Differential Coils ... theory.

[AdrianM]

Mick, I hope you do get a chance to try this yourself - I would like to know how it goes!

Tinkerer, have you considered making a widely separated twisted pair for your connecting cable (like in the photo of post#8 above)? I wound separate wires, about 10mm apart, around a 20mm x 100cm plastic pipe to get a "Barbers pole" multiple helix. This was using PVC materials and the capacitance this added to the coil was just 24pf. The plan is to then sleeve this with shrink wrap - or 25mm pipe to form the final search coil handle.

With twisted pair for the differential signal I think there's a vanishingly small need for an overall shield which adds considerably to the capacitance and introduces another variable as an adjacent false target. Like I said, with an unscreened twisted pair "cable" such as this, terminated at the far end, I couldn't measure any significant differential noise.

The one thing this kind of connection is evidently excellent at is eliminating capacitive coupling to ground or proximity with the operator. You can wrap your hands around the insulation of the unscreened wires and see no effect on the differential preamp output.

Still trying to visualize nature's trick of unwinding coils.

[Tinkerer]

Mick, I hope you do get a chance to try this yourself - I would like to know how it goes!

Tinkerer, have you considered making a widely separated twisted pair for your connecting cable (like in the photo of post#8 above)? I wound separate wires, about 10mm apart, around a 20mm x 100cm plastic pipe to get a "Barbers pole" multiple helix. This was using PVC materials and the capacitance this added to the coil was just 24pf. The plan is to then sleeve this with shrink wrap - or 25mm pipe to form the final search coil handle.

With twisted pair for the differential signal I think there's a vanishingly small need for an overall shield which adds considerably to the capacitance and introduces another variable as an adjacent false target. Like I said, with an unscreened twisted pair "cable" such as this, terminated at the far end, I couldn't measure any significant differential noise.

The one thing this kind of connection is evidently excellent at is eliminating capacitive coupling to ground or proximity with the operator. You can wrap your hands around the insulation of the unscreened wires and see no effect on the differential preamp output.

Still trying to visualize nature's trick of unwinding coils.

Helix pole.
At high power TX, this coil could and will do things. What exactly I don't know. If you cross the wires (Caduceus?) then the inductance should cancel. Will it ?

You can consider the cables a LC tank circuit. LC tanks make high Q filters. If you chose the right frequency of the filter it could be useful.
The wrong frequency might just kill your most important target response or it could enhance that one radio frequency.

I have 3 wires from the RX coil. 2 ends and the center tap. the 2 ends twisted do fine. The third leg does strange things and need to be handled with care. there are several ways it can be handled and each way gives different results.

I have not done yet exact tests, but need to do them to figure out which way is best.

Puzzles..............

Tinkerer

[AdrianM]

I'm pretty sure that if you wrap a pair of wires in parallel around a pencil (funny that, Helix is a make of pencil ) and then pull the pencil out - and pull the wires tight, you get a twisted pair just the same as if you spin a pair of wires in a drill.

This is what I was aiming at anyway, a BIG twisted pair! As for the common (centre tap), I ran both ends of the bifilar coil up the pole (repeating the four wires +-+- in sequence) and made the centre-tap join at the circuit end. I tried the join at both ends and it was 200ns faster in decay when joined at the top so that's how I use it. Makes no difference to the RF pickup either way.

The inductance-free wirewound resistor winding is one continuous length of resistance wire, folded in half and wrapped around an armature such that the same current travels in opposite directions, so cancelling the induced magnetic field (and thus creating a time machine/free energy source/wart remover or what ever else tickles your fancy judging by teh interweb)

We, of course, just get the usual small amount of transmission line inductance and capacitance between our search coil and preamp which will indeed be receptive to RF, but by the identical amount in each leg on the way to the difference amplifier. That's how it seems to me tonight at any rate.

[Tinkerer]

You gave me an idea that I have not tried yet.
If I make the center tap of 2 wires and pair each one with one end, then I have a balanced input again.
Worth a try.

On the Helix, I don't agree. A 20mm diameter coil with 20 turns does not have the same inductance as a 3mm coil with 20 turns. Just to mention a different point of view.

Tinkerer

[moodz]

Hello Mick, great to see another new poster on the Geotech forums! When I first glanced at moodz writeup I simply accepted that this would be so - but it doesn't seem to be the case.

The first thing that aroused my suspicions was when I realised, night after night, that the signal to noise ratio dropped when the Sun went down - skip I think it's called. I then went about systematically testing each part of the setup. Shorting the coil where the connecting lead joins on the search head totally silences the preamp stage - so the connecting lead and "above" is ruled out. I also check the balance of the differential preamp with a signal generator just to make sure that common mode signals were being rejected properly and they were. At this point I know it's all coming out of the coil itself. So finally, I added a parallel tuning capacitor across the coil and a pair of headphones to the output of the preamp and made myself a long-wave radio set!

OK, so the bandwidth was wide open but classic radio tuning sounds abound - plus some 50Hz, WiFi, Bluetooth - and worst of all (embarrassingly) my Tektronix TDS3014 scope. Everything but the kitchen sink basically.

So, back to theory - I appreciate that radio antenna systems employ a "ground plane" - either literally as the ground (or electrically large - compared to the wavelength being received - conducting surface) with a monopole sticking out of it, or a dipole where one arm provides a similar function. So radio signals are not necessarily referenced to ground (otherwise how would radio work in outer space )

With the differential search coil, despite the notional ground reference point at the centre tap, there are still two projecting arms "curled up" as a coil either side. The electrical length of these arms will accommodate a wide range of radio frequencies where the peak-to-peak distance puts a positive voltage on one end and a negative on the other. A differential voltage I believe. That is how it works, sort of, I think! (I never could get how nature can "unwind" a coil to pull off this sort of trick though). However, if I'm mistaken and nature can't "unwind" the coilI, it would be expected that pretty much the same voltage would appear at both ends. But then again, all kind of antennas are coiled up - particularly long wave ones! I'm sure some more words will be added by a RF expert...

If you are getting RF on the output of the differential amp it means that your circuit and / or coil is unbalanced. I have demonstrated the rejection capability of the diff coil vs commercial sheilded mono coil PI where the AC rf on the mono coil PI was 300millivolts and there was less than a 1 millivolt on the unsheilded diff circuit. Bringing a finger near one side of the diff amp produced similiar output to the mono PI ... so it only takes 10s of pf to unbalance the circuit. Mechanical and cable layout must be symmetrical.

moodz.

[AdrianM]

The ability of the differential amplifier to reject common mode signals is frequency dependent. Common mode RF signals shouldn't be allowed to reach the diff. amp in the first place - otherwise they'll get demodulated and that's what my problem was. The tricky bit is limiting the bandwidth such that the signal we're looking for isn't lost as well

[moodz]

The ability of the differential amplifier to reject common mode signals is frequency dependent. Common mode RF signals shouldn't be allowed to reach the diff. amp in the first place - otherwise they'll get demodulated and that's what my problem was. The tricky bit is limiting the bandwidth such that the signal we're looking for isn't lost as well

I will have to respectfully disagree with you there .... common mode rejection is frequency dependant to a point however amplifiers like the THAT1510 are good for at least 30 db of CMRR @ 1 Mhz ( and half that .. 15 db is fine ).

Demodulation would only occur if the input protection diodes were driven to conduction or the amplifier is saturating ( non linear ) this means that you either have several volts of CM signal or the amplifier is amplifying not the common mode signal but the difference RF signal caused by electrical unbalance at the input.

Contrary to popular opinion we do not want to limit the bandwidth to remove noise. The bandwidth of a PI impulse is (on my implementations ) around 1 Khz to 1 Mhz. Smaller / conductive targets respond at higher freqs, bigger/ deeper / magnetic targets respond at lower freqs. Band limiting / low pass filters cause amplitude and phase changes of the spectral components ... this is bad. Synchronous filtering ( all pass filter ) in conjunction with common mode rejection at the front end will pass the target response to for analysis with only the asynchronous / common mode noise removed.

Regards,

Moodz.

[AdrianM]

Thanks for the help with this here Moodz. I've used three OP37's in the classic instrumentation amplifier configuration and have verified that the performance is as expected.

However, in the scope shots below you will see there is full 10mv pk-pk differential noise that is a few cycles of around 100KHz directly measured at the coil (traces 3 and 4). This is amplified with a gain of 2000 giving a swing of +-10V on trace 1. The Mosfet driving the coil is disabled so I can just look at what the coil sees around it. In the other scope photo I've simply shorted the ends of the coil (down on the search head) and the noise goes. The gain is still 2000 and I've increased trace 1 to 100mv/div just to show how quiet everything but the coil is.

To me this eliminates everything from the full length of connecting lead all the way up through the diff. amp - leaving just the pickup in the coil. This has a very uniform winding and a natural balance in both halves as you'd expect from the twisted bifilar construction. The signal shown below happens to be dominant, but there's all sorts going on up to the bandlimit of the OPamps.

Night falls, and the phantom long-wave signals creep in

[moodz]

Hmmmm ... in the top CRO shot .... traces 3 and 4 are the two sides of the diff coil right ?

Trace 4 clearly has RF on it but Trace 3 does not ( or less at any rate ). If the coil is balanced any RF that appears would be on both traces ( but 180 degrees out of phase ).

disconnect the coil inputs from the amp and re-measure coil ouputs.

Moodz.

[mikebg]

Let we reinvent the TWIN LOOP

Let we (R) the TWIN LOOP!

Symbol (R) is used by (R)EMI group to note a combination of verbs meaning: Rename, Revise, Reinvent, Rediscover, Redesign etc.

The (R)EMI group ceased its action about one year ago because participants of this forum have enough knowledge to (R) the EMI (electromagnetic induction) used for metal detecting.

IMNHO this is not valid for some participants of this thread, because they hate to rename and to read attentively what is written by authors of TWIN LOOP TREASURE SEEKER.
The project TWIN LOOP TREASURE SEEKER can be improved. This was made several years ago in Canada by G. L. Chemelec and by Sharky. Search this forum for details.

Can we improve something more?
To answer, we should read the article attentively, rename the principle and search WEB.
The name of this sensor is "planar gradiometer", but this name is valid for RX loops only. The term "Differential coils" relates also to RX loops only. Let we search WEB and let read the original article by Crone's published in ETI 1989!
I made new drawing for principle of operation, because the drawing in article (see post #9) seems complicated for my primitive method of thinking.

[moodz]

Let we (R) the TWIN LOOP!

Symbol (R) is used by (R)EMI group to note a combination of verbs meaning: Rename, Revise, Reinvent, Rediscover, Redesign etc.

(R)ead and (R)echeck your comments .... the twin loop was a D configuration ... the coil we are discussing here is a monocoil pair configuration.

moodz.

[mikebg]

(R)ead and (R)echeck your comments .... the twin loop was a D configuration ... the coil we are discussing here is a monocoil pair configuration.

moodz.

Moodz, the TWIN LOOP is O configuration. It is also monocoil pair configuration.

[moodz]

Moodz, the TWIN LOOP is O configuration. It is also monocoil pair configuration.

Maybe you need glasses ... your own "evidence" diagrams you are calling monocoils are offset from each other .... read the dimensions on the diagram. over to you.

Moodz

[mikebg]

Maybe you need glasses ... your own "evidence" diagrams you are calling monocoils are offset from each other .... read the dimensions on the diagram. over to you.
Moodz

Moodz, I seriously not understand your terminology. The TWIN LOOP TREASURE SEEKER uses two monocoils because each coil operates as TX and RX. If we remove one of them, the device should keep to operate, but as a conventional monocoil PI metal detector. The second monocoil network is used to form planar gradiometer, having important advantages.

The dimensions in above diagrams are used only to illustrate change of mutual inductance M, which is important for calculation of total RX inductance L2 when we design L2A and L2B.

[mickstv]

The difference is that the moodz coil scans the same time period where as a twin coil has a short difference in time as scanned over the ground.

That's how I see it anyway.



Mick