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  • I can post a schematic, but in fact it will be two schematics - a delta against the original so that the existing boards can be utilised or upgraded, and a clean one. Fe channel correction performs so good, and you only need to violate two LM339-s to enjoy it. I don't know - maybe the best way to present everything would be to go through mods and explain what they do.

    Regarding EMI, I think this area is quite quiet, but truth is - I have no other rig to compare this one with. I'd say most of the chatter is self-produced, mostly when batteries are running on fumes. This would be a weak spot of this design. I'd say that an oscillator that would run more symmetric would kill most of the chatter. I kinda expected this - it is the PWM problem.

    In a moment of epiphany I realised a perfect mount for the device on the shaft. So it became complete. OK, a few washers wouldn't hurt, but a nut and a screw just proved the concept.

    I have a silly notion of giving names to all devices at home, and i christened this one "Raskov 2012". Have a look (temporary straps are still on the probe):
    Attached Files

    Comment


    • Originally posted by Davor View Post
      I can post a schematic, but in fact it will be two schematics - a delta against the original so that the existing boards can be utilised or upgraded, and a clean one. Fe channel correction performs so good, and you only need to violate two LM339-s to enjoy it. I don't know - maybe the best way to present everything would be to go through mods and explain what they do.

      Regarding EMI, I think this area is quite quiet, but truth is - I have no other rig to compare this one with. I'd say most of the chatter is self-produced, mostly when batteries are running on fumes. This would be a weak spot of this design. I'd say that an oscillator that would run more symmetric would kill most of the chatter. I kinda expected this - it is the PWM problem.

      In a moment of epiphany I realised a perfect mount for the device on the shaft. So it became complete. OK, a few washers wouldn't hurt, but a nut and a screw just proved the concept.

      I have a silly notion of giving names to all devices at home, and i christened this one "Raskov 2012". Have a look (temporary straps are still on the probe):
      Looks interesting.

      1. What exactly is the "perfect mount" and what is it made of?

      2. Also, how is the arm cradle and grip made?

      3. To evaluate EMI effect, I look at LM308 (or whatever chip you use) outputs for background noise signal, and compare the levels with the case where preamp inputs are shorted (preferably as close to chip as possible -- even jumper clips can pick up some stuff). Can you look and give approximate size of noise signals for those two cases?

      Regards,

      -SB

      Comment


      • The "perfect mount" is made using a "Pipe clamp with rubber and nut", a screw and a nut. I'm missing two washers to relieve strain on the box, but they'll come eventually. So I drilled a hole in a device box, put the screw into it it so that it protrudes outside, secured it with a nut, and screwed into a nut of a clamp. I can unscrew it after I detach the coil cable.

        Arm cradle and grip are made from a forearm crutch. Got two of them for ~13US$ with explanation that "granny does not need them any more". These come with a plastic arm cradle and grip made in one piece, and are supposed to withstand enormous weights. The rest of it are telescopic metal tubes that I discarded (for the time being). Instead I use a Φ22mm standard (literally) garden variety wooden broomstick (~2US$, new) that I trimmed a bit. Fits perfectly.

        Regarding noise ... I'm not equipped with a real lab at the moment, so all my measurements are done in situ with very basic equipment. With IGSL it means that Tx must operate in order to obtain negative voltage. What I observed while balancing a coil was a uniform noise floor at some distance (say 20dB) below the unbalanced minimum which was at below 1mVpp. For these to become meaningful, I'd have to play with spectrometer to make it scan with much narrower window, do some averaging and whatnot, but the point is - I am measuring with a laptop soundcard, and it is noisy by definition. It is fine at 1mV, but not so fine 30dB below.

        Comment


        • Originally posted by Davor View Post
          The "perfect mount" is made using a "Pipe clamp with rubber and nut", a screw and a nut. I'm missing two washers to relieve strain on the box, but they'll come eventually. So I drilled a hole in a device box, put the screw into it it so that it protrudes outside, secured it with a nut, and screwed into a nut of a clamp. I can unscrew it after I detach the coil cable.

          Arm cradle and grip are made from a forearm crutch. Got two of them for ~13US$ with explanation that "granny does not need them any more". These come with a plastic arm cradle and grip made in one piece, and are supposed to withstand enormous weights. The rest of it are telescopic metal tubes that I discarded (for the time being). Instead I use a Φ22mm standard (literally) garden variety wooden broomstick (~2US$, new) that I trimmed a bit. Fits perfectly.

          Regarding noise ... I'm not equipped with a real lab at the moment, so all my measurements are done in situ with very basic equipment. With IGSL it means that Tx must operate in order to obtain negative voltage. What I observed while balancing a coil was a uniform noise floor at some distance (say 20dB) below the unbalanced minimum which was at below 1mVpp. For these to become meaningful, I'd have to play with spectrometer to make it scan with much narrower window, do some averaging and whatnot, but the point is - I am measuring with a laptop soundcard, and it is noisy by definition. It is fine at 1mV, but not so fine 30dB below.
          Pipe clamp is nice! Better than the hose clamp which I've put to use .

          Laptop soundcard scope should be fine because typical noise voltage at LM308 output can be 20 to 40 mV with RX coil connected, and maybe 5 to 10 mV with RX coil shorted (actually short the differential inputs to the op amp). And the bandwidth is very narrow (couple hundred Hz max?), so won't exceed the soundcard bandwidth at all.

          You could probably even measure with a decent RMS AC voltmeter.

          -SB

          Comment


          • Hi SB

            Do you really mean short the input pins of the opamp? This usually makes the output slam over to one or other of the supply rails because it removes all feedback and the input offset voltage is then amplified by the open loop gain, which is a lot. What has been your experience?

            Gwil

            Comment


            • It wouldn't happen if you short the differential inputs, not directly the inputs of the opamp. Anyway, I'm not in a mood of doing any of these at the moment. I don't see how I can benefit from it. Instead, I am doing some field tests, and today was my first day at the beach. Got one almost deserted, but brim full of thrash. Found several pieces of Al foil, Al caps, no pulltabs a piece of lead, some odd piece of iron, and a piece of concrete with iron in it (at first I thought it was a hot rock of a kind).

              Anyway, when my IGSL starts chattering, it means that some old car with bad electric circuitry is passing by. Otherwise it behaves nicely. That means a permanent solution for this kind of chatter would be a noise blanker. I'll give it a thought.

              I also learned that I need to adjust the GEB gross adjustment because I was missing some quarter of a turn (I guess) at fine tuning for perfectly balancing the sea water. Also the red soil is somewhere at the very edge of my setting, so I really need to fix this. Bobbing the coil says it all - it is that easy. With sea water and red soil you can stick the ferrite up your ... you know, but it is a good reference for a start.

              Comment


              • Originally posted by Gwil View Post
                Hi SB

                Do you really mean short the input pins of the opamp? This usually makes the output slam over to one or other of the supply rails because it removes all feedback and the input offset voltage is then amplified by the open loop gain, which is a lot. What has been your experience?

                Gwil
                I think you have a point; In some tests, I possibly did short the actual pins without thinking, and I don't recall looking at the output of that amp. However, if the amp did swing to one rail, the effect would still probably be to effectively remove all signal from the input of the final LM308, which is AC coupled to the previous stage, so it would serve as some kind of comparison to the full RX-connected case. But I agree it is not really a useful test; a better comparison would be to disconnect the RX coil and short the connector terminals to see what kind of background circuit noise there is without the coil. However, even then the connector and resistors pick up significant EMI in my locale I think, so it is hard to really isolate the pure circuit noise.

                -SB

                Comment


                • Originally posted by Davor View Post
                  It wouldn't happen if you short the differential inputs, not directly the inputs of the opamp. Anyway, I'm not in a mood of doing any of these at the moment. I don't see how I can benefit from it. Instead, I am doing some field tests, and today was my first day at the beach. Got one almost deserted, but brim full of thrash. Found several pieces of Al foil, Al caps, no pulltabs a piece of lead, some odd piece of iron, and a piece of concrete with iron in it (at first I thought it was a hot rock of a kind).

                  Anyway, when my IGSL starts chattering, it means that some old car with bad electric circuitry is passing by. Otherwise it behaves nicely. That means a permanent solution for this kind of chatter would be a noise blanker. I'll give it a thought.

                  I also learned that I need to adjust the GEB gross adjustment because I was missing some quarter of a turn (I guess) at fine tuning for perfectly balancing the sea water. Also the red soil is somewhere at the very edge of my setting, so I really need to fix this. Bobbing the coil says it all - it is that easy. With sea water and red soil you can stick the ferrite up your ... you know, but it is a good reference for a start.
                  The relevance of EMI for most of us is that it can directly affect our depth tests. Perhaps not so much with your modifications, I don't know.

                  The problem seems to be that, because of that "chatter filter" we discussed at length, noise is not just "additive" to the target signal; rather, it causes breakup of weak target signals into smaller pulses; the smaller pulses cannot get past the "chatter filter", because the chatter filter has effectively a time gate that blocks short pulses.

                  So many people think there is something wrong with their circuit or coil when it could be EMI. I tested one of dfbowers TGSL mds in my workshop and get about 15 cm air depth. If I go 10 miles into the mountains, I get over 30 cm air depth.

                  In your case you get good depth, so probably EMI not a problem; I was just curious.

                  -SB

                  Comment


                  • Yeah, I get good depth even with wrong GEB I just didn't leave enough margin to account for highly mineralised soil: red soil soaked in sea salt. I couldn't get anything from the sea though - my GEB pot with poor margins didn't reach it. Waving a coil over the sea without a real ground balance produced a very funny series of tones.

                    Be assured that I noticed chatter already, and I know what you are referring to. I get it too when a car with bad electricity passes by, but I can wipe it off by adjusting Disc sense - in effect I am trading sensitivity for noiseless operation. I guess the original antichatter would wipe off small signals too because it is too slow in recovering. I get chatter, but it is a manageable nuisance.

                    There are two cures for EMI that I can think of.

                    Electronics tinkering way around this would be a noise blanker, but it would require either a tuned tank with high Q (impractical) or a delay line to be effective. Once I modded the old FT101 SW transceiver's noise blanker and got miraculous results. Can you imagine - they've put a diode in reverse so the original blanker never worked

                    The other would require a modified, differential coil, where Rx coil resembles figure 8 of a sort. There is a phasing problem with this approach, but not so much if Tx is running a differential coil too. This will have to be further examined. The idea is that differential coils are much more sensitive to the local signal than the far field, so in effect they are cancelling EMI. Guitar pickups are typically differential coils, so they don't pick up hum, ergo Humbucker pickups.

                    Comment


                    • Would be good if you now do make another coil, but this time shielded.
                      Good to compare between that and the one you are having now.
                      I have made maybe 60 TGSL's so far, maybe 70, not sure.
                      I have noticed chatters only in case when coil&machine were not matched good but not due any EMI you are talking about here.
                      TGSL design is pretty immune on EMI if done neat and properly.
                      Same case with IGSL.
                      Made only 4 IGSL's so far. The one with original Musketeer coil is rock stable and calm.
                      No EMI issues at all.
                      Yet another, with my hand made coil, tends to produce occasional chatters when Sense adjusted to maximum.
                      Is all about coil.
                      Video clip i uploaded on Youtube, showing IGSL testing, is made in my workshop; pretty noisy environment.
                      Next to the table where coil was placed, there is ADSL line; terrible source of noise!
                      For example; at same table i made video "Deus&chain" and if you watch that video you can hear Deus chattering all the time.
                      So IGSL is rock stable considering "outdoor" influences.
                      Next; i checked mods you are suggested, funny i can't get the results you are talking about here!?
                      On contrary; IGSL turns worse in behavior!?
                      Would be good if you draw schematic with your exact modifications so others here can check and test too.
                      Cheers!

                      Comment


                      • Originally posted by ivconic View Post
                        ...
                        For example; at same table i made video "Deus&chain" and if you watch that video you can hear Deus chattering all the time.
                        ...
                        Yeah, I noticed that. But also that chatter is much quieter than the target response and is not much of a problem. It also gives you some insight about the soil qualities. Proportional audio takes care of that. Funny, but that's kind of my goal. Believe it or not

                        The whole chain of channel gain stages (without preamp) has sensitivity at ~6µVpp set by the Disc sens setting at 3mV at maximum sensitivity. I wouldn't go any lower with disc sense to avoid confusing the comparators, but I know I can go with more gain. Considering the bandwidth of these stages, I know for sure I can do with more gain, and get better sensitivity, hence more depth. I can always adjust Disc sense in case of problems, otherwise - pedal to the metal. My perfect candidate for this to happen is a second gain stage, that has gain set at 20 (or less when diodes kick in). So that's what I'm playing with now. When I'm ready I'll post a video. I'm not in much of a hurry and I have other things to do as well so don't hold your breath till I do.

                        BTW, the cars that produce EMI on my li'l island are real racks, no registration or insurance... for ages, and it happens only at maximum sensitivity - not enough incentive to play with coil shielding at this point. Partly the problem was on my wrong GEB setup, but it gave me a valuable insight on what exactly chatter sounds like on my rig. 10k finetune single turn pot was a nice idea, but I need more than that to cover full span between GEB on ferrite and GEB on red soil or sea water. I'll put a 100p in parallel to the existing one so the 10k pot will cover more span, just like 20k pot would do now.

                        Otherwise, even without proper GEB I found various small objects at depth ~20cm on a beach in red soil saturated with salt. It works

                        Comment


                        • Originally posted by Davor View Post
                          Yeah, I noticed that. But also that chatter is much quieter than the target response and is not much of a problem. It also gives you some insight about the soil qualities. Proportional audio takes care of that. Funny, but that's kind of my goal. Believe it or not ....
                          Not funny at all, but true and smart! I agree.
                          Deus's chatters are least problem, because outdoor on real soil those chatters are not that noticeable nor annoying. Also can disappear when Sensitivity is lowered a bit.
                          IGSL is calmer (apparently) ... but also much weaker and less sensitive than Deus.

                          Comment


                          • Ergo the path to a much smarter IGSL. I LOVE the Fe channel and what it does. It is not just iron you "see" with it, but also Al foil by overlapping audio - when I set discrimination that way. With proportional audio I get a feeling of being immersed in a terrain of a kind, like the soil becomes transparent.

                            I think I can understand why you love Deus so much.

                            Comment


                            • Originally posted by Davor View Post

                              The whole chain of channel gain stages (without preamp) has sensitivity at ~6µVpp set by the Disc sens setting at 3mV at maximum sensitivity.
                              I was trouble shooting my a TGSL about a month ago and wound up injecting a 10 hz signal into the filter stages at the source of Tr4 and Tr5 and looking at the outputs of the LM308's. Once I got the problem fixed, the input sensitivity was about 25uV for .6Vpp at the 308's which was a huge improvement from original.

                              By "the Disc sens setting at 3mV at maximum sensitivity", are you refering to the output of the gain stages with 6 uV input?


                              I think having input sensitivity specs for a given output of the gain stages is good to have for determining how well the circuit is working. Sort of like a two way radio tech measuring .1 uV at the antenna connector for 20 db of quieting in a receiver. I know metal detectors and radios are not the same thing but the idea is the same.

                              Right now I have a same of two units to make comparisons on.

                              It looks like you are have a lot of success with your IGSL.

                              Jerry

                              Comment


                              • Originally posted by Davor View Post
                                Yeah, I get good depth even with wrong GEB I just didn't leave enough margin to account for highly mineralised soil: red soil soaked in sea salt. I couldn't get anything from the sea though - my GEB pot with poor margins didn't reach it. Waving a coil over the sea without a real ground balance produced a very funny series of tones.

                                Be assured that I noticed chatter already, and I know what you are referring to. I get it too when a car with bad electricity passes by, but I can wipe it off by adjusting Disc sense - in effect I am trading sensitivity for noiseless operation. I guess the original antichatter would wipe off small signals too because it is too slow in recovering. I get chatter, but it is a manageable nuisance.

                                There are two cures for EMI that I can think of.

                                Electronics tinkering way around this would be a noise blanker, but it would require either a tuned tank with high Q (impractical) or a delay line to be effective. Once I modded the old FT101 SW transceiver's noise blanker and got miraculous results. Can you imagine - they've put a diode in reverse so the original blanker never worked

                                The other would require a modified, differential coil, where Rx coil resembles figure 8 of a sort. There is a phasing problem with this approach, but not so much if Tx is running a differential coil too. This will have to be further examined. The idea is that differential coils are much more sensitive to the local signal than the far field, so in effect they are cancelling EMI. Guitar pickups are typically differential coils, so they don't pick up hum, ergo Humbucker pickups.
                                Yes, I've played a little with figure-8 type RX with centered TX; Aziz using "top hat" stacked coils; they should have potential against EMI -- not needed out in the wilderness. Another technique against EMI should be increasing TX power. I've played a little with resonant TX coil in that regard. All take much more experimenting.

                                -SB

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