7
Fig. 4.2. Output Rise Time Measurements.
CAUTION
Do not use a metal screwdriver for this
adjustment; there is a possibility of high-bias
voltage leakage on the printed circuit that could
cause a shock.
the slope of the leading edge is maintained over the
whole dynamic range of the preamplifier, and since
discriminator thresholds are typically set well below
1.0 V.
In Fig. 4.2, the rise time of the T output appears to
be longer than that for the E output but this is not
necessarily true. The 10% and 90% check points on
the E rise time are based on the E
final
level, which is
less than the initial overshoot. On the other hand,
the 10% and 90% check points on the T rise time
are based on the peak level of the initial overshoot,
which is relatively more than the final value for the
E output.
4.5. COMPENSATION ADJUSTMENT
A bandwidth compensation control is accessible to
the user of the 142 to optimize the rise time of the
T and E output signals for a change of detector
capacitance. Although this is not necessary for
energy measurements, a typical timing experiment
will provide the best timing resolution when the rise
time is optimized with this control. The adjustment
is accessible through an opening in the bottom of
the case and should be adjusted with a small plastic
screwdriver or a TV tuning tool.
When the 142 Preamplifier is shipped from the
factory, the compensation adjustment has been set
for the specified rise time resolution for a 0-pF input
capacity; in the 1428, the adjustment has been
made for a 100-pF input capacity; in the 142C, the
adjustment has been made for 400 pF. For optimum
results for other input capacities, the control should
be adjusted under actual operating conditions.
If the control has been adjusted for optimum
bandwidth for a specific input capacity and the input
circuit is then changed to provide less capacity,
control readjustment is necessary so that the
preamplifier will not oscillate. If the input capacity is
increased from the value for which the adjustment
has been made, the preamplifier should be stable
and should not oscillate.
In the 142B and C there is a ferrite bead on the lead
between the input capacitor and the first amplifier
stage. This bead will permit the use of input cable
lengths up to about 15 in. When the input cable
length is appreciably less than 15 in., this bead may
be removed and the rise time characteristics may
be improved by a factor of as much as 30%. Also,
the experimenter may use the bandwidth control to
underdamp the preamplifier to obtain even faster
rise times than those that are specified.
4.6. INPUT PROTECTION
A provision is built into the preamplifier to protect
the input FET stage from damage when high-
voltage transients are applied to its input. These
transients can result from any one or more of many
causes, including detector breakdown, moisture
condensation on the input connector, short circuits
or uncharged capacitance connected across the
input while bias is being applied through the
preamplifier, or disconnection of a bias voltage
without first reducing it gradually to zero.
The protection circuit is installed in the preamplifier
when the unit is shipped from the factory. Although
it offers protection to the FET, it also causes some
degradation of the noise performance of the
