Physics and Engineering of Radiation Detection

474 Chapter 8. Signal Processing

Sr

Detector

Output

Ss

Sr

Preamp

Output

Detector
Charge

Preamp
Output

Detector
Charge

(a)

(b)

Figure 8.1.9: Timing diagrams for oper-

ation of a pulsed reset charged sensitive

preamplifier (a) without and (b) with a

switch at the preamplifier input (see also

Fig.8.1.8). The diagram in part (b) is gen-

erally said to representdynamic recording,

since it does not permit loss of information

during feedback capacitor reset. All tim-

ings shown areactive-low, i.e., the level 0

corresponds to the active state.

built-in circuitry that sends a reset pulse to the transistor switch if the preamplifier
output reaches a predefined value (normally the upper limit of preamplifier’s range).
Fig.8.1.10 shows the working principle of such a preamplifier. It is also possible to
toggle the switch through an external circuit that does not take any input from
the preamplifier. Such a system is generally configured to time the operation at
regular intervals of time and therefore provides more flexibility in terms of further
data readout and processing.
The transistor based reset switches may suffer from time jitters, logic level edge
excursions, and digital charge injections from the logic unit. These problems can
be minimized, if not eliminated, by using the so calledoptical reset trigger.Inthis
scheme the resetting is done through light instead of electrical current. Certainly
such a device does not eliminate the need for timing circuitry but, by electrical
isolation from the preamplifier input, it does not cause any charge injection in the
circuit. The result is a much cleaner toggling of the switch as compared to the
electronically triggered switch. Fig.8.1.10(b) shows a typical optical feedback pulsed
preamplifier. Here a light emitting diode in the logic circuit inhibits the preamplifier
switch. The switch itself is a field effect transistor (FET).

8.2 SignalTransport

It may at first seem quite trivial to transport signal from the preamplifier, if the
detector has one, to other pulse processing units. One would think that all is needed
is a cable that takes the signal from one end to the other or a simple wireless
transmitter and receiver. However, as it turns out, there are a few points that must