Using trimmer to zero DD

[Mullihaka]

Someone at the Russian Web site had torn down an Garrett ACE-250. It had coplanar coils, of which one had a trimmer potentiometer connetcted between some turns.

I assume that the trimmer is used to zero the coils instead of moving them around.

Has anybody tried that with DD coils for a VLF detector? Obviously there a drawback, otherwise it would be "the" way to zero the coils, instead of making 0.01mm increments.

All hints appreciated.

[Mullihaka]

Picture attached

The coil is described here: http://metadec.narod.ru/sch_md.htm

[simonbaker]

Someone at the Russian Web site had torn down an Garrett ACE-250. It had coplanar coils, of which one had a trimmer potentiometer connetcted between some turns.

I assume that the trimmer is used to zero the coils instead of moving them around.

Has anybody tried that with DD coils for a VLF detector? Obviously there a drawback, otherwise it would be "the" way to zero the coils, instead of making 0.01mm increments.

All hints appreciated.

Double-D doesn't have convenient bucking coil, but probably very possible with extra loops. Maybe it reduces Q of coil a little so not desirable if you can take the time to null other ways. Just speculating - trimmer would be very handy. I think Aziz has a trimmer circuit with his coils, maybe he can explain.

Cheers,

-SB

[dfbowers]

Nautilus feedback method

Here is a convenient way of balancing either DD or Coplaner. It is used on the current Nautilus DMCIIb (Nautilus patent). In the older models it was used as a means of implementing ground balance or disc (not both at the same time). It can be found in the patents section.

Don

[ivconic]

Oh! This is the question of all questions! Reffering the coils subject.
It would save us of enormous effort when nulling coils, if we had possibillity to "electronicaly" null coils. Just swing few trimmers and job is done! Personally i am dreaming about that. But..seriously i have doubts that it can be done universally to all machines and coils. Suppose coils are not nulled manually just gathered togather...and suppose we achieve "electronic" null with some trimmer and few other components...well, those will be "nulled" somehow but will not have good performances at all. So i dont think is possible just to put trimm and do the job. Probably coils were nulled previoulsy, and trimmer serves just to fine readjust in small percentage some minor jitter or simillar.
This is just my presumption, i do not claim it is absolute truth. Also i would like to hear others on this.
This is very interesting question, right question; are there any other, "electronic" methods to null coils?

[Qiaozhi]

Oh! This is the question of all questions! Reffering the coils subject.
It would save us of enormous effort when nulling coils, if we had possibillity to "electronicaly" null coils. Just swing few trimmers and job is done! Personally i am dreaming about that. But..seriously i have doubts that it can be done universally to all machines and coils. Suppose coils are not nulled manually just gathered togather...and suppose we achieve "electronic" null with some trimmer and few other components...well, those will be "nulled" somehow but will not have good performances at all. So i dont think is possible just to put trimm and do the job. Probably coils were nulled previoulsy, and trimmer serves just to fine readjust in small percentage some minor jitter or simillar.
This is just my presumption, i do not claim it is absolute truth. Also i would like to hear others on this.
This is very interesting question, right question; are there any other, "electronic" methods to null coils?

I think your assumption is correct. The trimmers are used for final adjustment of the coils. This allows for phase-shift adjustment between TX and RX, and will affect both the ground balance and discrimination.

[dfbowers]

Using trimmer to zero DD

Yes, you are correct. This is only used as a final adjustment from any mechanical shifting of the windings. One thing that must be considered from the diagram above is the impedence matching. I believe that the Nautilus patent shows that the coil is tapped to reduce the loading on the Tx coil and the 39pf cap and 150k resistor might need to be changed a little to suit your application.

If you build a coil and set it in epoxy and the windings shift a little during curing, the circuit above can be used to bring the arrangement to a perfect balance by feeding back a phase and amplitude adjusted signal. It works wonderfully.

The Nautilus DMCIIb detector is stictly an old analog type of circuit with a Tx power control and external R & C coil balance pots that really help tweek the perfomance of the detector. With a relic hunter who knows what he is doing with it, he won't leave much behind for others.

Don