Targets frequency response

[B^C]

Gday Aziz,

Interesting Graph, not sure if i have it correct?

The top line--signal + noise-- is this the eddy current decay?

Excuse my ignorance.

[B^C]

Gday Mike,

Thanks for that, i may have found more information for you, i will read through it & if it seems useful i will post it for you.

[Aziz]

Gday Aziz,

Interesting Graph, not sure if i have it correct?

The top line--signal + noise-- is this the eddy current decay?

Excuse my ignorance.

Hi B^C,

any form of exponential signal decay ( f(t)=a*exp(-t/TC), TC=L/R, e.g. induced voltage on the RX coil or negative of it). It is just for noise analysis and modulation to see the signal frequency spectrum response. As it is quite a broad band spectrum, a broad band noise is also concerned. So it is a big challenge, to keep the broad band noise small and construct the original signal from the noise.
If the modulation of the signal is done properly, the spread noise spectrum can be kept small.
Following attachement shows the reconstruction of the decay curve using the lock-in amp.

Regards,
Aziz

[Aziz]

Even the signal below can be handled without any problems.

[B^C]

Gday Aziz,
Thanks for that, very interesting!.

I have been testing reading very small voltages from some coils & getting them to the computer.
I have "nearly" got a clean signal to the computer with 32760 samples but can't get rid of the remaining noise--not much hair left as i have near pulled it all out trying. --see attachment---

Any ideas on how to get rid of small variations, not sure exactly what is causing this??

[B^C]

OOP'S forgot attachment!

[Aziz]

Gday Aziz,
Thanks for that, very interesting!.

I have been testing reading very small voltages from some coils & getting them to the computer.
I have "nearly" got a clean signal to the computer with 32760 samples but can't get rid of the remaining noise--not much hair left as i have near pulled it all out trying. --see attachment---

Any ideas on how to get rid of small variations, not sure exactly what is causing this??

Hi B^C,

noises can have various sources.
See the following link for noise types and their characteristics:
http://www.educypedia.be/electronics/noise.htm

I don't know where this noise is coming from (too few info).
One noise source can be the dumping resistor (PI) which is converting the magnetic energy into heat (-> Thermal noise, Johnson noise, shot noise).

Another noise sources:
Amplifier stages, filter stages, measuring instrument itself, supply voltage, not capacitively decoupled op-amp supply voltages (when used on high bandwidth), not enough frequency compensated op-amps (on fast op-amps), inductively and/or capacitive coupled noise sources, ground-loops, etc...

You should use just lock-in amp's.

Aziz

PS: A spectrum analyser should locate the noise source probably.

[Aziz]

Hello B^C,

studying noise types on the link above, I found that there is more noise to find and detect than signals. It is probably one of the most complex problems in electronics.
But your acquired signals looks really pretty well. Noise is lower than 1 mV.
Under real conditions, we see often more noise.

It is interesting to see, what lock-in amps can do and not. I made another extreme noise test. It looks pretty well. My (surrealistic) goal is to measure nano volts on the coil. I am not sure, if this is ever possible.
Regards,
Aziz

[mikebg]

Re: mneteorites

TNX B^C for your posting about meteorites.
It contains information, which I can't find in the WEB. I have questions about results shown in the figures, concerning method used for measurements. However, before to ask you, I should browse WEB for answers, because it is for shame one to ask for something, which is written, explained and even animated in the WEB.
Unfortunately, more beauteful and more expensive meteorites have very little permeability. Their price however is larger than price of gold. See in the WEB that one gram meteorite can be sold at price greater than one gram gold. The most expensive meteorites can't produce signal in air at all. That forced me to use discontinuity in matrix for meteorite location and to use as sensor coplanar gradiometer (differential configuration of RX coils).
Fortunately, experts on magnetic tape recording, showed me recently that my idea to use air gap as demagnetizing factor and to change hysteresis cycle, is very stupid (to put it mildly). Air gap is ineffective passive demagnetizator. There is active method used for linearisation in magnetic recording of continous audio signals. Now engineers on audio recording study (instead me) patents of B. H. Candy, to explain me how he with short bidirectional pulses demagnetizes matrix.
I start studying magnetic tape recording to understand how HF bias erases magnetisation and how linearises magnetic properties of tape. May be the future PI detectors will have an additional sinusoidal HF generator at 150 kHz, in order to make magnetic soil transparent for EMI sensor (instead comprehensive TX pulses used in B.H.Candy's designs). This becomes ever more and more interesting.
Regards. Mike.

[TheWizard]

AZIZ
This HI freq. Noise in Pulse Induction is NOT problem
The big problem is 50/60 Hz and low freq. noise.....

[B^C]

Hi All,

I have setup an experiment of inducing various metal objects with the same magnetic flux intensity etc. I wanted to read the induction & rise time values & decay times of various objects to get a better understanding of things.
From what i have done the induction values, rise time etc change & is very responsive to area of contact & material but the Decay times are all near identical.

For some test Standards i used a 25mm square piece of Aluminium foil & also a single turn loop of AWG18 Magnet winding wire--1 Turn formed around an AA Battery giving an outside diameter of 17mm. This is the first Attachment, Green reading is the Aluminium Foil & the Red reading is the single turn of AWG18 wire.

I then tested a 1grm reasonably FLAT Gold Nugget vs the Single turn AWG18 loop, the results i found interesting & is the second attachment.
Flat Gold Nugget is Red, Single turn loop is Green.

The Third Attachment is the 1grm Flat nugget vs another 1grm Nugget that is 1mm narrower, Flat Nugget is Red, Narrower Nugget is Green.
I did this test to see how responsive the readings were compared to surface area of the the same type of target.
The Nuggets are the same length but the width is 1mm different. The test shows as things should be with the wider Nugget a little stronger, so the tests seem to be responsive to change.

Fourth Attachment is the 1grm Flat Nugget vs a very Fine Gold Ring, Nugget is Green & Gold Ring is Red.

I did a lot more testing on ferrous objects etc with good results, i won't attach them unless somebody will find them usefull.

You will see there is variation in the produced signal & the rise time of the different objects but the decay is almost identical, what's your thought's, besides i wasted a ****load of time?

I thought with the Decay being almost the same on all tests done that the Driving Test Coil was interfering with the Decay times. I found this not to be the case by moving the Test Coil Across the test pieces just like a Metal Detector & the same Decay time results applied.

Have i done this correctly, have a look at the attachments & you will see what i mean?

[TheWizard]

I see good device - 20 Mega Samples - LOL

What is GREEN, RED... explain pls

[Aziz]

Hi B^C,

the fourth picture showing the difference between ring and surface targets best.
As on the ring, the induced eddy currents have to go through the ring path. Defining a relative constant radius of the "coil".
However on the flat targets, there are more (concentric) paths induced in the flux area of the target. These different current path cancels some of the outer eddy current induced magnetic fields. Also the small radius current path have small magnetic fields result. Therefore the total response is smaller compared to ring targets.
On the other side, one would expect, that flat targets would consume more magnetic field energy due to eddy currents producing heat on the target. Probably, the magnetic field is collapsing to fast, to see the difference. It would be interesting to see a response of super conducting target (more decay time)!

To see the differencies on the decay curve better, its is probably practical to make the first derivation of the normalized signal curves and comparing this.
Otherweise, the differencies are to small to see.

Regards,
Aziz

PS: Nice instrument. I would dream of such an instrument.

[B^C]

Gday Aziz,

Yes, the instrument is ok, i have spent considerable time building things up & to eliminate all noise problems, your link to noise problems has helped very nicely. Now i have a very nice signal---Thanks!

I have been testing many things with surprising results & also how different coils respond to very small recieved signals--very interesting to see this now on computer. I need to setup things more accurately to show extended test results.
I have a good idea now on various diameter & types of coils vs there sensitivity from small target signals, i will setup up things better & do multiple tests to give accurate results & post them when i am finished--very interesting, this will take some time as i am now moving to a new location--Gold Area .
I will be setting up tests in actual Ground conditions to see target sensitivity at depth from various signals & frequencies & how this affects the recieved signals in differing ground conditions.

I should be able to give some informative information if i can test these things accurately.

Mrs B^C is now going, BLAH,BLAH,BLAH,BLAH so i will have to go pack up some things ready to move.


TheWizard,

Green--Red is COLOUR.

[TheWizard]

......Green--Red is COLOUR.......

What signals, how its make, why red is before green.... for me, yours pictures is blah blah....