Physics and Engineering of Radiation Detection

494 Chapter 8. Signal Processing

Ci

Cd

Rd

Ri

Signal In Signal Out

Signal In Signal Out

(a)

(b)

Low Pass Filter High Pass Filter

Low Pass Filter High Pass Filter

Figure 8.4.2: (a) Conceptual design of a band pass filter. (b) A

simple but realistic band pass filter consisting of a RC integrator and

a CR differentiator.

external circuit impedances. Therefore even if the preamplification is not required,
it is a good idea to connect the detector output directly to a preamplifier with unity
gain. It should however be pointed that in principle it is possible to avoid using a
preamplifier if the output signal is large enough to be transported and processed.
Whether a preamplifier is used or not, a main amplifier at a later stage is almost
always present. Generally one uses a shaper that amplifies the signal as well. In fact
almost all active shapers can also amplify the signal. The basic working principle
of such a combination is the same as we have already discussed in the sections on
preamplifiers.

8.6 Discrimination

Often it is desired to check whether the analog output signal is above a predefined
threshold or not. This is done through adiscriminatormodule, which accepts an
analog input and produces a digital output. It has a comparator circuitry that
compares the input voltage to a preset voltage. The preset voltage, also called the
discriminatorthreshold, can normally be adjusted through a potentiometer. If the
input pulse amplitude is greater than the preset threshold, the output logic level
becomes active, otherwise it remains in its previous state. Now it is also possible to
design a discriminator that has a high threshold level as well. The output of such a
module would become high only if the input lies between two predefined thresholds.
This concept is graphically depicted in Fig.8.6.1.
The output of a discriminator is binary. That is, it can be either high or low.
Definition of low and high is arbitrary because one can define any voltage level
to represent high and any other to represent low. For a system that is designed
to work on a certain logic definition, this arbitrariness does not pose a problem.
However in expandable systems, where it is desired that modules made by other