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  • #31
    Originally posted by Carl-NC View Post
    TDI EF=75us. Long TC targets are still detected in the EF sample, but should always be substantially weaker than the main sample. The result is it doesn't matter much.

    Hi Carl thanks for the reply.

    At the moment the circuit is only on one of those white plugin prototype boards but I'm transferring it to a breadboard design at the moment. I also have to make a better +/-5 volt power supply with the circuit running at 9 volts input I can't sync the 7660 to the TX pulses if I do for the output into the 5 volt reg is too low.



    Mick

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    • #32
      Hi Mick,

      I got bored and made this spreadsheet, its a sim of the TDI method, you might find it useful. You can play around with the sample points, co-efficients etc. and see how it affects the response to different TC targets. Not sure how valid it all is but hey its a bit of fun.

      Midas
      Attached Files

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      • #33
        Originally posted by Midas View Post
        Hi Mick,

        I got bored and made this spreadsheet, its a sim of the TDI method, you might find it useful. You can play around with the sample points, co-efficients etc. and see how it affects the response to different TC targets. Not sure how valid it all is but hey its a bit of fun.

        Midas

        Hi Midas, thanks that might come in handy depending on if I can get the design to work.



        Mick

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        • #34
          Hi Carl, I'm just after a little more guidance. So far the design is the same as the circuit I posted earlier I have changed the 10k EF pots to 500ohms and have both the Target and GB samples balanced into two separate integrators and the outputs showing a DC level depending on Target distance. Both sample outputs go in a positive direction measured from common (9 volts in my circuit). The GB channel is a lot lower in level because it's being sampled out further.

          The idea I had was to get the output of the GB/EF2 (being DC level) and inverting it with gain (I might be getting mixed up with inverting the GB, maybe it's got to be non inverting). Then the output of this stage would be fed to a variable resistor and on the other side have the non inverting BG/EF1. This would hopefully work as the ground balance. Do you think this would work or would you have any better suggestions.


          Cheers
          Mick


          Here's a couple of pictures for the detector build so far. If the design works correctly the final version will go onto a properly designed board.




          Comment


          • #35
            Originally posted by Mick-GD View Post
            The idea I had was to get the output of the GB/EF2 (being DC level) and inverting it with gain (I might be getting mixed up with inverting the GB, maybe it's got to be non inverting). Then the output of this stage would be fed to a variable resistor and on the other side have the non inverting BG/EF1. This would hopefully work as the ground balance. Do you think this would work or would you have any better suggestions.
            This should work. TDI applies the GB to a higher gain stage, then subtracts GB from the main channel in a subtraction diffamp. Not necessarily better, just different.

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            • #36
              Originally posted by Carl-NC View Post
              This should work. TDI applies the GB to a higher gain stage, then subtracts GB from the main channel in a subtraction diffamp. Not necessarily better, just different.

              Thanks Carl.

              I've done some more experimenting and the GB circuit is up and running. All that's left now is the output section to the speaker, but before I do this I'm going to rebuild it due to some instability in the output of the final diff amp and also set it up for running on 12-14 volts.

              Once I have these issues sorted I'll post some more details and test results.


              Mick

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              • #37
                Wouldn't EF manifest exactly the same way as opamp input offset does?

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                • #38
                  HI,

                  Mick MD, I have more problems with the preamps AD...

                  Here, the condition of integration in the traditional PI:


                  Regards
                  Taktyk
                  Attached Files

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                  • #39
                    While I'm waiting on my minipulse board, I'm also considering a GB add-on circuit for it.
                    There are two options, time variable sampling with constant amplification, and TDI-style time-constant samples with varying amplification for target vs GB.

                    Time variable is a simple solution that expects 1/t ground behaviour. I'm considering 3 samples, target, GB, and EF after some delay. Gain being constant, the T(EF) sample is a simple T(target)-T(GB)
                    The idea behind time-variable samples at constant gain is in the fact that ground responds at power law, hence for 1/t function, an integrated sample taken after a pause from 0..1 and with duration 1 (which is between time 1 and 2) is equal to a sample taken between time 2 and 4, and so forth.



                    What is left to do is add an EF sample with duration 1 at some later time.
                    It would require replacing the timing circuitry with something else (that I already have), but the analogue part would remain the same. The downside is that by exact 1..2 and 2..4 timing only exact 1/t ground will completely cancel.
                    In a real life implementation the above mentioned 1..2 and 2..4 would be replaced with, say, 10..20 and 20..40 microseconds. It translates to a simple counter with a mere time increment.
                    E.g. 10us delay, 10us positive sample (target), 10us negative (GB), 10us negative(GB again), 4 times 10us delay, 10us positive EF sample. 90us in total.
                    By varying a time increment the hole also shifts, but the ground cancellation mechanism remains the same. So in a way I may have discrimination of some kind as well.

                    I also have a solution TDI style, which would require additional gain block, some monoflops and a slight intervention to the existing integrator. It is a sort of no-brain-no-pain well trodden solution, but I'm in favour of a time varying solution. Guess it would work with HH as well.



                    BTW, a sexy formula from above is entered by clicking to "Insert image" and placing the TEX formula as following (together with the http-thing):

                    http://www.geotech1.com/cgi-bin/mimetex.cgi?\int_1^2 \! 1/t\,dt - \int_2^4 \! 1/t\,dt=0

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                    • #40
                      Really looking forward to your implementation. Especially the time varying solution.

                      Comment


                      • #41
                        It is cooking.
                        I had an epiphany that gave me an interesting idea on employing a Johnson counter to make a whole time base with only two chips. It requires a little refinement, but it is coming.

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                        • #42
                          hi davor i'm also interested in your gb solution, so far i have got -5, +5 on my mpp, its coming slowly, i'm finding the solder spots small and fiddly in some places especially where it should not be connected to the ground.
                          some of the newcomers may need the board to be physically bigger so there can be more space around the traces, i'll mention it to george when i do an update in the mini pulse thread.

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                          • #43
                            Im looking at Johnson counters now. Im really enjoying learning, albeit very slowly with lots of bumps and headslapping moments.

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                            • #44
                              Well, don't look much further, just observe all the nice things you can make with a popular CD4017

                              I'll post my time base solution soon. I was doing some casual math (thank you Wolfram Alpha) and it appears one simply can't avoid a null, or better say a "hole" in a response to remain within a PI lingo, if any of the GB schemes are to be applied. The best thing one can do is to simply alternate two time bases and observe what happens.
                              I also checked for a bit more complex schemes with periods +(1to2)-(2to4)+(4--(8-16) and it appears to be better - hole falls at lower t/tau, and transfer function is flattened.
                              I also cross-examined a TDI solution, and it is superior for below half t/tau, a solution which is preferable for gold. So in a case you are after gold on tough terrains, go for a TDI. The solution I'm suggesting is working the best for initial sample starting at 0.7 x t/tau (hole is just below 0.5 x t/tau) and in such case provides a peak at 1.3 x t/tau without any holes after that.

                              Long story short - there are no especially bad points to this scheme, only constraints also common to the other schemes. There is a possibility to turn this approach to a real gold extracting monster by mere placing the integrator to a very front end and gate the very coil. With initial sample starting at 5us it would enable detection of targets with tau of 1.5us to 20us (and so forth). With additional taps the null moves further towards zero.

                              I searched for an ideal weighting function to PI receivers, and instead I found that integration works fine as well.

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                              • #45
                                Here is the solution for GB as I see it. This would be a simple form with only one target and one GB pulse. I also designed a possibility of adjusting first pulse delay so that GB may assume some narrow range of values to compensate soils that are behaving different than 1/t law. All (other) pulses have exactly the same duration. A positive gate is supplied with pulses Q1 and Q8 via "wired OR" configuration for target and EF, while the same circuit is used for combining Q2 and Q3 for GB.
                                There is only one EF pulse and it should work because the gain is the same for all pulses, therefore EF duration is GB duration (2 pulses) minus target pulse duration (1 pulse) = 1pulse.
                                Duration of pulses is set with R1, while initial delay is not fixed to 1 exact pulse, but varied within, say, 0.8 to 1.2 pulse ratio using R2. T̶h̶i̶s̶ ̶e̶n̶a̶b̶l̶e̶s̶ ̶v̶a̶r̶i̶a̶t̶i̶o̶n̶ ̶o̶f̶ ̶p̶u̶l̶s̶e̶ ̶d̶u̶r̶a̶t̶i̶o̶n̶ ̶i̶n̶d̶e̶p̶e̶n̶d̶e̶n̶t̶ ̶f̶r̶o̶m̶ ̶v̶a̶r̶i̶a̶t̶i̶o̶n̶ ̶o̶f̶ ̶G̶B̶.̶ I realised that there is a dependence between R1 and R2 so there is no free lunch here
                                This is not tried yet, but should equally apply to HH and minipulse.
                                Attached Files
                                Last edited by Davor; 07-07-2014, 01:26 PM. Reason: a mistake

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