4
3. INSTALLATION
3.1. CONNECTION TO DETECTOR
A direct connection with 93
S
or 100
S
shielded
cable should be made between the detector and the
Input connector on the preamplifier. For best
results, the length of this cable must be as short as
possible. This will not only minimize the preamplifier
noise (due to the capacitive loading of the cable)
but will also maintain the stability of the
preamplifier. The complex impedance presented to
the preamplifier input that is due to transmission
line effects acting on the detector system
impedance can disrupt the stability of the whole
system. The interconnecting cable, which acts as
an impedance transformer, must be kept as short as
possible for the system to remain compatible with
the wide bandwidth of the preamplifier. Due to
vagaries in the detector system, a definite
maximum length cannot be specified but is typically
24 inches for the 142A and 15 inches for the 142B
and 142C.
Type RG-62/U cable is recommended for the
detector to preamplifier connection. This is 93
S
cable with a capacity of 13.5 pF/ft.
After the input cable has been installed, the
electronic noise performance of the preamplifier can
be predicted by adding the capacity furnished by the
detector to the capacity of the cable. The cable
capacity can be calculated from its length and its
rated capacity per foot. Figures 2.1 and 2.2 show
typical performance at two commonly-used
amplifier shaping time constants, based on the total
input capacitance.
3.2. ENERGY OUTPUT CONNECTION TO
MAIN SHAPING AMPLIFIER
The E output of the preamplifier can be used to
drive a long 93
S
Iine to a shaping main amplifier
and is designed to be directly compatible with
ORTEC main amplifiers. It can be used with any
shaping main amplifier if a power supply is also
used to furnish the preamplifier power requirements
that are available on all ORTEC main amplifiers.
3.3. TIMING OUTPUT CONNECTION TO
TIMING MODULES
The T output of the preamplifier can be used to
drive a long, terminated 50
S
cable to a timing
module. A typical timing module is an amplifier, fast
discriminator, or a time-to-amplitude converter.
When not being used, the T output should be
terminated in 50
S
.
For a positive detector bias voltage polarity, the T
output signal polarity is negative, since the timing
channel operates noninverting with respect to the
detector output. For ORTEC ruggedized surface-
barrier detectors which require a negative detector
bias polarity, the E output may be used as the
source of a negative timing pulse or the T output
can be inverted through a suitable amplifier.
3.4. INPUT OPERATING POWER
Power for the 142 Preamplifiers is supplied through
the captive power cord and 9-pin Amphenol
connector. This connector can be attached to the
mating power connector on any ORTEC main
amplifier or 114 Preamplifier Power Supply. The
preamplifier's power requirements are added to the
operating power, requirements of the amplifier or
power supply to which it is connected.
3.5. TEST PULSE
A voltage test pulse for energy calibration can be
accepted through the Test input connector on the
142 without the use of an external terminator. The
Test input of the preamplifiers has an input
impedance of 93
S
and its circuitry provides charge
injection to the preamplifier input. The shape of this
pulse should be a fast rise time (less than 10 ns)
followed by a slow exponential decay back to the
baseline (200 to 400
:
s). While test pulses are
being furnished to the Test input, connect either the
detector (with bias applied) or its equivalent
capacitance to the Input connector on the 142.
The Test input may be used in conjunction with a
pulser such as the ORTEC 419 or 448 to calibrate
the preamplifier E Output amplitude in terms of
