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HOW TO: Make a resonant trap circuit

Resonant trap for dual-tuned 23Na-1H surface coil

Information about the coils and trap:

  • Trap is to be located on the 23Na coil in place of one of the capacitors or another break in the circuit (not across the leads).
  • Trap will be tuned to 1H frequency (our 3T case is 123.23MHz).

Making the trap:

  • wire to be used is 16-24 gauge magnet wire (copper in our case).
  1. Wrap wire around a 0.5 cm - 0.75 cm plastic screw 3 or 4 times
    • NOTE: If coil loops are too big, there is too much noise.
    • NOTE: If coil loops are too small, the inductor will block too much current.
  2. Remove insulation from ends of inductor to be used for soldering
  3. Solder the capacitor to the inductor (20 pF by Andy Nnewihe's experience -- at Stanford)

Attaching and tuning the trap:

  • Solder the trap to the coil.
  • Make sure the coil is not a completed circuit, for testing the trap.
  • Attach the trap to the network analyzer for tuning.
  • Tune the trap to the frequency of interest. ω = 1/sqrt(LC)
    1. For large changes in frequency, change the capacitor.
    2. For small changes in frequency, carefully change/manipulate the inductor (i.e.: open/close rings, add/remove a winding, etc.).
  • Epoxy the setup only when the desired frequency is reached. Epoxy is difficult to remove cleanly.
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