th  - 1987 - 51st ENC Conference

MF43
DESIGN AND USE OF A PULSE SHAPER FOR HIGH RESOLUTION NMR.
Kermit M. Johnson* and Klaas Hallenga
Michigan State University, Department of Chemistry
East Lansing, MI 48824
Although the use of tailored excitation in high resolution NMR has
been suggested on several occasions, the method has been used only
sporadically due to the poor performance of existing circuitry.
The solution of the Bloch equations for perfect frequency
selective inversion by Hoult et al (I) and the possibility of computer
designed highly selective excitation makes it desirable to have pulse
shaping capability routinely available.
We have implemented a flexible pulse shaping circuit on our
spectrometer (Bruker WM-250 with Aspect 2000) which will deliver
amplitude and phase modulated pulses. The device uses two IK x 12 bit
buffers feeding two identical 12 bit D/A converters. The rate at
which the waveform is read out of the buffers is controlled by a
variable frequency clock.
The outputs of the DAC's control separately the amplitudes of
quadrature radio-frequency signals which are recombined to form a
shaped pulse which may thus vary in both amplitude and phase.
Additionally, this circuit could be easily modified to perform phase
shifting of the rf pulse after the method of Sears (2). The device is
easily loadable under computer control with any desired waveform.
Finally, no spectrometer hardware or software modifications were
required to use the pulse shaper.
Details of the circuitry, test results, and some applications to
ID and 2D NMR experiments will be demonstrated.
(I) M. S. Silver, R. I. Joseph and D. I. Hoult, J. Magn. Res. 59, 347
(198~).
(2) R. E. J. Sears, Rev. Sci. Instrum. 55, 1716 (1984).
a