Replacing a mono coil with a DD coil: How should the RX and TX coils be dimensioned?

[GeoMax]

Question for the community:

I want to modify a PI detector circuit designed for mono coils (RX/TX) to work with DD coils (RX & TX). The DD coil still needs to be fabricated, and the original mono coil for the PI circuit has an inductance of approximately L=1 mH. How should the RX and TX coils be configured in a DD setup so that they work with the detector circuit? While damping resistors might also need adjustment, I am not concerned with them here, as my question focuses solely on the inductance values and the ratio between RX and TX. Should both TX and RX be each L=1 mH? Or should there be a different ratio—perhaps TX=350 µH and RX=1 mH?

[nejdet]

Hello, if TX worked with 1 mH, I think it will not work correctly with TX=350 µH

[GeoMax]

Hello, if TX worked with 1 mH, I think it will not work correctly with TX=350 µH

Hi, Nejdet,

this I am not sure because of the following.
I have a detector—a low-power PI model with an original 1mH mono coil. I swapped the original coil for a different 320µH mono coil. The result was that the detector seemed to work better than it had with the 1mH mono coil.

[eduardo1979]

Hi Geomax.
In case my information is of any use to you; I have an original 11" DD search coil for Minelab GP series and these are the measurements:
TX = 310 uH = 0.5 Ohms
RX = 620 uH = 17 Ohms
Regards, Eduardo.

[GeoMax]

Hi Eduardo1979

is your MineLab coil the model "Commander" (white color round DD-Coil 11 Inch for GP Series)?

[eduardo1979]

Hi Eduardo1979

is your MineLab coil the model "Commander" (white color round DD-Coil 11 Inch for GP Series)?

[boilcoil]

When working with an inductively balanced coil system, you need to be aware of what the nature of the received signal from RX should be. I assume that you mean sampling during on-time, where this system really makes sense. If this signal must be necessarily in one quadrant, for example in the first quadrant, the RX coil must be unbalanced/displaced in such a way that the increase and decrease of the signal from it, caused by ferromagnetic (high magnetic permeability) and diamagnetic conductive targets (high levels of eddy currents), does not change the quadrant in which the signal being processed is (or to shift the DC sample from the real zero to the middle of the operating range, but in this case you will reduce the usability of samples taken by this block during off-time).
Since this system is, in practice, not completely balanced, the RX coil directly affects to the TX module, and this influence is proportional to both the mutual inductance coefficient of the coils (which depends on the imbalance/displacement), and the ratio of the self-inductances of RX and TX, as well as the degree of damping of both tracts. The optimal levels will have to be found experimentally. I have experimented with systems where both coils have the same inductance, as well as systems with radically different coils (500uH TX, 3mH RX). With an RX coil with higher inductance, you can achieve higher sensitivity, but at the cost of this, the other balances of the MD (especially GB) become more complicated/worse.
All of this depends on both the device's circuitry and the signal processing method, so you'll really have to find the optimal parameters experimentally.

[GeoMax]

Hi Boilcoil

Thanks for the helpful explanations. The detector circuit I want to convert to a DD coil system is very similar to the C-Scope CS6PI. This detector also comes with a standard 1mH mono coil. The CS6PI is an old Eric Foster design. Eric Foster also manufactured DD coils for his detectors in the past. Does anyone happen to know the RX and TX parameters of his DD coils?

[boilcoil]

The CS6PI is an old Eric Foster design. Eric Foster also manufactured DD coils for his detectors in the past.

Yes GeoMax, Eric was a genius - always trying new things and never afraid to step outside the box. It was like he caught ideas in the air, where others didn't notice them...