field test unit no 001 "model T"

[dougAEGPF]

The "holes" in the target response profiles will only be a problem if you make the secondary sample pulse too close to the main sample. A much later secondary sample will only affect targets with longer time constants, and I presume you're really focussing on targets with very short time constants (such as gold nuggets).

Some multi ounce Oz gold nuggets can have very long Tc's!!! For Oz some of us want a detector that can handle the worst ground but not sacrifice depth on some bigger, solid nuggets with longer TC's as happens with some other detectors timings on some of these nuggets!
DougAEGPF

[Qiaozhi]

There is a far more elegant solution involving DSP. Though I did give the cap method a go in a previous design. I have not tried the late sampling method suggested because it can create holes in the target response profiles.

The "holes" in the target response profiles will only be a problem if you make the secondary sample pulse too close to the main sample. A much later secondary sample will only affect targets with longer time constants, and I presume you're really focussing on targets with very short time constants (such as gold nuggets).

[moodz]

I said " there is" not "there will be". ..... The changes to the frontend actually just pipe the mag signals to the dsp instead of trying to block them. If you can't solve a problem in one place ... Solve it in another.

[UrbanFox]

There is a far more elegant solution involving DSP.

However, your statement was "I made a couple of changes to the front end and the Mag problem is solved now ....".

What advantage do you expect to get from DSP when you state you have already solved the problem?

[moodz]

Moodz, the only changes I can see being made to the front end to reduce static magnetic field effects is to use capacitive coupling. Is this what you have done?

There is a far more elegant solution involving DSP. Though I did give the cap method a go in a previous design. I have not tried the late sampling method suggested because it can create holes in the target response profiles.

[UrbanFox]

Hey thanks Qiaozhi ... I am kinda aware of that but I appreciate your tip.
I made a couple of changes to the front end and the Mag problem is solved now ....

Moodz, the only changes I can see being made to the front end to reduce static magnetic field effects is to use capacitive coupling. Is this what you have done?

[Qiaozhi]

I have just removed a large number (~14) nonsense and personally abusive posts from this thread. Please keep this type of behaviour out of Geotech!
This is a very interesting thread that is being dragged down into the gutter by a few individuals. If it continues then Geotech Admin will be forced to step in and start issuing infractions.

[Aziz]

Wow! That's an impressive result.
It's the end of ... *LOL*
Cheers,
Aziz

[moodz]

You can solve the earth field effect problem very easily by taking a second [much later] sample and subtracting it from the main sample. If set up correctly, the magnetic field from a large white board magnet can be completely cancelled.

Hey thanks Qiaozhi ... I am kinda aware of that but I appreciate your tip.
I made a couple of changes to the front end and the Mag problem is solved now ....

The shots below show the output ( yellow ) of the first stage with a 1 gram natural gold nugget ( ie not a ring or foil ) at 2cm from the coil.
The red trace is the TX OFF gate pulse edge.
I am getting 1 volt of deflection which is not bad IMHO .
This signal goes to the second stage where it is amplified again and DSPed etc etc etc

The coil is a mono, 0.4 ohm, 300uH

My "magic sauce' front end is damping this coil in around 5us ...

waving the magnet does not change this result.

[moodz]

When you have the coil stationary and a moving speaker magnet is sensed when 12 ft from the coil, but when the same magnet is stationary and the coil is moved you find the signal is no different whether the magnet is there or not, it is not that the coil is sensing the moving magnet, it is that the moving magnet is distorting the earth's magnetic field and the coil is within the distortion zone.

Trap for young players, Moodz.

Thanks... yeh I realise that .... the magnet at 12 feet is much weaker than the earths magnetic field however the "movement" of the field causes the detector to react. I will try your pipe experiment after I knock up the next circuit.

[Qiaozhi]

Thanks .... the detector is "very" sensitive to moving magnetic fields ... ie if the coil is moving it will detect the EMF ... if the coil is stationary it will detect a moving speaker magnet ( ferrite ... not particulary strong ) at just on 12 feet ... which I was quite impressed with ... because that would be a quite a small signal at the coil.

Even though the detector is responding to mag fields it is relatively easy to pick target responses ... the big plus is auto ground balance ... so what you gain on the swings you lose on the merrygoround LOL ... IMHO ... an earth field problem is much easier to solve than ground balance.

You can solve the earth field effect problem very easily by taking a second [much later] sample and subtracting it from the main sample. If set up correctly, the magnetic field from a large white board magnet can be completely cancelled.

[UrbanFox]

Moodz, some compulsory reading.......

http://www.finders.com.au/forum/viewtopic.php?p=61488

Moodz, another test for you....get your coil up in the air with the coil face vertical. Get a piece of 1 inch water pipe 4 ft long and swing it up and down with the pipe parallel to the coil face. Move further and further away while swinging the pipe up and down until the detector is barely responding. The moving pipe is disturbing the earth's field and the detector will sense this. With the pipe stationary at this distance and the coil moving the pipe is too far away to be sensed by the detector.

[UrbanFox]

Thanks .... the detector is "very" sensitive to moving magnetic fields ... ie if the coil is moving it will detect the EMF ... if the coil is stationary it will detect a moving speaker magnet ( ferrite ... not particulary strong ) at just on 12 feet ... which I was quite impressed with ... because that would be a quite a small signal at the coil.

When you have the coil stationary and a moving speaker magnet is sensed when 12 ft from the coil, but when the same magnet is stationary and the coil is moved you find the signal is no different whether the magnet is there or not, it is not that the coil is sensing the moving magnet, it is that the moving magnet is distorting the earth's magnetic field and the coil is within the distortion zone.

Trap for young players, Moodz.

[moodz]

Moodz, doesn't your detector cancel static magnetic fields ?????????????



Moodz, how are you testing this ?????

Is the magnet stationary and the coil moving ?????

Is the coil stationary and the magnet moving ???????



All the best with it. I am interested to know how you can make any comment re the potential depth performance until you fix up the earth's magnetic field. I consider you have one large problem, not a small one, but again. all the best with it.

Thanks .... the detector is "very" sensitive to moving magnetic fields ... ie if the coil is moving it will detect the EMF ... if the coil is stationary it will detect a moving speaker magnet ( ferrite ... not particulary strong ) at just on 12 feet ... which I was quite impressed with ... because that would be a quite a small signal at the coil.

Even though the detector is responding to mag fields it is relatively easy to pick target responses ... the big plus is auto ground balance ... so what you gain on the swings you lose on the merrygoround LOL ... IMHO ... an earth field problem is much easier to solve than ground balance.

[UrbanFox]

.... the current detector auto ground balances however it is very sensitive to magnetic fields

Moodz, doesn't your detector cancel static magnetic fields ?????????????

( eg..... and magnets of course )....

Moodz, how are you testing this ?????

Is the magnet stationary and the coil moving ?????

Is the coil stationary and the magnet moving ???????

one small problem

All the best with it. I am interested to know how you can make any comment re the potential depth performance until you fix up the earth's magnetic field. I consider you have one large problem, not a small one, but again. all the best with it.