PIXIE-4 User’s Manual
V2.69
©
XIA
2015. All rights reserved.
xxxvii
Figure 6.3: Trapezoidal filtering of a preamplifier step with L=1
s and G=0.4
s.
6.3 Baselines and Preamplifier Decay Times
Figure 6.4 shows an event over a longer time interval and how the filter treats the preamplifier
noise in regions when no
-ray pulses are present. As may be seen the effect of the filter is both
to reduce the amplitude of the fluctuations and reduce their high frequency content. This region
is called the
baseline
because it establishes the reference level from which the
-ray peak
amplitude V
x
is to be measured. The fluctuations in the baseline have a standard deviation
e
which is referred to as the
electronic noise
of the system, a number which depends on the rise
time of the filter used. Riding on top of this noise, the
-ray peaks contribute an additional
noise term, the
Fano noise
, which arises from statistical fluctuations in the amount of charge
Q
x
produced when the
-ray is absorbed in the detector. This Fano noise
f
adds in quadrature
with the electronic noise, so that the total noise
t
in measuring V
x
is found from
t
= sqrt(
f
2
+
e
2
)
(3)
The Fano noise is only a property of the detector material. The electronic noise, on the other
hand, may have contributions from both the preamplifier and the amplifier. When the
preamplifier and amplifier are both well designed and well matched, however, the amplifier’s
noise contribution should be essentially negligible. Achieving this in the mixed analog-digital
environment of a digital pulse processor is a non-trivial task, however.