Reduction of resistance value. If only front-chopped impulse voltages of
short duration ( 1 µsec) have to be measured, a further reduction of R is
possible if the impulse generator has high stored energy and the waveshaping
front resistors (R1 in Fig. 2.26) are of low value. The heat dissipation problem
is then solved only by the chopping. It is essential, however, to reduce the
inductive time constant L/R of the resistors as far as possible. For assessment, we have to refer to the equivalent circuit, shown in Fig. 3.32, and
the relevant transfer properties. The numerical evaluation of eqn (3.75), an
example of which is given in Fig. 3.35, shows the appearance of oscillations
in the USR with too low resistance values, although L/R was kept constant
as well as C
e and Cp. The reasons for this instability can easily be explained
using eqn (3.75). Although the damping factor expat) of the infinite series
remains constant, the hyperbolic functions will change to trigonometric ones,
depending upon the series number k. The most efficient term within the series
is the first one (k D 1). For this term, the transition takes place if bk becomes
complex. Hence,