This is a 3-channel NMR system.
- If the system is OFF, then follow the steps on UVA-NMR system start-up.
- Locate the NMR computer's screen (back of the rack), and change the mode to "NMR-tune" mode on the PDP software.
- Ensure that you identified the channel mapping between the Q-meter box and the target insert in the cave.
Once you identified it, please write it on the Physical Logbook with the date and time. Then, ask the "Target Expert" to add a uva-elog entry regarding that. - This is the 3-channel Q-meter box located on top of the UVA-NMR rack on the cryoplatform (see Figure 1).
Figure 1: Front-Side of the Q-meter box - Locate the back of the Q-meter box and identify where the lambda/2 cables connect to the Q-meters.
NMR signal cables = lambda/2 cables.
You can see there are lambda/2 cables connected to three Q-meters in the following picture.
Figure 2: Back-Side of the Q-meter box - Select the channel you want to tune/re-tune.
- The first step is the "Diode mode tune".
- Connect the "Diode" connecter (see Figure 1: there are three ports labeled as "Diode") to the Oscilloscop's CH2 using a BNC-coax cable.
You will see a horizontal (or nearly horizontal) line on the Oscilloscope screen. - While keeping an eye on the oscilloscope, disconnect that lambda/2 cable from the Q-meter. You should see the signal (horizontal line) jump up (above 3V).
- Then, while keeping an eye on the oscilloscope, connect the lambda/2 cable back to the Q-meter's port. If you see the signal drops down below 1V, then it indicates a good tune for now (need fine tuning).
- You can change the resolution of the oscilloscope to see the shape of the signal on the oscilloscope screen.
- There are two tuning parameters: length of the cable & capacitance of the system (Note: they are coupled)
Use a small screw-driver to change the capacitance of the variable-capacitor (check the Q-meter) for coarse adjustment and there is a knob for fine-tuning.
Looking at the line-segment appears on the oscilloscope screen, the length of the lambda/2 cable should be adjusted as,
i) If Left-hand-side is higher > "Lengthen" the cable
ii) If Right-hand-side is higher > "Reduce" the length of the cable
until we obtain a nearly horizontal (ideally up-side-down smiley face shape) line with the DC voltage less than ~1V (this value is ~3V for the NMR test stand in the counting-house). - Disconnect the BNC-coax cable from the oscillocope's CH2 and connect the BNC-coax cable from the signal (see Figure 3).
Figure 3: The BNC-2090 input channels (bottom left):
An example of CH2 is connected to the oscilloscope.
(You can see the "Diode" and "Signal" ports on the Q-meter box in the background) - Press "DC-convert" button (red-square) on the DIO-box panel, to get rid of any DC off-set when changing from Diode mode to Phase mode.
Figure 4: "DC-convert" button (red-square) - Change the Phase cable as,
i) If Left-hand-side is higher > "Lengthen" the cable
ii) If Right-hand-side is higher > "Reduce" the length of the cable
In order to see the signal on the oscilloscope, you will need to press the "DC-convert" button every time you change the phase cable length. - Once you complete the Step 15, then the signal should look like as it is shown in Figure 5.
Figure 5: An example signal after both "Diode" and "Signal" tunes. - Now the channel that you tuned is "ready" for a baseline. If you are planning to take the baselines for other channels, then proceed with tuning the other channels by following Steps 7 to 16.