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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).
- 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.
- Remove insulation from ends of inductor to be used for soldering
- 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.
Epoxy the setup only when the desired frequency is reached. Epoxy is difficult to remove cleanly.
- For large changes in frequency, change the capacitor.
- For small changes in frequency, carefully change/manipulate the inductor (i.e.: open/close rings, add/remove a winding, etc.).