Service manual

7
4. OPERATING INSTRUCTIONS
Two principal functions may be furnished by the
ORTEC Model 579 wideband Fast-Filter Amplifier,
depending on the details of the system in which it is
installed. The 579 may be used to linearly amplify
a small amplitude range into one that is better
suited to the requirements of a subsequent
instrument in the system; the gain can be set at any
level from X0.9 to X500 for this purpose. The
shaping time constants that select integration and
differentiation circuits in the 579 can be set to
normalize a pulse rise and decay time to optimize
timing measurements. In addition to these basic
functions, the 579 can also be used to invert the
pulse polarity if desired and to correct for the pole-
zero effect from the preamplifier in the system.
There are no typical control settings that can be
suggested for operation, since each application of
the 579 will require a different combination of
functions. After the system has been installed, use
an oscilloscope to observe the waveforms at the
input and output of the 579 and adjust its controls to
optimize its operation, considering the functions
that are required of it.
Generally speaking, the Integrate time constant can
be selected so that the rise time of the output
pulses is normalized at a rate that is slower than the
rise times of the input pulses. This function is of
greatest value when the pulses originate in a large
detector so that they generate a wide variety of rise
times and are difficult to observe for timing
measurements. The Differentiate time constant is
also selectable and determines the total interval
before the pulse returns to the baseline and allows
a new pulse to be observed. The combination of
integration and differentiation time constants also
contributes to the amount of electronic noise that is
seen in the system, so the resulting waveforms
should be considered from each of these points of
view and adjusted for best results.
Care must be used to determine the best BLR
threshold. The Busy LED on the front panel and
Busy Output on the rear panel facilitate adjustment.
The best way to adjust the BLR threshold is to first
select the desired gain and filter settings and adjust
the P/Z (pole-zero) with the detector connected to
the 579 input. Next, remove or block the radiation
source so that only background radiation strikes the
detector. Lower the BLR threshold by turning the
potentiometer counterclockwise until the BLR LED
begins to flicker.
A more accurate BLR threshold adjustment can be
made by connecting the 579 Busy Output to a
counter/timer, such as the ORTEC Model 871, and
adjusting the BLR threshold for approximately 200
cps at the Busy Output. The Busy Output indicates
the time when the output signal is not being
sampled by the gated baseline restorer.
If the BLR threshold is too low, there is a possibility
of excessive noise on the baseline and a baseline
shift at low counting rates. When the BLR threshold
is set too high, a premature baseline shift will occur
as count rate increases. These comments are
generally true for any instrument having a gated
baseline restorer.