field test unit no 001 "model T"

[Ferric Toes]

My recent lurker membership was cancelled without warning, and I'm not about to renew it. Best rewrite it (differently to avoid lifting your own post) and bring it here. I have some relevant maths in my archives, which I will also try and locate.

Eric.

[Aziz]

I have searched the Geotech forums for your alleged post, but it seems to have gone AWOL.
Or have you posted it on an off-site forum?

Oh well, the universe is big. So the internet (almost).
It's published on AEGP Forum.

http://australianelectronicgoldprospectingforum.com/diy-detectors-read-all-about-the-aziz-mpi-and-moodztinkerer-pi-detector/the-subtraction-of-two-samples-in-pi-technology

(membership required)

Aziz

PS: Thanks Eric. The subtraction method is fairly old. Probably older than me.

[Qiaozhi]

Ok, I have published the results somewhere in the metal detecting forums.
Have a look. And eat it! *LOL*
Aziz

I have searched the Geotech forums for your alleged post, but it seems to have gone AWOL.
Or have you posted it on an off-site forum?

[Ferric Toes]

Subtraction method is at least 44 years old and maybe 46. I wrote an article describing it in an italian archaeological magazine in 1968, but there was another earlier version, by another engineer, using tuned circuits in 1966. Sounds crazy, but yes a PI that used a narrow band tuned amplifier post sampling. Sampling, subtraction and integration is not equivalent to RC high pass filter as a swept frequency test will show.

Eric.

[Aziz]

Ok, I have published the results somewhere in the metal detecting forums.
Have a look. And eat it! *LOL*
Aziz

[Aziz]

Win a Prize!!!

Ok, ya wanna win a prize now? (bling!, bling!, bling!)
(Would my own personal impression satisfy you?)

The infamous subtraction method of two samples:
- High-pass filter?
- 1. Order? 20 dB/decade roll-off/on?
- Transfer characteristic?


Hey guys, this is really very very important to know it now. Don't tell me, that the issue has lasted for 30 years (the subtraction method) and nobody in the universe has not really analysed it yet.


Who is going to bite the bullet now?
(I mean to win the prize?)
Good luck to whom ever.

Aziz

[Aziz]

Oh man!, I give up.
You guys don't want to share good metal detecting science (except Eric and Moodz). You all want to make heaps of $$$$ profits $$$$ out of it. Ain't it?
No job offer now. "MadLabs Inc."(c)(r)(tm)() is driving mad.
Aziz

[Aziz]

Hi all,

anyone made the proof?

(I'm lazy. I'm too much lazy to do it by myself. Hey, it was a trick to motivate you to do it for me. *LOL*)

Ok, another trick - ooops - a motivation for you.
A job offer:
Anyone, who solves the proof can be a member of "MadLabs Inc."(c)(r)(tm). No earnings (hey, mad companies don't make any profits ).
Deal?

Cheers,
Aziz

[Aziz]

Hi Eric, thanks for your very valuable contribution.
Cheers,
Aziz

[Aziz]

Hey guys,

the "MadLabs Inc."(c)(r)(tm) will drive you mad and confuse you in the first instance. It's a tacticle mind operation to get you thinking of the issue.
Furthermore, it is forcing you to do some homework. You know, there is no "free lunch" in the evil world. Do some hard work for your "lunch".

If you have understood the issue correct, you also will understand the target signal loss with the subtraction method (the infamous detection hole).

The "MadLabs Inc."(c)(r)(tm) with its first medically certified mad engineer just want to help you.

Aziz,
the master of madman

[Ferric Toes]

Actually, there is no need to have a front end that is flat down to dc. Some of the industrial PI detectors that I designed in the 1990’s had a bandpass characteristic that rolled off rapidly below 200Hz, This was done by means of an active filter in the feedback loop. The purpose here was to give more attenuation to both 50Hz and 60Hz line frequencies, the levels of which can be high in an industrial environment. The active filter involved a couple of capacitors which could be described as being in the signal chain. Generally, industrial detectors are just required to sense the presence or absence of metal in a product and the decay characteristic is irrelevant.

However the requirements of gold nugget detectors are somewhat more stringent, especially if effective ground balance and/or discrimination is required. Whatever circuit elements are incorporated, IMHO they should not distort the received waveform, particularly so for accurate ground balance at all coil heights i.e. large changes in signal amplitude with a mono coil.

I find that with conventional dual sampling and analogue integration I can achieve all the earth’s field cancellation I need when using a 25in diameter coil and also be completely insensitive to maghemite “hot” rocks that have a level of remanent magnetism in addition to the viscous signal. At this point I will say that my preamp and integrator is quite different to what you see in popular PI schematics. Recently, I experimented with a dual differential 25in coil which would give additional high levels of earth’s field attenuation, but was primarily to cancel low frequency em noise. As far as earth’s field went the improvement was not noticeable because the main noise source is still the front end amplifier. Strictly speaking, not the amplifier but the 1K input resistor, and I know what to do about that.

I used to wave ferrite magnets over the coil to check earth’s field cancellation, but now use a low frequency signal generator to inject a 5Hz sine wave into the RX input. This way you can control the amplitude and set it such that the signal swing is well within the linear range of the amplifier/integrator. Ferrite magnets close to the coil can induce a superimposed magnetostrictive ringing on the RX waveform, but when only a few inches away the 1K noise predominates.

I am old school electronics but it is good to see new ideas, such as those which Moodz is developing, being investigated and applied in PI technology. Let’s encourage him and others to persevere with their work, particularly in reducing noise by means of novel front ends, new coil configurations, dsp, etc. After all, the performance of any detector is basically a S/N problem.

Eric.

[Aziz]

Hey UFox,

did you get a grip on the math proof? No problem, if you don't know how to do it. I don't blame you.
I do not give credit for PJ to do it as well. I don't blame him too.
Ok, who's the next in my guru list?
Robby_H and Rafferty? Or the Lord mastercat? It's your chance now guys!

Everybody feel free to contribute to the hot discussion (it ain't off-topic).
Cheers,
Aziz

[moodz]

Moodz, whether I seem very technical to you is irrelevant.



Moodz, if you want to use capacitive coupling and believe you are up there with the best, go to it, but personally, I don't think it is all over.

Actually it is highly relevant as either you are talking through your hat or you are actually interested in the subject of capacitive coupling and earth fields. If you are going to ask "dorothy dixer" questions then this is not the forum to air them.

(The term references American advice columnist Dorothy Dix's reputed practice of making up her own questions to allow her to publish more interesting answers. The only problem here is that you are not publishing the answers but casting doubt on legitimate lines of discussion.)

Anyway I must be on to something as this is the only thread you hang around .... so thanks for the vote of confidence.

As for being up there with the best ... I am better than the best LOL.

[UrbanFox]

UF you dont seem to be very technical as you are focusing on aspects like cap coupling which quite frankly is not even circuit theory 101 and bit lower than standard of discussion on this thread.

Moodz, whether I seem very technical to you is irrelevant.

...I am afraid its all over for the monocoil PI development .....

Moodz, if you want to use capacitive coupling and believe you are up there with the best, go to it, but personally, I don't think it is all over.

[moodz]

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.



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



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 see another "world beater" in the market place???

UF you dont seem to be very technical as you are focusing on aspects like cap coupling which quite frankly is not even circuit theory 101 and bit lower than standard of discussion on this thread.

60 db aint that hot but it conditions the signal before it hits the 140 db filter in the processor.

BTW problems are never solved ...they just have an answer in place till a better one comes along.

I am moving forward ... thanks for enquiring ... I am not making detectors its a PITA ... I will leave that to some other company.