

Andy

PS. If anyone has a sensitivity or ANY problem with any MD they built or bought and they know that the coil's cables Faraday shields or cable shields may form a loop somewhere, it would be interesting to hear from you.......and if you can fix all problems......finding the loop after building is difficult or impossible......unless you can separate everything at the electronics again.....

I have not seen a cable where the two inner shields are electrically insulated from each other -- but I assume they are insulated in your mic cable to satisfy your design.

I think the "USB" style cable does not really make a ground loop, does it? Unless you consider the TX grounded lead and the shield to make a loop because they are connected at both ends. But they have so little separation, could it matter? And the loop is outside the circuit.

However I like the idea of the TX pair and the RX pair being shielded from each other, although haven't seen it prove superior in practice (judging from dfbowers MDs). The USB cable does have a twisted pair for the non-power pair, which helps isolate.

One problem with heavy-duty shielded cables is they perhaps can look like a lot of metal to the search coils, and so I'd think it even more important to fasten them down securely to prevent any jiggling.

I have definitely gotten different behavior when I switched between USB and Belkin cable (couldn't get the Belkin to quiet down), but maybe some additional tweaking of the circuit would make them more similar.

It would be interesting to study the difference between grounding the TX shield to the grounded TX lead in the head or not.

-SB