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PI coil: self resonance frequency to aim at?

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  • PI coil: self resonance frequency to aim at?

    It has been stated, that the higher the self resonance of a PI coil, the smaller items can be detected.

    Is there any rule of thumb of self resonance frequency vs the size of the target? In other words, I'm looking for a table like: 500 kHz - 1 euro coins, 1 MHz - golden rings, 2 MHz - gold nuggets etc.

    All hints appreciated.

  • #2
    Search for posts by BBsailor - he's the man who knows! I seem to remember that about 700 Mhz upwards is a fast coil able to detect gold well.

    Regards, Jim.

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    • #3
      A coil with zero capacitance won't resonate at all, thus the lower the capacitance, the higher the coil's resonant frequency.

      The coil's in-circuit resonant frequency will always be much lower though because of circuit capacitance thus the need to select low capacitance components to ensure the flyback settles quickly.

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      • #4
        Originally posted by Mullihaka View Post
        It has been stated, that the higher the self resonance of a PI coil, the smaller items can be detected.

        Is there any rule of thumb of self resonance frequency vs the size of the target? In other words, I'm looking for a table like: 500 kHz - 1 euro coins, 1 MHz - golden rings, 2 MHz - gold nuggets etc.

        All hints appreciated.
        Mullihaka,

        You want the coil discharge time constant to be 5 times faster than the time constant of the target you are attempting to detect. Higher coil/cable self resonance frequencies allow higher values of damping resistors to be used. The coil discharge time constant is determined by dividing the coil inductance by the value of the damping resistor. The actual coil discharge occurs in three stages.

        Stage 1 includes any MOSFET peak voltage clamping
        Stage 2 includes the first op amp input resstor typically 1K ohms in parallel with the damping resistor down to 0.7V, the clamping voltage of the two input stage clamping diodes.
        Stage 3 iincludes only the value of the damping resistor below 0.7V.

        Another key factor is how quickly the first op amp stage comes out of saturation.

        Smaller gold targets typically have lower time constants so a fast coil with a higher self resonant frequency is needed. Also, try to limit the coax length to minimize capacitance.

        Look to putting a high voltage diode between the MOSFET and coil to minimize the MOSFET COSS (output capacitance).

        Fast coils require a combination of many techniques to reduce coil circuit capacitance. See this article. http://www.geotech1.com/cgi-bin/page...oils/index.dat

        bbsailor

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        • #5
          Yes, for a critically damped coil there is an equation that correlates the resonance frequency with target tau for peak response. My notes are at work. However, for most targets you will always get a stronger response for shorter sample delays, and reaching the peak delay means using an induction balanced system. For a mono coil, always strive for the shortest sample delay you can get, which means the highest resonance frequency.

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