New Concentric Co-planar Coil with Strong MF.

[Davor]

My thoughts went to that direction as well. There are too much "wet grass" etc. effects called upon with a common denominator: E-field coupling. I'm almost certain my Rx coils will be bifilar with center tap to the signal ground, and Tx will be fed via separate pair.

Now regarding the IB coil systems. I made a LTspice simulation of a DD coil system, with values according to my gut feeling. I tinkered a bit with coupling to achieve some 120dB Tx suppression, and copied everything to show situation with resonant Rx tank, and a non-resonant Rx coil. This arrangement is as such for future experiments purposes - I did not put any target there yet.
Even here It is obvious that the tank resonance screws up phase so badly that any discrimination at resonance is doubtful. Also it is obvious that below resonance there is not much going on that would be any different from a non-resonant case.

So, have fun and play with this. This arrangement will be easy to adapt for concentric coils as well, and of course with more life-like values.

[Tinkerer]

Hi Tinkerer:

My experiments (TGSL Experiments thread) seem to indicate that residual voltage is due to some non-magnetic coupling (capacitive?) in the wires of the coil and/or cable system. I got a much deeper null with a special Belden cable with shielded pairs than with a USB cable, for example. I do not know if that translated into detection superiority (couldn't test that). Also, both nulls happened to be well within the headroom of the pre-amp.

Magnetic coupling, in theory, should be able to be canceled quite well by shifting our coils I think. So maybe a little circuitry to cancel some of the "capacitive" coupling could be employed if needed to accomodate a coil whose residual null can't be knocked down enough by shifting the coils.

-SB

I agree, there are many factors that add up to generate the residual. Cable, shielding, inter-wire capacitance, are some of the factors. Add 2" of cable on the coil and be amazed how much residual that can produce.

Nulling the coils, means to obtain the minimum residual, but there will always be a residual.

Like everything else in metal detector design, it is a matter of compromises. We add here, only to loose there.
In case of doubt, we should fall back to the KISS principle.

Tinkerer

[simonbaker]

My thoughts went to that direction as well. There are too much "wet grass" etc. effects called upon with a common denominator: E-field coupling. I'm almost certain my Rx coils will be bifilar with center tap to the signal ground, and Tx will be fed via separate pair.

Now regarding the IB coil systems. I made a LTspice simulation of a DD coil system, with values according to my gut feeling. I tinkered a bit with coupling to achieve some 120dB Tx suppression, and copied everything to show situation with resonant Rx tank, and a non-resonant Rx coil. This arrangement is as such for future experiments purposes - I did not put any target there yet.
Even here It is obvious that the tank resonance screws up phase so badly that any discrimination at resonance is doubtful. Also it is obvious that below resonance there is not much going on that would be any different from a non-resonant case.

So, have fun and play with this. This arrangement will be easy to adapt for concentric coils as well, and of course with more life-like values.

Yes, a number of people have gone through that exercise, but always good to have another sim to play with, thanks.

It does illustrate why Tesoro (and others) probably go with off-resonance designs -- any little component parameter deviation (manufacturing tolerances) would create a hefty phase bias if the TX frequency was at RX tank resonance.

It would be interesting to know how much heat effects those component parameters during typical MD operation. The reason is that I am interested in trying an "on-resonant" design some day, which would require dealing with the phase bias. Once you know the phase bias, discrimination could be performed easily, unless thermal effects cause significant movement of that bias during typical operation.

You would probably need to use a different kind of oscillator also, since ground composition probably significantly affects the TX oscillator frequency in the current TGSL design where the search coil is part of the oscillator. I would favor perhaps a relaxation oscillator (or crystal type) driving a resonant TX tank, although I'm sure that has its own challenges such as amplitude fluctuations.

I would probably tackle an on-resonant design with the help of an on-board microprocessor to help manage the calibration issues.

Just one of those pipe-dreams on my list...

-SB

[Davor]

As a parallel resonance seen from a coil is in fact a series resonance with low impedance, the way around the troubles with resonance would be connecting a Rx coil to a low impedance pre-amp. You may play with the LTspice design here by lowering the 4k7 resistor (preamp input impedance stand-in), and see what happens to phase. Instant conclusions are that low impedance affects phase response, but in a mild way, and the voltage response remains mostly untouched.
To make a long story short: you may benefit from resonance pickup - without resonance - by connecting a Rx coil to a low impedance preamp.

Point with Tx coil being a part of an oscillator is not that much of a problem because whatever effect is dragging Tx frequency up or down, the same effect moves Rx resonance in the same fashion.

[simonbaker]

As a parallel resonance seen from a coil is in fact a series resonance with low impedance, the way around the troubles with resonance would be connecting a Rx coil to a low impedance pre-amp. You may play with the LTspice design here by lowering the 4k7 resistor (preamp input impedance stand-in), and see what happens to phase. Instant conclusions are that low impedance affects phase response, but in a mild way, and the voltage response remains mostly untouched.
To make a long story short: you may benefit from resonance pickup - without resonance - by connecting a Rx coil to a low impedance preamp.

Could you give some more detail there? Not sure what your point is.

Part of the appeal of an "on-resonant" design is the high gain of a high-Q RX resonant tank (using low resistance wire), which would seem to offer hope of improved signal-to-noise. However, I haven't analyzed far enough to know if such gains would evaporate due to some other factors.

Point with Tx coil being a part of an oscillator is not that much of a problem because whatever effect is dragging Tx frequency up or down, the same effect moves Rx resonance in the same fashion.

Good observation -- in fact, that might be a compelling reason to not use a frequency-stable oscillator, since the RX resonant frequency would then drift around relative to the TX freq!

-SB

[Davor]

Could you give some more detail there? Not sure what your point is.

Both parallel or series resonance are the same phenomenon, but the way you take a signal off it makes it series or parallel. In series you see a very low impedance, ideally only parasitic resistances of the tank components, and you are basically measuring current that passes through. Equivalent circuit of an ampere meter is a short circuit. Simultaneously you may measure voltage across a coil and from that point of view it is a parallel resonance tank. In both instances a coil has some voltage across and some current going through. As the voltage is a function of magnetic flux, you are better off if you allow current to flow - thus maximizing power transfer. To ensure current flow you may either connect a capacitor to a coil and achieve resonance, or you may connect it to a low impedance preamp.

Because of the phase problems in resonance, I think the way around it is using a low impedance preamp. If you go too low you'll have some phase shift, but uniformly so, and not a step function as in resonance.

High side of low impedance is also a low noise. Optimum input impedance of an opamp is calculated from voltage and current noise (Vn and In -> Ropt=Vn/In). With NE5534 you get Ropt at ~9kohm, OPA134pa would have Ropt ~ 1800ohm, while with LT1115 you get Ropt at ~750ohm. Hence, LT1115 is better for low Z operation, and NE5534 is best for quasi resonant input. Option B would be using a common base or gate preamp, or some impedance transformer -just like pre-pre for MC phono.

Check out Hiraga pre-pre for ideas on low input impedance
http://www.bonavolta.ch/hobby/en/audio/prepre.htm

You may play with LTspice simulation above for all cases mentioned here. Try terminating a non-resonant branch with, say, 100 ohm instead of 4k7 and see what happens. Your criteria are voltage and phase. As the coil voltage is a function of flux, unless you go with input impedance below the coil reactance your coil voltage will be the same.

[Carl-NC]

Optimum input impedance of an opamp is calculated from voltage and current noise (Vn and In -> Ropt=Vn/In).

Never heard this before... "optimum" for what?

[ivconic]

Never heard this before... "optimum" for what?

That's because you are one ... trial&error "ee" and nothing else!!!


P.S.
Don't give me evil eyes and don't ban me! Ban Aziz and Davor! They are guilty for everything!

[Davor]

Never heard this before... "optimum" for what?

It is the input impedance above which the sole contributor of noise is resistor noise. Below that the dominant source of noise is opamp.
In case your design calls for input impedance in neighborhood of 9k, NE5534 gives best bang per buck. Spending money on LT1115 will benefit you nothing, it will only lighten your purse.

[Carl-NC]

It is the input impedance above which the sole contributor of noise is resistor noise. Below that the dominant source of noise is opamp.
In case your design calls for input impedance in neighborhood of 9k, NE5534 gives best bang per buck. Spending money on LT1115 will benefit you nothing, it will only lighten your purse.

Opamp noise consists of voltage noise and current noise. If the input resistor is zero, then all you get is the opamp voltage noise. As the input resistor increases, at some point the 4kT*R noise overtakes the Vn noise. That exact point is when R = (Vn^2)/4kT.

If the input resistor = Vn/In, then the resistor has 4kTR thermal noise, but the opamp current noise through the resistor also contributes. Which one is larger depends on Vn and In, there is nothing magical about Vn/In.

As the input resistor increases, at some point the In*R noise overtakes the 4kTR noise. That exact point is when R = 4kT/(In^2). Of course, the Vn contribution remains constant.

So we have:

Vn dominates for R=0 up to (Vn^2)/4kT
4kTR dominates* for R=(Vn^2)/4kT up to 4kT/(In^2)
In dominates for R=4kT/(In^2) or larger

So the bottom line is, the optimum value of R is zero.
-----------------

*Depending on the opamp, 4kTR might never dominate.

[Davor]

... which is not very useful
BTW, Vin is usually measured at 100 ohm, not 0 ohm.
There is a nice application note explaining the subject of input impedance and choice of opamp: http://cds.linear.com/docs/Design%20Note/DN355f.pdf

[simonbaker]

Both parallel or series resonance are the same phenomenon, but the way you take a signal off it makes it series or parallel. In series you see a very low impedance, ideally only parasitic resistances of the tank components, and you are basically measuring current that passes through. Equivalent circuit of an ampere meter is a short circuit. Simultaneously you may measure voltage across a coil and from that point of view it is a parallel resonance tank. In both instances a coil has some voltage across and some current going through. As the voltage is a function of magnetic flux, you are better off if you allow current to flow - thus maximizing power transfer. To ensure current flow you may either connect a capacitor to a coil and achieve resonance, or you may connect it to a low impedance preamp.

Because of the phase problems in resonance, I think the way around it is using a low impedance preamp. If you go too low you'll have some phase shift, but uniformly so, and not a step function as in resonance.

High side of low impedance is also a low noise. Optimum input impedance of an opamp is calculated from voltage and current noise (Vn and In -> Ropt=Vn/In). With NE5534 you get Ropt at ~9kohm, OPA134pa would have Ropt ~ 1800ohm, while with LT1115 you get Ropt at ~750ohm. Hence, LT1115 is better for low Z operation, and NE5534 is best for quasi resonant input. Option B would be using a common base or gate preamp, or some impedance transformer -just like pre-pre for MC phono.

Check out Hiraga pre-pre for ideas on low input impedance
http://www.bonavolta.ch/hobby/en/audio/prepre.htm

You may play with LTspice simulation above for all cases mentioned here. Try terminating a non-resonant branch with, say, 100 ohm instead of 4k7 and see what happens. Your criteria are voltage and phase. As the coil voltage is a function of flux, unless you go with input impedance below the coil reactance your coil voltage will be the same.

Ok, sounds like your talking about minimizing noise, different topic from "on-resonant" design phase-shift problems.

I'm guessing that you are comparing these two configurations for noise?

-SB

[Carl-NC]

There is a nice application note explaining the subject of input impedance and choice of opamp: http://cds.linear.com/docs/Design%20Note/DN355f.pdf

From the app note:

"For low REQ, the op amp voltage noise dominates (as VN is the remaining term);"

This is true.

"for very high REQ, the op amp current noise dominates (as IN is the coefficient of the highest order REQ term)."

This is true.

"At middle values of REQ, the resistor noise dominates and the op amp contributes little significant noise."

This is not necessarily true, and may be utterly false. Depends on the opamp.

"This is the ROPTIMUM of the amplifier and can be found by taking the quotient of the op amp’s noise specs: VN/IN = ROPT."

Again, possibly utterly false. I think the author looked at a few opamps which met this criterion, and he created a rule-of-thumb. Vn/In is generally meaningless.

- Carl

[Wiltran]

CC coil

Hello Dave,

Made some photo’s from the coilwindig forms I used… they are from late 1980’s …so no computer software then, but the made coils worked fine.

To make a CC coil :

Start with the transmit coil, as seen on the large 22,5 cm form 97 turns give’s 14,5 KHz, (114 turns is 12.5 KHz.) Wire used is 0.25 mm and the coil is connected to the Tesoro transmitter so the cap from the oscillator has also influence at the transmit frequency !


The first receive coil, 18 turns, is added at the 22.5 cm coil, same wire and turn direction . Transmit coil and first receive coil both clockwise !

This 18 turns will , just like in a transformator, pick up a part from the transmit signal .
If the transmit signal is 15 volts then there is 15volt /97turns = ca. 0.15 volt per turn wire so the first receive coils 18 windings pick up 18x0.15 volt = 2,7 volt.

Now the trick is to null out this 2,7 volt … this must be done by the second small receive coil, As the distance to the transmit coil is greater the magnetic field is smaller, so less volts per winding.. so more turns are needed to get that 2,7 volt. AND that voltage needs to be in opposite faze , so the windings for the second receive coil have to be counter clockwise !
For a 10-12 cm coil form some 160-170 turns are needed, also 0.25 mm wire. The two receive coils are connected in series but picking up the volts in opposite faze's so the result is zero.

If you have to move the small receive coil towards the transmit coil (stronger magnetic field) then you have to add windings at the small coil….if you have to lift up the small receive coil (less magnetic field) than you have to reduce windings…

For the receive capacitor look at the first opamps output… select the cap for a nice sinus out.


Good luck !

Ap

And yes... some software to adjust the stuff....hmmm but we do not have that... so...here is the info I have... it works but it needs some experiments to get it right.. winding strength diameters from the coils.... all make’s differents... and I hope I have not made errors...

Apberg, would you please draw a schematic of this coil?

[ApBerg]

Apberg, would you please draw a schematic of this coil?

the cable used is a 2x2 pair screend cable, the ground connection for the receiver is made in the coil, this is needed this way for the twisted pair effect, .. the black wire is not used.. prevents earthloops...

If the small coils is picking up a much to large signal... just turn it upside down......if it needs to lift up to get a null.. there are to many turns , if it needs to be moved to the large coils for a null ... some more windings are needed.

Hope it is all ok ..... but if needed corrections are welcome...