DSPEC
®
Digital Gamma-Ray Spectrometer
12
pass filtering). Variable gain allows matching the range of gamma-ray signals to the range of the
ADC.
The shape and time duration of the shaping amplifier output pulse determine the electronic noise
and maximum data rate of the system. Analog systems typically use a Gaussian shape (like the
standard probability curve) or a modified triangular shape. These perform well, but the optimum
shape has an exponential rising edge, a flattop of duration about equal to the detector rise time,
and an exponentially falling edge with the same time constant as the rising edge. This shape is
referred to as a
flattop cusp
. Obtaining this shape is very difficult with analog electronics.
There is no advantage to adjusting the time duration of the output pulse to a value longer than
the minimum required. If the optimum duration is too long for the existing count rate, shorter
values are used to allow higher rates accompanied by somewhat degraded energy resolution.
Analog amplifiers typically allow two shapes and 5–10 durations (shaping times).
These systems have been in continuous development for over 30 years. Modern analog systems
include special circuits called
baseline restorers
to maintain the reference voltage level, and
pile-up rejectors
that reject signals that occur so close to another signal that they are distorted.
4.4. Analog vs. Digital
If the output of the preamplifier is sampled at a very high rate by a fast ADC, digital signal
processing can be used to perform all the functions performed by the analog shaping amplifier.
The output of the “shaping amplifier” exists as a sequence of numbers instead of a voltage
signal, but there is an exact equivalence to the analog system. The optimum shape and time
duration are the same. The tradeoff of noise and data rate are the same. The differences,
however, are many:
The digital system can generate the ideal flattop cusp. The shape of the rising and falling
edges (the “cuspiness”) are adjustable to match the detector’s noise characteristics or
improve data rate. The width of the flattop is adjustable to match the rise time.
Changing the shaping time in the analog system involves mechanical switches changing
several components. Consequently, the number of shaping times available is limited. The
shaping time in the digital system is determined by constants in the DSPEC computer code
and can be changed at will. Many more values are available.
The gain in analog systems depends on the value of electronic components and drifts with
temperature and time. Digital systems do not exhibit such changes. As the DSPEC is almost
totally digital, it drifts much less than analog systems.
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