Sweep coils

The available NMR instruments are classified according to the RF employed. Instruments can utilize different RFs. Examples are the 60-MHz and the 100-MHz instruments. Different frequencies require different field strengths. For example, a 60-MHz instrument requires a field strength of 14,092 G (the gauss (G) is a unit of field strength), while a 100-MHz instrument requires 23,486 G. In the traditional instruments, the field strength is varied over a very narrow range. To accomplish this variability, a pair of sweep coils are used as described below. 

Seven components magnet, sample holder, RF generator, RF detector, sweep generator, sweep coils, and data system. See Section 10.4.2 for the function of each. 

A CW NMR spectrometer (Figure 3.14) consists of a control console, magnet (typically electromagnets or permanent magnets are used), and two orthogonal coils (transmitter and sweep coils) of wire that serve as antennas for rf radiation. One coil is attached to an rf transmitter and the other coil is the rf amplifier (pickup) coil and is attached to the detection electronics. 

FIGURE 3.14 Schematic diagram of a CW NMR spectrometer. The tube is perpendicular to the z axis of the magnet A, sample tube B, transmitter coil C, sweep coils D, receiver coil E, magnet... 

Figure 2. Image-dissector photomultiplier. (1) Sweep-coil electronics (2) Photomultiplier power supply (3) Focus-coil electronics (4) Display (5) Signal amplifier. (69).

Fig. 1. Schematic diagram of NMR spectrometer (a) sample tube (5) r.f. input coil (t) magnet sweep coils ...

Fig. 1. 249.9-GHz FIR-ESR spectrometer. A, 9-T magnet and sweep coils B, phase-locked 250-GHz source C, 100-MHz master oscillator D, Schottky diode detector E, resonator and modulator coils F, 250-GHz quasioptical waveguide G, power supply for main coil (100 A) H, current ramp control for main magnet I, power supply for sweep coil (50 A) J, OC spectrometer controller K, lock-in amp for signal L, field modulator and lock-in reference M, Fabry-Perot tuning screw N, vapor-cooled leads for main solenoid O, vapor-cooled leads for sweep coil P, He bath level indicator Q, He transfer tube R, bath temperature thermometer S, " He blow-off valves. [From Lynch et al. (1988), by permission of the AIP.]...

Fig. 8.1.1 [Mal3] Principle of STRAFl imaging. The STRAFl plane is defined by a region of constant field gradient in the fringe field outside the magnet. The plane can be shifted through the sample by energizing sweep coils, or the sample is moved through the STRAFl plane for pointwise acquisition of projections.

The transmitter coil, which is placed at right angles to the sweep coils and is used to generate the exciting field Hi. 

Continuous wave NMR spectrometer (a) sample tube (b) magnet (c) sweep coils (d) receiver coil (e) transmitter (f) amplifier (g) oscilloscope (h) recorder... 

FIGURE 16.18 A diagram of a standard NMR spectrometer. The sweep coils vary the magnetic field in small amounts while radio waves bombard the sample. Any absorption of radio waves will be detected by the detector circuit. The samples are usually spun to minimize inhomogeneities. 

Field profile with sweep coil energised... 

It can be seen from eq. (8.4.1) that the resonance condition can be met either by changing the frequency of the secondary alternating magnetic field at constant magnetic field B, or the induction variation B at constant frequency. Both possibilities can be realized in modem NMR apparatus. Sometimes it is easier to change the field (by means of sweep coils c in Figure 8.8a) and use the constant frequency. The majority of devices work according to these principles actually, results do not depend on the choice of measurements. 

Figures 8.8 Schemes of NMR spectrometers, (a) An original old spectrometer scheme of the continuous wave (a sample tube b magnet c sweep coil d receiver coil e transmitter f amplifier g register unit h recorder), the electromagnet poles are also seen, (b) Section of a more advanced NMR spectrometer, the nitrogen and helium vessels with the superconducting magnetic coil are depicted. A sample is introduced from above however, the device (probe), inserted from below, realizes the connection of sample measurement coils with the register system. CM position of the sample investigated.

---