Sensor Coil

The purpose of the sensor coil is to acquire the proton precession signal from the sample, and to provide the amplifier with as much of the signal as possible.

4.1  Sensor coil requirements

4.2  How the coils are wired together

It’s important to wire the two coils together correctly. If wired incorrectly, the ambient noise will not be diminished, but amplified. The two coils are wound in opposite directions with respect to each other and connected in series as shown in figure 16.

Figure 16:  How to connect the two coils together.

Before permanently wiring them together, it’s a good idea to first temporarily connect them as you think they should go, then check with an oscilloscope whether they do indeed diminish environmental noise. One way to do this is, is to place the two coils inside the polarizing coil, then connect the polarizing coil to a signal generator. If connected correctly, this signal will be greatly reduced when viewing the output of the connected coils with an oscilloscope.

4.3  How to connect cable to coils

The sample coils are connected to the amplifier via a two conductor 22 AWG shielded cable. Figure 17 below indicates how the cable is connected to the coils.

Figure 17:  How to connect a shielded cable to the coils.

The cable shielding is connected to the ground of the amplifier, as well as the center of the two coils. The two conductors of the cable are connected to opposite ends of the coils.

4.4  Specifications of sensor coil

4.5  Sensor coils parts list

4.6  Construction of sensor coils

In constructing the sensor coils, it is important to keep in mind that the more identical the two coils are, the better they will be at canceling environmental noise. Especially, the total number of turns should be the same. One way to check how similar the coils are, is to weigh the coil forms before they are wound, then weigh them after.

Steps for constructing the sensor coils:

  1. Cut the acrylic to the dimensions given in the parts list.

  2. Using an acrylic solvent, glue the coil parts together as shown in figure 3. Ensure that the sample bottle can slide into the top of at least one of the coils (the rings on the top of the tube should not get in the way of the bottle).

  3. Weigh the two coil forms.

  4. Wind the coils in opposite directions using 22 AWG copper magnet wire. See figure 16 above.

  5. Weigh the two coils.

  6. Compare the weight differences between the wound coil and empty coil forms. As necessary, remove wire loops to make the weight differences as identical as possible.

  7. Use quick drying epoxy applied to the ends of the coil to prevent the windings from coming undone.

  8. Wire the coils together so that they cancel environmental noise. See section "How the two coils are wired together".

  9. Epoxy the two coils together at their base, as shown in figure 4, making sure the two coils are parallel to each other.

  10. Using a scrap piece of flat acrylic, epoxy on a stabilizer (ensures that the sample coils maintain a fixed position as the polarizing coil is tilted) as shown in figure 20.

  11. Put more epoxy around the coils as needed to make them more mechanically stable.

  12. Attach the 2 conductor shielded cable as described in section 4.3.

  13. Attach the spade terminals to the cable.

Figure 18:  Sample coil forms.

Figure 19:  Wound sample coils with sample bottle.

Figure 20:  Sample coils with stabilizer.

Figure 21:  Polarizing coil with sample coils inside, on tiltable platform.