New ground exclude?

[asolone]

this teoria is already applied in some coils, the cancellation is done by a trimmer connected to two windings of the coil and the receiver coil
Franco where are you writing? I am of padua italy

[Dave J.]

I think what FrancoItaly is describing is what we used to call a "bucker" system as used on some Fisher products back in the mid 1970's through early 1980's. An OTA feeds a reverse phase reactive signal into the receiver coil. The output of the reactive synchronous demodulator drives an integrator which then proportions the gain of the OTA. Models which had manual ground balance, you could adjust the phase of the of the feedback signal.

The 1260-X used both resistive and reactive buckers, and took the output signals from the integrators. Thus the receiver coil was maintained in null despite ground and targets. Troubleshooting was difficult because in a full quadrature bucker system, when it's working properly there's nothing to see on the oscilloscope.

An advantage of the system is that you can resonate the receiver coil and as long as it's not too far off resonance, it still ground balances. In the case of the 1260-X, the preamp was a Darlington transistor emitter follower running at gain less than unity. The advent of integrated circuit low noise amplifiers and CMOS IC demodulators led to the demise of parallel resonated receiver coils. However urban mythology is made of much more durable stuff, the superstitions that arose from the use of parallel resonant receiver coils are still being bandied about more than two decades since multifrequency metal detectors went mainstream and should have blown the superstitions out of the water.

[Qiaozhi]

I found these in my archive ->

[WM6]

Does anyone know, how this "Total Automatic Ground Compensation" coil was made (electric circuit)?

[Dave J.]

Thanks, George. ....It's not exactly a trade secret any more. For what it's worth, I don't recommend that anyone try to build a "bucker" based project, analysis is difficult and troubleshooting is also difficult.

The 1210-X is an example of a somewhat similar circuit topology but without the buckers, using a similar searchcoil. Had to use polypropylene caps for stability, and hardwired the searchcoil so customers couldn't plug in a searchcoil having a different inductance. Probably a schematic in the archives here somewhere.

[Carl-NC]

Does anyone know, how this "Total Automatic Ground Compensation" coil was made (electric circuit)?

I'm pretty sure it has a figure-8 RX coil, which inherently cancels the ground.

[Dave J.]

Inherently cancels the ground, sort of. There are really good reasons why the industry didn't adopt the idea. For starters, "Motion discrimination" was a much better solution to the problem.

[EDIT] Funny world we live in now. I can argue with my boss (and forum admin) in public view with people all the way to China listening in, and life will go on, I probably won't get canned.

[Qiaozhi]

Probably a schematic in the archives here somewhere.

Yes Dave, you're correct.
Here's a William Lahr schematic, and a PCB layout from ApBerg ->

[WM6]

I'm pretty sure it has a figure-8 RX coil, which inherently cancels the ground.

Thanks Carl and Dave for comments

Figure-8 RX coil could be logical conclusion, but looking more in entrails (not my photo), it is not something really like 8-design.

Maybe it is something alike: two RX coils in counter-phase? So I am asking for electric schema of this old unusual BH coil.

[FrancoItaly]

Repeat with simple words my thoughts, with the head of research in the air, away from the ground, the RX signal is in phase with the TX signal and has an amplitude, for example, 4V, and is sinusoidal in shape. This signal is rectified and smoothed and drives an amplifier (for example, an OTA). I approach the search head to the ground (suppose that there is no metal), and then the ground causes an increase of the RX signal amplitude and also a small change in the phase (depending on the type of soil). If we send a signal exactly out of phase by 180 degrees and amplitude proportional to the RX amplitude we can null the ground effect. Obviously, in the presence of metal there will be a variation of amplitude and phase in the RX signal.

[scrungy_doolittle]

Such compensation to reach a "null" in the induction balance can be found on the Quasar and Kpot Mole metal detector:

http://fandy.ucoz.org/QuasarARM/Quasar_ARM_SCH.pdf (the SIN_C signal, grid square 5D)
http://radiodom.ucoz.ru/_si/0/82158727.jpg (the DAC1 signal, on the lower left where the pre-amp is)

I've been wondering how that krot feature works. can you please elaborate on it?