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  • To make it correct, don't forget the coil inductance at ~11mH and a parallel capacitor at 6n8. I found these values somewhere, but I'm not sure if Rx tank being resonant at Tx frequency makes much sense.

    I'm curious about where this analysis will end.

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    • Originally posted by Davor View Post
      I'm curious about where this analysis will end.
      Here is the SPICE analysis:
      Attached Files

      Comment


      • If you drive the coil by minimum coupling with another voltage source driven coil (instead of the voltage source), you'll see a somewhat different picture, but much closer to your real goal

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        • Originally posted by Davor View Post
          If you drive the coil by minimum coupling with another voltage source driven coil (instead of the voltage source), you'll see a somewhat different picture, but much closer to your real goal
          Hi Davor,
          I agree with you, but I think the equivalent circuit of LC tank is enough correct. Because of minimal coupling, the resistance and reactance of voltage source V1 change the coil parameters negligible.

          SPICE shows that amplitude and phase characteristic of this section is correct designed according stupid conventional block diagram of metal detectors. That block diagram requires high phase stability of preamp. This is achieved by damping of tuned circuit. Jkdj means increased bandwidth, ie increased interferences and noise.

          Note that unlike other Tesoro preamp circuits, the third harmonic of mains frequency is suppressed more than 60dB.
          Let we make noise analysis..

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          • How to make PSPICE model DD coil and potentiometer?

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            • [QUOTE=Derx;155768]How to make PSPICE model DD coil and potentiometer?[/QUO

              A DD coil would be tough to model but the (linear) potentiometer is pretty simple. Here goes:
              *********************************
              * This is the potentiometer
              **** _____
              * 1--|_____|--2
              ****** |
              ****** 3
              *
              .SUBCKT potentiometer 1 2 3
              .param w=limit(wiper,1m,.999)
              R0 1 3 {Rtot*(1-w)}
              R1 3 2 {Rtot*(w)}
              .ENDS
              ********************************

              Comment


              • Originally posted by mikebg View Post
                Let we make noise analysis..
                I did. It is quite nice for a simple aperiodic frontend. Funny but I was musing about such approach, yet in a fully differential frontend. Low impedance to squeeze juice from coils at low noise conditions.
                I replaced generic opamps with NE5532 and gave them proper power, fired noise analysis and got ~6.6nV/sqrt(Hz) when normalised for gain. Not bad at all.

                I think the aperiodic approach is much better for amateur builds in case you are building your own coils, and do not care about exact inductances. It just works.
                Attached Files

                Comment


                • Nice work, Davor. I went and tuned the RC values to obtain a peak more closely to 20kHz and used some additional circuitry to rotate phase back to where its needed, but I have to believe that Tesoro had reason for doing what they did. Meaning, all I did was to probably fix a non-problem, and without understanding repercussions.

                  Question for you: What do you have to do to normalize the noise analysis to a particular gain?

                  Comment


                  • It is simple, you just divide noise at a frequency of interest with system gain at that particular frequency. LTspice does not do that in one pass, so you'll have to do AC analysis first and see the gain at frequency of interest. To do that put "1" as AC amplitude and check gain as numeric value, not dB, fix it in "manual limits" ... click on results window, "Plot settings" -> "Manual limits" and in "Left axis" find decibels and turn them to "linear". AC amplitude 1 is OK even for devices with very small input signals, because it treats every signal as a small signal, and it does not bother checking for saturation etc.

                    Comment


                    • Davor, the noise situation in RX is much better. The width of noise band in your simulation is not correct estimated between 1kHz and 100kHz because
                      TGT signal makes modulation of carrier wave (AIR&GND signal) with maximum 16 Hz.

                      Conventional block diagram contains a band pass filter after synchronous demodulator. In this case, the demodulator can not see noise in a band wide more than 20Hz.

                      However the correct noise value is not important for design of RX coil. Important is relative noise generation by coil resistance (relative to other noise generating components - resistors and opamp). My idea is to redesign RFA to operate with increased coil resistance without significant increase of circuit noise. That means decreased weight and price of RX coil. I will simulate the idea when I have time.
                      Attached Files

                      Comment


                      • Please note that my simulation presents noise density, so feel free to multiply these results with square root of your bandwidth, and there you have it. This approach is bandwidth invariant.
                        Actually, the situation here can't go much better since a 1k ohm resistor produces ~4nV/sqrt(Hz) of noise, and this contraption is only marginally worse, just because that is the very voltage noise of the op amp.

                        The lowest you could go would be the noise produced by the coil resistance, matched by the op amp input voltage noise, and that would be something!

                        IMHO this frontend can become better only if configured as a differential one to fight common mode noise. Otherwise - superb!

                        BTW, op amps tend to introduce some THD when pressed hard on the output. In this preamp we have an op amp heavily loaded with the feedback network, while in reality the op amp input voltage noise is much larger than the equivalent feedback resistance can produce. You can observe this from 2 angles: you can put there an op amp with better noise and gain better results, or, you can increase the feedback resistances and prevent THD with strong signals ... or just leave it be.

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                        • SPICE noise analysis shows that damping resistor R1 generates more noise than coil resistance. What will happen when we remove damping?
                          Attached Files

                          Comment


                          • add strayC = 6n8 and replace C3 to 10p and repeat test?

                            Comment


                            • I've seen somewhere a schematic with much larger "stray" so that the coil forms a near resonance tank. That could be right.
                              Originally posted by mikebg View Post
                              ...What will happen when we remove damping?
                              Try adding a common mode high voltage and of course high impedance source coupled capacitively (think of wet grass), and you'll see what this resistor is for. From what I've seen, the whole circuit is designed for an opamp of much better voltage noise than the one that is found in a commercial Lobo. Maybe they had some turbo special variety with somewhat better guts?

                              Anyway, I like it. Apart from the balanced operation that it is lacking, the rest of the Lobo's solution would be my current view of an ideal frontend. It has low impedance, and (most probably) aperiodic input circuitry optimised for low noise. Nice.

                              Comment


                              • Originally posted by Davor View Post
                                Try adding a common mode high voltage and of course high impedance source coupled capacitively, and you'll see what this resistor is for. From what I've seen, the whole circuit is designed for an opamp of much better voltage noise than the one that is found in a commercial Lobo. Maybe they had some turbo special variety with somewhat better guts?

                                Anyway, I like it. Apart from the balanced operation that it is lacking, the rest of the Lobo's solution would be my current view of an ideal frontend. It has low impedance (most probably) aperiodic input circuitry optimised for low noise. Nice.
                                Hi Davor:

                                What do you mean by aperiodic input circuitry?

                                Would you also compare this frontend to TGSL/IGSL (your balanced version) and explain differences and advantages?

                                Regards,

                                -SB

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