Teachspin - Earth's Field NMR: The Instrument

Earth's Field NMR

Perhaps the best way to explain our apparatus is to examine the simplified block diagram in Figure 1.

The high Q sample coil surrounds a 125 ml plastic bottle containing a liquid rich in either hydrogen or fluorine nuclei. The sample is placed in a uniform part of the Earth's magnetic field with the coil's axis aligned perpendicular to this field. The sample coil is connected to either a dc current regulated power supply or a high-gain, low-noise, tuned amplifier, with both switching and tuning electronically controlled. The output of the tuned amplifier is presented on a storage oscilloscope for observation and measurement. At the beginning of an experiment, the sample coil is connected to the dc power supply. This supplies a maximum of 3 amperes for a pre-determined length of time. The electronic circuit then disconnects the supply, quickly dissipates the stored energy, and connects the coil to the tuned amplifier. The sample nuclei (usually protons), having been polarized in a large magnetic field created by the power supply, find themselves oriented with their net magnetization perpendicular to the Earth's field. With the polarizing field now off, this magnetization precesses in the Earth's field, producing a time varying magnetic flux through the sample coil. The time varying flux creates an emf at the coil's terminals, which is magnified by the tuned amplifier. Although the fundamental ideas behind the instrument are simple, the actual design of the apparatus has important subtleties. As the photographs show, there are actually two coils around the sample. The outer or "halo" coil is a bucking coil, designed to significantly reduce random noise pick-up from the always present stray electromagnetic fields. The bucking coil has the same turns-area as the inner sample coil. The coils are connected in series but in opposition. The output of the two coils is connected to the receiver. Since the two coils are in opposition, local noise fields produce equal and opposite emf in each coil and the net noise emf at the receiver is theoretically zero. Because the precessing magnetization is a dipole field, it couples primarily to the sample coil. This common mod rejection is essential to the outstanding signal-to-noise ratio of the apparatus. The students can study this system by rearranging the bucking coil connections.
TeachSpin offers a special stand for the coils made of wood and brass. The stand allows the coils to be moved vertically as well as horizontally.because it is essential that the sample be located in a region where the Earth's magnetic field is uniform. This requires that there be no ferromagnetic materials nearby. The instrument uses a linear full-wave rectifier and low-pass filter as an amplitude detector. Its output is proportional to the maximum amplitude of the precession signal. This detector is particularly useful for signal averaging weak signals to enhance the signal-to-noise ration.
Turning off the polarization field with different damping configurations has some subtle yet interesting physics for advanced students to consider. The instrument allows for various damping resistors to be added. It is also possible to monitor both the sample coil current and voltage. The experimenter can also listen to the precessing nuclei. The nuclear precession signals are amplified and fed into an internal loud speaker. If the local magnetic field is reasonable uniform, the "ping" can be heard for several seconds. External speakers can be used for classroom demonstrations.

Learn about the Earth's Field NMR Gradient/Field Coil System.