Preamp

[Old cart]

Ivconic, excellent summary of the coil information. I have always thought a DD coil would be nice design for a PI but I don't see many designs using it so I dismissed it. I have had good results on my modified (very old) Garrett XL 500. It has a small 6" coil with low inductance 128uH and I have been able to reduce the sampling delay to about 14 uS after optimum damping the coil. This increased the depth on a US nickel and mans gold ring from 20 to 30 cm-50%. To summarize it would seem that the following are necessary for best performance, at least on low conductivity targets:

1. Critical coil damping
2a. Early sampling
2b. Well nulled coil or other means to minimize TX residual to prevent early amp stages from saturating.
3. Low coil inductance
4. Low coil capacitance
5. Short TX pulses ( just long enough for target TC)

Of of these I have implemented and proven 1,2a,3 and 4.

[Old cart]

The problem with induction balanced (IB) coils, is that a mechanical balance is easily achieved, however, any slightest movement during casting the coil or warping or even due to warming up in the sun upsets the balance to some degree.

The electronic balance adjustment re-adjusts this post manufacturing imbalance.

This is a circuit that matches or nulls any 2 coils.

For example: Take a TX coil and a compensation coil. Different inductance, different size, even different location. The circuit matches the 2 signals, current, phase, amplitude, so that adding coil signal a to coil signal b of opposite phase, results in 0.

Like: Take the signal from the RX coil, add it to the signal from a few turns of wire around the TX cable and we have a NULL.
Any target upsets the NULL and generates a target signal.

This circuit has been prototyped and field tried. It works. However, it can be improved a lot to reduce the noise and to simplify it.

That is an impressive circuit. Looks like a compensated voltage divider. The question I have is this. In a PI do we have to carefully balance out the TX signal or is it just fine to simply reduce the TX residual signal to the point where the first stage does not saturate?
In a more advanced detector I can see the value in a processor based auto calibrate feature that would compensate for nulls and maybe even adjust for different coil LCR.
In a simple design though maybe just getting the TX residual down to less than say 8 volts peak to peak divide by first stage gain ( 10 in this example) or 0.8 volts p-p would keep the first stage out of saturation and go along way towards allowing earlier sampling.

[Sean_Goddard]

WOW, I'm BLOWN AWAY!! EXCELLENT WORK ALL!!

I'm at work at the moment so can't digest what gold we have here, but I will have a proper discussion later when I get home. Ivconic, again, 100% on the mark, I will do as you suggest, take the suggestions and make a final design, but I need to look at the merits of each before making the final chopice. Teleno, hats off to you for the sim work. THat looks about the quietest option we can get. Again, I need ot ready the posts fully to see what we have here.

This is better then detecting itself, I see all these wondeful nuggets being revealed, great fun and an honour to see people who have so much knowledge. just because I don't mention your neam doesn't mean I don't appreciate your input.

What I'd like to see along with any ideas is a Pro's and Con's kist put up by others who want to comment on the design suggestions.

Great Start!!

[ivconic]

And the main point:

"... One other major asset of a DD coil is the fact the receive coil is isolated from the transmit coil. This helps in the fact that any low level noise that is generated by the transmit circuitry during the sampling time is isolated from the receive circuitry.
This isolation therefore reduces the combined noise that can negatively affect a target response. One final advantage of a DD coil is, by nature, a DD coil partially cancels the ground signal.
If the coils are properly aligned or positioned, most ground signal in the receive coil is eliminated. This results in a detector that has very little ground response, yet still responds with a strong signal from a buried object..."

[ivconic]

On page 2, Reg is explaining about coils;

http://www.nuggetshooter.com/articles/UnderstandingPIdetector2.html

[ivconic]

Reg Sniff:

"...One important factor that determines the sensitivity of the detector is sampling delay time. The sooner a sample can be taken, the stronger the signal that will be seen. In other words, it is quite possible to take a sample sooner and produce a stronger signal on a PI operating with less current than might be seen on a more powerful PI using much more current and having a longer delay. One simple way to allow earlier sampling is to reduce the coil current. In other words, there are a whole lot of other factors that need to be taken into account to determine what is the best combination. Ah, but someone who just read the previous information might simply say, pulse with a strong signal and then simply sample sooner to make the best detector. Well, unfortunately, the stronger the pulse, the more difficult it is to sample sooner because of the reasons mentioned before. A longer pulse or lower coil resistance will result in more coil current, which will affect how long it takes for the spike to decay. A larger inductance will also result in a longer decay time. In fact, it becomes almost impossible to obtain the very short delay times when using a very strong pulse of long duration.
One way to help shorten the delay time of the decaying pulse is to reduce the number of turns of wire in the search coil. However, the field strength of the coil produced when current flows in the coil is a function of both the current and the number of turns, so reducing the number of turns also reduces the field strength produced. So any reduction in number of turns directly relates to potential depth loss..."

Instead me struggling with my English, trying to explain exactly this, i find more easier simply to Copy&Paste shorter quote from splendid Reg's article.
Now, reading Reg's lines it becomes obviously why "double coil" balance at PI detector is confusing to understand and also why (in some cases) "stiff" balance is not mandatory.
All depends on what you want to achieve, what type of PI detector it is etc.
One thing is definitely for sure; using "double-coil" setup; TX and RX coil can be designed separately, with separate and different specs, to overcome drawbacks and problems which Reg Sniff mentioned above.
Therefore the "balance" in such coil setups will vary from case to case.
And than having all this on my mind i added in pre-pevious post as example; simple playing with TX amplitude and RX gain can significantly affect (and additionally "electronically" balance or disbalance) balance at such "double-coil" setup, DD or concentric.

[ivconic]

That is a good explanation. It would seem the DD would be easiest to null at least until Ivconic explains how to null electronically.

I see logical mistake in what i wrote; "...Concentric coil actually does not need so precise nulling as it is case with IB coil..."
This can be heavily misunderstood... my mistake, sorry!
I meant; " DD or concentric coil FOR PI detector actually does not need so precise nulling as it is case with VLF I/B coil..."

[WM6]

To correct post producing imbalance of DD coils can be done way more simple, than applying such a little excessive circuit.

But catch here is how to balance coil electronically in "automated" (digital) way, by applying adequate balancing algorithm in circuit.

[Monolith]

That is a good explanation. It would seem the DD would be easiest to null at least until Ivconic explains how to null electronically.

The problem with induction balanced (IB) coils, is that a mechanical balance is easily achieved, however, any slightest movement during casting the coil or warping or even due to warming up in the sun upsets the balance to some degree.

The electronic balance adjustment re-adjusts this post manufacturing imbalance.

This is a circuit that matches or nulls any 2 coils.

For example: Take a TX coil and a compensation coil. Different inductance, different size, even different location. The circuit matches the 2 signals, current, phase, amplitude, so that adding coil signal a to coil signal b of opposite phase, results in 0.

Like: Take the signal from the RX coil, add it to the signal from a few turns of wire around the TX cable and we have a NULL.
Any target upsets the NULL and generates a target signal.

This circuit has been prototyped and field tried. It works. However, it can be improved a lot to reduce the noise and to simplify it.

[Old cart]

Coil- help for the preamp

That is a good explanation. It would seem the DD would be easiest to null at least until Ivconic explains how to null electronically.

[Teleno]

Interesting. How do you balance the TX /RX coil? I know how to do this with DD coils but how about concentric? Ivconic mentioned electronic balance.

Figs. 9 to 13 http://www.geotech1.com/pages/metdet/info/coils.pdf

[Old cart]

Interesting. How do you balance the TX /RX coil? I know how to do this with DD coils but how about concentric? Ivconic mentioned electronic balance.

[Teleno]

Ivconic, I think that is great idea. If we can do a DD .... The input resistor can be dropped from 1 K to maybe 100 ohms and the feedback resistor decreased correspondingly to maintain the desired gain. This would take the noise down by factor of 10. ...

The Rx coil will need to be damped in any case, even more so that the Tx coil because it'll have more turns, therefore higher R, hgher L and higher C.

In the presence of a target, the Rx signal is the difference of two exponentials: the Rx coil own decay minus the decay of the target. A low damping R means the Rx coil decays slowly and sampling will need to be done later, reducing S/N.

Signals for an Rx coil balanced to 99.8% - Red: no target (Rx own decay), Blue: target (diff of exponentilas).

[ivconic]

Ivconic, I think that is great idea. If we can do a DD we may not need the protection diodes at all. The input resistor can be dropped from 1 K to maybe 100 ohms and the feedback resistor decreased correspondingly to maintain the desired gain. This would take the noise down by factor of 10. Using a TI49990 ( sadly being discontinued ) would give a front end limited mainly by resistor noise. This will also reduce ground noise. Does anybody have experience with DD's on PI detectors? What is the downside other than they must be nulled?
A concentric is also OK but I think they are harder to null. This is not a huge problem if you have a scope or a very broadband multimeter-but those are quite uncommon.

Exactly!
I had S/N ratio on my mind exactly when i suggested "two-loop" design instead "mono-loop".
Everything is turning again and again around S/N.
So; fundamental improvement (at any project, not only this one) is to improve S/N ratio.
Tx coil can be projected separately and RX coil also can be projected separately.
Each one to suit best specs for the particular design.
Minelab did so far most on this. Not really original idea. Already seen.
...
Concentric coil actually does not need so precise nulling as it is case with IB coil.
Coils can be additionally nulled "electronically" by manipulating with amplitude at TX or gain at RX! Ain't that sweet!

[Old cart]

Ivconic, I think that is great idea. If we can do a DD we may not need the protection diodes at all. The input resistor can be dropped from 1 K to maybe 100 ohms and the feedback resistor decreased correspondingly to maintain the desired gain. This would take the noise down by factor of 10. Using a TI49990 ( sadly being discontinued ) would give a front end limited mainly by resistor noise. This will also reduce ground noise. Does anybody have experience with DD's on PI detectors? What is the downside other than they must be nulled?
A concentric is also OK but I think they are harder to null. This is not a huge problem if you have a scope or a very broadband multimeter-but those are quite uncommon.