Cylinder Coils?

[Detectorist#1]

What is the schematic of constant current bipolar transmitter used from you?

[hykj888]

I have also tested the PMOS and NMOS combination to achieve bipolarity. However, due to their different characteristics, a single damping resistor meet the needs of two different types of Mosfets, resulting in one side having a perfect waveform while the other side is very poor. The picture I showed is a simplified circuit diagram. In practice, I also added a constant current circuit, using an instrumentation amplifier, which can achieve a perfect output from the preamifier.

[Detectorist#1]

Also, have in mind, the schematic of the bipolar transmitter in attached document - sch2 at page 4

[hykj888]

Sorry, I rarely go to the forum, regarding the DOD coil, my TX coil inductance is 300uh, RX are each 350uh, regarding bipolar transmission, I have tested many ways, these two are more satisfactory, it's just that the control is more. Please refer to the picture.

[Detectorist#1]

Hi Tinkerer,
Attached is the picture of some possible planar gradiometer configurations. Output signals of two RX coils are processing from differential amplifier. In "Fe" mode no TX pulses. This guarantee that if a target is recognized, this target is 100% sure from Fe. I think that differential signal processing of the signal from single coil reduces the overall sensitivity of MD and not guarantee 100% Fe recognition for small targets - or I'm not right? Planar Gradiometer Used in PI MD.pdf

[Tinkerer]

Hi Tinkerer,
If you uses only one Rx coil, you loose the possibility to make simple Fe discrimination by connecting of the two Rx coils as planar gradiometer.

I am not sure what you mean by "planar gradiometer"?
With a single coil you can use differential signal processing. Is that different?

[Detectorist#1]

Hi Tinkerer,
If you uses only one Rx coil, you loose the possibility to make simple Fe discrimination by connecting of the two Rx coils as planar gradiometer.

[Tinkerer]

More like 2 TX coils side by side, like OO, and a RX coil in the center, overlapping the TX coils for induction balance.
GPX 7000 uses something similar, but it is 2RX coils and one TX coil in the center.

The Current in the tandem TX coils can be both clockwise or one clockwise and the other counter clockwise. The magnetic fields f the 2 TX coils can be aiding or opposing.

Here is a visualization of the magnetic fields of a Tandem coil in opposing and in aiding configurations.

[GeoMax]

Hi hykj888,
can you also post the front-end and driver circuit for your DOD coil?

[Detectorist#1]

Hi hykj888,
Give please more information for the coils of your DOD sensor: dimensions, wires, turns, inductivity, SFR frequency of Tx coil and Rx coils, type of used screens.

[Detectorist#1]

Hi hykj888,
Thank you for the picture. I hope to repeat your construction with good sensitivity and only 2us sample delay!

[hykj888]

This is the DOD coil I made two years ago, which uses bipolar emission and can stably sample at 2us. It has very high sensitivity to small gold nuggets,

[Mdtoday]

MdToday, fair enough, just one more question about 'Forties" road, yeah i do have the yearly pass, but we can't use that now, is Forties road always closed by gate?, i think when i drive past it ,it's always open, but i'm not sure, and since we aren't allowed to drive inside now, how or by what road would you choose to get into that area, thanx mate?

Nova, yes, all closed up for fire ban and yes, Forties always gated. Have to go ridge road then back down into it.
Can park the car on Rocky Creek and walk in. Try using small coil and look for the gravel that has been ripped up when they planted.
There is a large area, just use the Forties as your reference is my advice. See attached.

cheers.

[Detectorist#1]

Next my design will be with oOo configuration with gradiometer connection of the two Rx coils and induction balanced Rx coils. Tx coil will be bifilar with two Tx switches or bipolar pulsed.

[Detectorist#1]

I prefer to use configuration with 2 Rx coils inside one big Tx coil. The two Rx coils are connected as planar gradiometer. The planar gradiometer can process weak signals even under a lot of noise. In "Fe" mode, no Tx pulses and with 15cm distance between Rx 300uH coils , Fe bottle caps are surely detected at 15cm. No false signal from non-Fe targets. In "All metals" mode, for stable operation without EFE problems, Tx coil have to be bifilar type or bipolar pulsed. In "ALL metal" mode US 25c coin is surely detected at 30-35cm.