Interesting Phenomenon with DD coil

[Detectorist#1]

Hi MD enthusiasts,
These days I make many experiments with using of DD coil for hybrid PI MD. With two pots connected for damping of TX and RX coil respectively, in one strange setting, I meet interesting results. All these experiments are with 15x12" SEF coil from DETECH company. TX coil is 300uH 0.42R and RX coil is 380uH 6.92R. When the value of the damping resistor of the TX coil is unusual low (130R or 62R, TX coil is over over damped), I see the output voltage of the first RX stage with gain of 10. When the damping resistor of the RX coil is set to 47R (very low value), the output voltage of the first stage go near to 0V after only 1us. This allows to recognize 25c USA surely from distance of 30cm instead of only 15cm distance when TX and RX coils are critically damped (with 390R resistor on TX coil and 1K damping resistor on RX coil). The peek TX current is appr. 2.8A. It will be good if someone have possibility to explain this phenomena.

[Detectorist#1]

The interesting things go on - for Fe target (beer's cap), I receive two high tones (with zero in the center of the coil) and for non-Fe target (25c USA), I receive only one high tone in the center of the coil. This is PI MD with inherent Fe/non-Fe recognition!

[eduardo1979]

Hello Detectorist. I really like your idea. It seems very interesting. I would like to be able to apply it to the Bipolar PI detector. Do you think it would be possible?
Regards,
Eduardo

[Detectorist#1]

Hi Eduardo,
I think, this idea have to be applied to bipolar PI detectors. Ready manufacturing DD coils have to be used with half-bridge transmit stage.
Attached is a file with some half-bridge possibilities for schematic.

[boilcoil]

I guess, because the RX coil is connected to the transmitter coil not only inductively, but also capacitively, according to the level of the RX potentiometer , the two components of the signal can be roughly dosed (the inductive one is mainly influenced by the position of the potentiometer). Maybe there should be a position of RX pot where these two components of the receiving energy from the TX are equal.
But specifically in this case - I suppose that due to the fact that these two components of the signal (cap. and ind.) are dephased at 180 degrees, and working with a strongly overdamped TX, as you describe, i.e. a rather flat falling characteristic of TX, i.e. a low value of dB/dt in the initial part (inductive component), but strong enough capacitive (dephased at 180), the general characteristic of the signal may fall much earlier.

I don't know if I'm right, but you can experiment with it by changing the pot value of the RX and see, after a certain value, if the NATURE of the output signal/oscillogram ( out of the first preamp) changes.

[eduardo1979]

Hello Detectorist#1.
Thanks for sharing your interesting experiments. I really like your ideas. I'm currently working on the design of the PCB for the Bipolar PI, as I'm doing it in my spare time and when everything is finished and tested, I'd like to try your idea. Best regards,
Eduardo.

[Detectorist#1]

Hi boilcoil,

First, thank you for your answer. I think you are right - in DD configuration the inductive and the capacitive connection have to be compensated. As confirmation of your explanation, I noticed strongly influence of the proximity of the wires (TX and RX) from the coil's connector to the PCB. Also I noticed influence of a near hand! As conclusion - RX wire inside the box have to be screened also and PCB traces and part's disposition on the PCB have to be designed carefully when inductive balance search coil is used. This is my first project with this type of search sensor. Thank you again!

[Detectorist#1]

After good separation between TX and Rx wires inside the box, I meet very good difference in the polarity of the output signals after SAT stage for Fe/non-Fe targets. This allows to not use parallel channel for the processing of the RX signals on TX-ON time ( I hope so) . This seriously increase the simplicity of the overall schematic. I will post the scheme after specifying of the things. Now, I have to make decision for the final indication of the target - only with different tone for different targets ( Fe/non-Fe) or different tone + different volume for different level of the RX signal. Also - use or not use different color LEDs indication for Fe/non-Fe.

[boilcoil]

Without a channel receiving information during the on-time interval (working only with the channel, operating during collapse time), you will be able to separate really ferro/nonferro only the normal targets - either with sufficiently high eddy currents or with high magnetic permeability. But for very small or highly resistive targets, you will also need information during the active pulse time.

[Detectorist#1]

OK. I will do many additional tests before the final decision. Thank you!

[Riss]

I'm afraid that after the tests in real conditions ( Soil, hot rocks, ceramics and seawater ) you will feel like a lover who, during a serenade under the balcony, is poured with a bucket of ice-cold water. Of course – you will react like a Woody Woodpecker after another crash by plane that he makes (back to the drawing board again!). Excuse me, but I could not find the link to this particular video with Woody – I watched it too many years ago... Next comes another bucket of ice-cold water on the head...https://www.youtube.com/watch?v=7SWvm5VdSF0 . With a high degree of probability you will not listen to me, you will share the results on the forum and I will start laughing like this -https://www.youtube.com/watch?v=7SWvm5VdSF0

[Detectorist#1]

Hi Riss,
Many thanks for your very kind words! Go further with your help for the colleagues.

[boilcoil]

Ha, ha, Bulgarian cooperation...

[Detectorist#1]

Hi boilcoil,
People are different!

[boilcoil]

Excuse me, it was a pretty stupid joke. Excuse me again.