Advanced Measurement Technology 142AH Manual de funcionamiento y servicio - Página 13

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Fig. 4.2. Typical Simultaneous E and T Output

Signals.

due to slewing effects and large signal bandwidth

considerations within the charge loop, the large-

signal rise time is slightly longer than that

specified at the E output for an amplitude of 1.0V

unterminated (0.5 V terminated). Again, however,

this has no impact on a typical timing experiment

since 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

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 adjustment is

accessible to the user of the 142AH. This control

can be adjusted through the bottom of the case

without opening the unit. It is used to tune the

preamplifier to the particular detector that is

connected in order to provide the fastest optimal

rise time of the T and E output signals for the best

Fig. 4.3. Output Rise Time Measurement.

timing resolution when the rise time is optimized

with this control. Because of the high voltages

that are present inside the case when the

preamplifier is operating, a small plastic

screwdriver or a TV tuning tool should be used for

this adjustment.

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.

final

When the 142 AH Preamplifier is shipped from

the factory, the compensation adjustment has

been set for the specified rise time resolution for

a 0-pF input capacity. 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.