EG&G ORTEC 485 User manual

100 MIDLAND ROAD
OAK RIDGE, TENN. 37830
PHONE [phone]GGB
TWX 81G-572-1G7B
INSTRUCTION
MANUAL
485
AMPLIFIER
n
(z:oi\/ii=»ArsiY

INSTRUCTION MANUAL
485
AMPLIFIER
Serial No.
Purchaser
Date issued.
I Ci; CD F=1 (=> CD F=l AT" E CD
100 MIDLAND RDAD
DAK RIDGE, TENN. 37830
PHDNE [615) 482-4411
TWX 81D-572 -1078
© ORTEC Incorporated 1968
Printed In U.S.A.

TABLE OF CONTENTS
Page
WARRANTY
PHOTOGRAPH
1. DESCRIPTION
1 1
1.1 General Description
1 1
1.2 Pole-Zero Cancellation
1 1
1.3 Active Filter
1
3
2. SPECIFICATIONS
2 1
2.1 General Specifications
2 1
2.2 Amplifier Specifications
2 1
3. INSTALLATION
3 1
3.1 General Installation Considerations
3
1
3.2 Connection to Preamplifier
3 1
3.3 Connection of Test Pulse Generator
3 1
3.4 Connection to Power - Nuclear Standard Bin, ORTEC 401A/402A
3 2
3.5 Output Connections and Terminating Considerations
3 2
3.6 Shorting the Amplifier Output
3 2
4. OPERATING INSTRUCTIONS
4 1
4.1 Controls and Connectors
4 1
4.2 Initial Testing and Observation of Pulse Waveforms
4 2
4.3 General Considerations for Operation with Semiconductor Detectors
4 2
4.4 Operation in Spectroscopy Systems
4
7
4.5 Typical System Block Diagrams
4 10
5. CIRCUIT DESCRIPTION
5 1
5.1 General Block Diagram
5
1
5.2 Circuit Description
5
1
5.3 Circuit Modifications for Special Applications
5 3
6. MAINTENANCE
6
1
6.1 Test Equipment Required 6 1
6.2 Pulser Modifications for Overload Tests 6
2
6.3 Pulser Tests and Calibration 6
4
6.4 Tabulated Test Point Voltages
SCHEMATIC
485-0101-S1 ORTEC 485 Amplifier Schematic

LIST OF FIGURES AND ILLUSTRATIONS
Page
Figure 1.1 Differentiation In a Non-Pole-Zero Cancelled Amplifier
Figure 1.2 Differentiation (Clipping) in a Pole-Zero Cancelled Amplifier
Figure 1.3 Pulse Shapes for Good Signal-to-Noise Ratios
Figure 1.4 ORTEC 485 Active Filter
Figure 4.1 Measuring Amplifier and Detector Noise Resolution
Figure 4.2 Resolution Spread Versus External Input Capacity
Figure 4.3 Amplifier and Detector Noise Versus Bias Voltage
Figure 4.4 Measuring Resolution with a Pulse Height Analyzer
Figure 4.5 Measuring Detector Current-Voltage Characteristics
Figure 4.6 Silicon Detector Back Current Versus Bias Voltage
Figure 4.7 High Resolution Alpha Particle Spectroscopy System
Figure 4.8 High Resolution Gamma Spectroscopy System Using a Lithium Drifted
Germanium Detector
Figure 4.9 Scintillation Counter Gamma Spectroscopy System
Figure 4.10 High Resolution X-Ray Spectroscopy System
Figure 4.11 Gamma Ray-Gamrtia Coincidence Experiment - Block Diagram
Figure 4.12 Gamma Ray-Charged Particle Coincidence Experiment - Block Diagram
Figure 4.13 Gamma Ray Pair Spectrometer - Block Diagram
Figure 5.1 485 Amplifier Block Diagram
Figure 5.2 ORTEC 485 Attenuator Networks
Figure 6.1 Pulser Modification for Overload Tests
Figure 6.2 Pulser Pole'zero Cancellation
Figure 6.3 Method for Measuring Nonlinearity
Figure 6.4 Method for Measuring Crossover Walk
1-2
1 -2
1 -4
1-5
4-3
4-3
4
4
4
4
4
4
4
4
4
4
4
5
5
6
6
6
6
4
5
6
6
7
8
9
10
11
11
12
1
2
1
2
3
3

A NEW STANDARD TWO-YEAR WARRANTY FOR ORTEC ELECTRONIC INSTRUMENTS
ORTEC warrants its nuclear instrument products to be free from defects in workmanship and materials, other
than vacuum tubes and semiconductors, for a period of twenty-four months from date of shipment, provided
that the equipment has been used in a proper manner and not subjected to abuse. Repairs or replacement, at
ORTEC option, will be made without charge at the ORTEC factory. Shipping expense will be to the account
of the customer except in cases of defects discovered upon initial operation. Warranties of vacuum tubes and
semiconductors, as made by their manufacturers, will be extended to our customers only to the extent of the
manufacturers' liability to ORTEC. Specially selected vacuum tubes or semiconductors cannot be warranted.
ORTEC reserves the right to modify the design of its products without incurring responsibility for modification
of previously manufactured units. Since installation conditions are beyond our control, ORTEC does not
assume any risks or liabilities associated with methods of installation other than specified in the instructions,
or installation results.
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
ORTEC instruments not in warranty may be returned to the factory for repairs or checkout at modest expense
to the customer. Standard procedure requires that returned instruments pass the same quality control tests as
those used for new production instruments. Please contact the factory for instructions before shipping
equipment.
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 we may assist in damage claims and in providing replacement equipment if necessary.

-i V,- .
■,Vj.;' .
ORTEC®
^ MODEL 485 €
AMPLIFIER
COARSE GAIN
16
FINE GAIN
PZ-TRIM
POS UNIPOLAR
t> ■#•
NEG BIPOLAR
INPUT
OUTPUT
@
HH
i■ M-A
-I. - ?5'- - ■
*.?!■■ ■ •

1 -1
ORTEC 485 AMPLIFIER
1. DESCRIPTION
1.1 General Description
The ORTEC 485 Amplifier is a functional design utilizing integrated circuits to provide a general purpose
amplifier at minimum cost. The low input noise, dynamic gain range, and pulse shaping networks
allow operation with semiconductor detectors and scintillation detectors in a wide variety of applications.
The performance capability of the 485 coupled with its low cost will allow a wide range of uses in such
fields as research, counting rooms, monitoring applications, and instructional laboratories.
The instrument has a single linear output which can be switch selected for either unipolar or bipolar
pulse shape. The first differentiation network has variable pole-zero cancellation which can be adjusted
to match preamplifiers with greater than 40/itsec decay times. Excellent overload performance is accom
plished by the use of pole-zero techniques. In addition, the amplifier contains an active filter shaping
network which optimizes the signal-to-noise ratio and minimizes the overall resolving time.
The 485 can be used for crossover timing when used in conjunction with an ORTEC 407 Crossover
Pickoff or a 420 Timing Single Channel Analyzer. The 420 Timing Single Channel Analyzer output has
a minimum of walk as a function of pulse amplitude and incorporates a variable delay time on the out
put pulse to enable the crossover pickoff output to be placed in time coincidence with other outputs.
The 485 has complete provisions, including power, for operating any ORTEC solid state preamplifier
such as the 109A, 113, and 118A. Preamplifier pulses should have a rise time of 0.25;usec or less to
properly match the amplifier filter network, and a decay time of greater than 40)usec for proper pole-zero
cancellation.
The input impedance is 1000 ohms. 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 resistance.
The output impedance of the 485 is about 0.5 ohm. The output can be connected to other equipment
by either a single cable going to all equipment and shunt terminated at the far end (and series ter
minated at the amplifier if reflections are a problem) or separate cables to each instrument with each
cable series terminated at the amplifier. (See Section 3.5.)
Gain changing is accomplished by constant impedance T attenuators. By using this technique, the
bandwidth of the feedback amplifier stages involved in gain switching remains essentially constant
regardless of gain and therefore rise time changes with gain switching (which cause crossover walk) are
limited to small capacitance effects across the attenuators.
The 485 is one NIM standard module wide. The unit has no self-contained power supply; power is ob
tained from a NIM standard Bin and Power Supply such as the ORTEC 401A/402A. The 485 design
is consistent with other modules in the ORTEC Modular Nuclear Instrumentation Series; i.e., it is
not possible to overload the bin power supply with a full complement of ORTEC modules in the bin.
Since twelve 485 amplifiers can be contained and powered in one Bin and Power Supply, the 485 is
particularly suited to experiments requiring many amplifiers.
1.2 Pole-Zero Cancellation
Pole-zero cancellation is a method for eliminating pulse undershoot after the first differentiating network.
The technique employed is described by referring to the waveforms and equations shown in Figure 1.1
and 1.2. In a non-pole-zero cancelled amplifier, the exponential tail on the preamplifier output signal
(usually 50 to 500/tsec) causes an undershoot whose peak amplitude is roughly:
Undershoot Amplitude _ Differentiation Time
Differentiated Pulse Amplitude Preamplifier Pulse Decay Time

1 -2
(V ^max ®
200409
Undershoot
First
Charge loop clipping P" "
with
network
undershoot
Equations:
-t
Emax ^
1 s
F X = f I (s); Laplace Transform
'^max 1 1 I > /. r-
-t
^max — Ti - To /^. u -r d
To e -T^e = Ci where Ti = R i Cj
T o-T,
Figure 1-1. Differentiation in a Non-Pole-Zero Cancelled Amplifier
><K <1
So (t) e
T
K
ii
S— —VNA*
>R3 Rj i
r -1-^
«! (t)
No Undershoot
Pole-zero
, cancelled _ Clipped pulse
Charge loop x - vvithout
clipping
output
network undershoot
1 K
Pole zero cancel by letting s + —— s + ■
T o R2C1 or:
^max^'^'^G(t)=e, (t)
s -I-
^ X = E\ (s); Laplace Transform
-max 1 " Ri -rR2
^^T^ '""RiRjC,
Emax Efy^gx
Ri -r R2 1
s + „ „ „ s +
R
R1 R2
£, (s); where Rp = ^^
]R2 Cj RpCi
-t
200410
Figure 1-2. Differentiation (Clipping) in a Pole-Zero Cancelled Amplifier

1-3
For a l^isec differentiation time and a 50jusec preamplifier pulse decay time, the maximum undershoot
is 2% and decays with a BOfisec time constant. Under overload conditions, this undershoot is often
sufficiently large to saturate the amplifier during a considerable portion of the undershoot, causing
excessive deadtime. 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.
Pole-zero cancellation is accomplished by the network shown in Figure 1.2. The pole ( ^ ^ ) due to
b -(• 1 /Tq
the preamplifier pulse decay time is cancelled by the zero (S + K/R^Ci) of the network. In effect, the
dc path across the differentiation capacitor adds an attenuated replica of the preamplifier pulse to just
cancel the negative undershoot of the clipping network.
Total preamplifier - amplifier pole-zero cancellation requires that the preamplifier output pulse decay
time be a single exponential decay and matched to the pole-zero cancellation network. The variable
pole-zero cancellation network allows accurate cancellation for all preamplifiers having 40^isec or
greater decay times. The network is factory adjusted to BO^isec which is compatible with all ORTEC
FET preamplifiers. Improper matching of the pole-zero cancellation network will degrade the overload
performance and cause excessive pile-up distortion at medium counting rates. Improper matching causes
either an undercompensation (undershoot is not eliminated) or an overcompensation (output after the
main pulse does not return to the baseline and decays to the baseline with the preamplifier time con
stant). The pole-zero trim is accessible from the front panel of the 485 and can easily be adjusted by
observing the baseline with a monoenergetic source or pulsar having the same decay time as the preamp
lifier under overload conditions.
1.3 Active Filter
When only grid current and shot noise (gate current and drain thermal noise for an FET) are considered,
the best signal-to-noise ratio occurs when the two noise contributions are equal for a given pulse shape.
Also at this point, there is an optimum pulse shape for the optimum signal-to-noise ratio. Unfor
tunately, this shape (the cusp shown in Figure 1.3) is not physically realizable and very difficult to
simulate. A pulse shape that can be simulated (the Gaussian in Figure 1.3) requires a single RC
differentiate and n equal RC integrates, where n approaches infinity. The Laplace transform of the
transfer function is: ^ ^
GS =,= X n °o
t^Rc] h Rc]
where the first term is the single differentiate and the second term is the n integrates. The 485 active
filter attempts to simulate this transfer function with the simplest possible circuit.
The 485 active filter circuit is shown in Figure 1.4. The major attraction of the active RC filter is the
simple synthesis of a complex pulse shape resulting in a significant reduction in size, complexity and
cost. For a given resolving time (RC), the time response of the filter network depends only on K (see the
circuit equations in Figure 1.4). For K = 1, the transfer function simplifies to:
1 1
eo_ R^C^ _ R^C^
which is an n = 2 approximation to the Gaussian pulse shape (see Figure 1.3).
For K = 2 (the actual case for the ORTEC filter), the transfer function becomes:
Bq, R'^C^ R^C^
ei
■J c2
r-. - 2s 2 i; i+jir- i-i1 i=V^
S + + ^ ^ ' I Icj. ' I ' V
RC R^C^

1 -4
CUSP
-t/RC
t/RC
t >0
t < 0
GAUSSIAN
(S+l/RC) (S+l/llC)
n
GAUSSIAN APPROXIMATION
FOR n=2
1
(S+l/RC) (S+1/RC)^
200362
ACTIVE FILTER
V rcA rc/
j=xFT
Figure 1-3. Pulse Shapes for Good Signal-to-Nolse Ratios

1-5
In this case, the complex roots cause an underdamped effect which reduces the resolving time and results
in a more symmetrical pulse shape (see Figure 1.3).
The 485 is manufactured with 1/isec time constants (R = 1000f2, C = lOOOpF, C/K = 500pF). These
time constants can be changed to suit the experiment. See Section 5.3 for details on this modification.
R R
e: e„
-Wv-
C -L
*
£
^ K
Equations:
K
SC K
®o - ea — ea
K "K + SRC
ei - ea ea - eo ea - Bq
SC
Eliminating ea and solving for the transfer function:
R^
eo
®i . 2S . K
+ ^ +
RC R2C2
Figure 1.4. ORTEC 485 Active Filter

2-1
2. SPECIFICATIONS
2.1 General Specifications
2.1.1 The 485 is intended for use with a Nuclear Standard Bin such as the ORTEC 401A/402A. Two
watts of power is required for the operation of the 485 in the quiescent condition. The 401A/-
402A can be operated on either 115 or 220 volts ac, 50-60 Hz; if it is used with 220 volts ac,
the manual for the 401A/402A must be referred to in order to ensure that correct connections
have been made before operation on 220 volts ac is attempted. The instrument is supplied from
the factory wired for 115 volts ac operation. The power input connector to the 401A/402A is a
NEMA standard 3-wire grounding type.
Preamplifier power of ±12V and ±24V is available on the 485 rear panel connector, CN3. All
signal inputs and outputs are on BNC connectors which are mounted on the front panel.
2.1.2 The instrument is intended for rack mounting in a 401A/402A Nuclear Standard Bin, but the
Nuclear Standard Bin is suitably packaged for cabinet installation if desired. The weight of the
485 is 2 pounds and its outside dimensions are approximately 8.75 inches high by 1.35 inches
wide by 9.75 inches deep.
2.2 Amplifier Specifications
INPUT: Input pulse should have less than 0.25Msec rise time for best filter action, 40;usec minimum
decay time for pole-zero cancellation, ^2\l maximum, 6V maximum to prevent saturation
before differentiation, 1000 ohms impedance, dc-coupled
OUTPUT: Positive or bipolar, 0-1OV linear with 11.5V saturation into 1000 ohms. 0-9V linear with 10V
saturation into 100 ohms, 0.5 ohm output impedance
SHAPING: Active network filter resulting in approximately Gaussian shape, peak amplitude at 1.5/isec
for unipolar and l.ljusec for double clip, crossover at 2.5;usec for bipolar
MAXIMUM GAIN: 640
GAIN CHANGE RANGE: 107:1 total, 32:1 in factors of 2 by switches with 1% accuracy, 3:1 continuous
fine control
LINEARITY: ±0.15% over specified linear range
NOISE: 10/uV at maximum gain and single clip, 12/xV at maximum gain and double clip, both referred
to input
SHORT CIRCUIT LIMITS: The amplifier will sustain a direct short on the output for an indefinite
period for counting rates up to 10^ Hz
COUNTING RATE: Less than 0.5% gain shift and 0.25% resolution spread fullwidth at half maximum
for a pulser peak above a 50K cts/sec ' ^ 'Cs background
OVERLOAD: Recovery within 2% of rated output from 600 times overload in 2.5 non-overloaded pulse
widths (25iisec) at maximum gain and specified input
CROSSOVER WALK: ±3 nsec for 10:1 dynamic range with 1;usec bipolar shaping (including amplifier
and ORTEC 420 Timing Single Channel Analyzer)
OPERATING TEMPERATURE: 0 to 50^0
TEMPERATURE STABILITY: 0.02%/oc
POWER REQUIRED: (Supplied by 401A/402A Power Supply) +24 V, 35 mA; -24 V, 20 mA; +12 V,50
mA;-12 V, 70 mA.

3-1
3. INSTALLATION
3.1 General Installation Considerations
The 485, used in conjunction with a 401A/402A Bin and Power Supply,is intended for rack mounting;
therefore, it is necessary to ensure that vacuum tube equipment operating in the same rack with the
485 has sufficient cooling air circulating to prevent any localized heating of the all-semiconductor
circuitry used throughout the 485. The temperature of equipment mounted in racks can easily exceed
120OF (50OC) unless precautions are taken.
3.2 Connection to Preamplifier
The preamplifier output signal is connected to the 485 BNC connector CN1, labeled INPUT. The input
impedance seen at the input is 1000 ohms and is dc-coupled to ground; therefore, the output of the
preamplifier must be either ac-coupled or have no dc voltage under no signal conditions.
The 485 incorporates pole-zero cancellation in order to enhance the overload characteristics of the
amplifier. Thistechnique requires matching the network to the preamplifier decay time constant in order
to achieve perfect compensation. The network is variable and factory adjusted to 50^tsec to match all
ORTEC FET preamplifiers. If another preamplifier is used or more careful matching is desired, the trim
is accessible from the front panel. Adjustment is easily accomplished by using a monenergetic source
and observing the amplifier baseline after each pulse overload condition.
Preamplifier power of-h24V, -t-12V, -12V, and -24V is available on the preamp power connector, CN3.
When using the 485 with a remotely located preamplifier (i.e., preamplifier-to-amplifier connection
through 25 feet or more of coaxial cable), ensure that the characteristic impedance of the transmission
line from the preamplifier output to the 485 input is matched. Since the input impedance of the 485
is 1000 ohms, sending end termination will normally be preferred; i.e., the transmission line should be
series terminated at the output of the preamplifier. All ORTEC preamplifiers contain series terminations
which are either 93 ohms or variable.
3.3 Connection of Test Pulse Generator
3.3.1 Connection of Pulse Generator to the 485 Through a Preamplifier
The satisfactory connection of atest pulse generator such as the ORTEC 419 or equivalent depends
primarily on two considerations: (1) the preamplifier must be properly connected to the 485 as
discussed in Section 3.2, and (2) 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.
3.3.2 Direct Connection to Pulse Generator to the 485
Since the input of the 485 has 1000 ohms input impedance, 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 input
of the 485 is dc-coupled to the first amplifier stage. The ORTEC 204 or 419 Test Pulse Generators
are designed for direct connection. When either of these units is used, it should be terminated
with a 100 ohm terminator at the amplifier input. (The small error due to the finite input imped
ance of the amplifier can normally be neglected.)
3.3.3 Special Test Pulse Generator Considerations for Pole-Zero Cancellation
The pole-zero cancellation network in the 485 is factory adjusted for a 50^isec decay time to match
ORTEC FET preamplifiers. When a tail pulser (such as the 204 or 419) is connected directly to one
of the amplifier inputs, the pulser should be modified to obtain a 50^isec decay time if overload
tests are to be made (other tests are not affected). See Section 6.2 for details on this modification.

3-2
If a preamplifier is used and a tail pulser connected to the preamplifier pulser input, similar
precautions are necessary. In this case, the effect of the pulser decay must be removed; i.e.,
a step input should be simulated. Details for this modification are given in Section 6.2.
3.4 Connection to Power - Nuclear Standard Bin, ORTEC 401A/402A
The 485 contains no internal power supply; therefore, it must obtain power from a Nuclear Standard
Bin and Power Supply such as the 401A/402A. Turn power off when inserting or removing modules.
The ORTEC Modular Nuclear Instrument Series is designed so that it is not possible to overload the bin
power supply with a full complement of modules in the Bin. This may not be true when the Bin
contains modules other than those of ORTEC design, and in this case, check the power supply voltages
after insertion of the modules. The 401A/402A has test points on the power supply control panel to
monitor the dc voltages.
3.5 Output Connections and Terminating Considerations
The 485 linear output can be switch selected for either a unipolar or a bipolar output. The unipolar
output should be used for high resolution spectrometry applications with semiconductor detectors. The
bipolar output should be used in applications requiring high counting rates or crossover timing. Typical
system block diagrams for a variety of experiments are described in Section 4.
The source impedance of the 0-10 volt standard linear outputs of most ORTEC modules is approxi
mately 1 ohm. Interconnection of linear signals is thus non-critical, since the input impedance of circuits
to be driven is not important in determining the actual signal span, e.g., 0-10 volts, delivered to the
following circuit. Paralleling several loads on a single output is therefore permissible while preserving
the 0-10 volt signal span. Short lengths of interconnecting coaxial cable (up to approximately 4 feet)
need not be terminated. However, if a cable longer than approximately 4 feet is necessary on a linear
output, it should be terminated in a resistive load equal to the cable impedance. Since the output
impedance is not purely resistive and is slightly different for each individual module when a certain
given length of coaxial cable is connected and is not terminated in the characteristic impedance of the
cable,oscillations will occasionally be observed. These oscillations can be suppressed for any length of
cable by properly terminating the cable, either in series at the sending end or in shunt at the receiving
end of the line. To properly terminate the cable at the receiving end, it may be necessary to consider
the input impedance of the driven circuit, choosing an additional parallel resistor to make the combina
tion produce the desired termination impedance. Series terminating the cable at the sending end may be
preferable in some cases, where receiving end terminating is not desirable or possible. When series
terminating at the sending end, full signal span, i.e., amplitude, is obtained at the receiving end only
when it is essentially unloaded or loaded with an impedance many times that of the cable. This may
be accomplished by inserting a series resistor equal to the characteristic impedance of the cable internally
in the module between the actual amplifier output on the etched board and the output connector. This
impedance is in series with the input impedance of the load being driven, and in the case where the
driven load is 900 ohms, a decrease in the signal span of approximately 10% will occur for a 93-ohm
transmission line. A more serious loss occurs when the driven load is 93 ohms and the transmission
system is 93 ohms. In this case, a 50% loss will occur. BNC connectors with internal terminators are
available from a number of connector manufacturers in nominal values of 50, 100, and 1000 ohms.
ORTEC stocks in limited quantity both the 50 and 100 ohm BNC terminators. The BNC terminators are
quite convenient to use in conjunction with a BNC tee.
3.6 Shorting the Amplifier Output
The output of the 485 is ac-coupled with an output impedance of about 0.5 ohm. If the output is
shorted with a direct short-circuit and the amplifier counting rate exceeds 10,000 counts per second,
the output stage will eventually heat up sufficiently to destroy itself (about one minute for 10 cts/sec).
The amplifieroutput may be shorted indefinitely without catastrophic damage at rates below lO" cts/sec.

4-1
4. OPERATING INSTRUCTIONS
4.1 Controls and Connectors
4.1.1 Description
FINE GAIN: A single-turn fine gain control is provided for a dynamic range of 3:1. The Fine Gain
factor is selectable from X3 through X10.
COARSE GAIN: Coarse gain control is provided by a rotary switch. The binary selector switch
covers the range of X2 to X64.
UNIPOLAR-BIPOLAR: Either unipolar or bipolar output pulses are switch selectable. The gains
are matched in both modes to approximately ±2.5%.
PZ-TRIM: A trim potentiometer is provided to adjust the pole-zero cancellation network for
varying preamplifier decay times.
INPUT: A front panel BNC connector for preamplifier pulses having either polarity. Preamplifier
pulses should have less than 0.25;usec rise time and a 50/isec decay time.
POS-NEG: A switch to accommodate either positive or negative input signals from a preamplifier.
OUTPUT: A front panel BNC connector providing a low impedance shaped output, with a test
probe adjacent for oscilloscope monitoring.
PREAMP: A connector providing ±12 and ±24 volts for preamplifier power. This furnishes direct
compatibility for all ORTEC transistor and FET preamps.
4.1.2 Front and Rear Panel Connector Data
Connector
See Dwg
485-0101-S1
Generic
Designation
Test
Point
Output
or Input
Impedance
Shape and Amplitude
Limitations
CN-1 INPUT No 1000J2 Positive or negative. Less
than 0.25^tsec rise time and
SOjusec decay time is rec
ommended, although faster
rise and slower decay times
can be handled. 6V maxi
mum linear, 12V absolute
maximum
CN-2
CN-3
OUTPUT
PREAMP
POWER
TP-1
No
0.5J2
dc
Positive or bipolar, 0-10V
linear with 11.5V satura
tion into lOOOrZ, 0-9V
linear with 10V saturation
into 100J2, approximate
Gaussian shape
Pin 1: Gnd
Pin 6: -24V
Pin 7: +24V
Pin 9: -12V
Pin 4: +12V

4-2
4.2 Initial Testing and Observation of Pulse Waveforms
Refer to Section 6 for information on testing performance and observing waveforms at front panel test
points.
4.3 General Considerations for Operation with Semiconductor Detectors
4.3.1 Calibration of Test Pulser
The ORTEC 204, 419, or 480 Precision Pulse Generators, or equivalent, may easily be calibrated
so that the maximum pulse height dial reading (1000 divisions) is equivalent to a 10-MeV loss in a
semiconductor radiation detector. The procedure utilizing the 419 is as follows:
(1) Connect the detector to be used to the spectrometer system; i.e., preamp, main amplifier,
and biased amplifier.
(2) Allow particles from a source of known energy (a-particles, for example) to fall on the
detector.
(3) Adjust the amplifier gains and the bias level of the biased amplifier to give a suitable output
pulse.
(4) Set the pulser PULSE HEIGHT potentiometer at the energy of the a-particles striking the
detector (e.g., for a 5.1 MeV a-particle, set the dial at 510 divisions).
(5) Turn on the pulser and use the NORMALIZE potentiometer and the attenuators to set the
output due to the pulser to the same pulse height as the pulse obtained in (3) above.
(6) The pulser is now calibrated; the dial reads in MeV if the number of dial divisions is divided
by ICQ.
4.3.2 Amplifier Noise and Resolution Measurements
As shown in Figure 4.1, the preamplifier, amplifier, pulse generator, oscilloscope, and a wide-band
rms voltmeter such as the Hewlett-Packard 400D are required for this measurement. Connect a
suitable capacitor to the input to simulate the detector capacitance. To obtain the resolution
spread due to amplifier noise:
(1) Measure the rms noise voltage (Ej-^pg) at the amplifier output.
(2) Turn on the pulse generator and adjust the pulser output to any convenient readable voltage,
Eq, as determined by the oscilloscope.
(3) The full width at half maximum (fwhm) resolution spread due to amplifier noise is then
2.660 Epppg E(jja|,
N (fwhm)
Eo
where Ejjjai is the pulser dial reading in MeV and the factor 2.660 is the correction factor
for rms to fwhm (2.35) and noise to rms meter correction (1.13) for average-indicating
voltmeters such as the Hewlett-Packard 400D. A true rms voltmeter does not require the
latter correction factor.

4-3
(DETECTOR
OR
CAPACITOR)
200162
LINEAR AMP
ORTEC
VOLTMETER
RMS
PREAMPLIFIER
ORTEC
PULSE
GENERATOR
ORTEC
OSCILLOSCOPE
Figure 4.1. Measuring Amplifier and Detector Noise Resolution
The resolution spread will depend upon the total input capacitance, since the capacitance degrades
the signal-to-noise ratio much faster than the noise. A typical resolution spread versus external
input capacitance for the 118A-435A system is shown in Figure 4.2. The same general curve is
applicable with the 109A-485 system, although the intercept on the keV axis will be higher.
100
0)
O
JC.
S
X
10
485 AMPLIFIER
118A PREAMPLIFIER
^ M
sec . b
'ir
j MMC
y
f
y
X
a
1 oiiTac
200073
10 100
EXTERNAL INPUT CAPACITY (pf)
1000
Figure 4.2. Resolution Spread Versus External Input Capacity

4 - 4
4.3.3 Detector Noise Resolution Measurements
The same measurement described in Section 4.3.2 can be made with a biased detector instead of
the external capacitor used to simulate the detector capacitance. The resolution spread will be
larger because the detector contributes both noise and capacitance to the input. The detector
noise resolution spread can be isolated from the amplifier noise spread if the detector capacity is
known, since
Ndet^ + Namp^ ~ '^total^
where N^Q^^gl resolution spread and Ngmp's f^e amplifier resolution spread with the
detector replaced by its equivalent capacitance.
The detector noise tends to increase with bias voltage, but the detector capacitance decreases, thus
reducing the resolution spread. The overall resolution spread will depend upon which effect is
dominant. Figure 4.3 shows curves of typical total noise resolution spread versus bias voltage,
using the data from several ORTEC silicon semiconductor radiation detectors.
.8
</)
I-
_j
o
>
(Si
o
z
(fi
s.
cr
0
A - ORTEC BA-030-007-300
B - ORTEC BA-025-050-100 .
C- ORTEC BA-025-100-100
U- UK1tU W
E - ORTEC B/I
1-045^50-100
E
■>—
D
.c
—
F
.A
0
MTce
200159
25
100
125
50 75
BIAS VOLTAGE
Figure 4.3. Amplifier and Detector Noise Versus Bias Voltage
4.3.4 Amplifier Noise and Resolution Measurements Using a Pulse Height Analyzer
Probably the most convenient method of making resolution measurements is with a pulse height
analyzer as shown by the set-up illustrated in Figure 4.4.
The amplifier noise resolution spread can be measured directly with a pulse height analyzer and the
mercury pulser as follows:
(1) Select the energy of interest with an ORTEC 419 Pulse Generator, and set the Active
Filter Amplifier and Biased Amplifier GAIN and BIAS LEVEL controls so that the energy
is in a convenient channel of the analyzer.
(2) Calibrate the analyzer in keV per channel, using the pulser (full scale on the pulser dial is
10 MeV when calibrated as described in Section 4.3.1.).

4-5
(3) The amplifier noise resolution spread can then be obtained by measuring the full width at
half maximum of the pulser spectrum.
The detector noise resolution spread for a given detector bias can be determined in the same manner
by connecting a detector to the preamplifier input. The amplifier noise resolution spread must be
subtracted as described in Section 4.3.3. The detector noise will vary with detector size, bias
conditions, and possibly with ambient conditions.
DETECTOR
OR
CAPACITOR
200091
ORTEC
LINEAR
AMPLIFIER
PULSE
GENERATOR
ORTEC
ORTEC
BIASED
AMP
PREAMP
ORTEC
ORTEC
PULSE
STRETCHER
MULTICHANNEL
PULSE HEIGHT
ANALYZER
Figure 4.4. Measuring Resolution with a Pulse Height Analyzer
4.3.5 Current-Voltage Measurements for Silicon and Germanium Detectors
The amplifier system is not directly involved in semiconductor detector current-voltage measure
ments, but the amplifier serves well to permit noise monitoring during the measurements. The
detector noise measurement is a more sensitive method of determining the maximum detector
voltage which should be used, because the noise increases more rapidly than the reverse current at
the onset of detector breakdown.
Figure 4.5 shows a typical setup required for current-voltage measurements. The ORTEC 428
Bias Supply is used as the voltage source. Bias voltage should be increased slowly, and reduced
when noise increases rapidly as a function of applied bias. Figure 4.6 shows several typical
current-voltage curves for ORTEC silicon detectors.
When it is possible to float the microammeter at the detector bias voltage, the alternate method of
detector current measurement shown by the dashed lines in Figure 4.5 is preferable. The detector
is grounded as in nominal operation and the microammeter is connected to the current monitoring
jack on the 428 Detector Bias Supply.

4-6
I MICRO- I
' «"MCTCD I
I ^ I'l 1*1 b h
O RTEC
DETECTOR
BIAS SUPPLY
w I > I
I I
CURRENT MONITORING JACKS
DETECTOR
BYPASS
CAPACITOR
ORTEC ORTEC
RMS
PREAMPLIFIER
AMPLIFIER
VOLTMETER
MICRO-
AMMETER
ORTEC
200093
Figure 4.5. Measuring Detector Current-Voltage Characteristics
I r I I
ORTEC BA-030-007-300
ORTEC BA-025-050-100
ORTEC BA-025-100-100
ORTEC BA-030-200-100
ORTEC BA-045-450-100
50 70
BIAS VOLTAGE
Figure 4.6. Silicon Detector Back Current Versus Bias Voltage