NMR Techniques at Liquid Helium Temperatures - Low ...

6 Equipment tests 44
z (cm)
11
10
9
8
7
6
5
4
3
2
1
0
-1 0 1
r (cm)
6 mm
Top NMR coil
H = 31.8 mT
L
f = 1.03 MHz
L
3 He-B
3 He-A
3 He-B
Bottom NMR
H = 29.8 mT
L
f = 0.965 MHz
L
Bottom plate
with orifice
Figure 6.5: The old experimental setup for NMR measurements of 3 He during
the years 2005 to 2007. The difference to present setup is the absence of the coil
formers and the cryogenic preamplifiers. Instead, the pick-up coils are wound
directly on the quartz glass sample cylinder and the signal is read through a weak
inductive coupling provided by a couple of turns of wire around the coil windings.
is better shielded and less prone to vibrations due to better isolation and vibrationpreventing
design, e.g. semi-rigid cables, screwed connectors, and air bearings.
Some of the connections throughout the circuitry were also remade before the
actual measurements.
In spite of the drifts and the jumps we were able to measure the approximate
base noise of the signal at different Vexc and Vgs. Step-by-step testing of all
the devices was done to find the source of the noise. With only excitation generator,
compensator, lock-in amplifier, room temperature amplifier, and the bias
voltage source connected, the noise level was found to be below 1 µV. With the
same settings, after connecting the resonance circuit, but not the low temperature
preamplifier, the noise was 2-4 µV and the jumps emerged (10-70 µV). The signal