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  • ground vs target

    With the basic VLFs on here like IDX IGSL etc..


    If you pump the ground, the speed of pump will give a frequency out of the geb detector proportional to the rate of pump. Say you pump once per second you get a 1Hz result at the GEB integrating filter//


    Correct or not?

    S

  • #2
    Actually you are detecting a phase, timed amplitude. In case your, say, GEB channel is phase shifted by 90° you'll obtain 0V out, regardless of the amplitude. Moving phase a little over you get positive response, a little below - negative.

    Target will respond by constant phase against the Tx and a varying amplitude, and this variation is determined by your waving a coil. Hence, the low Hz responses are due to your waving action.

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    • #3
      I get the target stuff - 6 to 11Hz dependant on shaft length and sweep speed


      The Geb stuff is my question.

      I know we need to arrange the phase/timing of gating the geb det to integrate equal amounts of + and - ground signal so it averages out.

      My question is if you were to sweep - and the gnd is flat and uniform matrix - no target.

      You will not get a 6 to 11Hz GEB signal ? Like the Disc ch on a target..


      So why do we have a filter peaking where targets live at 9Hz on the GEB channel??



      s

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      • #4
        Different targets appear as vectors at different angles. So does ground. The only difference is we align GEB channel at 90° against this ground vector, hence the ground signal is notched out - regardless how strong it appears at coil. Not evaporated, but notched. Every other angle vector will have some voltage change at GEB channel.

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        • #5
          I know that too.

          My question was. What frequency does the ground signals come in at.


          I know target signals come in at ~9Hz during a sweep


          {For those who dont know.
          On a motion machine with a target, the det inputs are a sample of Tx plus the Rx return.

          Det output is the difference in frequency between the two inputs.

          example..
          Tx 7,000Hz
          Rx signal from target during sweep is 7,009 Hz

          The delta between them 9Hz. This what comes out of the det.
          This is why we have an Intermediate frequency filter centered at 9 Hz.}


          My question was. What frequency does the ground signals come in at.


          S

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          • #6
            Ideally, the ground signal is constant, so it looks like a DC offset.

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            • #7
              Aha, so the other half of the question is,


              If the ground is uniform during a sweep and the user does'nt bob the coil there wont be 9Hz frequency component from GES det !

              So Im confused why some designs on here have the GEB filter centered on 9Hz too?




              S

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              • #8
                It is because all vectors (i.e. target responses) are dismantled into two components, one aligned with GEB phase (say X), and another one aligned with Disc phase (say Y). Vectors tend to do that
                This means that ground signal gets amplitude modulated at, say 9Hz, and you wish to pass exactly that through. Just like in AM radio. You pass the AC component of the signal envelope through.

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                • #9
                  Sincerely, Carl is a pleasure to meet you, please I need your help, connect the circuit of writcher treasure, which is a two box detector IB, which is piblicado on your site. But the transmitter operates at 180 Hz and 180 kHz, not the receiver does not recognize any signal and I need to know what happens, poque the arm following the scheme, please help me, I put every item listed. Thank you.

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                  • #10
                    Originally posted by juaka2010 View Post
                    Sincerely, Carl is a pleasure to meet you, please I need your help, connect the circuit of writcher treasure, which is a two box detector IB, which is piblicado on your site. But the transmitter operates at 180 Hz and 180 kHz, not the receiver does not recognize any signal and I need to know what happens, poque the arm following the scheme, please help me, I put every item listed. Thank you.
                    I don't know how you tried to attach the image, but it ended up as several pages of text that caused the message to load very very slowly. This has been removed.

                    Please scroll down the page and used the Manage Attachments button when adding images.

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                    • #11
                      This means that ground signal gets amplitude modulated at, say 9Hz, and you wish to pass exactly that through
                      Does it?


                      "Why do we have 9Hz filter in GEB circuit?" We all know why in the disc channel.


                      If we think that, having balanced out the ground, there is no need for the 9Hz filter.
                      Altough if we think that some target signal may leak through - and we try to pass it and harness the signal energy to boost sens?

                      This would reduce the sensitivity to a detect as the geb input to U4 on IDX or U7 on IGSL would have target signal too. The diff amp would see less of a difference between the channels to trigger on.



                      I think the Geb filter wants a LP cutting off at about 4Hz.

                      This way you get rid of ground signals as before, you also kill any break through of target signals on GEB - leaving a clean level on geb input to U4 on IDX or U7 on IGSL.

                      The ratio of target input to geb input levels on U4 on IDX or U7 on IGSL will be greater and give more sens.



                      S

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                      • #12
                        As I see it:

                        There is a ground signal in the receive coil which is amplitude modulated by variations in coil height above the surface. If the Ground Balance control is set correctly the output of the GEB phase sensitive detector averages to zero, and amplitude changes in the ground signal have no effect (hence the term "Ground Exclusion"). Any signals from targets having a phase different from that of the ground produce an output that is passed on to the following filter/amplifier stages. The output of the GEB channel is therefore an "Everything But The Ground" signal.

                        Does this make sense?

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                        • #13
                          Originally posted by Gwil View Post
                          As I see it:

                          There is a ground signal in the receive coil which is amplitude modulated by variations in coil height above the surface. If the Ground Balance control is set correctly the output of the GEB phase sensitive detector averages to zero, and amplitude changes in the ground signal have no effect (hence the term "Ground Exclusion"). Any signals from targets having a phase different from that of the ground produce an output that is passed on to the following filter/amplifier stages. The output of the GEB channel is therefore an "Everything But The Ground" signal.

                          Does this make sense?
                          This is correct.

                          Comment


                          • #14
                            Exactly

                            GEB channel will produce 0V for a vector that is aligned with ground, but full voltage for vector perpendicular to ground vector. That's why TGSL, IGSL, etc. take GEB voltage for level indication - metals are more or less perpendicular vectors against the ground vector.

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                            • #15
                              Any signals from targets having a phase different from that of the ground produce an output that is passed on to the following filter/amplifier stages. The output of the GEB channel is therefore an "Everything But The Ground" signal.

                              Does this make sense?
                              It does make sense - but if you consider the context of the question i.e. IGSL for example.


                              Here, the GEB channel has its det gated, phase-aligned to ave out the gnd signal - I see that.

                              If having integrated the gnd signal to a small dc offset, there is a bit of target signal there too -great we pass that thru the 9Hz 'window' and get more signal here! We all sign autographs and go home.

                              Nope, passing target residuals thru GEB reduces Sens.


                              U7 on IGSL (instrumentation amplifier) it is a difference amp - If we put Target on one input and small target on the other we reduce gain. I would not expect to see any target on the geb, (unless its polarity was 180 deg out of phase (inverse) of targets on disc ch.



                              Ideally with the diff amp arrangement as the sense no sense on these machines,
                              if we had two Disc channels, each 180 apart so true differential, with a Geb on each of them - we maximise sens to a target as we fully utilise the Diff amp gain - in a target case. Plus neither channel has any gnd signal ..


                              S

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