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field test unit no 001 "model T"

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  • The problem with magnetic ground is that it decays with a 1/t law. This means that the initial decay is fast, but then slows down at later times. This can be seen if you use a log amplifier for the front end. The output of the log amplifier shows an exponential decay, while that from a gold ring shows a linear straight line decay. To get magnetic ground to give a linear decay, you have to have a logarithmic timescale as well as a log compression of the amplitude.

    The total measured decay time of a 5oz (160gm) nugget is 700uS to the point where it disappears into the noise. This is not "time constant" in the accepted sense. The total decay of a 30oz (940gm) nugget is >2000uS by comparison. Taking the 5oz nugget the approximate true time constant would be 700/5 (disregarding unseen signal in the noise) = 140uS. The log amplifier (and theory) shows that the signal only becomes a single exponential after 1 time constant. Prior to that it is a sum of exponentials. To get best S/N ratio, early sampling is employed and this part of the decay is faster and higher amplitude than the later invariant decay. Except in the case of a ring. Maths get tricky, but do the experiments. Maybe on a simulator program.

    This doesn't answer the capacitor v dual sampling and subtraction debate, but is relevant to it. I always avoid capacitors early in the signal chain as there are always unwanted overshoots and undershoots when dealing with pulses, and a lack of preserving a true dc base level.

    Nothing is straightforward, and there is bound to be some loss of the wanted signal in getting rid of the ground response.

    Eric.

    Comment


    • Originally posted by Qiaozhi View Post
      ....
      Go on, don't be a spoil sport ... give us the proof.
      No! Where are all the metal detecting Einstein's there?
      I'm addressing all the self defined gurus like UFox, PJ, Robby_H, Rafferty and Co with it.

      Aziz

      Comment


      • Originally posted by Aziz View Post
        No! Where are all the metal detecting Einstein's there?
        I'm addressing all the self defined gurus like UFox, PJ, Robby_H, Rafferty and Co with it.

        Aziz

        Your the one who says he knows it all. Waiting to see your answer smarty. Qiaozhi has you over a barrel !!!

        Comment


        • Originally posted by PATCHES JUNIOR View Post
          Your the one who says he knows it all. Waiting to see your answer smarty. Qiaozhi has you over a barrel !!!
          Oh well, the proof is a mind**** (=beyond my scope).
          But I want you and your best mates to solve this little and very trivial quiz. Or run quickly to your master and ask for a help.

          Aziz

          Comment


          • Originally posted by Aziz View Post
            Oh well, the proof is a mind**** (=beyond my scope).
            But I want you and your best mates to solve this little and very trivial quiz. Or run quickly to your master and ask for a help.

            Aziz
            Aziz, go back and read Eric's second last paragraph in his post. Are you saying capacitive coupling is the way to go?

            Comment


            • Originally posted by UrbanFox View Post
              Aziz, go back and read Eric's second last paragraph in his post. Are you saying capacitive coupling is the way to go?
              Hey UFox, you're not playing the infamous quiz game. Go back and read my post again.

              Aziz

              Comment


              • Originally posted by Aziz View Post
                Hey UFox, you're not playing the infamous quiz game. Go back and read my post again.

                Aziz
                Aziz, I do believe you are too wrapped up in theoretical models. Why would a designer go to the trouble of extra components and subtraction if cutting a line and adding a capacitor and a couple of extra components would achieve the same result.

                Go back and read Eric's second last paragraph again and take note of what he is saying, It might be time for you to do a few tests in the real world and identify why blokes like Eric, Minelab, and many others, don't use AC coupling early in the signal chain.

                Let's see you justify the usability of a technique that everyone else has dropped, and explain why we should accept modeling that is not representative of results achieved in real world. So, are you suggesting Eric is wrong?

                Comment


                • Originally posted by UrbanFox View Post
                  Aziz, I do believe you are too wrapped up in theoretical models. Why would a designer go to the trouble of extra components and subtraction if cutting a line and adding a capacitor and a couple of extra components would achieve the same result.

                  Go back and read Eric's second last paragraph again and take note of what he is saying, It might be time for you to do a few tests in the real world and identify why blokes like Eric, Minelab, and many others, don't use AC coupling early in the signal chain.

                  Let's see you justify the usability of a technique that everyone else has dropped, and explain why we should accept modeling that is not representative of results achieved in real world. So, are you suggesting Eric is wrong?
                  Go back and read my post again.

                  Comment


                  • Originally posted by Aziz View Post
                    Go back and read my post again.
                    What are you trying to say, Aziz? Spit it out!

                    If you wish to use design concepts that others deserted a long time ago then go for it. Don't expect me to waste time playing your silly games.

                    Comment


                    • Originally posted by UrbanFox View Post
                      What are you trying to say, Aziz? Spit it out!

                      If you wish to use design concepts that others deserted a long time ago then go for it. Don't expect me to waste time playing your silly games.
                      Hey ....get a grip UF ... Aziz is correct ... DC coupling is used where the designer is unable to resolve coupling issues or does not understand the signal domain he is working with. FYI ... I reworked that part of the design and put the capacitive coupling back in ....it has knocked another 60db off the Earth field response. The back end DSP filter was a 10th order but could not deal with overload conditions for magnets very close to the coil ... so thanks for that UF ... you "convinced" me to make the design even better.

                      Comment


                      • Originally posted by moodz View Post
                        Hey ....get a grip UF ... Aziz is correct ...
                        Hey....get a grip Moodz.... if you want to believe Aziz has all the answers then go for it. I am finding it interesting just watching from the sideline.

                        DC coupling is used where the designer is unable to resolve coupling issues or does not understand the signal domain he is working with.
                        Moodz, I can totally accept what you say. If a designer doesn't understand what he is doing then anything is possible, I guess.

                        FYI ... I reworked that part of the design and put the capacitive coupling back in ....it has knocked another 60db off the Earth field response. The back end DSP filter was a 10th order but could not deal with overload conditions for magnets very close to the coil ... so thanks for that UF ... you "convinced" me to make the design even better.
                        Gee, another 60 db, and you already had the problem solved before that. Sounds bluddy miraculous Moodz!

                        Anyway, glad to hear you are going forward, Moodz. How long before we actually see this detector in the market place???

                        Comment


                        • Originally posted by UrbanFox View Post
                          Hey....get a grip Moodz.... if you want to believe Aziz has all the answers then go for it. I am finding it interesting just watching from the sideline.



                          Moodz, I can totally accept what you say. If a designer doesn't understand what he is doing then anything is possible, I guess.



                          Gee, another 60 db, and you already had the problem solved before that. Sounds bluddy miraculous Moodz!

                          Anyway, glad to hear you are going forward, Moodz. How long before we see another "world beater" in the market place???

                          UF you dont seem to be very technical as you are focusing on aspects like cap coupling which quite frankly is not even circuit theory 101 and bit lower than standard of discussion on this thread.

                          60 db aint that hot but it conditions the signal before it hits the 140 db filter in the processor.

                          BTW problems are never solved ...they just have an answer in place till a better one comes along.

                          I am moving forward ... thanks for enquiring ... I am not making detectors its a PITA ... I will leave that to some other company.

                          Comment


                          • Originally posted by moodz View Post
                            UF you dont seem to be very technical as you are focusing on aspects like cap coupling which quite frankly is not even circuit theory 101 and bit lower than standard of discussion on this thread.
                            Moodz, whether I seem very technical to you is irrelevant.

                            Originally posted by moodz View Post
                            ...I am afraid its all over for the monocoil PI development .....
                            Moodz, if you want to use capacitive coupling and believe you are up there with the best, go to it, but personally, I don't think it is all over.

                            Comment


                            • Originally posted by UrbanFox View Post
                              Moodz, whether I seem very technical to you is irrelevant.



                              Moodz, if you want to use capacitive coupling and believe you are up there with the best, go to it, but personally, I don't think it is all over.
                              Actually it is highly relevant as either you are talking through your hat or you are actually interested in the subject of capacitive coupling and earth fields. If you are going to ask "dorothy dixer" questions then this is not the forum to air them.

                              (The term references American advice columnist Dorothy Dix's reputed practice of making up her own questions to allow her to publish more interesting answers. The only problem here is that you are not publishing the answers but casting doubt on legitimate lines of discussion.)

                              Anyway I must be on to something as this is the only thread you hang around .... so thanks for the vote of confidence.

                              As for being up there with the best ... I am better than the best LOL.

                              Comment


                              • Hey UFox,

                                did you get a grip on the math proof? No problem, if you don't know how to do it. I don't blame you.
                                I do not give credit for PJ to do it as well. I don't blame him too.
                                Ok, who's the next in my guru list?
                                Robby_H and Rafferty? Or the Lord mastercat? It's your chance now guys!

                                Everybody feel free to contribute to the hot discussion (it ain't off-topic).
                                Cheers,
                                Aziz

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