Service manual

1
ORTEC MODEL 575A AMPLIFIER
1. DESCRIPTION
1.1. GENERAL
The ORTEC 575A Amplifier is a single-width NIM
module that features a versatile combination of
jumper-selectable pulse-shaping characteristics.
The amplifier has extremely low noise, a wide gain
range, and excellent overload response for
universal application in high-resolution
spectroscopy. It accepts input pulses of either
polarity. Input may originate from germanium or
silicon semiconductor detectors, from scintillation
counters with either fast or slow scintillators, from
proportional counters, from pulsed ionization
chambers, from electron multipliers, etc.
The unit has an input impedance of ~1000
and
accepts either positive or negative input pulses with
rise times <650 ns and fall times >30
s. Three
integration and differentiation time constants are
jumper-selectable on the printed wiring board
(PWB) to provide optimum shaping for resolution
and count rate. The differentiation network has
variable pole-zero cancellation that can be adjusted
to match preamplifiers with decay times >30
s.
The pole-zero cancellation drastically reduces the
undershoot after the differentiator and greatly
improves overload and count rate characteristics. In
addition, the amplifier contains an active, filter-
shaping network that optimizes the signal-to-noise
ratio and minimizes the overall resolving time.
The amplifier has unipolar and bipolar BNC outputs.
The unipolar output is used for spectroscopy in
systems where dc coupling can be maintained from
the 575A to the analyzer. A BLR (baseline restorer)
circuit is included in the unit for improved
performance at all count rates. Baseline correction
is applied only during intervals between input pulses
and automatically selects a discriminator level to
identify input pulses. The unipolar output dc level is
within the range of
5 mV to +5 mV. This output
permits the use of the direct-coupled input of the
analyzer with a minimum amount of interface
problems.
The 575A can be used for constant-fraction timing
when operated in conjunction with an ORTEC 551,
552, or 553 Timing Single-Channel Analyzer. The
ORTEC Timing Single-Channel Analyzers feature
a minimum of walk as a function of pulse amplitude
and incorporate a variable delay time on the output
pulse to enable the timing pick-off output to be
placed in time coincidence with other signals.
The 575A has complete provisions, including power
distribution, for operating any ORTEC solid-state
preamplifier. Normally, the preamplifier pulses
should have a rise time of 0.25
s or less to
properly match the amplifier filter network and a
decay time >30
s for proper pole-zero
cancellation. The input impedance is 1000
.
When long preamplifier cables are used, the cables
can be terminated in series at the preamplifier end
or in shunt at the amplifier end with the proper
resistors. The output impedance is ~0.2
, and the
output can be connected to other equipment by a
single cable going to all equipment. The cable must
be shunt-terminated at the far end. (See Section 3
for further information).
1.2. POLE-ZERO CANCELLATION
Pole-zero cancellation is a method for eliminating
pulse undershoot after the differentiating network.
In an amplifier not using pole-zero cancellation
(Fig. 1) the exponential tail on the preamplifier
output signal (usually 50 to 500
s) causes an
undershoot whose peak amplitude is roughly
determined from :
undershoot amplitude
differentiated pulse amplitude
=differentiation time
peramplifier pulse decay time
For a 1-
s differentiation time and a 50- s pulse
decay time the maximum undershoot is 2%, and
this decays with a 50-
s time constant. Under
overload conditions this undershoot is often
sufficiently large to saturate the amplifier during a
considerable portion of the undershoot, causing
excessive dead time. This effect can be reduced by
increasing the preamplifier pulse decay time (which
generally reduces the counting rate capabilities of
the preamplifier) or compensating for the
undershoot by providing pole-zero cancellation.