Transmitter coils

Transient Time-domain signal (FID) acquired in an FT experiment. Transmitter Coil of wire and accompanying electronics from which Rf energy is applied to the NMR sample. 

Figure 10.3 Diagram of an NMR experiment. The sample lies between the poles of a powerful magnet, and is spun rapidly around its long (x) axis in order to compensate for any unevenness (inhomogeneity) of the magnetic field. Radio frequency receiver coils form a helix around the x-axis, and transmitter coils spiral around the y-axis.

The second RF field may be introduced into the high-resolution NMR spectrometer probe either by winding an extra transmitter coil (11) or by double-tuning the existing transmitter coil. (12, 13)... 

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... 

In addition to the difficulty of finding the optimal r, the JCP experiment also suffers from extreme sensitivity to the HaHa match. Moreover, the original experiment required both rf fields (Si and H) to be highly homogenous, preferably created by the same transmitter coil, to keep the same ratio of the two fields within the whole active volume of the sample147. To reduce the sensitivity of the enhancement to the HaHa condition and cross-relaxation time, a modified, refocused JCP experiment (RJCP) was suggested151. In this experiment the spin-lock field on the silicon resonance is interrupted at r = j for a duration corresponding to a conventional 90° pulse, and later (r = /) the spin-lock field on protons is similarly interrupted both fields then remain on until r = 2//. 

The principle of operation of the EM method is shown in Figure 10. The basic instrument consists of two coils and an electronics module. The transmitter coil is separated from the receiver coil by a specified distance. When energized, the transmitter coil induces circular eddy current loops into the... 

F or the purposes of a discussion related to an experimental spectrometer, it will be assumed that the construction of its probe is based on the common, crossed-coil146 arrangement, in which the static magnetic field (H0) is applied along the z axis, a transmitter coil is oriented... 

In c.w.-n.m.r. spectroscopy, a relatively weak, but rapidly oscillating, magnetic field is produced on the x axis by the application of a continuous, low-powered radiofrequency (r.f.) to the transmitter coil(s). As this radiofrequency approaches the resonance frequency, the magnetization vector is very slightly tipped out of the z axis, and precesses about this axis. When this frequency of precession is matched by the r.f. applied (the resonance condition), some of the individual, nuclear moments undergo transitions to the less-stable energy-level represented by precession about the — z direction, accompanied by absorption of energy from the transmitter coil. 

Figure 3.3. Orientation of (a) the receiver coil (attached to ammeter A) around the y axis and (f>) the transmitter coil (attached to rf oscillator) around the x axis.

From the foregoing discussion we can list the basic components of an NMR spectrometer. There will be a magnet to generate B0, an rf oscillator to generate B, in the transmitter coil, a receiver coil to pick up the signal, the electronics (including a computer and plotter) to turn the signal into a... 

In a pulsed-mode NMR experiment, which is performed at both constant magnetic field and constant rf frequency, rf radiation is supplied by a brief but powerful computer-controlled pulse of rf current through the transmitter coil. This monochromatic (single-frequency) pulse, centered at the operating frequency v0, is characterized by a power (measured in watts and controlling the magnitude of B,) and a pulse width (tp), the duration of the pulse measured in microseconds. However, as a direct consequence of the uncertainty principle [Eq. (1.6)], this brief pulse acts as if it covers a range... 

Passage of a brief but powerful pulse of rf current through the transmitter coil (whose axis lies in the xy plane) gives rise to a secondary oscillating magnetic field (Bi) perpendicular to and precessing around B0. The frequency of B is set to match the precessional frequency of the specific isotope (e.g., I3C) to be observed. 

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