grounded vs ungrounded RX coil

[dfbowers]

Hi Don:

I'm interested in what the heck wet grass does.

What do the false signals from wet grass sound like?

Do you actually brush the wet grass with the coil, or can you get the false signals just passing above it?

You mentioned that your ungrounded RX coils have slightly less air depth. Is it possible a drop in sensitivity could account for not picking up the wet grass, or is the wet grass a big honking signal that clearly is knocked out by the ungrounded coil config?

People often mention "ground capacitance" and I'm trying to imagine a model for that -- mikebg showed his idea of it. The voltages across the coils are small compared to static charge, but maybe if water suddenly is spread on the coil base perhaps it does something capacitance-wise. But I would think once wet, the effect would not repeat much using a capacitance model. But if the dew carries a varying electrostatic charge, then I could see continued false signals.

I could also see water as a conductor making little targets of a particular phase. But then ungrounding the coils should not affect the signals (unless loss of sensitivity).

I don't really trust my Spice model for grounded vs. ungrounded RX coil. We should look at the signals on the bench and make sure no overall target phase shift or such happens. Maybe you already checked that out. Of course the fact the ungrounded RX coil works well in your MD means it probably is essentially the same.

By the way, did you happen to compare grounded vs. ungrounded RX coils on wet sand on the beach?

Regards,

-SB

Simon,

Here are a few details on my grounding experiment. Attached are the two grounding schemes that I have tried. Coil details are as follows:


* 5 complete coils.

* 4 of them have been wired per the "TGSL coil making" pdf.

* One of them that I made has not been tested in wet grass (the one I
sent you) but is wired per the TGSL coil making pdf.

* One has been wired without any active parts of the coil attached to the
shields in any way (see attached). This one is quiet in wet grass..

* That leaves me with 3 coils that behave badly in wet grass.

* All 4 of my 27cm coils airtest at over 30cm for 1e. (one is a smaller DD
test coil).

* For all practical purposes, there is no real difference is detection depth between grounding schemes.. just ungrounded is quieter.. I have about 10 hours of
actual use on the ungrounded one at the beach.. I found 115 coins total in a week.



* All are shielded the same, have the same type of shell, epoxy and cable.

See attached links to videos for wet grass demonstration..

Sensitivity was at minimum in both videos.
GB on ferrite.
Disc set to MIN.

Don


Links to videos.

http://www.youtube.com/watch?v=XhRboS639no (Grounded)

http://www.youtube.com/watch?v=7UBOca3btMw (Ungrounded)

[simonbaker]

Simon,

I can do a short video with sound if you would like to see the differences between grounded Rx and ungrounded.. The differences in noise are fairly obvious and the differences in sensitivity are not. I have been using an ungrounded coil all this week and I will stick with it. I dug 94 coins and a boat load of trash (bottle caps, juice boxes condom wrappers, pop tops, etc..) I dug 4 coins (3 quarters and 1 penny) that were really deep and corroded like they were there for a few years. Deep holes! No jewelry yet .

This morning I took my X5 down (Boo!!!) I really like the DD coils for the TGSL now compared to concentric.

Anyway, I have a grounded coil with me and will give that a try tonight at the waters edge to see if the differences are like wet grass.

Don

Everything you show is useful if not too much trouble.

You're getting some nice hits there at the beach -- I'll wish for some nice rings for you. Isn't digging sand nice! Just tricky to avoid the public sometimes...

Keep enjoying...

-SB

[simonbaker]

Simon,
All MOS inputs have diode input protection built in. With a 5K1 series resistor in each leg it is extremely unlikely that static would cause damage to the inputs. So no protection is required. Incidentally, diodes do not produce noise if they are not conducting. Even then not much unless they are zeners.

So far, I don’t think anyone has done a simulation of the effects of capacitive coupling in the cable to the head. So I thought I would have a go. I did a LTspice simulation giving a 10pF capacitive coupling from the ‘hot’ side of the Tx coil to both sides of the Rx coil. It would also give information about direct pickup of static on an unscreened Rx coil – because the capacitive pickup is spread throughout the coil.

The circuit is given in the attachment as a .txt file so I could upload it. I hope its OK. You will have to change the extension from .txt to .asc to run it.

The input has been potted down by R1 and R2 to get the signals to be roughly the same amplitude.

I thought that perhaps 10pF was a bit on the low side but with a 10V input it gave a 250mV output at the op-amp. Increasing the caps to 25pF gave a massive 550mV! These signals were leading the input signal by about 90º and would considerably alter the phase of the signal we are trying to null out, and that would vary depending on the type of cable used.

I tried removing the 10pF feedback cap C7 and the figure then dropped from 250mV to just 1mV, which I think is an acceptable figure.

I am wondering whether that 10pF was put in the original design to compensate for some particular aspect of the original coils and cable. If so, it would be interesting to see whether its removal would help any members.

On the basis of that simulation, there would appear to be little advantage in screening either Rx coils or cable.

Thanks for info about protected inputs -- sounds good.

Interesting, I was recently experimenting with a 10 pF capacitor between the two coils, just on one side, but didn't get what I expected, so still in progress. I'll look at your simulation.

I'd like also to simulate TX-RX coupling due to the coil shields somehow. I feel the shields are kind of like a target and make a coupling of a certain phase. Not sure. Need to somehow take some real data comparing shielded and unshielded coils. Maybe you have some ideas on that.

The 10pf in the feedback path does make a low-pass filter, but what else it's for I'm not sure. The LF353 preamp seems very marginally stable, as any pulse seems to set off oscillations that die out. I'm aware of it, but haven't thought about it too deeply.

Regards,

-SB

[dfbowers]

Hi Don:

I'm interested in what the heck wet grass does.

What do the false signals from wet grass sound like?

Do you actually brush the wet grass with the coil, or can you get the false signals just passing above it?

You mentioned that your ungrounded RX coils have slightly less air depth. Is it possible a drop in sensitivity could account for not picking up the wet grass, or is the wet grass a big honking signal that clearly is knocked out by the ungrounded coil config?

People often mention "ground capacitance" and I'm trying to imagine a model for that -- mikebg showed his idea of it. The voltages across the coils are small compared to static charge, but maybe if water suddenly is spread on the coil base perhaps it does something capacitance-wise. But I would think once wet, the effect would not repeat much using a capacitance model. But if the dew carries a varying electrostatic charge, then I could see continued false signals.

I could also see water as a conductor making little targets of a particular phase. But then ungrounding the coils should not affect the signals (unless loss of sensitivity).

I don't really trust my Spice model for grounded vs. ungrounded RX coil. We should look at the signals on the bench and make sure no overall target phase shift or such happens. Maybe you already checked that out. Of course the fact the ungrounded RX coil works well in your MD means it probably is essentially the same.

By the way, did you happen to compare grounded vs. ungrounded RX coils on wet sand on the beach?

Regards,

-SB

Simon,

I can do a short video with sound if you would like to see the differences between grounded Rx and ungrounded.. The differences in noise are fairly obvious and the differences in sensitivity are not. I have been using an ungrounded coil all this week and I will stick with it. I dug 94 coins and a boat load of trash (bottle caps, juice boxes condom wrappers, pop tops, etc..) I dug 4 coins (3 quarters and 1 penny) that were really deep and corroded like they were there for a few years. Deep holes! No jewelry yet .

This morning I took my X5 down (Boo!!!) I really like the DD coils for the TGSL now compared to concentric.

Anyway, I have a grounded coil with me and will give that a try tonight at the waters edge to see if the differences are like wet grass.

Don

[pebe]

(Of course take that sim with a big grain of salt... no physics to speak of.)

Very interesting your tests with ungrounded RX coil.

Do you think the common mode overload issue could happen? If our shields really can get big static jolts (oxymoron?) could it knock out the chip because neither wire is grounded? I was thinking of possible diode protection, but it is not a place you want to put noise generators like diodes if possible.

Of course I have no idea how the shields are working. Are they protecting against an actual charge/discharge, or from the emf pulse from a static discharge, or something else? And how does the insulating effect of the housing factor in?

I do see reason to believe that the grounded RX coil could be unbalanced if a charge zapped both leads of the coil, thus responding to the discharge/charge. Well, we know it's unbalanced don't we.

Interesting stuff.

-SB

Simon,
All MOS inputs have diode input protection built in. With a 5K1 series resistor in each leg it is extremely unlikely that static would cause damage to the inputs. So no protection is required. Incidentally, diodes do not produce noise if they are not conducting. Even then not much unless they are zeners.

So far, I don’t think anyone has done a simulation of the effects of capacitive coupling in the cable to the head. So I thought I would have a go. I did a LTspice simulation giving a 10pF capacitive coupling from the ‘hot’ side of the Tx coil to both sides of the Rx coil. It would also give information about direct pickup of static on an unscreened Rx coil – because the capacitive pickup is spread throughout the coil.

The circuit is given in the attachment as a .txt file so I could upload it. I hope its OK. You will have to change the extension from .txt to .asc to run it.

The input has been potted down by R1 and R2 to get the signals to be roughly the same amplitude.

I thought that perhaps 10pF was a bit on the low side but with a 10V input it gave a 250mV output at the op-amp. Increasing the caps to 25pF gave a massive 550mV! These signals were leading the input signal by about 90º and would considerably alter the phase of the signal we are trying to null out, and that would vary depending on the type of cable used.

I tried removing the 10pF feedback cap C7 and the figure then dropped from 250mV to just 1mV, which I think is an acceptable figure.

I am wondering whether that 10pF was put in the original design to compensate for some particular aspect of the original coils and cable. If so, it would be interesting to see whether its removal would help any members.

On the basis of that simulation, there would appear to be little advantage in screening either Rx coils or cable.

[simonbaker]

Fairly certain.. So far, I have built 5 coils.. completely.. and have tested 4 of them in wet grass. All coils work well but the only one that works correctly in wet grass is the ungrounded one. All have the same type of epoxy. I can build coil #6 just to verify and most likely will..

Don

Hi Don:

I'm interested in what the heck wet grass does.

What do the false signals from wet grass sound like?

Do you actually brush the wet grass with the coil, or can you get the false signals just passing above it?

You mentioned that your ungrounded RX coils have slightly less air depth. Is it possible a drop in sensitivity could account for not picking up the wet grass, or is the wet grass a big honking signal that clearly is knocked out by the ungrounded coil config?

People often mention "ground capacitance" and I'm trying to imagine a model for that -- mikebg showed his idea of it. The voltages across the coils are small compared to static charge, but maybe if water suddenly is spread on the coil base perhaps it does something capacitance-wise. But I would think once wet, the effect would not repeat much using a capacitance model. But if the dew carries a varying electrostatic charge, then I could see continued false signals.

I could also see water as a conductor making little targets of a particular phase. But then ungrounding the coils should not affect the signals (unless loss of sensitivity).

I don't really trust my Spice model for grounded vs. ungrounded RX coil. We should look at the signals on the bench and make sure no overall target phase shift or such happens. Maybe you already checked that out. Of course the fact the ungrounded RX coil works well in your MD means it probably is essentially the same.

By the way, did you happen to compare grounded vs. ungrounded RX coils on wet sand on the beach?

Regards,

-SB

[simonbaker]

Hi guy's

I have had a very similar problem with a VLF detector that I build from scratch it was meant to be a simple detector for kids I wanted it to be simple to use reliable, light and have at minimum 12 hours of battery life well this detector has ended up to become a real challenge when it came to the coils it runs at 22Khz I intentionally used that frequency to locate gold the first problem was to null the coils having done that it would detect (in air) a gold ring at +25 cm but once the coil shielded it would only detect at about 10 cm now normally from what I understand at low frequencies there is no need to shield the coil but at high frequencies you should shield I did some testing outdoors with the shield big problems would detect wet grass with no shield works very well no detection of wet grass but only works for about one minute then you have to re-adjust the gain. Hope that this info will be of some use to you.

Regards Xavier

James Joyce lives...

[dfbowers]

Well inside you don't have the earth's inductance so you won't find the same properties but take a piece of ferrite to null the coil is one way that I read about doing it I will see if I can find the article about it but I'm sure that there is some one on this forum that will be able to explain how it,s done and what ferrite you must use.

Regards Xavier

Xavier,


If inductance had something to do with it, then I would not expect things to change with wet vs. dry grass.. It's only wet vegetation that I have had issues with.. but, my problems have been solved by changing my coil grounding configuration. So, It appears as though the shield function as intended.. Now, I am only trying to figure out why, and what others have experienced when building their own coils.

Don

[Xavier]

Well inside you don't have the earth's inductance so you won't find the same properties but take a piece of ferrite to null the coil is one way that I read about doing it I will see if I can find the article about it but I'm sure that there is some one on this forum that will be able to explain how it,s done and what ferrite you must use.

Regards Xavier

[dfbowers]

In that case this is somewhat strange, because Tesoro coils are not affected by wet grass, and their coils are grounded in the same way as your 4 coils that do have the problem. Maybe you could hook up the detector to the scope and wave some wet grass across the front of the coil, or how about moving your hand close to the coil? Do either of these tests have any effect on the RX signal? If so, I would still suspect the electrostatic shield.

The only thing that causes false siganls at all is wet grass (outside). On the bench inside it is stable.. even when applying a wet sponge.
All coils work just fine when the grass is dry. My only other thought was the type of epoxy (Thermally conductive) since is is using for encasing electronics.. but that would not explain why only the ungrounded Rx works fine.

Don

[Xavier]

Hi guy's

I have had a very similar problem with a VLF detector that I build from scratch it was meant to be a simple detector for kids I wanted it to be simple to use reliable, light and have at minimum 12 hours of battery life well this detector has ended up to become a real challenge when it came to the coils it runs at 22Khz I intentionally used that frequency to locate gold the first problem was to null the coils having done that it would detect (in air) a gold ring at +25 cm but once the coil shielded it would only detect at about 10 cm now normally from what I understand at low frequencies there is no need to shield the coil but at high frequencies you should shield I did some testing outdoors with the shield big problems would detect wet grass with no shield works very well no detection of wet grass but only works for about one minute then you have to re-adjust the gain. Hope that this info will be of some use to you.

Regards Xavier

[Qiaozhi]

Fairly certain.. So far, I have built 5 coils.. completely.. and have tested 4 of them in wet grass. All coils work well but the only one that works correctly in wet grass is the ungrounded one. All have the same type of epoxy. I can build coil #6 just to verify and most likely will..

Don

In that case this is somewhat strange, because Tesoro coils are not affected by wet grass, and their coils are grounded in the same way as your 4 coils that do have the problem. Maybe you could hook up the detector to the scope and wave some wet grass across the front of the coil, or how about moving your hand close to the coil? Do either of these tests have any effect on the RX signal? If so, I would still suspect the electrostatic shield.

[dfbowers]

Are you 100% certain that the ungrounded coil was the cure for the wet grass problem? Because this is usually due to a bad electrostatic shield.

Fairly certain.. So far, I have built 5 coils.. completely.. and have tested 4 of them in wet grass. All coils work well but the only one that works correctly in wet grass is the ungrounded one. All have the same type of epoxy. I can build coil #6 just to verify and most likely will..

Don

[mikebg]

SPICE can't help to solve problem.

Ungrounded RX coil needs symmetry of front end circuitry. Another solution is to ground center tap of RX coil.
The task of shielding is to suppress the influence of variable capacitances shown in the attached equivalent circuit. Although SPICE can simulate variable capacitances, no benefit from this, because it can not show to the designer how and what must be shielded.
The equivalent circuit is valid for non symmetrical grounding of the TX and RX tank circuit. However, in electronics often used symmetry to avoid shielding. The easiest way for symmetry is to use center tapped coils and connecting the center tap to COM rail.

[simonbaker]

Excellent.. Thank you Simon. After making 3 coils that could not handle the wet grass wired with a grounded Rx, I was motivated to make an ungrounded one, which completely solved the wet grass problems. I will most probably make subsequent coils ungrounded just because of that.

I was mostly looking for unseen issues that an unground coil could cause but so far, I just don't see any. I would like to see if we could get an explaination from some of the more seasoned members on the forum on why a grounded Rx circuit would be more preferable.

Thanks!

Don

(Of course take that sim with a big grain of salt... no physics to speak of.)

Very interesting your tests with ungrounded RX coil.

Do you think the common mode overload issue could happen? If our shields really can get big static jolts (oxymoron?) could it knock out the chip because neither wire is grounded? I was thinking of possible diode protection, but it is not a place you want to put noise generators like diodes if possible.

Of course I have no idea how the shields are working. Are they protecting against an actual charge/discharge, or from the emf pulse from a static discharge, or something else? And how does the insulating effect of the housing factor in?

I do see reason to believe that the grounded RX coil could be unbalanced if a charge zapped both leads of the coil, thus responding to the discharge/charge. Well, we know it's unbalanced don't we.

Interesting stuff.

-SB