pi preamp

[Carl-NC]

(off-topic posts moved or deleted)

liudengyuan, Geotech is the wrong forum for electronic fish lures. I suggest more general electronics forums, two good ones are:

https://www.electro-tech-online.com

https://forum.allaboutcircuits.com

Also, there is little anyone here can do if you are blocked from accessing internet sites.

[Ferric Toes]

Hello Mr Ferric!
Whether a monopolar or bipolar transmitter is used here ? Or is the upper coil connected to the gnd on one side and directly to the other amplifier input on the other ?

Hi Orbit, the bottom coil is monopolar TX and RX. The top coil is exactly the same, but acts as a second RX. The electronics takes the signal from the top coil and feeds it into the other, opposite polarity input, of the front end opamp. Interference signals, such as from overhead power lines, induces an equal signal in both coils which the + and - inputs cancel. The noise coil needs to be damped like the TX/RX coil and also back to back protection diodes must be in place, as the TX transients may damage the opamp. Spacing of the coils need to be about 1/3rd of the coil diameter so that cancellation of the target signal is not significant. This arrangement also cancels earth's field effects when this type of coil bounces around, as on a vehicle.



Eric.

[liudengyuan]

Hi Orbit, the bottom coil is monopolar TX and RX. The top coil is exactly the same, but acts as a second RX. The electronics takes the signal from the top coil and feeds it into the other, opposite polarity input, of the front end opamp. Interference signals, such as from overhead power lines, induces an equal signal in both coils which the + and - inputs cancel. The noise coil needs to be damped like the TX/RX coil and also back to back protection diodes must be in place, as the TX transients may damage the opamp. Spacing of the coils need to be about 1/3rd of the coil diameter so that cancellation of the target signal is not significant. This arrangement also cancels earth's field effects when this type of coil bounces around, as on a vehicle.

[ATTACH]49503[/ATTACH]

Eric.

Is it possible to draw a schematic or image so that we can really know how to make such a coil, thank you very much

[liudengyuan]

Is there only one tx coil at the bottom or another rx coil? Is the top coil connected to the bottom coil?

[Orbit]

Hi Orbit, the bottom coil is monopolar TX and RX. The top coil is exactly the same, but acts as a second RX. The electronics takes the signal from the top coil and feeds it into the other, opposite polarity input, of the front end opamp. Interference signals, such as from overhead power lines, induces an equal signal in both coils which the + and - inputs cancel. The noise coil needs to be damped like the TX/RX coil and also back to back protection diodes must be in place, as the TX transients may damage the opamp. Spacing of the coils need to be about 1/3rd of the coil diameter so that cancellation of the target signal is not significant. This arrangement also cancels earth's field effects when this type of coil bounces around, as on a vehicle.

[ATTACH]49503[/ATTACH]

Eric.

Thank you mr Eric! I assumed it was so but I wasn't sure, i have never tried anything like that .All the best!

[Carl-NC]

Is it possible to draw a schematic or image so that we can really know how to make such a coil, thank you very much

Here is one possibilty:

[liudengyuan]

Here is one possibilty:

[ATTACH]49507[/ATTACH]

Thank you. Can this coil solve the mineralization reaction on the ground? Do the damping resistances of the two coils have exactly the same resistance value? What will happen if the resistance values are different?

[Ferric Toes]

Thank you. Can this coil solve the mineralization reaction on the ground? Do the damping resistances of the two coils have exactly the same resistance value? What will happen if the resistance values are different?

Carl's circuit is the one I used. I used the same value of damping on both coils as they both have the same inductance and resistance. However, the bottom coil's resonant frequency will be a bit lower because of the added capacitance of the switching transistor. For the applications I had, there was no need to sample early, so any slight mismatch that caused overshoot or residual ringing, did not matter. The resistors around the opamp should be equal as shown. The arrangement does not cancel any ground mineralisation.

Eric.

[liudengyuan]

Carl's circuit is the one I used. I used the same value of damping on both coils as they both have the same inductance and resistance. However, the bottom coil's resonant frequency will be a bit lower because of the added capacitance of the switching transistor. For the applications I had, there was no need to sample early, so any slight mismatch that caused overshoot or residual ringing, did not matter. The resistors around the opamp should be equal as shown. The arrangement does not cancel any ground mineralisation.

Eric.

According to your experience, how about the high voltage resistance of this dual coil, can it eliminate the influence of high voltage? Since the ground signal cannot be eliminated, how can it be eliminated? Thank you

[liudengyuan]

Tried this coil today, it lowered the detection distance of the machine, can not rule out the ground madness, did not go to the test near the high voltage line

[liudengyuan]

Without strong software support, it is impossible to make good machines,

[Carl-NC]

According to your experience, how about the high voltage resistance of this dual coil, can it eliminate the influence of high voltage? Since the ground signal cannot be eliminated, how can it be eliminated? Thank you

You normally eliminate ground by subtractive sampling. This has been discussed on the forum and there are some circuit ideas posted.

[6666]

Have a look at Moodz ZP1 design.

[liudengyuan]

hi