rd  - 1962 - ENC Conference

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 pro-
grammer 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 third 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.
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