ground balance capability

[Davor]

i m from syria
...
davor forgive me your idea "Simple PI GB schemes convert the all-positive signal to part positive, part negative, and a hole in the middle" confuse me a bit
btw i did a air test with a ferrite peice and obtain no response

In Syria there should be no problem with soil. The only kind of soil that PI detectors react to are the magnetically viscous ones, and your country is not famous for having it. Waving ferrite proves this point - PI detectors are utterly deaf against the fast response targets like ferrites, or wet salts, and for such you don't need any GB for a PI. Only viscous soils require a GB with a PI detector.
For that reason I urge you to check what's wrong with your shielding, and it is always good to send photos.

[mohalothman]

hi guys
sorry to reply late, i was busy
any way i assure you guys that the problem is not a shielding issue or any thing else, i'm certain that the MPP sense the soil here.
i put a wet soil in a plastic container waved in front of the coil and i got a strong response.
here i attached pictures for my coil shielding setup and sample timing.
now i'm working on ground balance i m trying to achieve GB in a way which differ from this :
http://www.ipaustralia.com.au/applic.../AU2010101019/
the new one will not reduce the device sensitivity at all, i let you know when i finish

[Davor]

First sample delay is a bit too close to the response knee, which seem a bit overdue. It is possible your damping resistor is too low, and flyback bleeds in. With flyback also proximity to almost anything also bleeds in.
You may experiment with damping up to the critical point, but your first step should be increasing a first sample delay.

[Qiaozhi]

hi guys
sorry to reply late, i was busy
any way i assure you guys that the problem is not a shielding issue or any thing else, i'm certain that the MPP sense the soil here.
i put a wet soil in a plastic container waved in front of the coil and i got a strong response.
here i attached pictures for my coil shielding setup and sample timing.
now i'm working on ground balance i m trying to achieve GB in a way which differ from this :
http://www.ipaustralia.com.au/applic.../AU2010101019/
the new one will not reduce the device sensitivity at all, i let you know when i finish
[ATTACH]40777[/ATTACH]
[ATTACH]40778[/ATTACH]
[ATTACH]40779[/ATTACH]
[ATTACH]40780[/ATTACH]
[ATTACH]40781[/ATTACH]

It looks to me like your soil is highly mineralized.

[homefire]

It looks to me like your soil is highly mineralized.

Sampling at 20uS I would say his ground has to be quite hot to produce a response.

Wrapping your Shield Conductor wire around that much of the coil is bound to produce some target response. Combine that with the Ground and it may be your issue.

Try just a couple of turns. .

[mohalothman]

First sample delay is a bit too close to the response knee, which seem a bit overdue. It is possible your damping resistor is too low, and flyback bleeds in. With flyback also proximity to almost anything also bleeds in.
You may experiment with damping up to the critical point, but your first step should be increasing a first sample delay.

i delayed the first sample up to 115us to reject the ground signal totally, that is insane i know
the picture is here:
the damping resistor is 820 ohm and the peak of coil flyback voltage is nearly 435V (i use irf840 instead irf740)
the coil flyback voltage is here:
here the soil i tested:
it was dry and i tested more than one amount of soil. the MPP can sense the soil from 7cm depth (with sample delay 25 us)

[mohalothman]

Wrapping your Shield Conductor wire around that much of the coil is bound to produce some target response

the Shield Conductor wire is a thin one and not make a close loop so it has almost neglected effect
see the two pictures down for the same coil the first with shield and the second without shield u can conclude that the shield does not really affect the residual coil voltage
(note: the interference in the second picture is just occur when i blogged my oscilloscope with my PC)

[Qiaozhi]

i delayed the first sample up to 115us to reject the ground signal totally, that is insane i know
the picture is here: [ATTACH]40782[/ATTACH]

When you put the bag of soil in front of the coil, do you actually see the preamp output signal move across by 115us?
That's hard to believe.

[mohalothman]

with the scale 2v/div i can't see any change because it is in mV, if i lower the scale down to 5 mv/div maybe i'll notice a different
any way i would to explain that the soil here have a long time constant and this make me a bit confused

[homefire]

Did it really take 820 ohm damping . What is your coil resistance by chance ?

[Qiaozhi]

with the scale 2v/div i can't see any change because it is in mV, if i lower the scale down to 5 mv/div maybe i'll notice a different
any way i would to explain that the soil here have a long time constant and this make me a bit confused

Try it with a metal target, such as a large coin. You will see that the rising edge of the preamp output moves along to the right. Does the same thing happen with the soil sample?

[mohalothman]

Did it really take 820 ohm damping . What is your coil resistance by chance ?

the coil resistance is about 2.5 ohm
the damping resistor is 820 ohm no doubt about its value

[mohalothman]

Try it with a metal target, such as a large coin. You will see that the rising edge of the preamp output moves along to the right. Does the same thing happen with the soil sample?

actually the soil affect the decay of preamp output like a small piece of iron or a small coin. maybe tomorrow i'll provide you a pictures illustrate this
my point was that u can't notice change in preamp output far away let us say after maybe 40 us

[homefire]

I assume your Coil is within the 300 - 500 uH range ?

[mohalothman]

yeh it is 400 uH