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Gas-tube spark arrestor as RF pulse-modulator?!

Discussion in 'Electronic Design' started by Bill Beaty, Mar 15, 2013.

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  1. Bill Beaty

    Bill Beaty Guest

    I find some SMT mystery components on a few-watts RF pulse board for
    an NMR probe driver. Little white ceramic cylinders with gold end
    caps, maybe 0.1" diameter. But they're wired *across* a 50-ohm pcb
    trace, and driven by some 500V FETs via a resistor chain and one HV
    capacitor. WTF?

    Huh. What if these are spark gaps? If the designer chose several
    different voltages, then one DC pulse could switch the 800MHz signal
    through and back out. A higher DC pulse would short the RF to ground
    through a different spark gap. But why not just modulate the RF at
    signal level using diode switching?
     
  2. Bill Beaty

    Bill Beaty Guest

  3. A single coil NMR? (Where the same coil does the drive and
    detection.)

    I so then they could be to protect the detctor amp during the RF
    pulse. You can get some wicked voltages during the pulse. An early
    NMR coil/ amp that I made would sometimes arc over to a near by
    ground.

    George H.
     
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