NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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sealing the aluminium can surrounding the coil and submerging
it directly into liquid nitrogen.
5.4 THE PROBE HEADS
5.4.1 THE DESIGN OF THE TUNED CIRCUITS
A tuned PNMR transmitter circuit is required to produce
a high power H1 field in the sample from an rf pulse. The
Fourier transform of a rectangular rf pulse forms a distribution
of frequencies about the driving frequency the width of which
is of the order of the inverse of the pulse width. By making H1
large it follows from equation (1.9) that the pulse length is
short. - All nuclei will then be excited equally, even in solids
having broad spectral distribution functions.
After the exciting rf pulse the tuned receiver tank circuit
has to recover quickly to enable the small nuclear transient signal
to be amplified and detected. The importance of having only a
short 'dead time' during which the receiver is blocked arises
because that part of the signal near zero time makes a large contrib-
ution to the wings of the transformed lineshapes. If the signal
induced in the receiver coil of a series resonant circuit is
V sin wt, then the voltage appearing at the receiver input is
QV sin wot where Q, the quality factor, is given by wö /R. The
ringing in this circuit decays exponentially with a time constant
(66)
2Q/wo. It follows that the two conditions of maximum signal
and minimum dead time are incompatible and a compromise has to be
adopted.