grounded vs ungrounded RX coil

[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

[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

[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)

[Qiaozhi]

Hi Don,

OK - now I can see a difference between your TGSL "original" wiring and the wiring of a commercial Tesoro coil. In the commercial version there are only 3 wires used in the cable. The fourth wire remains disconnected. This would be equivalent to breaking the connection for the black wire (TX Finish) at both ends of the cable. Perhaps you could try this in your next coil and let use know the results.

[dfbowers]

Hi Don,

OK - now I can see a difference between your TGSL "original" wiring and the wiring of a commercial Tesoro coil. In the commercial version there are only 3 wires used in the cable. The fourth wire remains disconnected. This would be equivalent to breaking the connection for the black wire (TX Finish) at both ends of the cable. Perhaps you could try this in your next coil and let use know the results.

Qiaozhi,

Do you mean just disconnecting the "hitch hiker" wire (the redundant Tx finish wire)? I suppose I could do that by just snipping the connection from one of my existing coils.. So the cable shield would just carry the Tx finish connection. Is that what you mean?

Don

[Qiaozhi]

Qiaozhi,

Do you mean just disconnecting the "hitch hiker" wire (the redundant Tx finish wire)? I suppose I could do that by just snipping the connection from one of my existing coils.. So the cable shield would just carry the Tx finish connection. Is that what you mean?

Don

That's correct.

[dfbowers]

That's correct.

I'll give it a try..

[Monk]

FYI

[simonbaker]

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)

That's interesting demo, certainly shows your point.

I didn't realize your TX coil was not grounded at the coil shields either - interesting.

The funny thing is that your "ungrounded" coils are the wiring I would have expected naively -- the TGSL standard grounding always puzzled me but apparently is the Tesoro way. Only concern would be common mode spikes, but others feel that is not an issue with LF353.

I'm still interested if you were able to test both coil types over wet sand. Wet sand (like when surf recedes) is a mess for my Compadre, but an area I'd like to be able to hunt.

Your making a real case for "ungrounded" coils. And you appear to be in your element at the beach... I'll pretend I'm not sweating at a hot desk....

Thanks for the tests and videos.

Cheers,

-SB

[dfbowers]

FYI

Monk,

So you are suggesting that disconnecting the extra Tx wire buys us nothing.. correct?

Don

[dfbowers]

That's interesting demo, certainly shows your point.

I didn't realize your TX coil was not grounded at the coil shields either - interesting.

The funny thing is that your "ungrounded" coils are the wiring I would have expected naively -- the TGSL standard grounding always puzzled me but apparently is the Tesoro way. Only concern would be common mode spikes, but others feel that is not an issue with LF353.

I'm still interested if you were able to test both coil types over wet sand. Wet sand (like when surf recedes) is a mess for my Compadre, but an area I'd like to be able to hunt.

Your making a real case for "ungrounded" coils. And you appear to be in your element at the beach... I'll pretend I'm not sweating at a hot desk....

Thanks for the tests and videos.

Cheers,

-SB

Simon,

I did test both coil in the wet surf. Both coils reacted when touching salt water, but the ungrounded one would still be my pick as the reaction was minimized somewhat. I was able to nearly reduce all reaction to hunting over wet, salty sand by bumping up the disc a little... around where thin foil is rejected.

Now, where I had the most trouble was the area between where the sand was completely wet and completely dry.. but the TGSL was still very usable.

Don

[Monk]

So you are suggesting that disconnecting the extra Tx wire buys us nothing.. correct?

I don't understand you question. What do you mean by "extra Tx wire"?

[dfbowers]

Monk,

So you are suggesting that disconnecting the extra Tx wire buys us nothing.. correct?

Don

One of the earlier post suggested that maybe the difference between the "3 wire + cable shield" and the "4 wire + cable shield" had some effect on grounding. I used the 4th wire for the connection to the cold end of the oscillator and Tesoro does not. Electrically, I think it's the same thing.. just a redundant wire (The black lead inside the cable).

[Qiaozhi]

One of the earlier post suggested that maybe the difference between the "3 wire + cable shield" and the "4 wire + cable shield" had some effect on grounding. I used the 4th wire for the connection to the cold end of the oscillator and Tesoro does not. Electrically, I think it's the same thing.. just a redundant wire (The black lead inside the cable).

Although I cannot see why it would make a difference, it's something to try.

[simonbaker]

Simon,

I did test both coil in the wet surf. Both coils reacted when touching salt water, but the ungrounded one would still be my pick as the reaction was minimized somewhat. I was able to nearly reduce all reaction to hunting over wet, salty sand by bumping up the disc a little... around where thin foil is rejected.

Now, where I had the most trouble was the area between where the sand was completely wet and completely dry.. but the TGSL was still very usable.

Don

Ok, encouraging, sounds better than my Compadre.

-SB