rd  - 1962 - ENC Conference

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drive both a 4-place counter and a circuit producing the 180 ° pulses. The number of 180 ° pulses desired can be selected with the 4-place preset (3). This preset drives a flip-flop which shuts off the 180 ° pulses until it is reset by reset B. The 4-place preset (2) is used to select the number of periods T after which the whole program sequence will repeat itself. A manual button and selector switch are provided so that the sequence will only repeat when the manual button is pushed. The 4-place preset (i) is used to select the number of the 90 ° or 180 ° pulse position with reference to which the scope trigger will be positioned. The switch showing 90 ° and 180 ° positions is used to select this pulse reference. A reversing switch is also provided so that the 90 ° and 180 ° gates may be interchanged. The gate widths are derived from univibrators which have both a coarse and fine control. In Fig. 9, the pulse and gate sequences at the various positions in the logic circuit are shown. For this particular sequence, the programmer has been set to reset on every ten basic timing units T. A normal Carr-Purcell sequence has been selected with one 90 ° gate followed by three 180 ° gates° The scope trigger has been selected to be in advance of the thi<strong>rd</strong> 90 ° pulse which actually does not occur but is indicated by the dotted line. The letters preceding each sequence refer to the positions in the block diagram of Fig. 8 at which the respective voltages may be observed. In Fig. 10, an actual oscilloscope display of a Carr-Purcell sequence is shown. In Fig. ii, the same sequence is presented except that the scope sweep amplitude has been set to display a single echo width. The scope trigger advance was referenced to the 90 ° pulse position and a value selected in terms of the clock period which would allow the complete echo to be displayed. Succeeding echoes in the Carr- Purcell sequence were then selected with the 4-place preset (i) up to a maximum of the number selected on 4-place preset (3) in the block diagram of Fig. 8. In addition to its use in displaying details of the echo signals, the flexibility of a programmer in providing a pulse at any position in the spin-echo sequence makes it very useful when used in conjunc- tion with integrating circuits for the observation of weak signals. -4-
References I. F. Bloch, Phys. Rev. 7_~0, 460 (1946). 2. J. Schwartz, R.S.I. 2__88, 780 (Oct. 1957). 3. J. C. Buchta, H. S. Gutowsky and D. E. Woessner, R.S.I. 2_99, 55, (Jan. 1958). 4. S. Meiboom and D. Gill, R.S.I. 2_99, 688 (Aug. 1958). 5. R. J. Blume, R.S.I. 3_22, 554 (May, 1961). 6. D. F. Holcomb and R. E. Norberg, Phys. Rev. 9_~8, 1074 (1955). 7. R. J. Blume, R.S.I. 3_~2, 1016 (Sept. 1961). 8. E. L. Hahn, Phys. Rev. 8_~0, 580 (1950). 9. H. Y. Carr and E. M. Purcell, Phys. Rev. 9_~4, 630 (1954). -5-
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rd  - 1962 - ENC Conference

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