4 quadrant VLF discrimination

[Davor]

Actually target + here means a positive phase of a differentiated response after the demodulating switchers. I'm talking about the demodulated signal in a frequency range ~10Hz.

So this is not about target + and ground -. It all happens at earlier stages. Ground signal is either canceled or not, but in both cases I assume it doesn't produce response in a demodulated and motion compensated differentiated GEB channel due to the demodulating switcher being driven by a 90° shifted signal.

This topic is about making sense of a signal that may have phase reversal due to the use of a differential coil.

[Sergey_P]

the first link - the theory, the next - a practice
extracting GEB by means of phase - child's play.
GEB, GB - the G signal does not compensate … and the received signal with the phase of G - resets. (target + G - gives an error)…
I'm sorry, good luck…

[Davor]

Don't be hypnotised by my simplification to a Cu channel. The example above considers a two channel MD with separate Cu and Fe channels, just like IGSL, and the example was about only one channel.

BTW, you again just pasted links without emphasis on what to seek there. Took me an hour to sift through. Please put a word, say, "Verator3 metal detector" etc.

I think extracting GEB by means of phase shifting a switcher drive is much better over a wide dynamic range than partial addition/subtraction in gain blocks. It is less prone to opamp offsets, compression etc. However Verator is a nice idea, and has several points extremely well thought of ... I have some other favorites. No offense. I prefer proportional audio.

...

So here goes the visual representation of the example above, yet with addition of what happens in Fe channel as well. Input signal is considered to originate from a differential coil...

[Sergey_P]

you have 2 quadrant RX and 2 quadrant CX… no 4 quadrant (pic)
alternative to the method of phases for GB -> http://www.md4u.ru/viewtopic.php?f=7...4cafed5af084e0 , http://www.md4u.ru/viewtopic.php?f=14&t=5702 , http://www.md4u.ru/viewtopic.php?f=14&t=5918 , http://www.md4u.ru/viewtopic.php?f=14&t=4383 ...

[Davor]

5 zones => http://md4u.ru/download/file.php?id=4238
DD and concentric coil does not compensate for the soil
This concentric coil (http://md4u.ru/viewtopic.php?f=77&t=3866&start=0 ) – compensates
reception area of target = zone TX * zone RX ?

There are some experiential comparisons between a concentric and a DD that give an advantage for a DD over a concentric over a difficult soil. My view is that a sharp response change of a DD is responsible for all the benefits, and mostly because of the motion compensated Rx-es that tend to wipe out blunt responses.

I am not entirely clear on the concentric coils calculations yet, because I'm aware of the magnetic field peaking near the Tx coil. The most of the induction calculations assume a magnetic dipole and uniform magnetic fields, while in reality field concentrates near wires. Guess the calculations for the balance etc work fine in practice if you keep a bit distance against the Tx coil. I like the proposed concentric coil because it maintains the Tx coil integrity, Q factor, etc.

Reception area is a vector product of Rx and Tx. In case you use differential coils there are zones perfectly illuminated by both Rx and Tx, yet you have a sharp minimum ... due to the vector product.

...

Now about the 4 quadrant...
Let's observe a non-Fe channel of a regular 2 quadrant Rx.
We have a GEB channel that has a zero response for ground vector and a positive response for all metals - considering a positive coil phase.
We have also a Disc channel which has a response ~perpendicular to the GEB. It will have response positive for some metals and negative for others.
They both go to the comparators to be tested against a threshold value, and these form an analog AND function. If both Geb and Disc values are above the threshold, a Geb value is passed through, and detector beeps.

It is important to notice here that it is not necessary that Disc channel passes the threshold to obtain a correct response, but instead it must be positive. In a blurry area between a zero and a threshold value, it is the GEB that must overcome a threshold for a clear response.

To obtain a 4 quadrant Rx we need an analog XOR function, and in practice it is a balanced mixer or a 4 quadrant multiplier. There is a slight problem with both - with Disc and Geb being of the same order of magnitude, the output would be squared, unless... Disc is made binary.
So when Disc is positive the output of such XOR would be equal Geb, and when Disc is negative the output would be -Geb.

Easy way to make this is to supply Disc to a comparator and compare its value with zero. The comparator binary output is supplied to a switch which choses between a Geb and inverted Geb. Such signal is further compared with a threshold value, and passed to the tone circuitry. And we have a solution for discrimination of the opposite quadrants for a non-Fe channel in a 4 quadrant solution. It may even be done as a hardware add-on for the existing rigs.

To get a regular 2 quadrant Rx from this 4 quadrant solution, only the inverted Geb signal is level shifted below zero, and it will work exactly as expected. A Disc polarity switch will operate between a non-inverted Geb value and some negative value. If both Geb and Disc values are positive, a Geb value is passed through when it overcomes a threshold, and detector beeps. Such operation will be needed when using regular coils.

I'll make a model of this soon, but this concept is open for debate.

And yes, I already pimped my IGSL Disc channels gains so that they behave more binary-like, and it results with better sensitivity of borderline materials such as Al foil.

[ivconic]

I get that. However at the same time the discussion also seems to be about the physical makeup of the coil itself. The graphs that have been presented, show the received signal has a positive and a negative component to it as the coil sweeps over a target. Each is a mirror image of the other only in opposite polarity as in the graph that golfnut provided.

This may be a gross over simplification of the issue, but I am trying to grasp the practical aspects of this. If the purpose is to have a theoretical discussion on 4 quadrant Rx, then I will sit back and read and see where it leads us.

Jerry

[Jerry]

There is a very good reason why everyone does not have one already, and that is the main topic of this thread: 4 quadrant Rx.

I get that. However at the same time the discussion also seems to be about the physical makeup of the coil itself. The graphs that have been presented, show the received signal has a positive and a negative component to it as the coil sweeps over a target. Each is a mirror image of the other only in opposite polarity as in the graph that golfnut provided.

This may be a gross over simplification of the issue, but I am trying to grasp the practical aspects of this. If the purpose is to have a theoretical discussion on 4 quadrant Rx, then I will sit back and read and see where it leads us.

Jerry

[Sergey_P]

DD coil actually has 3 zones

5 zones => http://md4u.ru/download/file.php?id=4238
DD and concentric coil does not compensate for the soil
This concentric coil (http://md4u.ru/viewtopic.php?f=77&t=3866&start=0 ) – compensates
reception area of target = zone TX * zone RX ?

[Davor]

There is a very good reason why everyone does not have one already, and that is the main topic of this thread: 4 quadrant Rx.

[Jerry]

A concentric coil is not much of a sensor for hunting in difficult soils, and it doesn't use Tx coil's field efficiently. It is fairly easy to produce and balance, but is not as sharp as DD or this differential coil. Other differential coil configurations may not be as sharp, but all of them will outperform every concentric coil in ground suppression, and only a top hat will match them in EMI suppression.

I have and use both DD and concentric coils of various sizes for several detectors. I am familiar with their strength's and weakness's. Each has it's place for certain types of hunting. I will follow the development of the differential coil with interest.

Jerry

[Davor]

A concentric coil is not much of a sensor for hunting in difficult soils, and it doesn't use Tx coil's field efficiently. It is fairly easy to produce and balance, but is not as sharp as DD or this differential coil. Other differential coil configurations may not be as sharp, but all of them will outperform every concentric coil in ground suppression, and only a top hat will match them in EMI suppression.

[Jerry]

What would be the advantage of this approach over just using a concentric coil?

Jerry

[Davor]

A DD coil actually has 3 zones. A middle zone which is an overlap of the Rx and Tx coils is giving positive response, while the other 2 zones give negative, and these are ignored in a traditional 2 quadrant Rx. In fact, there are also 2 additional zones going left and right from the coils to the open space that give positive response, and these may add only a bit of confusion when you detect a large object.
Points where DD fails are the fact that maximum field is not exactly at the detection maximum but translated on Tx side, and Rx/Tx coils crossing over. The former means you are not using the maximum of your probe's area, and the latter means that you'll hardly ever be able to zero your coil entirely. On the + side it works perfectly with 2 quadrant Rx and it copes well with the ground.

Now the tanga string coil from the above example ...
- Tx coil encircles a whole area of the coil, and the maximum is where it is supposed to be. It is circular and may have the highest Q with regard to the engaged quantity of copper
- Rx coil is differential, and you surely can achieve balance and a real zero
- Rx is situated symmetrically inside and has a reasonable spacing against the Tx coils to further reduce capacitive couplings.
- It has an abrupt phase reversal at the very middle. Furthermore, when such coil is modeled as a collection of current conducting wires that have magnetic field concentrated right around them, and not as a magnetic dipole, you'll notice that the middle path comprises twice Rx wires with the same direction, hence the reception peaks right next to the very middle where phase reverses, ergo very sharp target indication. (Aziz may have final word on this)
- as a differential coil, it will suppress most of the ground vector, so it must work fine on the difficult soils or under water.
- it eliminates EMI, so it will enable higher gains and detection depths.

Want more?

[Jerry]

You have lost me so far. A DD type coil has an area right down the center which is sensitive to picking up targets. The drawing seem to show something other than that. Also, using both sides of the DD for one big round pattern with notch in the middle can give more targets under the coil at the same time for the detector to deal with. I understand the notch, but we have that in reverse right now with the DD. Which is excellent for sifting through trash.

Not knocking thinking out side the box at all, just thought I would toss that out for consideration.

Interesting subject.

Jerry

[Tinkerer]

target – point
soil – space (V)
-----
E(target) ~ S*N
E(soil) ~ V
----
1.
E(target)cx = E(target)rx
E(soil)cx = E(soil)rx
2.
E(target)cx << E(target)rx
E(soil)cx = E(soil)rx

Sergey_P
Thank you for the excellent explanation and drawings.

Tinkerer