ORTEC 673 Service manual

Model 673
Spectroscopy Amplifier
and
Gated Integrator
Operating and Service Manual
Printed in U.S.A. ORTEC Part No. 675590 0202
®
Manual Revision B

$GYDQFHG 0HDVXUHPHQW 7HFKQRORJ\ ,QF
a/k/a/ ORTEC , a subsidiary of AMETEK , Inc.
®®
WARRANTY
ORTEC* warrants that the items will be delivered free from defects in material or workmanship. ORTEC makes
no other warranties, express or implied, and specifically NO WARRANTY OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE.
ORTEC’s exclusive liability is limited to repairing or replacing at ORTEC’s option, items found by ORTEC to
be defective in workmanship or materials within one year from the date of delivery. ORTEC’s liability on any
claim of any kind, including negligence, loss, or damages arising out of, connected with, or from the performance
or breach thereof, or from the manufacture, sale, delivery, resale, repair, or use of any item or services covered
by this agreement or purchase order, shall in no case exceed the price allocable to the item or service furnished
or any part thereof that gives rise to the claim. In the event ORTEC fails to manufacture or deliver items called
for in this agreement or purchase order, ORTEC’s exclusive liability and buyer’s exclusive remedy shall be release
of the buyer from the obligation to pay the purchase price. In no event shall ORTEC be liable for special or
consequential damages.
Quality Control
Before being approved for shipment, each ORTEC instrument must pass a stringent set of quality control tests
designed to expose any flaws in materials or workmanship. Permanent records of these tests are maintained for
use in warranty repair and as a source of statistical information for design improvements.
Repair Service
If it becomes necessary to return this instrument for repair, it is essential that Customer Services be contacted in
advance of its return so that a Return Authorization Number can be assigned to the unit. Also, ORTEC must be
informed, either in writing, by telephone [[phone]] or by facsimile transmission [[phone]], of the
nature of the fault of the instrument being returned and of the model, serial, and revision ("Rev" on rear panel)
numbers. Failure to do so may cause unnecessary delays in getting the unit repaired. The ORTEC standard
procedure requires that instruments returned for repair pass the same quality control tests that are used for
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transit handling and must be shipped PREPAID via Air Parcel Post or United Parcel Service to the designated
ORTEC repair center. The address label and the package should include the Return Authorization Number
assigned. Instruments being returned that are damaged in transit due to inadequate packing will be repaired at the
sender's expense, and it will be the sender's responsibility to make claim with the shipper. Instruments not in
warranty should follow the same procedure and ORTEC will provide a quotation.
Damage in Transit
Shipments should be examined immediately upon receipt for evidence of external or concealed damage. The carrier
making delivery should be notified immediately of any such damage, since the carrier is normally liable for damage
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provided in making damage claims and in providing replacement equipment, if necessary.
Copyright © 2002, Advanced Measurement Technology, Inc. All rights reserved.
®is a registered trademark of Advanced Measurement Technology, Inc. All other trademarks used
*ORTEC
herein are the property of their respective owners.

iii
CONTENTS
STANDARDWARRANTY .................................................................... ii
SAFETY WARNINGS AND CLEANING INSTRUCTIONS ...........................................iv
SAFETY INSTRUCTIONS AND SYMBOLS ...................................................... v
1. DESCRIPTION ..........................................................................1
1.1. GENERAL ......................................................................... 1
1.2. GATED INTEGRATOR SPECTROSCOPY ................................................1
1.3. POLE-ZEROCANCELLATION ......................................................... 2
1.4. ACTIVEFILTER..................................................................... 4
1.5. GATEDINTEGRATOR ............................................................... 4
2. SPECIFICATIONS .......................................................................6
2.1. PERFORMANCE .................................................................... 6
2.2. CONTROLS ........................................................................ 6
2.3. INPUTS ...........................................................................7
2.4. OUTPUTS ......................................................................... 7
2.5. REARPANELCONNECTORS ......................................................... 7
2.6. ELECTRICALANDMECHANICAL ...................................................... 7
3. INSTALLATION.......................................................................... 7
3.1. GENERAL ......................................................................... 7
3.2. CONNECTIONTOPOWER ...........................................................8
3.3. CONNECTIONTOPREAMPLIFIER .....................................................8
3.4. CONNECTIONOFTESTPULSEGENERATOR ........................................... 8
3.5. SHAPINGCONSIDERATIONS .........................................................9
3.6. USEOFGATEINPUT ................................................................ 9
3.7. LINEAROUTPUTCONNECTIONSANDTERMINATINGCONSIDERATIONS....................9
3.8. SHORTINGOROVERLOADINGTHEAMPLIFIEROUTPUTS ............................... 10
3.9. INHIBITOUTPUTCONNECTION ......................................................10
3.10. BUSYOUTPUTCONNECTION....................................................... 10
3.11. CRMOUTPUTCONNECTION .......................................................10
4. OPERATION...........................................................................10
4.1. INITIAL TESTING AND OBSERVATION OF PULSE WAVEFORMS ...........................10
4.2. FRONTPANELCONTROLS..........................................................10
4.3. INPUTS ..........................................................................11
4.4. OUTPUTS ........................................................................ 11
4.5. STANDARD SETUP PROCEDURE ....................................................12
4.6. POLE-ZEROADJUSTMENTFORRESISTIVE-FEEDBACKPREAMPLIFIER ................... 12
4.7. BLRTHRESHOLDADJUSTMENT .....................................................14
4.8. GATEDINTEGRATORSETUP .......................................................14
4.9. OPERATION WITH SEMICONDUCTOR DETECTORS .....................................15
4.10. OPERATION IN SPECTROSCOPY SYSTEMS .......................................... 17
4.11. OTHER EXPERIMENTS ............................................................ 18
5. MAINTENANCE ........................................................................21
5.1. TESTEQUIPMENTREQUIRED ....................................................... 21
5.2. PULSERTEST* ....................................................................21
5.3. SUGGESTIONSFORTROUBLESHOOTING.............................................22
5.4. FACTORY REPAIR .................................................................22
5.5. TABULATEDTESTPOINTVOLTAGES ................................................. 22

iv
DANGER Opening the cover of this instrument is likely to expose dangerous voltages. Disconnect the
instrument from all voltage sources while it is being opened.
WARNING Using this instrument in a manner not specified by the manufacturer may impair the
protection provided by the instrument.
CAUTION To prevent moisture inside of the instrument during external cleaning, use only enough liquid
to dampen the cloth or applicator.
SAFETY WARNINGS AND CLEANING INSTRUCTIONS
Cleaning Instructions
To clean the instrument exterior:
 Unplug the instrument from the ac power supply.
 Remove loose dust on the outside of the instrument with a lint-free cloth.
 Remove remaining dirt with a lint-free cloth dampened in a general-purpose detergent and water
solution. Do not use abrasive cleaners.
 Allow the instrument to dry completely before reconnecting it to the power source.

v
SAFETY INSTRUCTIONS AND SYMBOLS
This manual contains up to three levels of safety instructions that must be observed in order to avoid
personal injury and/or damage to equipment or other property. These are:
DANGER Indicates a hazard that could result in death or serious bodily harm if the safety instruction is
not observed.
WARNING Indicates a hazard that could result in bodily harm if the safety instruction is not observed.
CAUTION Indicates a hazard that could result in property damage if the safety instruction is not
observed.
Please read all safety instructions carefully and make sure you understand them fully before attempting to
use this product.
In addition, the following symbol may appear on the product:
ATTENTION – Refer to Manual
DANGER – High Voltage
Please read all safety instructions carefully and make sure you understand them fully before attempting to
use this product.

vi

1
Fig. 1. Example of the Throughput Improvement Using the
Gated Integrator Technique.
Fig. 2. Distortion Due to Charge Collection Time Effects
When Using Semigaussian Output at Short Shaping Time
Constants.
ORTEC MODEL 673 SPECTROSCOPY AMPLIFIER
AND GATED INTEGRATOR
1. DESCRIPTION
1.1. GENERAL
The ORTEC Model 673 Spectroscopy Amplifier and
Gated Integrator is a double-width NIM module with
a versatile combination of switch-selectable pulse-
shaping and output characteristics. It features
extremely low noise, wide gain range, and excellent
overload response for universal application in high-
resolution spectroscopy. It accepts input pulses of
either polarity that originate in germanium or silicon
semiconductor detectors, in scintillation counters
with either fast or slow scintillators, in proportional
counters, in pulsed ionization chambers, in electron
multipliers, etc.
The 673 is two amplifiers in one, having both a
semi-gaussian unipolar and a gated integrator
output. Optimum low- and high-rate energy resolution
can be obtained with improved throughput and
excellent energy resolution. The 673 can be used
with resistive feedback or pulse (transistor) reset
preamplifiers.
1.2. GATED INTEGRATOR
SPECTROSCOPY
A gated integrator is an essential element in a high-
throughput system. A major application of the gated
integrator, (GI), is in gamma-ray experiments
involving large volume HPGe detectors since ballistic
deficit effects caused by long charge collection
times are eliminated. When used following a
conventional resistive feedback or pulse-reset
preamplifier, optimum throughput of approximately
a factor of four can be achieved by us
as compared to conventional semigaussian shaping.
The increase in throughput is achieved with only
minimal increase in energy resolution (Fig. 1).
Charge collection time effects are of significant
importance when using large volume HPGe
detectors at high energies. Detector current pulses
having equivalent total charges but different rise
times produce different output pulse heights when
processed by a charge-sensitive preamplifier and a
semigaussian filter amplifier. This results in the
1
distortion of the spectrum in direct proportion to the
pulse amplitude or energy. This distortion is most
pronounced at short shaping time constants and
with large volume detectors. Experiments performed
with a small 10% efficient HPGe detector at 0.5 µs
shaping time, using the 1.33 MeV line of Co,
60
reveal a significant amount of distortion when using
conventional semi-gaussian shaping (Fig. 2). An
equivalent experiment using a shorter shaping time
of 0.25 µs and GI shaping, shows the dramatic
improvement in energy resolution due to elimination
of charge collection time effects (Fig. 3).
The 673 has an input impedance of ~500  and
accepts either positive or negative input pulses with
rise times <650 ns and fall times >40 µs. Six
integrate and differentiate time constants are
switch-selectable to provide optimum shaping for
resolution and count rate. The first differentiation
network has variable pole-zero cancellation that can
T.H. Becker, E.E. Gross, R.C. Trammell, “Characteristics of
1
High-Rate Energy Spectroscopy Systems With Time-Invariant
Filters,”IEEE Nucl. Sci., NS-28 598 (1981)

=
differentiation time
preamplifier pulse decay time
undershoot amplitude
differentiated pulse amplitude
2
Fig. 3. Elimination of Charge Collection Time Effects
With the Gated Integrator.
be adjusted to match preamplifierswith decay times proper pole-zero cancellation. (Pole-zero
>40 µs. The pole-zero cancellation drastically cancellation is not required when using a pulse-
reduces the undershoot after the first differentiator reset preamplifier). The input impedance is 500  .
and greatly improves overload and count rate When long preamplifier cables are used, the cables
characteristics. In addition, the amplifier contains an can be terminated in series at the preamplifier end
active filter shaping network that optimizes the or in shunt at the amplifier end with the proper
signal-to-noise ratio and minimizes the overall resistors. The output impedance is about 0.1  at
resolving time. Both unipolar and gated integrator the front panel connectors and 93  at the rear
outputs are provided simultaneously on the front panel connectors. The front panel outputs can be
and rear panels. connected to other equipment by a single cable
The unipolar output should be used for spectroscopy far end of the cabling. If series termination is
when dc-coupling can be maintained from the 673 desired, the rear panel connectors can be used to
to the analyzer. A BLR (baseline restorer) circuit is connect the 673 to other modules. See Section 3
included in the unit for improved performance at all for further information.
count rates. Baseline correction is applied during
intervals between input pulses only and a front
panel switch selects a discriminator level to identify
input pulses. The unipolar output dc level can be
adjusted in the range from -100 mV to +100 mV.
This output permits the use of the direct-coupled
input of the analyzer with a minimum amount of
interface problems.
The gated integrator, (GI), output is obtained by
integrating the integrating the entire unipolar signal.
As a result of this integration, the problems
associated with charge collection effects are
removed even when operating at short shaping time
constants. Another benefit of the GI technique is the
ability to maintain peak position and energy
resolution over a wide dynamic range of input count
rate.
Internal pulse pileup (a second pulse arriving before
the first pulse has been completed) is sensed
internally. The 673 includes an Inhibit output BNC
connector on the rear panel that can be used to
inhibit measurement of the result of a pulse pileup
when it occurs.
The unit can be used for constant-fraction timing
when operated in conjunction with an ORTEC 551,
552, or 553 Timing Single-Channel Analyzer
(TSCA). These TSCAs 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 pickoff output to be placed in time
coincidence with other signals.
The 673 has complete provisions, including power,
for operating any ORTEC solid-state preamplifier.
Normally, the preamplifier pulses should have a rise
time of 0.25 µs or less to properly match the
amplifier filter network and a decay time >40 µs for
going to all equipment and shunt terminated at the
1.3. POLE-ZERO CANCELLATION
Pole-zero cancellation is a method for eliminating
pulse undershoot after the first differentiating
network. In an amplifier not using pole-zero
cancellation (Fig. 4), the exponential tail of a
resistive feedback preamplifier output signal
(Usually 50 to 500 µs) causes an undershoot whose
peak amplitude is roughly determined from:
For a 1 µs differentiation time and a 50 µs pulse
decay time the maximum undershoot is 2%, and
this decays with a 50 µs 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 using pole-zero cancellation.

3
Fig. 4. Differentiation in an Amplifier Without Pole-Zero Cancellation.
Fig. 5. Differentiation in a Pole-Zero Cancelled Amplifier.
Pole-zero cancellation is accomplished by the cancellation for all preamplifiers having 40 µs or
network shown in Fig. 5. The pole [s + (1/T )] due to greater decay times. Improper matching of the
0
the preamplifier pulse decay time iscanceled by the pole-zero network will degrade the overload
zero of the network [s + (K/R C )]. In effect, the dc performance and cause excessive pileup distortion
21
path across the differentiation capacitor adds an at medium counting rates. Improper matching
attenuated replica of the preamplifier pulse to just causes either an undercompensation (undershoot is
cancel the negative undershoot of the differentiating not eliminated) or an overcompensation (output
network. after the main pulse does not return to the baseline
Total preamplifier-amplifier pole-zero cancellation time constant). The pole-zero adjust is accessible
requires that the preamplifier output pulse decay on the front panel and can easily be adjusted by
time be a single exponential decay and matched to observing the baseline on an oscilloscope with a
the pole-zero cancellation network. The variable mono-energetic source or pulser having the same
pole-zero cancellation network allows accurate
but decays to the baseline with the preamplifier

()
[]
()
Gs s
sRC
sRC
n
n
() (/ ) /,
=+×+→∞
11
1
4
Fig. 6. Pulse Shapes for Good Signal-to-Noise Ratios
decay time as the preamplifier under overload
conditions. The adjustment should be made so that
the pulse returns to the baseline in the minimum
time with no undershoot.
1.4. ACTIVE FILTER
When only FET gate current and drain thermal
noise are considered, the best signal-to-noise ratio
occurs when the two noise contributions are equal
for a given input pulse shape. The gaussian pulse
shape (Fig. 6) for this condition requires an
amplifier with a single RC differentiate and n equal
RC integrates where n approaches infinity. The
Laplace transform of this transfer function is
where the factor is the single differentiate and the
second factor is the n integrates. The active filter
approximates this transfer function.
Figure 6 illustratesthe results of pulse shaping in an
amplifier. Of the four pulse shapes shown the cusp
would produce minimum noise but is impractical to
achieve with normal electronic circuitry and would
be difficult to measure with an ADC. The true
gaussian shape deteriorates the signal-to-noise
ratio by only about 12% from that of the cusp and
produces a signal that is easy to measure, but
requires many sections of integration (n  ). With
two sections of integration of the waveform
identified as a gaussian approximation can be
obtained, and this deteriorates the signal-to-noise
ratio by about 22%. The ORTEC active filter
network in the 673 amplifier provides the fourth
waveform in Fig. 6; this waveform has characteristics
superior to the gaussian approximation, yet obtains
them with four complex poles. By this method the
output pulse shape has a good signal-to-noise ratio,
is easy to measure, and yet requires only a practical
amount of electronic circuitry to achieve the desired
results.
1.5. GATED INTEGRATOR
The GI output is formed by integrating the entire
output signal from a gaussian prefilter (Fig. 7). The
prefilter output, which is similar to conventional
unipolar semigaussian shaping, is inverted by
amplifier A1 before being processed by the
inverting gated integrator section. While a signal is
being processed, switch S1 is closed; S2 and S3
are open. At the end of the pulse processing time,
S1 is opened while S2 and S3 are closed. Any
charge stored on the integrating capacitor will be
discharged at this time, forcing the GI output to
zero. The total processing time is determined as
eight to ten times the shaping time constant, 2 .
Since the time-to-peak of the GI output occurs much
later than the peak of the gaussian signal, (Fig. 8),
there is more time to integrate the total charge
collected in the detector for a given shaping time.
The time-to-peak is an important parameter since
nuclear spectroscopy analog-to-digital converters,
ADCs, use a peak detect circuit to begin the data
conversion cycle. Also, a peak stretcher is used as
a buffer to the ADC. Therefore, this new pulse
processing technique permits the use of shorter

5
Fig. 7. Simplified Block Diagram of the 673 Spectroscopy Amplifier and Gated Integrator.
Fig.8. Gated Integrator, (GI), Output and
Unipolar Output.
Fig. 9. Typical Resolution and Baseline Stability vs Counting Rate for the GI Output of the 673 Using 0.25 )
)
s
Shaping Time.
shaping time constants for the gaussian prefilter output provides the optimum energy resolution with
and eliminates the charge-collection time effects. essentially no shift in the peak centroid (Fig. 10).
The resulting shorter total processing time improves
the throughput by eliminating
occurred at the longer shaping time constants.
Peak centroid and energy resolution remain
relatively constant over a very wide dynamic range of
input counting rates when using the gated integrator
(Fig 9). The highest count rate performance can be
achieved when a gated integrator and pulse-reset
preamplifier is used. This ensures that the total
system does not ”lock up”at extremely high
counting rates. A gated integrator can also be used
with conventional resistive-feedback preamplifiers
if the counting rate is limited to <~125k cps at Co.
60
This count rate limitation is due to the average dc-
offset from pulse pileup at the preamplifier output.
For low- to medium-rate experiments, the unipolar

6
Fig. 10. Typical Resolution and Baseline Stability vs Counting Rate for the Unipolar (Semigaussian) Ouput of
the 673 Using 2 µs Shaping Time.
2. SPECIFICATIONS
2.1. PERFORMANCE
GAIN RANGE Continuously adjustable, X1 through
X1500.
PULSE SHAPING Unipolar, gaussian on all ranges
with peaking time equal to 2.2 2 and pulse width at
0.1% level, equal to 2.9 times the peaking time.
GI PULSE SHAPING Time variant gated integrator.
INTEGRAL NONLINEARITY <±0.05% (0.025%
typical) using 2-µs shaping.
NOISE <4 µV referred to the input using 3-µs
shaping; gain >100, unipolar output.
TEMPERATURE INSTABILITY
Gain
 0.0075%/ ( C, 0 to 50 ( C.
DC Level <±10 µV/ ( C, 0 to 50 ( C.
UNIPOLAR COUNT RATE INSTABILITY The
1.33-MeV gamma ray peak from a Co source,
60
positioned at 85% of analyzer range, typically shifts
<0.024%, and its FWHM broadens 14% when its
incoming count rate changes from 0 to 100,000
counts/s using 2 µs shaping. The amplifier will hold
the baseline reference up to count rates in excess
of 150,000 counts/s.
GI THROUGHPUT AND RESOLUTION The Gated
Integrator allows operation at short time constants
which permits higher throughput rates while
maintaining excellent resolution. Typical results for
a 10% HPGe detector using a Co source and
60
200,000 counts/s input:
Output Time Dead Max Resolution
Constant Time Throughput
Unipolar 0.5 µs 5 µs 74 k c/s 7.5 keV
GI 0.25 µs 5µ
) s 74 k c/s 2.3 keV
OVERLOAD RECOVERY Recovers to within 2% of
rated output from X300 overload in 2.5
nonoverloaded unipolar pulse widths, using
maximum gain.
2.2. CONTROLS
FINE GAIN Ten-turn precision potentiometer for
continuously variable direct-reading gain factor of
X0.5 to X1.5.
COARSE GAIN Six-position switch selects feedback
resistors for gain factors of 20, 50, 100, 200, 500,
and 1 k.
INPUT ATTENUATOR Jumper on printed wiring
board selects an input attenuation factor of 1 or 10
(gain factor of X1 or X0.1).
POS/NEG Toggle switch selects Pos or Neg input.
SHAPING TIME Two six-position switches select
time constant for active-filter-network pulse shaping;
selections are 0.25, 0.5, 1, 2, 3, and 6. Switch
settings should be set equally for normal operation.
PZ Two potentiometers to adjust pole-zero
cancellation for decay times from 40 µs to

. Fine
PZ corresponds to approximately 10% of coarse PZ.

7
BLR Toggle switch selects a source for the gated
baseline restorer discriminator threshold level from
one of three positions:
Auto The BLR threshold is automatically set to an
optimum level as a function of the signal noise level
by an internal circuit. This allows easy setup and
very good performance.
PZ Adj The BLR threshold is determined by the
threshold potentiometer. The BLR time constant is
greatly increased to facilitate PZ adjustment. This
position may give the lowest noise for conditions of
low count rate and/or longer shaping times.
Threshold The BLR threshold is set manually by
the threshold potentiometer. Range, 0 to 300 mV
referred to the positive output signal. The BLR time
constant isthe same as for the Auto switch setting.
DC Screwdriver potentiometer adjusts the unipolar
output baseline dc level; range, +100 mV to -100 mV.
2.3. INPUTS
LINEAR Positive or negative signal through either
front panel or rear panel BNC connectors. Accepts 2.6. ELECTRICAL AND MECHANICAL
pulses with rise times in the range from 10 to 650 ns
and decay times from 40 to 2000 µs; Z x 1 k  , dc-
in
coupled; linear maximum 1 V (10 V with attenuator
jumper set at X0.1); absolute maximum 20 V.
GATE Rear panel BNC connector accepts standard
positive NIM signal to inhibit pileup rejector circuit
during reset interval of a pulse-reset preamplifier.
2.4. OUTPUTS
UNI Front panel BNC with Z <1  and rear panel
0
BNC with Z = 93  . Short-circuit proof; full scale
0
linear range 0 to +10 V; active-filter-shaped and dc-
restored; dc level adjustable to ±100 mV. (2.70 x 8.714 in. front panel) per TID-20893 (Rev).
GI Front panel BNC with Z <1  and rear panel
0
BNC with Z = 93  . Short-circuit proof; full scale
o
range 0 to +10 V; dc level ±5 mV.
2.5. REAR PANEL CONNECTORS
REAR PANEL CONNECTORS
BUSY Rear panel BNC with Z < 10  provides a
0
+5 V logic pulse for the duration that the input pulse
exceeds the baseline restorer discriminator level.
Connect to the ORTEC MCA Busy Input for dead
time correction.
UNI INH Rear panel BNC with Z < 10  provides a
0
nominal +5 V logic signal when an internal pulse
pileup occurs; to be used for an MCA
anticoincidence input to prevent storage of pileup
data in the spectrum when using the unipolar output.
GI INH Rear panel BNC with Z < 10  provides a
0
nominal +5 V logic signal when an internal pulse
pileup occurs; to be used for an MCA
anticoincidence input to prevent storage of pileup
data in the spectrum when using the GI output.
CRM (Count Ratemeter) Rear panel BNC furnishes
a nominal +5 V logic signal for every linear input
pulse, width 300 ns; to be used as an input to a
ratemeter or counter.
PREAMP POWER Rear panel standard ORTEC
power connector; Amphenol 17-10090; mates with
captive and noncaptive power cords on all standard
ORTEC preamplifiers.
POWER REQUIRED +24 V, 125 mA; -24 V,
105 mA; +12 V, 150 mA; -12 V, 75 mA.
WEIGHTS
Net 1.4 kg (3 lb).
Shipping 3.2 kg (7 lb).
DIMENSIONS NIM-standard double-width module
3. INSTALLATION
3.1. GENERAL
The 673 operates on power that must be furnished
from a NIM-standard bin and power supply such as
the ORTEC 4001/4002 Series. The bin and power
supply is designed for relay rack mounting. If the
equipment is to be rack mounted, be sure that there
is adequate ventilation to prevent any localized
heating of the components that are used in the 673.
The temperature of the equipment mounted in
racks can easily exceed the maximum limit of 50 ( C
unless precautions are taken.

8
3.2. CONNECTION TO POWER
The 673 contains no internal power supply and
must obtain the necessary dc operating power from
the bin and power supply in which it is installed for
operation. Always turn off power for the power
supply before inserting or removing any modules.
After all modules have been installed in the bin and
any preamplifiers have also been connected to the
Preamp Power connectors on the amplifiers, check
the dc voltage levels from the power supply to see
that they are not overloaded. The ORTEC
4001/4002 Series Bins and Power Supplies have
convenient test points on the power supply control
panel to permit monitoring these dc levels. If any
one or more of the dc levels indicates an overload,
some of the modules will need to be moved to
another bin to achieve operation.
3.3. CONNECTION TO PREAMPLIFIER
The preamplifier output signal is connected to the
673 through the appropriate Input BNC connector
on the front or rear panel. The input impedance is
~500  and is dc-coupled to ground; therefore the
preamplifier output must be either ac-coupled or
have approximately zero dc voltage under no-signal
conditions.
The 673 incorporates pole-zero cancellation in
order to enhance the overload and count rate
characteristics of the amplifier when used with
resistive-feedback preamplifiers. This technique
requires matching the network to the preamplifier
decay time constant in order to achieve perfect
compensation. The pole-zero adjustment should be
set each time the preamplifier or the shaping time
constant of the amplifier is changed. For details of
the pole-zero adjustment see Section 4.6. An
alternate method is accomplished easily by using a
monoenergetic source and observing the amplifier
baseline with an oscilloscope after each pulse under
approximately X2 overload conditions. Adjustment
should be made so that the pulse returns to the
baseline in a minimum amount of time with no
undershoot. Pole-zero adjustment is not needed
when using a transistor-reset preamplifier.
Preamplifier power at +24 V, -24 V, +12V and -12 V
is available through the Preamp Power connector on
the rear panel. When the preamplifier is connected,
its power requirements are obtained from the same
bin and power supply as is used for the amplifier,
and this increases the dc loading on each voltage
level over and above the requirements for the 673
at the module position in the bin.
When the 673 is used with a remotely located
preamplifier (i.e., preamplifier-to-amplifier connection
through 25 ft or more of coaxial cable), be careful
to ensure that the characteristic impedance of the
transmission line from the preamplifier output to the
673 input is matched. Since the input impedance of
the 673 is ~500  , sending-end termination will
normally be preferred; the transmission line should
be series-terminated at the preamplifier output. All
ORTEC preamplifiers contain series terminations
that are either 93  or variable; coaxial cable type
RG-62/U or RG-71/U is recommended.
3.4. CONNECTION OF TEST PULSE
GENERATOR
THROUGH A PREAMPLIFIER The satisfactory
connection of a test pulse generator such as the
ORTEC 419 Precision Pulse Generator or equivalent
depends primarily on two considerations; the
preamplifier must be properly connected to the 673
as discussed in Section 3.3, and the proper input
signal simulation must be applied to the preamplifier.
To ensure proper input signal simulation, refer to
the instruction manual for the particular preamplifier
being used.
DIRECTLY INTO THE 673 Since the input
impedance of the 673 is ~500  , the test pulse
generator will normally have to be terminated at the
amplifier input with an additional shunt resistor. In
addition, if the test pulse generator has a dc offset,
a large series isolating capacitor is also required
since the 673 input is dc-coupled. The ORTEC test
pulse generators are designed for direct connection.
When any one of these units is used, it should be
terminated with a 100  terminator at the amplifier
input or be used with at least one of the output
attenuators set at In. (The small error due to the
finite input impedance of the amplifier can normally
be neglected.)
SPECIAL CONSIDERATIONS FOR POLE-ZERO
CANCELLATION When a tail pulser is connected
directly to the amplifier input, the PZ Adj should be
adjusted if overload tests are to be made (other
tests are not affected). See Section 4.6 for the pole-
zero adjustment. If a preamplifier is used and a tail
pulser is connected to the preamplifier test input,
similar precautions are necessary. In this case the
effect of the pulser decay must be removed; that is,
a step input should be simulated.

9
3.5. SHAPING CONSIDERATIONS 3.7. LINEAR OUTPUT CONNECTIONS
The shaping time constant on the 673 is switch-
selectable in steps of 0.25, 0.5, 1, 2, 3, and 6 µs.
The choice of the proper shaping time constant is
generally a compromise between operating at a
shorter time constant for accommodation of high
counting rates and operating with a longer time
constant for a better signal-to-noise ratio. For
scintillation counters the energy resolution depends
largely on the scintillator and photomultiplier, and
therefore a shaping time constant of about four
times the decay-time constant of the scintillator is a
reasonable choice (for Nal, a 1 µs shaping time
constant is about optimum). For gas proportional
counters the collection time is normally in the 0.5 to
5 µs range and a 2 µs or greater time constant
selection will generally give optimum resolution. For
surface barrier semiconductor detectors, a 0.5 to
2 µs resolving time will generally provide optimum
resolution. Shaping time for HPGe detectors will vary
from 1 to 6 µs, depending on the size, configuration,
and collection time of the specific detector and
preamplifier. When a charge-sensitive preamplifier
is used, the optimum shaping time constant to
minimize the noise of a system can be determined
by measuring the output noise of the system and
dividing it by the system gain. Since the 673 has
almost constant gain for all shaping modes, the
optimum shaping can be determined by measuring
the output noise of the 673 with a voltmeter as each
shaping time constant is selected.
The 673 provides both unipolar and gated integrator
outputs. The unipolar output pulses should be used
in applications where the best signal-to-noise ratio
(resolution) is most important, such as high-
resolution spectroscopy using semiconductor
detectors. Use of the unipolar output with baseline
restoration will also give excellent resolution at high
counting rates. The gated integrator output should
be used to obtain a higher throughput. Another
benefit of the gated integrator is the ability to
maintain peak position and energy resolution over
a wide dynamic range of input count rate.
3.6. USE OF GATE INPUT 673 for the cable connection. Use 93
 cable to
The Gate Input on the rear panel accepts a positive
logic pulse (TTL compatible) that keeps the gated
integrator from producing an output as long as it is
positive. It does not affect the unipolar output. The
main purpose is to receive a reset signal from a
reset-type preamplifier such as the ORTEC
Transistor-Reset Preamplifier.
AND TERMINATING CONSIDERATIONS
Since the 673 unipolar output is normally used for
spectroscopy, the 673 is designed with a great
amount of flexibility in order for the pulse to be
interfaced with an analyzer. A gated baseline
restorer, (BLR), circuit is included in this output for
improved performance at all count rates. A switch
on the front panel permits the threshold for the
restorer gate to be determined automatically,
according to the input noise level, or manually, with
a screwdriver adjustment. The switch also has a
center PZ Adj setting that can be used to eliminate
the BLR effect when making pole-zero adjustments.
The unipolar output dc level can be adjusted from
-0.1 to +0.1 V to set the zero intercept on the
analyzer when direct coupling is used. Typical
system block diagrams for a variety of experiments
are described in Section 4.
Three general methods of termination are used.
The simplest of these is shunt termination at the
receiving end of the cable. A second method is
seriestermination a the sending end. The third is a
combination of series and shunt termination, where
the cable impedance is matched both in series at
the sending end and in shunt at the receiving end.
The combination is most effective, but this reduces
the amount of signal strength at the receiving end
to 50% of that which is available in the sending
instrument.
To use shunt termination at the receiving end of the
cable, connect the <1  output of the 673 (on the
front panel) through 93  cable to the input of the
receiving instrument. Then use a BNC tee connector
to attach both the interconnecting cable and a 100 
terminator at the input connector of the receiving
instrument. Since the input impedance of the
receiving instrument is normally 1000  or more, the
effective instrument input impedance with the 100 
terminator will be of the order of 93  , and this
matches the cable impedance correctly.
For series termination use the 93  output of the
interconnect this into the input of the receiving
instrument. The 1000  (or more) normal input
impedance at the input connector represents an
essentially open circuit, and the series impedance
in the 673 now provides the proper termination for
the cable.

10
For the combination of series and shunt termination, sensed in the 673, and the pulse can then be used
use the 93  output on the rear panel of the 673 and to prevent the analyzer from measuring and storing
use 93  cable. At the input for the receiving a false amplitude. The signal is dc-coupled and
instrument use a BNC tee to attach both the signal rises from 0 to about +5 V for a time equal to 6 2 ,
cable and a 100  resistive terminator. Note that the
signal span at the receiving end of this type of
circuit will always be reduced to 50% of the signal
span furnished by the sending instrument.
For customer convenience, ORTEC stocks the
proper terminators and BNC tees, or they can be
ordered from a variety of commercial sources.
3.8. SHORTING OR OVERLOADING THE
AMPLIFIER OUTPUTS
All outputs of the 673 are dc-coupled with an output
impedance of ~0.1  for the front panel connectors
and 93  for the rear panel connectors. If the output
is shorted with a direct short circuit, the output stage
will limit the peak current of the output so that the
amplifier will not be harmed. When the amplifier is
terminated with 100  , themaximum rate allowed to
maintain the linear unipolar output is [200,000
cps/ 2 (µs)] x [10/V (V)].
0
3.9. INHIBIT OUTPUT CONNECTION
The GI and Uni Inhibit outputs on the rear panel are
intended for application at the anticoincidence input
of the analyzer. An output pulse is generated
through this connector when a pulse pileup is
starting when a pileup occurs. The Uni Inh is used for
the unipolar output and the GI Inh for the GI output.
3.10. BUSY OUTPUT CONNECTION
The signal through the rear panel Busy output
connector rises from 0 to about +5 V at the onset of
each linear input pulse. Its width is equal to the time
the input pulse amplitude exceeds the BLR
discriminator level, and is extended automatically
by the generation of an Inhibit output signal. It can be
used to provide MCA live time correction, to control
the generation of input pulses, to observe normal
operation with an oscilloscope, or for any of a variety
of other applications. Its use is optional and no
termination is required if the output is not being used.
3.11. CRM OUTPUT CONNECTION
One NIM-standard positive logic pulse is generated
to correspond to each linear input pulse into the
673. The pulses are available through the CRM
(Count Rate Meter) output BNC on the rear panel
and are intended for use in a count rate meter or
counter to monitor the true input count rate into the
amplifier. Its use is optional and no termination is
required if the output is not being used.
4. OPERATION
4.1. INITIAL TESTING AND OBSERVATION
OF PULSE WAVEFORMS
Refer to Section 6 for information on testing
performance and observing waveforms at front
panel test points. Figure 11 shows some typical
unipolar and gated integrator output waveforms.
4.2. FRONT PANEL CONTROLS
FINE GAIN Ten-turn precision potentiometer for
continuously variable direct-reading gain factor of
X0.5 to X1.5.
COARSE GAIN Six-position switch selects feedback
resistors for gain factors of 20, 50, 100, 200, 500,
and 1k.
INPUT ATTENUATOR Jumper on printed wiring
board selects an input attenuation factor of 1 or 10
(gain factor of X1 or X0.1).
POS/NEG Toggle switch selects Pos or Neg input.
SHAPING TIME Two six-position switches select
time constant for active-filter-network pulse shaping;
selections are 0.25, 0.5, 1, 2, 3, and 6. Switch
settings should be set equally for normal operation.
PZ Two potentiometers to adjust pole-zero
cancellation for decay times from 40 µs to  . Fine
PZ corresponds to approximately 10% of coarse PZ.

11
Fig. 11. Typical Effects of Shaping-Time Selection on
Output Waveforms
BLR Toggle switch selects a source for the gated
baseline restorer discriminator threshold level from
one of three positions:
Auto The BLR threshold is automatically set to an
optimum level as a function of the signal noise level
by an internal circuit. This allows easy setup and
very good performance.
PZ Adj The BLR threshold is determined by the
threshold potentiometer. The BLR time constant is
greatly increased to facilitate PZ adjustment. This
position may give the best energy resolution for
conditions of low- to medium-count rate and/or long
shaping times.
Threshold The BLR threshold is set manually by
the threshold potentiometer. Range, 0 to 300 mV
referred to the positive output signal. The BLR time
constant isthe same as for the Auto switch setting.
DC Screwdriver potentiometer adjusts the unipolar
output baseline dc level; range, +100 mV to -100mV.
4.3. INPUTS
LINEAR Positive or negative signal through either
front panel or rear panel BNC connectors. Accepts
pulses with rise times in the range from 10 to 650 ns
and decay times from 40 to 2000 µs; Z x 1 k  , dc-
in
coupled; linear maximum 1 V (10 V with attenuator
jumper set at X0.1); absolute maximum 20 V.
GATE Rear panel BNC connector accepts standard
positive NIM signal to inhibit pileup rejector circuit
during reset interval of a pulse reset preamplifier.
4.4. OUTPUTS
UNIPOLAR Front panel BNC with Z < 1  and rear
o
panel BNC with Z = 93  . Short-circuit proof; prompt
o
full scale linear range 0 to +10 V; active-filter-shaped
and dc-restored; dc level adjustable to ±100 mV.
GI Front panel BNC with Z < 1
 and rear panel
o
BNC with Z = 93  . Short-circuit proof; full scale
0
range 0 to +10 V; dc level ± 5 mV.
BUSY Rear panel BNC with Z < 10
 provides a
0
+5 V logic pulse for the duration that the input pulse
exceeds the baseline restorer discriminator level.
Connect to the ORTEC MCA Busy Input for dead
time correction.
UNI INH Rear panel BNC with Z < 10
 provides a
0
nominal +5 V logic signal when an internal pulse
pileup occurs; to be used for an MCA
anticoincidence input to prevent storage of pileup
data in the spectrum when using the unipolar output.
GI INH Rear panel BNC with Z < 10
 provides a
0
nominal +5 V logic signal when an internal pulse
pileup occurs; to be used for an MCA
anticoincidence input to prevent storage of pileup
data in the spectrum when using the GI output.
CRM (Count Ratemeter) Rear panel BNC
furnishes a nominal +5 V logic signal for every

12
Fig. 12. Typical Waveforms Illustrating Pole-Zero
Adjustment Effects; Oscilloscope Trigger, 673 Busy
Output, Co Source with 1.33-MeV Peak Adjusted
60
~9 V; Count Rate, 3 kHz; Shaping Time Constant, 2 µs.
linear input pulse; width 300 ns; to be used as an of the pulses returns to the baseline without
input to a ratemeter or counter. overshoot or undershoot (Fig. 12).
PREAMP POWER Rear panel standard ORTEC
power connector, Amphenol 17-10090; mates with
captive and noncaptive power cords on all standard
ORTEC preamplifiers.
4.5. STANDARD SETUP PROCEDURE
a. Connect the detector, preamplifier, high voltage
power supply, and amplifier into a basic system and
connect the amplifier unipolar output to an
oscilloscope. Connect the preamplifier power cable
to the Preamp power connector on the rear panel of
the 673. Turn on power in the bin and power supply
and allow the electronics of the system to warm up
and stabilize.
b. Set the 673 controls initially as follows:
Shaping 2 µs
Coarse Gain 50
Fine Gain 1.000
Internal Jumper X1.0
BLR PZ Adj
Thresh Fully clockwise
Pos/Neg Match preamplifier output polarity
c. Use a Co calibration source, set about 25 cm
60
from the active face of the detector. The unipolar
output pulse from the 673 should be about 8 to
10 V, using a preamplifier with a conversion gain of
170 mV/MeV.
d. Readjust the Gain control so that the higher
peak from the Co source (1.33 MeV) provides an
60
amplifier output at about 9 V.
4.6. POLE-ZERO ADJUSTMENT FOR
RESISTIVE-FEEDBACK PREAMPLIFIER
The pole-zero adjustment is extremely critical for
good performance at high count rates. This
adjustment should be checked carefully for the best
possible results. When using a transistor reset-type
preamplifier, the Coarse PZ Adj should be set to full
counterclockwise.
USING HPGe SYSTEM AND Co
60
a. Adjust the radiation source count rate between
2 kHz and 10 kHz.
b. Observe the unipolar output with an oscilloscope.
Adjust the PZ Adj. Control so that the trailing edge
The oscilloscope used must be dc-coupled and must
not contribute distortion in the observed waveforms.
Oscilloscopes such as Tektronix 453, 454, 465, and
475 will overload for a 10 V signal when the vertical
sensitivity is <100 mV/cm. To prevent overloading
the oscilloscope, use the clamp circuit shown in
Fig. 13.

13
Fig. 13. A Clamp Circuit that Can Be Used to Prevent
Overloading the Oscilloscope Input.
Fig. 14. Pole-Zero Adjustment Using a Square Wave Input to the Preamplifier.
USING SQUARE WAVE THROUGH
PREAMPLIFIER TEST INPUT
A more precise pole-zero adjustment in the 673 can
be obtained by using a square wave signal as the
input to the preamplifier. Many oscilloscopes include
a calibration output on the front panel and this is a
good source of square wave signals at a frequency
of about 1 kHz. The amplifier differentiates the
signal from the preamplifier so that it generates
output signals of alternate polarities on the leading
and trailing edges of the square wave input signal,
and these can be compared as shown in Fig. 14 to
achieve excellent pole-zero cancellation.
Use the following procedure:
a. Remove all radioactive sources from the vicinity
of the detector. Set up the system as for normal
operation, including detector bias.
b. Set the 673 controls as for normal operation; this
includes gain, shaping, and input polarity.
c. Connect the source of 1 kHz square waves
through an attenuator to the Test input of the
preamplifier. Adjust the attenuator so that the
673 output amplitude is about 9 V.
d. Observe the unipolar output of the 673 with an
oscilloscope triggered from the 673 Busy output.
Adjust the PZ Adj control for proper response
according to Fig. 14. Use the clamp circuit of
Fig. 13 to prevent overloading the oscilloscope
input.
Figure 14A shows the amplifier output as a series of
alternate positive and negative gaussian pulses. In
B, C, and D, of this figure the oscilloscope was
triggered to show both positive and negative pulses

14
Fig. 15. BLR Threshold Variable Control Settings.
simultaneously. These pictures show more detail to
aid in proper adjustment.
4.7. BLR THRESHOLD ADJUSTMENT
After the amplifier gain and shaping have been
selected and the PZ Adj control has been set to
operate properly for the particular shaping time, the
BLR Thresh control can be used to establish the
correct discriminator threshold for the baseline
restorer circuit. Normally, the toggle switch can be
set at Auto and the threshold level will be set
automatically just above the noise level. If desired,
the switch can be set at Thresh and the manual
control just below the switch can then be used to
select the level manually as follows:
a. Remove all radioactive sources from the vicinity
of the detector. Set up the system as for normal
operation, including detector bias.
b. Set the BLR switch at PZ Adj or Thresh and turn
the control fully clockwise for 300 mV.
c.1. LED Method: Adjust the front panel threshold
trimpot so that the LED (red light emitting diode) is
on about half the time.
c.2. Scope Method: Observe the unipolar output on
the 100 mV/cm scale of the oscilloscope using
5 µs/cm horizontal deflection. Trigger the
oscilloscope with the Busy output from the 673.
d. Reduce the control setting until the baseline
discriminator begins to trigger on noise; this
corresponds to about 200 counts/s from the Busy
output. Adjust the trigger level according to the
information in Fig. 15.
If a ratemeter or counter-timer is available, it can be
connected to the Busy output and the threshold
level can then be adjusted for about 200 counts/s.
4.8. GATED INTEGRATOR SET UP
a. Set both pulse shaping switches to the same
time constant.
b. Adjust the Unipolar output PZ and BLR threshold
as described in Sections 4.6 and 4.7.
c. Measure the Unipolar dc-output voltage with the
input removed. Adjust to 0 ± 1 mV using the front
panel control.
d. Connect the GI output to the ADC and then
make the final gain adjustment. The best resolution
should be obtained with the BLR switch in the PZ
position. The pole-zero adjustment is especially
critical at high rates (>50 k counts/s input) when
using a resistive-feedback preamplifier. The preamp
pole-zero needs to be checked and fine-tuned if
necessary.
e. Test resolution again with the BLR control in the
Auto and/or Thresh position. These are the “high”
restore positions. Improved resolution compared to
PZ usually indicates interferences picked up in the