Pyramid D100 User Manual

D100

Digital Pulse Processor

User Manual

Pyramid Technical Consultants, Inc.
1050 Waltham Street Suite 200, Lexington MA 02421 USA

US: TEL: [phone] ♦ FAX: [phone] ♦ EMAIL: [e-mail]
Europe: TEL: [phone]

PSI System Controls and Diagnostics

1 Contents

Safety Information ......................................................................................................................................................5

Models...........................................................................................................................................................................7

Scope of Supply............................................................................................................................................................8

Optional Items .............................................................................................................................................................9

Power supplies..........................................................................................................................................................9

Signal cables and cable accessories.........................................................................................................................9

Data cables...............................................................................................................................................................9

Fiber-optic loop........................................................................................................................................................9

Intended Use and Key Features ...............................................................................................................................10

Intended Use...........................................................................................................................................................10

Key Features...........................................................................................................................................................11

Specification...............................................................................................................................................................12

Installation .................................................................................................................................................................15

Mounting.................................................................................................................................................................15

Grounding and power supply .................................................................................................................................15

Connection to signal source ...................................................................................................................................16

Basic setup .........................................................................................................................................................16

Signal cables ......................................................................................................................................................17

Signal cables ......................................................................................................................................................17

Getting Started with the PSI Diagnostic host software..........................................................................................19

Installing the PSI Diagnostic Program ..................................................................................................................19

Running the PSI Diagnostic Program....................................................................................................................19

Running the PSI Diagnostic program.....................................................................................................................21

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Acquiring spectra with the PSI Diagnostic program..............................................................................................23

Principle of Operation...............................................................................................................................................26

Wideband digital pulse processing.........................................................................................................................26

D100 Circuit Overview...........................................................................................................................................27

Optimizing the Pulse Processing Algorithm...........................................................................................................30

Overview of the FPGA algorithm...........................................................................................................................30

Regions of interest .............................................................................................................................................30

Data processing stages .......................................................................................................................................31

Pulse processing user parameters..........................................................................................................................32

Shaping Time.....................................................................................................................................................32

Peak Width.........................................................................................................................................................32

Shaping Samples................................................................................................................................................32

Derivative Discriminator....................................................................................................................................32

Pulse Discriminator............................................................................................................................................32

Pile Up ...............................................................................................................................................................33

Decay Time........................................................................................................................................................34

Gain, Offset........................................................................................................................................................35

Connectors .................................................................................................................................................................36

Front panel connectors...........................................................................................................................................36

Photomultiplier control ......................................................................................................................................36

Test I/O ..............................................................................................................................................................36

Pulse signal input ...............................................................................................................................................37

Ground lug .........................................................................................................................................................37

Rear panel connectors............................................................................................................................................37

Power input ........................................................................................................................................................37

Fiber-optic direct data stream output .................................................................................................................37

Fiber-optic loop communications ......................................................................................................................37

Controls and Indicators............................................................................................................................................38

Front panel controls...............................................................................................................................................38

Rear panel controls.................................................................................................................................................38

Address switch ...................................................................................................................................................38

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Front panel indicators............................................................................................................................................39

LED block..........................................................................................................................................................39

Rear panel indicators .............................................................................................................................................39

Fault-finding .............................................................................................................................................................. 40

Maintenance...............................................................................................................................................................42

Returns procedure.....................................................................................................................................................43

Support.......................................................................................................................................................................44

Declaration of Conformity ...................................................................................................... ..................................45

Revision History.........................................................................................................................................................46

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2 Safety Information

This unit is designed for compliance with harmonized electrical safety standard EN61010­1:2000. It must be used in accordance with its specifications and operating instructions.
Operators of the unit are expected to be qualified personnel who are aware of electrical safety
issues. The customer’s Responsible Body, as defined in the standard, must ensure that operators
are provided with the appropriate equipment and training.

The unit is designed to make measurements in Measurement Category I as defined in the

standard.

CAUTION. The D100 does not generate high voltages, but is designed to control a high voltage
supply for a photomultiplier, which may typically be rated from 1000 to 2000 V DC. The user
must therefore exercise appropriate caution when using the device and when connecting cables.
Power should be turned off before making any connections.

CAUTION. The D100 features a DC-coupled input. It must only be used with photomulipliers
that produce a ground-referenced signal.

The unit must not be operated unless correctly assembled in its case. Protection from high
voltages could be impaired if the unit is operated without its case. Only Service Personnel, as
defined in EN61010-1, should attempt to work on the disassembled unit, and then only under
specific instruction from Pyramid Technical Consultants.

The unit is designed to operate from +24VDC power, with a maximum current requirement of
200 mA. A suitably rated power supply module is available as an option. Users who make their
own power provision should ensure that the supply cannot source more than 1500 mA.

A safety ground must be securely connected to the ground lug on the case.

Some of the following symbols may be displayed on the unit, and have the indicated meanings.

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PSI System Controls and Diagnostics

Direct current

Earth (ground) terminal

Protective conductor terminal

Frame or chassis terminal

Equipotentiality

Supply ON

Supply OFF

CAUTION – RISK OF ELECTRIC SHOCK

CAUTION – RISK OF DANGER – REFER TO MANUAL

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3 Models

D100 Digital pulse processor unit.

-CxRy Input amplifier feedback option capacitance x pF, resistance y kohm.

Example:

D100-C100R1.0 D100 with 100 pF and 1.0 kohm parallel feedback.

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4 Scope of Supply

D100 model as specified in your order.

USB memory stick containing:
User manual
PSI Diagnostic software guide
Software installation guide
PSI diagnostic software files
USB drivers and utilities

Optional items as specified in your order.

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5 Optional Items

5.1 Power supplies

+24 VDC 1.5 A PSU (universal voltage input, plug receptacle for standard IEC C8 socket) with
output lead terminated in 2.1mm threaded jack.

5.2 Signal cables and cable accessories

Cable, coaxial, BNC to BNC, 1 m, for input signal.

Cable, four-pin mini-DIN to unterminated, 3 m, for temperature sensing and pulser LED control.

Cable, six pin mini-DIN to unterminated, 3 m, for HV PSU control.

5.3 Data cables

Fiber-optic cable, 5m, 1 mm plastic, ST terminated.

Fiber-optic cable, 25m, 200 µm silica, ST terminated.

5.4 Fiber-optic loop

A100 serial port to fiber-optic loop adaptor.

A200 USB to fiber-optic loop adaptor.

A300 Ethernet to fiber-optic loop adaptor.

A500 intelligent multi-loop cell controller with Ethernet interface and direct input channels for
up to ten parallel D100 data streams.

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6 Intended Use and Key Features

6.1 Intended Use

The D100 is intended for the counting and spectrometry of single event current pulses generated
by sodium iodide scintillator/photomultiplier detectors operated in pulse-counting mode. The
D100 has design features which make it tolerant of electrically noisy environments, but the place
of use is otherwise assumed to be clean and sheltered, for example a laboratory or light industrial
environment. The unit may be used stand-alone, or networked with other devices and integrated
into a larger system. Users are assumed to be experienced in the general use of precision
electronic circuits for sensitive measurements, and to be aware of the dangers that can arise in
high-voltage circuits.

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6.2 Key Features

DC-coupled, wide-band analog signal stages.

Fast ADC for direct digitization of detector signal.

FPGA-based digital signal processing of detector signal.

On-board real-time pulse-height analysis / histogram generation.

On-board real-time pulse shape analysis for pile-up detection and invalid pulse rejection.

Pulse-pair resolution for sodium iodide detectors down to 500 nsec with individual pulse energy
spectrometry, or down to 50 nsec for counting.

Real-time data stream output via dedicated fiber-optic channel for region of interest counts.

Bi-directional fiber-optic communications channel for all control and sensing.

Built-in control for photomultiplier HV supply.

Built-in temperature sensing for detector stabilization.

Built-in control for LED test pulser.

Fast fiber-optic serial interfaces built-in. Selectable baud rates.

Can be operated in a fiber-optic serial communication loop with up to fourteen other devices.

100BaseT Ethernet available through the A300 and A500 interfaces.

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7 Specification

Inputs One, DC-coupled

Input impedance < 0.1 ohm in standard configuration.

Input analog channel

< 50 nsec

risetime

Digitization 14 bit, 80 MHz.

Pulse pair resolution <= 500 nsec with pulse height analysis of both pulses

< = 50 nsec with both pulses counted

Deadtime < = 50% of pulses rejected at 500 kHz average input rate

Pulse size analysis 1024 channel on-board histogramming

Pulse shape analysis Pulses accepted for processing based upon width.

Baseline compensation for pulse decay tail

Power input +24 VDC (+/-2 V), 150 mA typical without drain on 5 V output,

200 mA with maximum drain on 5 V output.

Power output +5 VDC (+/- 0.1V), 200 mA maximum

Controls Address: sixteen position rotary switch (01 to 0A valid)

Indicators Four status LEDs: power, activity, network, device

Case Stainless steel folded case.

Case protection rating The case is designed to rating IP43 (protected against solid

objects greater than 1mm in size, protected against spraying
water).

Weight 0.27 kg (0.6 lb).

Operating environment 0 to 35 C (15 to 25 C recommended to reduce drift and offset)

< 70% humidity, non-condensing
vibration < 0.1g all axes (1 to 1000Hz)

Shipping and storage
environment

-10 to 50C
< 80% humidity, non-condensing
vibration < 2 g all axes, 1 to 1000 Hz

Dimensions (see figures 1 and 2).

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M3 Ground Lug

PMT Control

6-Pin Mini DIN

TEST
4-Pin Mini DIN

Signal Input
BNC

Status LEDs

Power

Direct

Xmit

Rcv

Address

+24VDC Power In

2.1mm Jack
Fiber-optic Direct

ST Bayonet

Address
Selector

Fiber-optic RX
ST Bayonet

Fiber-optic TX

ST Bayonet

Figure 1. D100 chassis end panels. Dimensions mm.

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4X

3.66 Mounting Hole

77.5

80.2

140.0

147.1

158.1

62.0

3.6

0.74

28.2

Figure 2. D100 case side and plan views. Dimensions mm.

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8 Installation

8.1 Mounting

The D100 may be mounted in any orientation, or may be simply placed on a level surface. A
fixed mounting to a secure frame is recommended in a permanent installation for best
performance. Four clear holes are provided in the mounting flange on a 140 mm by 62 mm
rectangular pattern (see figure 2) for fixing to surfaces with a corresponding pattern of M3
threaded holes or studs.

We recommend that the signal connection to photomuliplier base should be of minimum possible
length, and not exceeding 2 m to prevent possible pulse shape degradation due to capacitive
load. Good quality 50 ohm coaxial cable should be used.

The mounting position should allow sufficient access to connectors and cable bend radii. 60 mm
minimum clearance is recommended at either end of the device.

Best performance will be achieved if the D100 is in a temperature-controlled environment. No
forced-air cooling is required, but free convection should be allowed around the case.

The D100 must not be mounted in a place with high flux of ionizing radiation, as this could lead
to soft errors and possible incorrect processing of signals. To keep the potential error rate below
one event per day, we recommend a maximum background dose rate of 1 mSv / hr.

8.2 Grounding and power supply

A secure connection should be made using a ring lug, from the M3 ground lug to local chassis
potential.

+24 VDC power should be provided from a suitably-rated power supply with the following
minimum performance:

Output voltage +24 +/- 0.5 VDC

Output current 500 mA minimum, 1500 mA maximum

Ripple and noise < 100 mV pk-pk, 1 Hz to 1 MHz

Line regulation < 240 mV

The D100 includes an internal automatically re-setting PTC fuse rated at 1.1 A. However the
external supply should in no circumstances be rated higher than the D100 connector limit of 5 A,
and a maximum of 1.5 A is recommended.

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8.3 Connection to signal source

8.3.1 Basic setup

Figure 3 shows a typical installation for a single sodium iodide / photomultiplier in schematic
form.

HV PSU

HV

HV control

Comms

NaI

Photomultiplier

Pulser LED

Base

Temperature sense and pulser

Signal

D100

Data stream

+24V in

A500

Figure 3. Schematic D100 installation

The fiber-optic communication channel is a looped system. Up to 15 devices, which could all be
D100s, or a mix of various devices) can be connected to a host computer via the A100, A200 or
A300 interfaces.

CAUTION. The D100 has a DC-coupled input, and thus must not be used with
photomultipliers that have the signal sitting on a high-voltage. Connecting the D100 input to a
high voltage will result in serious damage and will void warranty.

The D100 is optimized for DC-coupling direct to the anode. AC-coupled operation, with a
blocking capacitor in the photomultiplier base, is also possible, albeit with some degradation in
spectrometry performance. Contact your supplier or Pyramid Technical Consultants, Inc. for
information about firmware updates to optimize performance in AC-coupled systems. If you are
in any doubt about the suitability of your photomultiplier, please consult your supplier or
Pyramid Technical Consultants, Inc. before attempting to use the D100.

If you are using a photomuliplier, the base must not include a pre-amplifier, because the D100 is
designed to receive the anode current pulses directly. The high voltage supply may be external
or integrated with the base. The dynode voltage division may be resistive or active.

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If the anode is connected to ground with a load resistor in the photomultiplier base (RL in figure

3), then this should have a value of 1 kohm or greater. A 50 ohm load resistor is common in
systems where the signal is intended to be transmitted via 50 ohm coax to the 50 ohm input of a
remote current amplifier. Use of a 50 ohm load resistor with the D100 will degrade the pulse
shapes considerably, however, and is not recommended.

The D100 should be located as close to the source of the signal as possible. If space permits, it
can be connected directly to the photomultiplier base with a suitable through connector. Long
signal cables increase the chances of seeing unwanted signals and noise. A maximum length of
2m is advised. Longer cables may be used, but the energy resolution and ability to discriminate
good pulses from bad may be degraded.

8.3.2 Signal cables

Good quality coaxial cable such as RG174U or RG-58U should be used, terminated in a BNC
connector at the D100 end, and a connector to suit the photomultiplier base at the other. The
cable impedance is not critical, but the length should not exceed 2 metres to avoid degradation of
the pulse shapes. A direct connection of the D100 to the base is possible using a BNC through
connector where space permits.

8.3.3 Multi-channel systems

Combining the D100 with the A500 real-time cell controller allows multi-detector systems to be
configured, with data correlation across all the channels. The A500 provides five
communications loops, each of which can service up to fifteen devices. However, by using the
fast data output on the D100, you can configure a multi-channel system that allows simultaneous
pulse energy analysis and timing. One A500 can service up to ten fast data stream inputs, and so
control and coordinate the data from ten parallel D100s. Each incoming pulse in each detector is
analyzed in real time into one of up to 50 pre-defined energy regions of interest by the D100.
The result is transmitted immediately as a byte of digital data via the fast data channel to the
A500. By grouping the incoming data from all D100s at known spatial locations into time bins,
the A500 is able to build up a time, space and energy-resolved picture.

Very large detector arrays can be configured by linking groups of ten A500s under one master
A500, and then grouping these in another layer if required. Arrays of hundreds of detectors can
be operated in this way. Figure 4 illustrates a configuration for coordinated data acquisition
from 100 scintillation detectors.

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HV PSU

PhotomultiplierNaI Base

PhotomultiplierNaI Base

PhotomultiplierNaI Base

PhotomultiplierNaI Base

PhotomultiplierNaI Base

PhotomultiplierNaI Base

D100

HV PSU

D100

HV PSU

D100

HV PSU

D100

HV PSU

D100

HV PSU

D100

Comms

loop

Fast data
channels

A500

A500

A500

A500

A500

A500

A500

Ethernet

HV PSU

PhotomultiplierNaI Base

D100

HV PSU

PhotomultiplierNaI Base

D100

HV PSU

PhotomultiplierNaI Base

D100

HV PSU

PhotomultiplierNaI Base

D100

A500

A500

A500

A500

Figure 4. Example multi-channel system using the A500 real-time cell controller

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PSI System Controls and Diagnostics

9 Getting Started with the PSI Diagnostic host software

Before installing the D100 in its final location, and if it is the first time you have used a D100,
we recommend that you familiarize yourself with its operation on the bench. You can check the
unit powers up correctly, establish communications, and operate peripheral devices such as HV
supplies. The Pyramid Technical Consultants, Inc. PSI Diagnostic host software, provided with
your D100, provides a simple means of doing this.

9.1 Installing the PSI Diagnostic Program

The PSI Diagnostic is a stand-alone program which allows you to read, graph and log data from
the D100, and set all the important acquisition control parameters. For some applications it may
be adequate for all of your data acquisition needs.

The D100 is supplied with software programs and drivers on a USB memory stick. We
recommend that you copy the files into a directory on your host PC. Check the Pyramid
Technical Consultants, Inc. web site at www.ptcusa.com for the latest versions.

The program runs under the Microsoft Windows operating system with the 2.0 .NET framework.
This has to be installed before the PSI Diagnostic. Most new PCs have .NET 2.0 already
installed. The installer will warn if it is not already installed, and direct you to Microsoft web
site where it can be downloaded at no charge.

Install the PSI Diagnostic by running the PTCDiagnosticSetup.msi installer, and following the
screen prompts. Once the program has installed, you can run the Diagnostic at once.

9.2 Running the PSI Diagnostic Program

1) Inspect the unit carefully to ensure there is no evidence of shipping damage. If there appears
to be damage, or you are in doubt, contact your supplier before proceeding.

2) Connect +24 V DC power but no other connections. The power LED should illuminate when
the power is applied, and the other three LEDs should flash.

4) Set the address rotary switch to position “1” (address 1).

5) Run the PTCDiagnostic host program on a Windows PC.

6) Connect a fiber optic loop adaptor (A100, A200, A300 or A500) to the PC. Connect to the
loop adaptor to the D100 via fiber-optic cable. There is no need to connect to a detector at this
stage.

Note that it is possible to connect a D100 via a simple RS-232 to fiber-optic through converter
such as the Group 3 Model FTR. However this means that the fiber-optic loop and thus D100
have to work at 115k bps rather than the normal 10M bps. To allow this, you would need to
open the D100 case and fit a link to JPR2 option position 1. We do not recommend this
configuration for general use, due to the much reduced data rate.

7) Start the PSI Diagnostic. It will search the available communication channels on the PC and
present a search list. If you are using an A300 or A500 the list does not already include its IP
address, enter this into the edit box followed by :100 for the network port (for example

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PSI System Controls and Diagnostics

192.168.100.100:100), and click “Add”. The network port is fixed at :100 in A500s. Future
versions may lift this restriction in case there are conflicts on particular networks.

Figure 5. PSI Diagnostic Search Utility

Check the box next to the ports you want included in the search. Click “start” and the program
will search for loops and devices on all checked options.

The LAN Broadcast search uses information from the PC operating system to search for A500s
over the whole of your local network. This will generally take longer than going directly to a
known IP address.

A few seconds after you click the “Start” button, the program should find the D100 (plus any
intelligent loop controllers such as the A500). Expanding the tree in the System window will
reveal the all devices that were discovered. Clicking on any discovered device will open its
specific window.

8) Now you can explore the D100 PTCDiagnostic screens.

9.3 Running the PSI Diagnostic program

Clicking on the D100 entry in the System window search tree brings up the D100 screen with the
data tab showing. This is where energy spectra will display, where you can see readbacks for
temperatures, and set the DAC output that programs a high voltage supply for a multiplier.

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Figure 6. D100 window selected from the discovered devices

Clicking on the setup tab will display the controls for the FPGA digital pulse analysis, described
in section 11, plus other parameters which control how the PSI Diagnostic handles and displays
the data.

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Figure 7. D100 window setup tab showing FPGA algorithm parameters GUI display options

The Bias Scale parameter should be set so that the high voltage supply output scales correctly for
the 0 to 2.048 output from the D100. The value is the output voltage per volt of demand. Check
that the program voltage to the HV supply appears at the appropriate pins (see section 12.1)
when the bias voltage is set.

CAUTION. Take care when connecting and using high voltage supplies. Do not attempt a
direct measurement of the high voltage unless you have the appropriate test equipment and
have received the appropriate training

If you have a suitable high voltage probe, you can check that your high voltage PSU output
responds correctly to the demand from the D100, before connecting up to the photomultiplier.

Clicking on the Device tab shows details of the communication channel, installed software and
serial number. On the right it exposes the application code upload utility. A new firmware code
file can be uploaded from the host PC at any time.

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PSI System Controls and Diagnostics

Figure 8. D100 window device tab showing communication status, version numbers and code
upload utility.

9.4 Acquiring spectra with the PSI Diagnostic program

If you have the parts available to hand, you can connect up a scintillator and photomultiplier on
the bench as shown in figure 3. It is best if you can use parts that are known to work, rather than
untested ones. Check that the high voltage can be enabled and is stable. Finally, with a suitable
test radiation source, you can acquire an energy spectrum, and calibrate the energy scale.

22

Figure 9 shows an example

Na gamma spectrum. It is the result of binning the pulse areas into
1024 equal regions. Clicking and dragging in the spectrum window defines a region of interest
(not to be confused with the real-time histogramming regions of interest described later). The
software assumes you have highlighted a defined peak, and attempts to fit a Gaussian curve plus
a sloping background to the data. The position and full-width half maximum of the fitted peak
are displayed. Right-clicking brings up a menu, and you can select the calibrate option to use
this peak in an energy calibration, presuming you know its true energy. If you repeat this
process with a second peak, a linear calibration will be performed immediately.

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PSI System Controls and Diagnostics

Figure 9. 22Na spectrum: defining peak positions for an energy calibration.

Once you have two or more peaks in the calibration table, you can apply the calibration, and the
x axis will convert to an energy scale in keV. The cursor now reads out in energy. You can
define multiple regions of interest, and the software will display the total counts accumulated
into each of them.

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PSI System Controls and Diagnostics

Figure 10. 22Na spectrum after energy calibration. Multiple regions of interest selected.

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10 Principle of Operation

10.1 Wideband digital pulse processing

Traditional photomultiplier pulse-processing energy analysis systems generally feature a Pre­amplifier close to, or in, the photomultiplier base. Depending on signal to noise considerations,
the preamplifier may be a fast current amplifier or a charge integrator. The preamplifier signal is
transmitted to a shaping amplifier which produces an output whose height reflects the amount of
charge delivered by the photomultiplier. For energy spectrometry the pulse height is measured
by an analog to digital converter, for example in a multi-channel analyzer. For simple pulse
counting, the pulse is compared with low and possibly high limits in a single channel analyzer
and scaled.

The D100 uses a different approach. The basic pulse shape from the multiplier anode is
preserved and digitized at high rate. All pulse size and shape analysis then occurs in a FPGA
(field-programmable gate array) device using numerical processing algorithms. The algorithms
are executed in real time on a pulse by pulse basis, and can include various characterizations in
addition to the traditional pulse size analysis. These can include pulse width analysis and pulse
shape analysis for pileup rejection, and dynamic background correction.

PMT

PMT

Integrating
amplifier

D100

Fast
amplifier

Shaping
amplifier

ADC

FPGA

Pulse

height

analyzer

Figure 11. Comparison of traditional pulse processing (above) and D100 pulse processing
(below).

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10.2 D100 Circuit Overview

y

39pF

499R

x

50R

50R

39pF

R

R

39pF

F

50R

39pF

50R

F

ADC

80 MHz

14 bit

LEDs

Microcontroller

DC-DC

converters

FPGA

DAC

+24 V in

Communication
loop

Data stream out

PMT HV program
PMT gate

Test pulser LED
Temperature sense

Figure 12. D100 block schematic.

The amplifier stage is a fast current to voltage converter with 499 ohm feedback. The circuit
allows optional series input resistance at x and different feedback at y in order to adapt to
different input signal characteristics. The values shown are those for readout of sodium iodide
scintillators. A 50 ohm high speed differential amplifier then provides voltage gain and
conditions to the signal for the ADC input. The gain of this stage is given by 0.01*RF. The
nominal value of RF is 316 ohm. ADC conversions are streamed to the FPGA which executes
the pulse processing algorithms in the digital domain. There are auxiliary outputs for the high
voltage program (0 to +2.048 V), the gate control (5V differential) and a test pulser LED (5 V
through 2 kohm). There is an input for a remote AD596 temperature sensor.

The input stages do provide a small amount of integration of the signal, in order to improve
signal to noise ratio and ensure stability. However the basic shape of the sodium iodide
scintillator / photomultiplier output is preserved so that it can be used by the discrimination
algorithms. Figure 13 shows a circuit simulation result which illustrates this behavior.

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/div

mV

Y2

300

250

200

150

100

50

0

-50

-100

Y1

300

250

200

150

100

uA

50

0

-50

-100
0 100 200 300 400 500 600 700 800 900

Time/nSecs 100nSecs

Figure 13. Circuit simulation, current and voltage waveforms in the D100 input stages, 30 pF
input load.

The red curve is a simulated sodium iodide detector output current pulse, which typically
includes a very fast rise followed by an exponentially falling tail with decay constant around 250
nsec. The blue curve is the current that flows into the amplifier, after some integration by a 30
pF load capacitance, for example from one foot of coaxial cable. The dark green curve is the
voltage output of the first stage, and the orange curve is the signal fed to the ADC.

Capacitive load on the input causes unwanted integration and pulse distortion by any, which is
why this should be minimized. Figure 14 shows the simulation result with 180 pF of load
capacitance, corresponding to about 2 meters (6 feet) of coaxial cable. The distortion appearing
in the pulse shape makes the task of the pulse overlap discrimination algorithms more difficult.

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/div

mV

Y2

300

250

200

150

100

50

0

-50

-100

Y1

300

250

200

150

100

uA

50

0

-50

-100
0 100 200 300 400 500 600 700 800 900

Time/nSecs 100nSecs

Figure 14. Circuit simulation, current and voltage waveforms in the D100 input stages, 180 pF
input load.

D100 User Manual D100_UM_081027 Page 29 of 46

PSI System Controls and Diagnostics

11 Optimizing the Pulse Processing Algorithm

11.1 Overview of the FPGA algorithm

The energy required to create a scintillation photon in sodium iodide crystal is nearly constant at
25 eV per photon. Combined with the linear response of the photomultiplier, this means that the
energy of the original ionizing particle or photon is encoded in the amount of charge in the pulse
delivered from the anode of the multiplier. The task of the D100 is thus to integrate the current
pulses, making corrections as necessary for background offsets and overlapping pulses. The
algorithm running in the D100 FPGA is designed to extract the best energy resolution even when
there is the potential for degradation due to pulse overlap at high input rates. Pulses which have
been compromised by overlap are not included in the energy spectrum, but they can still be
counted.

Note that all processing at the FPGA level is done on raw binary numbers for speed. Conversion
to an absolute energy scale in eV is a function of the host software.

11.1.1 Regions of interest

When pulses have been discriminated and integrated, the D100 can assign them in real time to a
region of interest (ROI) according to their integral. This process is also called histogramming. It
is of course equivalent to putting the pulses into energy bins.

The normal D100 operation mode delivers 1024 channels of energy resolved spectral data to the
host over the bi-directional message channel between the D100 and the host. This allows a
conventional energy spectrum display, and is convenient for making the translation to an
absolute energy scale.

An alternate, fast pathway is available that allows pulses to be discriminated and delivered to the
A500 processor in real-time via dedicated fibers. It is only available when you are using the
A500 real-time controller to interface the D100. Up to ten D100 boards can be placed in this
mode per A500. The regions of interest for this fast energy discrimination mode are defined in
the host software in a standard file interface and downloaded to the D100. The file is a sequence
of 50 integers separated by carriage return, line feed. The entries must be in ascending order.
The first must be 0 and the last 65535, so covering the full available energy range. The regions
of interest are contiguous but do not have to be the same width.

Note that these ROIs are not the same as the regions that can be defined in the energy spectrum
in the PSI Diagnostic host software.

D100 User Manual D100_UM_081027 Page 30 of 46

PSI System Controls and Diagnostics

11.1.2 Data processing stages

Pile up usec

Pile-up

detection

Derivative discrim

Peak width min, max

Background

correction

Decay time %

Rise &

Width

detector

Integration

Shaping time
Pulse discrim

Validation

(Rejected
completely)

Validation

(Rejected
from ROI)

ROI

assignment

Scaler

Current

accumulator

Current denom

Current time usec

Figure 15. Processing stages in the D100 pulse processing

The incoming stream of ADC samples is first corrected for any background resulting from any
inherent offset of the photomultplier and the input amplifiers, and then for the tails of prior
pulses, based upon a standard model of the pulse shape. The width detector looks for the sudden
increase in slope that means a pulse is starting. As the pulse passes through its peak and falls
again, its width at half maximum is calculated and compared with a minimum and maximum
value. If the result of these three discriminations is positive, then the integration is accepted and
continues until the specified shaping time is reached. Otherwise the pulse is rejected. It is not
assigned to a region of interest, because any attempt to gain energy information would have been
too compromised by the overlap. However it is counted, as a rejected pulse.

The D100 counts the number of pulses accepted and pulses which were rejected for not meeting
the width requirements or for violating the shaping time or pile up time constraints. Pulses
which are too small to exceed the pulse or derivative discriminator are not counted, however, as
they are generally due to noise. By monitoring the counters for accepted and rejected pulses you
can optimize the resolution verses efficiency tradeoff as you tune the pulse processing
parameters.

There is a parallel data path to a current accumulator function. This accumulates and averages
all ADC conversions over a specified time period. It provides an independent, somewhat crude,
but robust indication of the input count rate. Its main function is to give an indication of detector
saturation.

D100 User Manual D100_UM_081027 Page 31 of 46

PSI System Controls and Diagnostics

11.2 Pulse processing user parameters

The adjustable parameters have the following functions:

11.2.1 Shaping Time

This parameter controls the behavior of the numerical integrator. If the rising edge of a pulse is
detected at time t, the D100 will numerically integrate the background corrected ADC data until
time = t + Shaping Time. You would normally set this to match the nominal pulse width from
the photomultiplier, plus a small allowance for integration in the D100. Larger shaping time
will give more accurate pulse energies because more of the exponential pulse tail will be
included. However if two pulses occur within Shaping Time of each other, both pulses will be
discarded, so there is an inevitable trade-off against count rate capability. Shaping Time has a
granularity of 12.5 nsec. A typical value for sodium iodide is 0.3 µsec, but you can try values in
the range 0.1 to 1.0 µsec to search for an optimum compromise of resolution against throughput
at your typical maximum count rate.

11.2.2 Peak Width

The D100 measures the width of the pulses at half maximum. If the width falls outside of the
range provided by the Min and Max parameters, it is discarded. The range is inclusive; if the
width is exactly equal to Min or Max, it is counted as valid. Typical values for sodium iodide
are Min = 0.05 µsec and Max = 1.50 µsec but you may wish to try smaller ranges to reject more
pulses and thus improve energy resolution.

11.2.3 Shaping Samples

This parameter is reserved for future development. You may set it to zero.

11.2.4 Derivative Discriminator

This parameter is sets the leading edge slope that must be seen for a pulse to be recognized.
Pulses which do not reach this value will be neither analyzed nor counted. It should be fixed for
a given combination of scintillator and photomultiplier. A value of 10 (increase in counts per
conversion) or more is appropriate for sodium iodide with most photomultipliers.

11.2.5 Pulse Discriminator

This is the height above background that a pulse must reach to be recognized as a valid pulse.
Pulses which do not reach this value will be neither analyzed nor counted. It should be fixed for
a given combination of scintillator and photomultiplier. A value of 10 to 15 counts is
appropriate for sodium iodide with most photomultipliers, but you may need to increase this if
there is a high background noise level.

D100 User Manual D100_UM_081027 Page 32 of 46

PSI System Controls and Diagnostics

300
250
200
150
100

ADC values

50

0

-50

Differential
Discriminator (sl ope)

Pulse
Discriminator

30
25
20
15
10
5
0

-5

Slope

Figure 16. Detail of the ADC conversions at the start of a pulse

11.2.6 Pile Up

If two pulses are within Pile Up Time of each other, the first is counted and analyzed, the second
is not. Typical values for sodium iodide are from 0.2 usec to 2.0 usec. You can set pile up time
equal to shaping time if you wish.

Figure 17 illustrates how two pulses with varying degrees of overlap mare handled. The cyan
bar represents the shaping time, and the yellow bar is the pile up time.

Both pulses
counted and analyzed

Both pulses
counted, only first
pulse analyzed

Neither pulse
counted or ana l yzed

Figure 17. Pile up rejection conditions.

The second pulse in figure 17 is still able to extend the deadtime. For example if Pile Up = 3
usec, Shaping Time = 2 µsec, and if the first pulse is at t = 0, the second at t = 2.5 µsec, and a
third arrives at at t = 5 µsec, then only the first pulse is energy analyzed and assigned to a region
of interest. All three pulses are counted, however. In other words, the D100 implements a
paralyzable deadtime model.

11.2.7 Decay Time

Although pulses with gross overlap are identified and rejected for analysis by the pile up
parameter, there can still be errors in the integration if a pulse starts on the extended tail of a
prior pulse. For a given scintillator type, the pulses decay at a specific rate. By knowing this
rate, we can subtract that decay away, and therefore reduce the interference between pulses.

D100 User Manual D100_UM_081027 Page 33 of 46

PSI System Controls and Diagnostics

A

The decay time correction works by assuming that every pulse is sitting on the tail of a prior
pulse which is decaying exponentially. The decay time parameter is the fraction of that signal
which is the fraction to which it falls in following ADC conversions, expressed as a percentage,
ie

DecayTime

=

S Sn+1

100

S

1

−

nn

This can be related to the time constant of the exponential decay because we know the time
interval between successive conversions occurring at 80 MHz is 12.5 nsec. Thus

τ

5.12−

= eDecayTime

where τ is the time constant in nsec. For example, if τ is 250 nsec, then Decay Time should be

0.951, or 95.1%.

Figure 18 shows how the effect of the tail of a preceding pulse is handled by this process. The
overlap is exaggerated for clarity, and the pulses are normalized to unit height. In practice
pulses with this amount of overlap would be discriminated out by the Pile Up test. Figure 19
shows how the correct integrated area of the second pulse is recovered by the subtraction.

1.6

1.4

1.2
1

0.8

0.6

0.4

0.2
0

0 20406080100120140

First point of
assumed decay tail

ssumed tail

Figure 18. Overlapping pulses showing assumed continuation of the first pulse under the
second.

1.6

1.4

1.2
1

0.8

0.6

0.4

0.2
0

0 20406080100120140

Figure 19. Effect of overlapping prior pulse on integrated area of second pulse removed by
subtracting the assumed decay tail.

D100 User Manual D100_UM_081027 Page 34 of 46

PSI System Controls and Diagnostics

11.2.8 Gain, Offset

The D100 has a standard default 1024 bin diagnostic histogram that is available before any
regions of interest are sent by the host. Binning into this diagnostic histogram is according to the
following equation:

Bin = (Pulse integral - Offset)*Gain / 32

If the resulting value of bin lies outside the range 0 to 1023, it is forced to the relevant limit.

11.2.9 Current Time

This is the time window that the current accumulator sums over, in microseconds. You should
set it to achieve a good compromise between response time and noise immunity. A value of
1000 is typical.

D100 User Manual D100_UM_081027 Page 35 of 46

PSI System Controls and Diagnostics

12 Connectors

12.1 Front panel connectors

12.1.1 Photomultiplier control

Six pin mini-DIN female. To mate with Conec MD-60, Singatron 62000-6P or equivalent.

View looking at connector (= solder side of mating connector)

1 HV enable out -ve (differential with 2)
2 HV enable out +ve (differential with 1)
3 HV program out (0 to +2.048 V)
4 Analog gnd (reference for 3)
5 +5 VDC power out (200 mA max)
6 Digital gnd (reference for 5)

A differential pair programmable enable output is provided as a differential (RS-485 style) pair
on pins 1,2. This is typically used to gate off the multiplier for some external event by turning
off the high voltage.

+5 V power is provided to power a modular high voltage supply. These typically require 50 to
100 mA at 5V.

12.1.2 Test I/O

Four pin mini-DIN female. To mate with CUI MD-40, Conec 301A10479X or equivalent.

View looking at connector (= solder side of mating connector)

1 Test LED pulser output +ve (+5V)
2 Test LED pulser output rtn
3 PMT temperature sensor input (AD592 +ve)
4 PMT temperature sensor input (AD592 -ve)

The test pulser LED is returned to digital ground on pin 2 via 2 kohm and a switching transistor
controlled by the FPGA. For a typical LED the forward current in the on state will be 1.5 mA.

D100 User Manual D100_UM_081027 Page 36 of 46

PSI System Controls and Diagnostics

The AD592 is a precision temperature transducer from Analog Devices with 0.5 C absolute
accuracy. It acts as a current source generating 1 uA C

-1

. The device should be mounted on the

photomultiplier in a position that measures the temperature that affects the multiplier gain.

+

-

3 2 1

Pin 2 n/c

AD596
View from below

12.1.3 Pulse signal input

BNC socket (female). To mate with standard signal BNC.

Core: signal

Outer: screen

12.1.4 Ground lug

M3 threaded stud. To mate with M3 ring lug.

12.2 Rear panel connectors

12.2.1 Power input

2.1 mm threaded jack. To mate with Switchcraft S761K or equivalent

Center pin: +24VDC

Outer: 0V

12.2.2 Fiber-optic direct data stream output

Fiber-optic transmitter, ST bayonet. Light gray shell.

12.2.3 Fiber-optic loop communications

Transmitter / receiver pair, ST bayonet. Transmitter light grey shell, receiver dark grey shell.

Xmit Rcv

D100 User Manual D100_UM_081027 Page 37 of 46

PSI System Controls and Diagnostics

13 Controls and Indicators

13.1 Front panel controls

None.

13.2 Rear panel controls

13.2.1 Address switch

16 position rotary switch setting device address. Choice of address is arbitrary, but each device
in a fiber-optic loop system must have a unique address.

Setting Function

0 (Reserved to loop controller)
1-15 Available address settings.

Indicated on switch as hexadecimal values 1-F.

D100 User Manual D100_UM_081027 Page 38 of 46

PSI System Controls and Diagnostics

13.3 Front panel indicators

13.3.1 LED block

Four green LEDs.

Device

Network
Activity
Power

Power: Lit to indicate +5 V power is present in the unit.

Activity: Lit to indicate device is initiated and is processing pulses

Network: Lit to indicate messages are being transacted on the fiber-optic communication loop

Device: Blinks to indicate processor is booted and working normally.

13.4 Rear panel indicators

None

D100 User Manual D100_UM_081027 Page 39 of 46

PSI System Controls and Diagnostics

D100 User Manual D100_UM_081027 Page 40 of 46

14 Fault-finding

Symptom Possible Cause Confirmation Solution

No communication with host
system

Transmit and receive fiber­optic lines crossed.

Check connections. Correct connections.

Fiber-optic loop is broken Trace connections around the

loop.

Correct connections.

No function No 24 V power – faulty PSU Check power status LED,

check PSU voltage

Replace PSU

No 24 V power – bad jack

connector

Check power status LED.
Power should be present and
not affected if connector or
cable is moved.

Replace connector

Poor energy resolution Shaping time too small Resolution improves if value

is increased.

Increase the Shaping time.

Pile Up time too short. Resolution improves if value

is increased.

Increase the Pile up time.

Photomultiplier gain too low Resolution improves if gain is

increased.

Increase gain.

Cable from photomuliplier to

D100 too long.

Resolution improves with
shorter cable.

Reduce the cable length if
possible, otherwise increase
the Shaping time.

Unstable energy calibration Unstable high voltage Check HV stability with Replace supply.

PSI System Controls and Diagnostics

D100 User Manual D100_UM_081027 Page 41 of 46

suitable probe. Try another
supply.

Excessive photomultiplier

temperature variation.

Monitor temperature with
D100 probe.

Insulate or isolate the
photomultiplier.

High proportion of rejected
pulses.

Pile up time too long. Reduce pile up time,

consistent with sufficient
energy resolution.

Photomultiplier gain

suppression by high rate.

Loss reduced if gain
increased.

Run at higher multiplier gain.

PSI System Controls and Diagnostics

15 Maintenance

The D100 does not require routine maintenance. There are no user-serviceable parts inside.

The D100 is fitted with a 1.1 A automatically resetting positive temperature coefficient (PTC)
fuse in the 24 VDC input. No user intervention is required if the fuse operates due to
overcurrent. The fuse will reset when the overcurrent condition ends.

D100 User Manual D100_UM_081027 Page 42 of 46

PSI System Controls and Diagnostics

16 Returns procedure

Damaged or faulty units cannot be returned unless a Returns Material Authorization (RMA)
number has been issued by Pyramid Technical Consultants, Inc. If you need to return a unit,
contact Pyramid Technical Consultants at [e-mail]

- model

- serial number

- nature of fault

An RMA will be issued, including details of which service center to return the unit to.

, stating

D100 User Manual D100_UM_081027 Page 43 of 46

PSI System Controls and Diagnostics

17 Support

Manual and software driver updates are available for download from the Pyramid Technical
Consultants website at
[e-mail]. Please provide the model number and serial number of your unit, plus
relevant details of your application.

www.ptcusa.com. Technical support is available by email from

D100 User Manual D100_UM_081027 Page 44 of 46

PSI System Controls and Diagnostics

18 Declaration of Conformity

D100 User Manual D100_UM_081027 Page 45 of 46

PSI System Controls and Diagnostics

19 Revision History

The release date of a Pyramid Technical Consultants, Inc. user manual can be determined from
the document file name, where it is encoded yymmdd. For example, M10_UM_080105 would
be a M10 manual released on 5 January 2008.

Version Changes

D100_UM_080407 First general release

D100_UM_080926 Update base flange mounting hole diameters.

D100_UM_081027 Add Declaration of Conformity

D100 User Manual D100_UM_081027 Page 46 of 46