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

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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
new-production instruments. Instruments that are returned should be packed so that they will withstand normal
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
in shipment. Packing materials, waybills, and other such documentation should be preserved in order to establish
claims. After such notification to the carrier, please notify ORTEC of the circumstances so that assistance can be
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