Targets frequency response

[B^C]

Hi to all,
Let's we determine what is FREQUENCY RESPONSE before to continue with exercises.
1. [Wikipedia] FREQUENCY RESPONSE is the measure of any system's spectrum response at its output, to a signal of varying frequency
(but constant amplitude) at its input.
2. [Steve Smith] FREQUENCY RESPONSE is the magnitude and phase changes that sinusoids experience when passing through a linear system. Usually expressed as a function of frequency. Often found by taking the Fourier transform of the IMPULSE RESPONSE. (This is in [2] page 636. Frequency response is described in [2] page 177).
3. [My reinvention] FREQUENCY RESPONSE is complex transfer function of a linear system in frequency domain. It can be expressed by two frequency functions:
a) Re(f) and Im(f) in rectangular coordinates, or
b) Magnitude vs. Frequency and Phase vs. Frequency in polar coordinates.
Trasfer functions in time domain are STEP RESPONSE and IMPULSE RESPONSE.
4. [My explanation] For a linear system, frequency response is its own characteristic because it is independent on amplitude of excitating signal. For example, the gain of an amplifier is its own characteristic, supposing it is linear (if gain is independent on input amplitude).

Fellows, I know that the terms "frequency response" and "impulse response" are misleading, but they are in English and
this forum is in English. In the nopnenglish rest of Europe is used correct term "Spectral characteristic" or "Spectrum response" or "Frequency characteristic" etc. In SPICE is used the term "AC analysis".
To avoid misleading, in my next postings I will use the term "spectral characteristic" instead "frequency response" because for analysis we assume that the sensing network is linear. The only advantage of term FREQUENCY RESPONSE is that it is valid also and for nonlinear system.

Mike,

I still can't see how this relates to a Passive target imbeded tightly in a conductive matrix that has been there for millions of years. When the only things that will change the frequency are harmonics, Doppler shift, or stimulated emissions (none of which would apply in this case senario) as stated previously.
There are a number of conductors including:

1. <LI type=a>amount of negative charge on the clay particle; <LI type=a>clay content; <LI type=a>concentrations of salts in the soil solution; and, <LI type=a>soil moisture content <LI type=a>temperature, and
2. bulk density.

This is not to mention ironstone etc in mineralised soils, in free air conditions i can understand we could setup a harmonic response but for gold that has been buried for millions of years or locked between fault lines or in quartz viens to setup harmonics of a particular target in this enviroment doesn't seem applicable.
To induce gold to the magnitude to setup a harmonic response, what sort of signal is going to be required to ignore or punch through the matrix to do it.

Like they say seeing is believing, you will need to produce more than words to get people interested though.
Put some formulas up & then everybody will want to go further, anybody can write words & i am am example of this. If you put up some mathematics then people can knock it on the head or take it further.
Otherwise there is nothing to learn, if someone walks up to you on the street & says he can create a hole & fall through it, we don't all sit around & take lessons. On the other hand if this is a proven FACT then everybody takes interest & wants to learn.

[B^C]

Bruce C,
It is useful for you to be student in the forums because:
1. You will grow younger, despite in this thread we study knowledge from 19-th century.
2. You will grow able to find errors in Bruce Candy's articles, patents and designs. Please read carefully old postings in
the technology forums since last 9 years (avoid postings from Sean Goddart).
3. You will be able to use LTSPICE and similar methods for computer aided analysis and design. They will help you to understand why comrehensive TX pulses in Minelab mashines are delusion.
4. You will find that the articles in your old Bible are useless for design purposes.
5. You will know which metal detector to buy for prospecting and how to use it effectivelly.
6. You will understand that Chinese engineers are not important danger for Minelab. Dangerous are Dave Emery and Carl's
"Commercial Design Groups". China and Bulgaria can be favourable producers.
To start with selfeducation, please download LTspice (or SWCAD III) and read [2] and [7]. Afther you post in this thread your answers to exercise, I will send you via email an interesting publication. You will ascertain the fact that you already can find in it errors.
Yours Mike.

Mike,

Answers to:
1)
Hahaha, have to love that, you promise much my friend, you will need to start your lessons from the beginning of technology to be able to make quantified steps to bring us up to date.
Leaving old technology out of the picture gives us nothing to take example from. We must be able to compare technologies side by side to make judgement to which is better suited to the application at hand.

2)
What was written in Bruce Candy's articles & what Bruce Candy does in reality may be two different things, it's no good telling everybody exactly what your doing or they'll all do it.

3)
Delusion or not Minelab machines are the top dog, "actual" field conditions prove this time & time again. If you can place something in my hands that is better then i will use it, until then it is all hypothetical jargon written on forums.

4)
Not sure about that one, knowledge is the key to understanding, can you supply me with actual factual Formulas to verify your statement? That is the only way to prove something wrong before design.

5)
That's easy, Minelab only, nothing competes at this stage, i have tried plenty.

6)
Not concerned about Dangers to Minelab, good luck to all who can out do them, just wish they would hurry along before Minelab introduce there new Subsurface Visualisation Technology that will stand this market on it's head.
Some previews have aleady circulated about this, it is a quantum leap ahead & will bring us all plenty to think about.
The only good thing that China makes is Fireworks--end of story, give them time & they will improve though.

If you can prove "mathematically" what your saying is RELEVANT to the detection of PASSIVE targets then i will sit down & learn something.
At the moment all we have is a lot of mumbo jumbo written on the forum with no actual proof any of this is relevant to the application.
I have posted what is relevant & can post another 500 pages on the subject with formulas to prove FACTS.

Your turn .

[B^C]

1

[B^C]

Mike,

PHS was me, i was in a hurry & didn't log on, had to go test something out, i meant to put PHYS on the bottom being short for PHYSICS. I will log on in the future to avoid confusion.

[B^C]

Gday Mike,

I will read your posts & reply but i thought i would would leave a couple of quotes by the man himself first.

Any intelligent fool can make things bigger, more complex,
and more violent. It takes a touch of genius, and a lot of courage,
to move in the opposite direction.

Albert Einstein

Technological progress is like an axe in the hands
of a pathological criminal.

Albert Einstein

If you can speak of technical things only in technical terms,
you do not understand them.

Albert Einstein

[mikebg]

Frequency Response = Spectral Characteristic

Hi to all,
Let's we determine what is FREQUENCY RESPONSE before to continue with exercises.
1. [Wikipedia] FREQUENCY RESPONSE is the measure of any system's spectrum response at its output, to a signal of varying frequency
(but constant amplitude) at its input.
2. [Steve Smith] FREQUENCY RESPONSE is the magnitude and phase changes that sinusoids experience when passing through a linear system. Usually expressed as a function of frequency. Often found by taking the Fourier transform of the IMPULSE RESPONSE. (This is in [2] page 636. Frequency response is described in [2] page 177).
3. [My reinvention] FREQUENCY RESPONSE is complex transfer function of a linear system in frequency domain. It can be expressed by two frequency functions:
a) Re(f) and Im(f) in rectangular coordinates, or
b) Magnitude vs. Frequency and Phase vs. Frequency in polar coordinates.
Trasfer functions in time domain are STEP RESPONSE and IMPULSE RESPONSE.
4. [My explanation] For a linear system, frequency response is its own characteristic because it is independent on amplitude of excitating signal. For example, the gain of an amplifier is its own characteristic, supposing it is linear (if gain is independent on input amplitude).

Fellows, I know that the terms "frequency response" and "impulse response" are misleading, but they are in English and
this forum is in English. In the nopnenglish rest of Europe is used correct term "Spectral characteristic" or "Spectrum response" or "Frequency characteristic" etc. In SPICE is used the term "AC analysis".
To avoid misleading, in my next postings I will use the term "spectral characteristic" instead "frequency response" because for analysis we assume that the sensing network is linear. The only advantage of term FREQUENCY RESPONSE is that it is valid also and for nonlinear system.

[mikebg]

Answer for unregistered PHS
You are welcome PHS,
To understand what I'm going to emit, you should read [2] and [7]. Your posting is very important for me. It forced me to plan exercise for analysis of modulation, caused by sensor movement above target. Please register in this forum to become able to receive private messages. In next my posting I will shed light on term "Frequency Response". Mike.

[mikebg]

Answer for B^C

Bruce C,
It is useful for you to be student in the forums because:
1. You will grow younger, despite in this thread we study knowledge from 19-th century.
2. You will grow able to find errors in Bruce Candy's articles, patents and designs. Please read carefully old postings in
the technology forums since last 9 years (avoid postings from Sean Goddart).
3. You will be able to use LTSPICE and similar methods for computer aided analysis and design. They will help you to understand why comrehensive TX pulses in Minelab mashines are delusion.
4. You will find that the articles in your old Bible are useless for design purposes.
5. You will know which metal detector to buy for prospecting and how to use it effectivelly.
6. You will understand that Chinese engineers are not important danger for Minelab. Dangerous are Dave Emery and Carl's
"Commercial Design Groups". China and Bulgaria can be favourable producers.
To start with selfeducation, please download LTspice (or SWCAD III) and read [2] and [7]. Afther you post in this thread your answers to exercise, I will send you via email an interesting publication. You will ascertain the fact that you already can find in it errors.
Yours Mike.

[Aziz]

Hello B^C,

your attached information is quite interesting to me. Many thanks!
I will check this on the coming week out.

Aziz

MIKEBG
You're not going to emit one frequency at a "passive target" and get a different frequency back except for harmonics, Doppler shift, or stimulated emissions (none of which would apply in this case senario).

PHS

[B^C]

Time Domain Data Attached:

[B^C]

Gday mikebg:

I am as dumb as a horses *** at the best of times being a full time Prospector but i still don't see any frequency response change.
Some Formulas may help.
I may of course be reading & summarising things differently--or totally wrong!
Maybe there is confusion & a lot of different technologies are being combined instead of concentrating on one technology at a time?.

For my sake & maybe others i will attach a few things so we all can comment without confusion & maybe you can run us through a few things & formulas to start off with & progress from there.

Adding a few numbers to the equations to the attached formulas & showing the results will certainly help a lot of people out.
I'm sure there are quite a few who would like to learn & many need to start from the begining.
The following information may be helpful to start things off, i have just blown the cobwebs of my metal detector bible, i'll attach some things..

If we are going to get lessons we may as well start at the beginning to benefit many other readers who just can't catch on to the stage things were started at--me included, then again i may be the only dumb prick that can't get it!.

I'm sure there's many on this forum who could make this interesting with there comments & Formulas to prove the facts please.

Back to school, boys!

I'll be interested & an eager participant, maybe start off with the formulas for:

Magnitude of Magnetic Field B=

The Magnitude of Magnetic Field at an axial point P a distance X from the centre of the loop is given by B=

Magnitude of flux through a loop is:

Magnitude of induce current in the coil while the electric current is changing:

Then keep progressing, i'm sure you'll get a ****load of questions, it will be very informative.

I have attached Frequency Domain Data in this post, i will post another with Time Domain Data next.

[mikebg]

Frequency Responses

Hi B^C,
The only wrong thing in your posting is your term "Target frequency". Target may be identifyed by its characteristic frequency, and even by its resonance frequency (if metal is ferrous). It has no own frequency.
Your expression:
"I suppose that's why we Discriminate via Phase shift rather than Target Frequency", means that my "Lesson" and "Exercise" are written too briefly.
Because the rest of your posting is correct, I will explain it in great detail.
Fig. 2 in the "Lesson" is valid for both linear and nonlinear systems. If a network is linear, it responds with the same Frequency as excitation. Then the magnitude and phase of received signal can be represented by a point M in the impedance line shown in fig. 3. If we change TX frequency from zero to infinity, the point M moves on arc in fig. 3 from origin to point 100% and the phase of response changes from 0 to -90 deg. The arc represents in complex plane response of an eddy current for all frequencies.

What represents the straight lines OA and OB in fig. 1? The answer is - response for fundamental frequency. If the system is nonlinear, it also responds with the same frequency as TX, but adds harmonics. For example, if we excitate an ferrous object with transmit TX frequency 2kHz, we receive also this fundamental frequency 2kHz, but there is in output also remarcable magnitude of second harmonic 4kHz, and especially of third harmonic 6kHz because the third harmonic is attributable to magnetizm. The complex plane can,t represent correct response of nonlinear properties as magnetizm. The straight lines OA and OB in fig. 1 are only an approximation, which shows that phase of fundamental frequency not changes with change of TX frequency. We will use them in future exercises to analyse frequency response of ferrous conductivity.

For a linear system, we can plot a normalized transfer impedance with relative magnitude 100% as shown in fig. 3. For a nonlinear system, transfer impedance in complex plane may represent only the response for fundamental frequency. For example, if we increase TX ampereturns twice, this no means that the signal will increase twice moving point M in twice distance from origin on straight lines OA or OB. Hysteresis cycle shows, that magnetism has saturation and this limits the maximal possible magnetic signal. The point M shifts less than twice if the magnitude of TX field increases twice, but the oppsite happens when the magnitude of TX field decreases. Very important thing for a nonlinear system is BIAS. We will discuss this in future exercises.
* * *

[B^C]

Are there published anywhere frequency responses of metal detected targets: different coins, rings, hot rocks, junk etc.? I can,t find in WEB even one frequency characterstic of target in complex (impedance) plane! Mike ***

Gday mikebg,

Not much written on the subject due to the fact that the Eddy Currents which are confined to the Conductive zones, set up there own secondary magnetic fields having the SAME Frequency but not necessarily the same phase as the primary inducing field.

By inducing a ground Matrix or Target does not change the frequency of the target response signal. There are not to many things in Nature that can be induced with one Frequency & we have another Frequency come out of it.
I suppose that's why we Discriminate via Phase shift rather than Target Frequency.

I may be wrong like always & i am all ears waiting for comments to the contrary.

[Aziz]

I think, we have to consider also the capacitance of the coils and the target.
Hence it follows that, particularly high-frequency domain responses could be taken more better into account due to dominating capacitive effects on high frequencies.

This applies especially to PI MD's. We have more harmonic parts of high freq. domains both on the coils and the target.

Also the ground has increased capacitance effects.

This capacitance is seen as a parallel connected C to coil L in series with resistance R.
.--R--L--.
|^^^^^|
*---C---*
|^^^^^|
°^^^^^°
^=space (can't use real spaces here)

This would explain the real physical background more better.