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  • 4 quadrant VLF discrimination

    I must make it going, so why not here in a fresh thread.
    So far I mostly re-invented the wheal - so to speak, because most of my revelations were already invented and patented, and many of them did not fare well on the market.
    I think this might be different.
    As far as I know, which is not much, there are no VLF MD-s on the market that do the 4 quadrant discrimination for real. I've seen only some detectors that present the vector of the target graphically and you have to see it to know where it belongs. Please correct me if I'm mistaken.
    My educated guess is that this kind of Rx will narrow the gap between VLF and PI by enabling the use of differential Rx coils. These coils eliminate EMI and most of the ground signal by their very nature, yet only one half of them is useful with a traditional 2 (or 1) quadrant Rx, and that makes them odd in comparison with the commonly used coils. One side of it works, and the other side is mute.

    Now, what is the big deal with these quadrants? Here goes a little intro...

    In a garden variety of a MD of nowadays you'll find 2 or more "channels" usually called GEB and Disc. These are phase aligned with ground vector, Al foil etc. thus dividing a full circle of all phases that the received signals can have in a pie-like quarters that are recognised in a phase comparator part. In a case that GEB is + AND Disc. is + you have a beeep. If either or both are "-" there is no beep. Simple as that. That's a 1 quadrant discrimination, and is usually tuned to non-iron targets.
    Fine. 2 quadrant is the same, but the extra quadrant is tuned to iron.
    The catch is that when a coil picks a signal from a, say, golden ring, it can come as +GEB AND +Disc, but also as -GEB AND -Disc. In order to have any indication at all for a -GEB AND -Disc situation one must build a receiver that accepts phases from the opposite quadrants. Not adjacent but opposite.

    Such a receiver that gives the same indication for the opposite quadrants would work perfectly well with a differential coil, because you'd have only a narrow notch in a very middle between the Rx coils' halves, that may come handy for pinpointing.

    So, how about that?

  • #2
    In my understanding, so called "4 quadrant" means 360 degrees.
    So, if i am right: by "4 quadrant" you actually mean on processing 360 degrees of signal phase?
    Do i understand this correctly?

    Comment


    • #3
      more or less. The same way you rotate criteria of the discrimination in a classic Rx, you have the same here, but you just extend the processing to the opposite angle as well. Yes, in fact it is 360°but may have holes, notches, whatever.

      Now, why bother? Imagine a coil just as in the picture below and the target response. Observe how the response suddenly reverses in a middle where you have ample Tx field. Now, just a step further, imagine that you can use the whole target path, positive and negative,, and that it gives a properly discriminated response. Now a final effort - see how abrupt is the signal change in the middle - that's your small target sniffer, and your pinpointer. And best of all - it is perfectly balanced, and all the wires are in the same plane.
      Attached Files

      Comment


      • #4
        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
        Attached Files

        Comment


        • #5
          I looked at my IDX in XY as did Don and others. You see this type of thing.

          At first I thought the signals remain in top RHS for non ferrous and top LHS for Fe.

          This is often where patents leave it.

          But on second look the signals do extend to diagonally opposite corner/state too.
          Attached Files

          Comment


          • #6
            Originally posted by Sergey_P View Post
            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

            Comment


            • #7
              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

              Comment


              • #8
                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?

                Comment


                • #9
                  What would be the advantage of this approach over just using a concentric coil?

                  Jerry

                  Comment


                  • #10
                    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.

                    Comment


                    • #11
                      Originally posted by Davor View Post
                      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

                      Comment


                      • #12
                        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.

                        Comment


                        • #13
                          Originally posted by Davor View Post
                          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 ?
                          Attached Files

                          Comment


                          • #14
                            Originally posted by Davor View Post
                            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

                            Comment


                            • #15
                              Originally posted by Jerry View Post
                              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

                              Comment

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