field test unit no 001 "model T"

[UrbanFox]

Hey ....get a grip UF ... Aziz is correct ...

Hey....get a grip Moodz.... if you want to believe Aziz has all the answers then go for it. I am finding it interesting just watching from the sideline.

DC coupling is used where the designer is unable to resolve coupling issues or does not understand the signal domain he is working with.

Moodz, I can totally accept what you say. If a designer doesn't understand what he is doing then anything is possible, I guess.

FYI ... I reworked that part of the design and put the capacitive coupling back in ....it has knocked another 60db off the Earth field response. The back end DSP filter was a 10th order but could not deal with overload conditions for magnets very close to the coil ... so thanks for that UF ... you "convinced" me to make the design even better.

Gee, another 60 db, and you already had the problem solved before that. Sounds bluddy miraculous Moodz!

Anyway, glad to hear you are going forward, Moodz. How long before we actually see this detector in the market place???

[moodz]

What are you trying to say, Aziz? Spit it out!

If you wish to use design concepts that others deserted a long time ago then go for it. Don't expect me to waste time playing your silly games.

Hey ....get a grip UF ... Aziz is correct ... DC coupling is used where the designer is unable to resolve coupling issues or does not understand the signal domain he is working with. FYI ... I reworked that part of the design and put the capacitive coupling back in ....it has knocked another 60db off the Earth field response. The back end DSP filter was a 10th order but could not deal with overload conditions for magnets very close to the coil ... so thanks for that UF ... you "convinced" me to make the design even better.

[UrbanFox]

Go back and read my post again.

What are you trying to say, Aziz? Spit it out!

If you wish to use design concepts that others deserted a long time ago then go for it. Don't expect me to waste time playing your silly games.

[Aziz]

Aziz, I do believe you are too wrapped up in theoretical models. Why would a designer go to the trouble of extra components and subtraction if cutting a line and adding a capacitor and a couple of extra components would achieve the same result.

Go back and read Eric's second last paragraph again and take note of what he is saying, It might be time for you to do a few tests in the real world and identify why blokes like Eric, Minelab, and many others, don't use AC coupling early in the signal chain.

Let's see you justify the usability of a technique that everyone else has dropped, and explain why we should accept modeling that is not representative of results achieved in real world. So, are you suggesting Eric is wrong?

Go back and read my post again.

[UrbanFox]

Hey UFox, you're not playing the infamous quiz game. Go back and read my post again.

Aziz

Aziz, I do believe you are too wrapped up in theoretical models. Why would a designer go to the trouble of extra components and subtraction if cutting a line and adding a capacitor and a couple of extra components would achieve the same result.

Go back and read Eric's second last paragraph again and take note of what he is saying, It might be time for you to do a few tests in the real world and identify why blokes like Eric, Minelab, and many others, don't use AC coupling early in the signal chain.

Let's see you justify the usability of a technique that everyone else has dropped, and explain why we should accept modeling that is not representative of results achieved in real world. So, are you suggesting Eric is wrong?

[Aziz]

Aziz, go back and read Eric's second last paragraph in his post. Are you saying capacitive coupling is the way to go?

Hey UFox, you're not playing the infamous quiz game. Go back and read my post again.

Aziz

[UrbanFox]

Oh well, the proof is a mind**** (=beyond my scope).
But I want you and your best mates to solve this little and very trivial quiz. Or run quickly to your master and ask for a help.

Aziz

Aziz, go back and read Eric's second last paragraph in his post. Are you saying capacitive coupling is the way to go?

[Aziz]

Your the one who says he knows it all. Waiting to see your answer smarty. Qiaozhi has you over a barrel !!!

Oh well, the proof is a mind**** (=beyond my scope).
But I want you and your best mates to solve this little and very trivial quiz. Or run quickly to your master and ask for a help.

Aziz

[PATCHES JUNIOR]

No! Where are all the metal detecting Einstein's there?
I'm addressing all the self defined gurus like UFox, PJ, Robby_H, Rafferty and Co with it.

Aziz

Your the one who says he knows it all. Waiting to see your answer smarty. Qiaozhi has you over a barrel !!!

[Aziz]

....
Go on, don't be a spoil sport ... give us the proof.

No! Where are all the metal detecting Einstein's there?
I'm addressing all the self defined gurus like UFox, PJ, Robby_H, Rafferty and Co with it.

Aziz

[Ferric Toes]

The problem with magnetic ground is that it decays with a 1/t law. This means that the initial decay is fast, but then slows down at later times. This can be seen if you use a log amplifier for the front end. The output of the log amplifier shows an exponential decay, while that from a gold ring shows a linear straight line decay. To get magnetic ground to give a linear decay, you have to have a logarithmic timescale as well as a log compression of the amplitude.

The total measured decay time of a 5oz (160gm) nugget is 700uS to the point where it disappears into the noise. This is not "time constant" in the accepted sense. The total decay of a 30oz (940gm) nugget is >2000uS by comparison. Taking the 5oz nugget the approximate true time constant would be 700/5 (disregarding unseen signal in the noise) = 140uS. The log amplifier (and theory) shows that the signal only becomes a single exponential after 1 time constant. Prior to that it is a sum of exponentials. To get best S/N ratio, early sampling is employed and this part of the decay is faster and higher amplitude than the later invariant decay. Except in the case of a ring. Maths get tricky, but do the experiments. Maybe on a simulator program.

This doesn't answer the capacitor v dual sampling and subtraction debate, but is relevant to it. I always avoid capacitors early in the signal chain as there are always unwanted overshoots and undershoots when dealing with pulses, and a lack of preserving a true dc base level.

Nothing is straightforward, and there is bound to be some loss of the wanted signal in getting rid of the ground response.

Eric.

[Tinkerer]

If the two techniques are mathematically equivalent, then (as I mentioned in my previous post) the ground rejection will require some compromise between gold targets of differing time constants. You cannot change the laws of physics.

Go on, don't be s spoil sport ... give us the proof.

A novel solution to the very long TC target decay, is to use the "forced decay curve", a method that accelerates the decay, so that the decay curve reaches 0 in a very much shorter time than it's natural TC.
This allows for much higher pulse repetition rates, without loosing any sensitivity to long target TC nuggets.

Tinkerer

[Qiaozhi]

Capacitive coupling: High-pass filter (-20 dB/decade = 1. order hp-filter)
Subtracting late sample: Mathematically equivalent with high-pass filter (-20 dB/decade = 1. order hp-filter)

Who dares to show us the mathematical proof? (UFox?)
(I won't do this )
Aziz

If the two techniques are mathematically equivalent, then (as I mentioned in my previous post) the ground rejection will require some compromise between gold targets of differing time constants. You cannot change the laws of physics.

Go on, don't be a spoil sport ... give us the proof.

[Qiaozhi]

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

The problem then is to find an acceptable compromise. Any technique designed to eliminate the worst ground will inevitably result in sacrificing depth for other targets.

[Aziz]

Capacitive coupling: High-pass filter (-20 dB/decade = 1. order hp-filter)
Subtracting late sample: Mathematically equivalent with high-pass filter (-20 dB/decade = 1. order hp-filter)

Who dares to show us the mathematical proof? (UFox?)
(I won't do this )
Aziz