Targets frequency response

[Tinkerer]

Tinkerer, please explain the problem, how to simulate it and where is the output point. I see only butiful TX pulses.

Thank you for the interest. I attach the picture of the traces. On top you can see the formulas I use.

L1 is the TX coil
L3 is the Bucking coil the sum of TX+BU is the balanced coil.

Adding the target response to the balanced coil trace (red) we see what the actual output of the RX coil is.

This is an actual real circuit that works. I try to reproduce the traces I see on the oscilloscope.

When we balance the coils, sometimes we get a little movement when potting the coils and the balance is upset. I have used a pot at R5 as current bypass to correct unbalance.

R5 has several functions. It is damping L3, to avoid HF oscillations.

However, when simulating R5 for best damping, I noticed that it also influences the response of long TC targets, as HP filter.

You got the Flyback pulse on your picture. I prefer to look at the currents as this is what I see at the RX pre-amp input.

[Aziz]

Davor,

brilliant!

[mikebg]

Looking at the TC2 of the ground response above, I just realized that a problem I have with a damping resistor R5, might be turned into an advantage.

Below is a simulation circuit. The inductor L3 with Resistor R5 form a high pass filter. Choosing the right cutoff frequency it might be used to mitigate the T2 response.

A frequency problem. Any help?

Tinkerer, please explain the problem, how to simulate it and where is the output point. I see only butiful TX pulses.

[Davor]

You can observe both systems' GB as very similar in terms of GB if you only observe a GB point as a combination of offset and a slope with zero crossing in time. Same goes with targets as well.

I'd say that the biggest problem of GB is not GB per se, but the offset that screws everything else and acts as if ground response is not sufficiently attenuated.

In case of IB, offset is dictated by residual unbalanced Tx field (air field), and once it is removed, you can observe Rx zero crossing at some point in time, a delay. Fun part is that this zero crossing delay is strongly related to the target tau, and less so with VLF frequency. At specific frequency it translates to a certain phase, but essentially it is a consequence of delay, you know the famous e^jwτ=cos(wτ)+j*sin(wτ) Being first order system the angles can stretch only within -90° and +90°. Finding zero crossing determines target ID. Provided there is no offset involved.

In PI you have essentially the same thing. You have exponential decay at rate e^-t/τ and half life at τln(2), and there you have it - your zero crossing... provided you can determine what the curve does in infinity. A first approximation would be subtracting EF.

Now something completely different. Main difference between CW and pulse systems is that in CW systems the residual target response is truncated in the opposite going half period, thus eliminating offset in a process, and reducing the amplitude of extreme long taus. Hence the viscosity effects are happening in the same phase playground as everything else.

In PI the zero crossing of viscosity affected materials happens beyond the EF point, thus screwing the offset and also zero crossing reading of other materials. A perfect nemesis.

There are two nemeses in CW systems though. Ground response may happen due to the two contributors, e.g. ferrites and salts, and rigs seldom attenuate both, but the other one is one completely overseen: 2nd harmonic. It effectively shifts offset between positive and negative going half periods.

Solutions? In CW systems you can fight 2nd harmonic and introduce ground balance that is separate from discrimination criteria. In PI you can introduce bipolar pulsing, and perhaps use some weighting function that would result in half life that is different from τln(2).

[Aziz]

At what ground, conductive or permeable? Please explain something more.

GB in PI architectures (off-time sampling) is overall simpler than other GB architectures. It doesn't matter, what type of ground you have. Only the magnetic viscous susceptible matter is the challange (time delayed magnetisation response). The magnetic relaxation effect is quite trivial. Conductive ground is trivial.

Aziz

[mikebg]

Exactly! PI is wasting a lot of TX power. On the other hand, PI makes simple GB possible.
Aziz

At what ground, conductive or permeable? Please explain something more.

[Tinkerer]

Cutoff frequency and timeconstants of ground

Addition to post #206:
Conductive ground also has series of timeconstants as described above in post #98:
http://www.geotech1.com/forums/showt...9656#post99656
John Corbin measured enough exact two of ground timeconstants. Note that measured by him ratio is almost 9 times
http://www.geotech1.com/pages/metdet...byn/corbyn.pdf

Looking at the TC2 of the ground response above, I just realized that a problem I have with a damping resistor R5, might be turned into an advantage.

Below is a simulation circuit. The inductor L3 with Resistor R5 form a high pass filter. Choosing the right cutoff frequency it might be used to mitigate the T2 response.

A frequency problem.

Any help?

[Aziz]

...
Note that this classification is subject to received frequency band only, not to the transmitted by own TX spectrum. If the spectrum radiated by transmitter contains frequencies that the receiver does not handle and does not need, it means that the TX wastes energy.
...

Exactly! PI is wasting a lot of TX power. On the other hand, PI makes simple GB possible.
Aziz

[mikebg]

CORRECTIONS OF PAPERS

To say that there are metal detectors working in the frequency domain is nonsense. The frequency domain is only a poweful tool for analysis and design. Every metal detector (see connected to it oscilloscope :-) works in time domain.
According to frequency domain, metal detectors are only two types:

1. Narrow band metal detectors, in which the receiver processes spectral data from a narrow bandwidth of frequencies (for example 16Hz), and

2. Wideband (broad band) metal detectors, in which the receiver processes information from a wider bandwidth (for example an octave or more).

Note that this classification is subject to received frequency band only, not to the transmitted by own TX spectrum. If the spectrum radiated by transmitter contains frequencies that the receiver does not handle and does not need, it means that the TX wastes energy. Each ham designer of QRP amateur radio will tell you that such one transmitter is not created by member of this brotherhood.
Using Frequency domain, we can correct certain documents posted in this thread. Here is an example, but instead corrections, I recommend you to write papers again.
You can see the original of this document in post # 27:

http://www.geotech1.com/forums/showt...1424#post71424

[Qiaozhi]

Cutoff frequency and timeconstants of ground

Addition to post #206:
Conductive ground also has series of timeconstants as described above in post #98:
http://www.geotech1.com/forums/showt...9656#post99656
John Corbin measured enough exact two of ground timeconstants. Note that measured by him ratio is almost 9 times
http://www.geotech1.com/pages/metdet...byn/corbyn.pdf

Note:
In the Corbyn article, there is some text missing from the bottom of the first page.
The scanned document says "Fig. 2 shows the case where a magnetic flux of Weber is normal to a loop of radius a and ..."

It should go on to say "... effectively falls to zero in time . If L is ...". Which then continues on the second page.

[mikebg]

Cutoff frequency and timeconstants of ground

Addition to post #206:
Conductive ground also has series of timeconstants as described above in post #98:
http://www.geotech1.com/forums/showt...9656#post99656
John Corbin measured enough exact two of ground timeconstants. Note that measured by him ratio is almost 9 times
http://www.geotech1.com/pages/metdet...byn/corbyn.pdf

[mikebg]

One year has passed since (R)EMI group ceased its activities in July morning.
Ex members of (R)EMI group felt that I have lack of knowledge. They were dissatisfied with my work as a spokesman, but I, in turn, was unhappy with their work organization because nobody helped me in compiling and editing postings. They only answered my questions. They stopped to answer me a year ago because they thought that in the 21st century

The learning process requires mostly student working, than work of teachers.

However, I have gained knowledge and skill to see the shortcomings of my postings.
For example, not even a brief explanation of the formulas to above figure. I had to point out that D is the diameter of the TX coil. The above formula is universally valid, but the below formula is simplified because it is assumed that no ferrite soil properties, ie ur = 1.
Not specified how by measurement of fc we can calculate timeconstants of soil.
Not specified how to design optimal D (TX coil diameter) by measuring phase lag of GND signal with a given TX coil.
However I know the answers

[mikebg]

Cutoff frequency and timeconstant of ground

Ground signal, TX frequency and diameter of TX loop

The conductive ground also has a cutoff frequency. It depends on the diameter of the TX loop.

What happens when you change diameter of TX coil, can be analyzed with a Normalized Impedance in complex plane.
Cutoff frequency of ground is inversely proportional to the square of the diameter according the formula below. However the attached figure and formula are valid for nonmagnetic ground only.
What happens when ground has mineralization (ferromagnetizm), is explained in the posting #18.
When synchronous demodulator is adjusted to eliminate the ground signal (ground balance), it must remain sensitive enough to target signal. The task of the designer is to select one TX frequency and such diameter of TX loop, to obtain an appropriate phase difference between the ground signal and target signal. The most relevant difference is quadrature phase, ie 90 degrees. But if you think about how to do this at the impedance of a mineralized ground shown in posting # 18, you'll understand why in Australia for metal detector is difficult to find gold nuggets.

[pebe]

Hi Tinkerer,
Thanks for your most comprehensive reply. It makes sense to me now.

pebe

[Tinkerer]

I am familiar with RC and LR time constants (TC), but don't see how they are relevant in your text. Are you referring to 'time delay' or am I missing something? And why 'fundamental'?

Hi pebe,

the "fundamental part, blue text, is a quote from Mikebg, he may answer that.

The TC or time constants I refer to, are for one part the time constant or TC of the coil, which is the LR you mention.

The other time constant is of the target. With the PI type of detector, a target behaves much like a coil. It does take some time to induce the maximum eddy currents like a LR time constant.

Then it takes some time for the eddy currents to dissipate again. This time is what the traditional PI uses to read the magnetic field caused by the eddy currents. (Time Domain)

When we refer to a time constant of a target, we usually consider it to be the time that it takes for the amplitude of a target response to fall by about 63%.
To put it in numbers, if a target response signal has an amplitude of 100uV
10us after switch off of the TX pulse, and 10us later the signal amplitude is only about 37uV and still 10us later it will be about 15uV.
This means that probably, about 30 to 40us after switch off of the TX pulse, the target signal will be lost in the noise. S/N

Few PI detectors are capable of sensing a target that has a TC less than 5us.

Tinkerer