Time-share modulation

Another development in modulation as applied to NMR is time-share modulation (Anderson, 1962 Grunwald, et al., 1962 Baker, et al., 1965 Boden, et al., 1968). This is a variation of amplitude modulation (AM) in which the transmitter produces an rf pulse while the receiver is pulsed off. Then, when the rf pulse is off, the receiver is turned on to detect the NMR free precession signal. Although time-shared modulation is a hybrid between cw and pulse methods, we discuss it because the apparatus for pulse NMR can be used for it with trivial modifications. 

All modulation schemes produce sidebands that are removed from the main signal by the frequency Av of the modulating signal. Of course the frequency separation between the centerband and the sidebands must be at least as large as the spectral region you wish to observe. 

In time-share modulation, illustrated below, rf switches are employed to alternately turn on the transmitter and the receiver. (A slight delay between turning the transmitter off and the receiver on may be necessary to eliminate any pulse feedthrough into the receiver.) In this method of modulation, 

When field modulation is applied to cw NMR, the spectrum is subject to distortion if the amplitude of the modulating field becomes an appreciable fraction of the line width. Small 

Pulse FT NMR does not suffer from the same problems as the standard cw NMR but it suffers from another the early 

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E. B. Baker, L. W. Burd, and G. N. Root, "Time-sharing modulation at 200kc applied to broad and narrow line NMR for base-line stability," Rev. Sci. Instrum. 36, 1495-1498 (1965). 

Time-sharing can be used in a broader context than just providing the necessary modulation. For example, it can be... 

The remarkable stability and eontrollability of NMR speetrometers penults not only the preeise aeeiimulation of FIDs over several hours, but also the aequisition of long series of speetra differing only in some stepped variable sueh as an interpulse delay. A peak at any one ehemieal shift will typieally vary in intensity as this series is traversed. All the sinusoidal eomponents of this variation with time ean then be extraeted, by Fourier transfomiation of the variations. For example, suppose that the nomial ID NMR aequisition sequenee (relaxation delay, 90° pulse, eolleet FID) is replaeed by the 2D sequenee (relaxation delay, 90° pulse, delay i -90° pulse, eolleet FID) and that x is inereased linearly from a low value to ereate the seeond dimension. The polarization transfer proeess outlined in die previous seetion will then eause the peaks of one multiplet to be modulated in intensity, at the frequeneies of any other multiplet with whieh it shares a eoupling. 

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Study of the cellular modulating effects of NO donors is new and exciting, and many questions remain to be answered. For example, do all NO donors share these actions equally, or do potency differences exist among the various chemical classes Time- and dose-dependent aspects (e.g., the potential for tolerance development) also remain to be defined, but these studies strongly suggest that the therapeutic utility of NO donors may expand beyond their traditional hemodynamic uses to include cytoprotec-tive, antiproliferative, and other actions as well. 

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