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Final Design Review Report:

Electronics Design for a Tomographic Photometer

ECE 4850 Design III

Thursday, April 16, 2009

Steven Burr Keith Bradford

Instructor’s Approval ____________________________________ ___________

Dr. YangQaun Chen Date

Department of Electrical & Computer Engineering

Utah State University Sponsor’s Approval ___________________________________ __________

Dr. Charles Swenson Date

Center for Space Engineering

Utah State University

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Abstract A tomographic photometer in development by students at Utah State University.

The photometer will be able to retrieve two dimensional profiles of the ionosphere

in the 1356 Å band by means of tomographic reconstruction (similar to a CAT scan only in the atmosphere). Such profiles would be the first of their kind and of great worth to the science community and public in general as well as improving satellite communications. The profiles would increase our understanding of Equatorial Plasma Bubbles (EQBs), which disrupt satellite communications especially GPS signals. The mechanical design of the photometer has been completed. A design solution for integrating the Photomultiplier Tube (PMT) and a brushless linear DC motor (BLDC) with an external system. This report contains the detailed design of an optical-electrical system featuring a PMT and photon counting system. It also contains design for the control of a BLDC motor and integration and test into the system.

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Table of contents

List of tables....................................................................................................................- 3 -

List of figures..................................................................................................................- 3 -

Acknowledgements.........................................................................................................- 3 -

1.0 Introduction.........................................................................................................- 4 -

1.1 Problem statement and design objectives ...........................................................- 5 -

2.0 Review of Conceptual and Preliminary Design..................................................- 6 -

2.1 Problem Analysis ................................................................................................- 8 -

2.2 Summary of specifications..................................................................................- 9 -

2.3 Technical Approach ..........................................................................................- 10 -

3.0 Basic Solution Description ...............................................................................- 14 -

4.0 Performance Optimization and Design of System Components ......................- 14 -

6.0 Final Scope of Work Statement ........................................................................- 16 -

6.1 Lessons learned and suggestions for future activities.......................................- 19 -

7.0 Other Issues.......................................................................................................- 20 -

8.0 Project Budget and Costs ..................................................................................- 21 -

9.0 Project Management Summary.........................................................................- 22 -

10.0 Conclusion ........................................................................................................- 22 -

REFERENCES .............................................................................................................- 24 -

Appendices....................................................................................................................- 25 -

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List of tables Table 1. Measured Pulse Properties....................................................................................................................... - 10 - Table 2. Motor Measurements................................................................................................................................ - 17 - Table 3. Budget and Component List...................................................................................................................... - 21 - Table 6. Field of View............................................................................................................................................. - 33 - Table 7. Orbital Parameter Spreadsheet ................................................................................................................ - 34 - Table 8. Dynamic Range Spreadsheet .................................................................................................................... - 35 - Table 9. Motor Parameter Spreadsheet.................................................................................................................. - 39 -

List of figures Figure 1. Plasma Bubbles in the Ionosphere _____________________________________________________ - 4 - Figure 2. Photometer Box____________________________________________________________________ - 5 - Figure 3. Workings of a Photomultiplier Tube [2]_________________________________________________ - 7 - Figure 4. Photon System Concept Diagram ______________________________________________________ - 8 - Figure 5. Measured Current Pulse (axis units are not correct, peak ~50nA) ___________________________ - 10 - Figure 6. Current Pulse Thresholding and Distribution [2] ________________________________________ - 11 - Figure 7. Photon Counting System [2]_________________________________________________________ - 12 - Figure 8. Measured Pulse Heights ____________________________________________________________ - 13 - Figure 9. Measured Pulse Heights and Histogram _______________________________________________ - 13 - Figure 10. Amplifier and Detection System _____________________________________________________ - 14 - Figure 13. Measured Pulses of Amplifier System_________________________________________________ - 18 - Figure 14. Comparator Output to Digital System ________________________________________________ - 19 - Figure 15. Project Schedule _________________________________________________________________ - 22 - Figure 16. Photon Counting System Schematic __________________________________________________ - 26 - Figure 17. Motor Controller Schematic ________________________________________________________ - 27 - Figure 18. Photometer Assembled ____________________________________________________________ - 28 - Figure 19. Photometer Optical-Mechanical System_______________________________________________ - 29 - Figure 20. Code Example for MSP430 Photon Counter Interface____________________________________ - 30 - Figure 21. M-Code for Measurement Script_____________________________________________________ - 36 - Figure 22. Test Setup for Measuring Pulses_____________________________________________________ - 38 -

Acknowledgements

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A special thanks to Texas Instruments for donating parts for the project.

1.0 Introduction

Figure 1. Plasma Bubbles in the Ionosphere

The ionosphere plays a very important part

in satellite and ground communications. Equatorial

Plasma Bubbles (EQBs) and other ionospheric

processes known as space weather can cause

disruption to any kind satellite communications,

which can cause problems for society. This process occurs when a region of low plasma densities

is topped by a region of high density, the lower plasma flows upward and creates a bubble. This

process creates a “lens” for radio waves and distorts communications signals, similar to the way

a bubble of oil in a glass of water distorts images on the other side. The new Wide Area

Augmentation System (WAAS) was developed to guide planes with use of the GPS system was

initially thought that accuracies would not be affected in the higher latitudes from EQBs.

However, the system was recently found to come under direct effects from EQBs as they spread

up to higher latitudes and reduce the accuracies of the system [1]. Even most financial

transactions could be effected as they use GPS for a time stamp. As society comes to rely more

upon satellite communications we need to fully understand the space weather by which our

communications are being affected. It has been proposed that a tomographic can help fill this

void of understanding about the atmosphere and yield new discoveries since the measurements it

is taking are unique. Upon launch the photometer would be the first remote atmospheric imager

to retrieve a two dimensional cross section of the ionosphere. These profiles of the atmosphere

could then be used to improve modeling and the prediction of space weather. If the density of the

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atmosphere were known corrective action could be taken such as improving correction and

accuracy of GPS systems. Warnings could also be issued by the science community and action

taken in the event of extreme space weather.

A photometer will take line of sight optical measurements of the plasma in the 1356 Å

oxygen band. As the instrument’s field of view will rotate in the orbital plane, the integrated

samples can be reconstructed topographically to form a two dimensional cross section of the

ionosphere. To take measurements of the atmosphere the photometer employs a rotating tube

with a frameless BLDC motor in which light passes through. A

mirror on the outside enables it to look in any direction along

the orbital plane. The light then passes through five filtering

mirrors that filter out most unwanted light and isolate the 1356

Å oxygen band. This light is then collected by a

PhotoMultiplier Tube (PMT). The photometer electronics

provide an interface between the motor and PMT and its orbital

vehicle or any computers used during demonstration, testing

and verification.

Figure 2. Photometer Box

1.1 Problem statement and design objectives

The problem involves interfacing components in an electrical optical system as well as

integration into a larger system such as a sounding rocket. A few design objectives are listed

here:

• Ability to count photons, which means one photon entering the PMT gets 1 count, pulses

coming from the PMT that are not from photons must be filtered out.

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• The individual counts must be counted with respect to time (counts/sec) and have the

ability to be monitored by an external system

• The BLDC must also have an interface for control via an external system for the rotating

mirror.

2.0 Review of Conceptual and Preliminary Design

Originally the design was for a mission on a student satellite, the program was cancelled.

It was decided that the project may be flown on a future satellite or sounding rocket mission. In

the aerospace industry in is desirable to have a high Technology Readiness Level (TRL), which

is a scale of 1 to 9 of how well developed a technology or subsystem is. The completion of this

project would bring a TRL of level 6 which means the design has been demonstrated with a

prototype. The prototype design does not work in the ultraviolet (UV) spectrum since the optics

and PMT are well over $30,000. To mitigate the cost and the risk of damage the components that

work in the visible range are substituted in place of the UV components. These cost only 300$

and are electrically interchangeable. Upon verification and testing of the electronics the visible

components may be replaced for those that work in the UV range and the design could then be

used on a space mission.

Many of the design specifications are known, such as the PMT power and signal

requirements. Power usage is an issue because it also increases cost since power is expanded by

adding solar cells or batteries Another consideration is in prototyping is finding parts that can

work in the industrial temperature range or mil-spec parts. These parts can be adapted for use in

the hostile space environment.

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The system collects light by means of a rotating mirror that is rotated by the BLDC

motor, this enables it to look out along the orbital path in 360 degrees. The light then hits a series

of filter mirrors which isolate a wavelength of light in the UV. The filtered light then enters a

PMT. Most photons incident on the PMT's windows are converted to electrons, which hit

subsequent dynodes which cause a shower of electrons and collected by an anode. Multiple

spreadsheets in the appendix are included to calculate the instruments field of view,

Figure 3. Workings of a Photomultiplier Tube [2]

One photon causes a shower of electrons, which produce a current spike; these spikes can then

be counted by electronic methods and used to find a count of photons entering the detector. This

photon count can then be used as the measurement for the tomographic reconstruction.

The BLDC motor needs a controller to drive it since it needs a pulse width modulated

(PWM) signal for power control and commutation As part of the optical specifications the

motor has been specifically selected to not produce jitter also known as cogging torque. This

jitter causes the rotating mirror to point in the wrong direction which can increase noise in the

optical measurements. Although no requirement exists for jitter it must be kept to a minimum. A

figure of the total system can be seen below. BLDC control is a project in of itself if the

controller is hand built. Although it is desirable to customize the controller to the application, for

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this project a development controller will be used to test the motor and get desired characteristics

such as power, torque and jitter. Then if the customer desires, a custom controller may be built to

these specifications.

Figure 4. Photon System Concept Diagram

2.1 Problem Analysis

Photon counting only works for low light levels but in comparison it has a very high

signal to noise ratio. Individual photons can be detected if the electronics are designed

appropriately. A photon hits the material of the window of the PMT and knocks off and electron.

The electron then heads for the next metal plate in the tube due to a positive voltage of a few

hundred volts placed on the plates. When the electron hits the next plate it knocks off several

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electrons and multiplies the effect. This effect continues with each plate multiplying the electrons

where they can be collected on the last plate as a current pulse. This current pulse can then be

amplified and detected using digital electronics. All pulses from the PMT do not correspond to

photons, these come from electrons, ions and cosmic rays. Electrons can jump off from the lower

stages of the PMT and cause a pulse that is lower than height than a photon. Ions and cosmic

rays have a higher voltage than normal photons due to their higher energies and thus have a pulse

height that is higher than that of a photon. Pulse height discrimination is needed to 'filter' out

these noisy pulses so that only photons are counted. This pulse can then be converted to a digital

pulse using a retriggerable one shot to maintain timing and then passed on to a digital counting

system.

2.2 Summary of specifications

• 1 photon to 1 5 volt pulse – the optoelectronic system must be able to convert the pulses

from the PMT and convert them to 5 volt pulses to be read by digital electronics.

• Thresholding – The system must be able to distinguish between dark pulses, radiation

pulses and regular photons.

• Counting and output– Must be able to count photons with respect to time or counts per

second, this is done with timer

• Motor control, needs to be able to rotate the motor at a rate between 1-2 Hz at the motors

voltage of 24v, the motor system cannot draw more than 1.5A

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2.3 Technical Approach

The approach was to model the system in software, the build the system and compare the

results with the real world values. Measurements were taken with an oscilloscope and current

amplifier to characterize the signal. The current preamplifier had filter modes to determine

source frequencies of noise. Several parameters were noted such as:

• Pulse heights

• Pulse width

• Frequency

• Noise

Figure 5. Measured Current Pulse (axis units are not correct, peak ~50nA)

Table 1. Measured Pulse Properties

Typical photon current pulse measured before amplification Frequency 5 MHz Pulse Width 5 ns Pulse Height N/A nA

Typical photon current pulse measured after amplification Frequency 200 to 500 kHz Pulse Width 2 to 5 µs Pulse Height 500 uA

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A useful source was Hamamatsu's "Photon Counting Using Photomultiplier Tubes", which give

a systems level perspective of photon counting systems and how to overcome common problems.

Wayne Sanderson an expert from the Space Dynamics Laboratory (SDL) was drawn upon for

expert opinion. Previous designs related to photon counting systems were discussed. Most of

these designs had two things in common: a transimpedance amplifier and method of

discriminating voltages. The amplifier system was then patterned after this design with a few

additions. The amplifier was changed to a two stage design using the same op amp. One side

functions as a preamp converting current to voltage and the other side pulls the voltage up to the

5V level. It was decided that a comparator would be best to filter pulses by their height.

Figure 6. Current Pulse Thresholding and Distribution [2]

The ability to filter out high pulses in addition to low pulses was added so the system can keep

the only the pulses that come from measured photons. The figure above shows how voltage

thresholding can be beneficial in eliminating noisy pulse signals from the system. A 74321

retriggerable one shot is then used to make sure that if two pulses come in they are accurately

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registered by the counter. It was decided that the best system for counting the digital output

would be an MSP430 development board. It has the ability to read voltages with and A/D

converter and also output the count on an LCD screen or RS232 port. Low power is also a

consideration and the MSP430 family is built to conserve power which made it an excellent

choice.

Figure 7. Photon Counting System [2]

2.4 Decision Analysis

A few specifications needed to be finalized before the design could continue. The gains

of the op amps and the comparator voltages. It was decided that the best method of design would

be to model the system then build the system. A digital storage oscilloscope was used to take

measurements and run them through a Pspice simulation and develop and simulate a model of

the system. The manufacturer of the op amps provides a Pspice model but it was not detailed

enough to work in the nano-amp range. A different approach using calculation and hand tuning

was used. The approximate gain was found and then a variable resistor was used to fine tune the

gain to it's appropriate range. Although this method is simple it works and a better pulse shape

can be obtained than calculations from a simulation. The data was also used to find the relative

thresholding voltages to filter out the high and low pulses that did not correspond to photons.

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The data was imported into Matlab and a peak detection algorithm was used to find the peak of

each pulse. A pulse height distribution was then used to find the relative high and low voltages

for the comparators.

Figure 8. Measured Pulse Heights

Figure 9. Measured Pulse Heights and Histogram

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3.0 Basic Solution Description

The basic solution of the system is the op amps to provide a gain of 10^8 to get the pulse

heights to 5volts. Comparators in a window circuit configuration can be used to discriminate the

pulse heights, the approximate voltages are shown. The voltages will be provided by a two 1k

potentiometers, this will also allow for easy optical calibration of the system. The output from

the comparators will be from 0-5 volts and will then be fed out to a digital system for counting.

Figure 10. Amplifier and Detection System

4.0 Performance Optimization and Design of System Components

After trouble with the first Pspice simulation, it was decided to build the preamplifier and

measure the characteristics of the amplifier. A simple hand calculation of a transimpedance

amplifier yields V_out = I_in * R, so the gain of the amplifier is all in selecting the right resistor

value. The preamplifier needs to go from 50nA of the current pulses to 150mA. This places the

gain 3x10^5 hence the resistor value in the 300k range. The circuit was then built and the

appropriate resistor value was substituted in, but the shape of the current pulse was changed.

Instead of a nice pulse the circuit was ringing, which means the circuit is unstable. This creates

several problems for the rest of the system including false counts and reduced bandwidth

(counts/sec).

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It was then discovered that the coax cable had enough stray capacitance to drive the

circuit unstable due to reflection in the coaxial transmission line. A document from Texas

Instruments called "Compensate Transimpedance Amplifiers Intuitively" [4], helped avoid the

ringing by compensation. A capacitor placed in parallel with the gain resistor can match the

impedance of the line. This can be applied to photodiodes, but the same principle applies to

transmission lines and matching the impedance of the line and the high voltage power supply

connected to the PMT. From equation 6 in the document we get

Figure 11. Measured Ringing of Current Pulse

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Our desired frequency is at least 200kHz so substituting in the parameters and solving for C_d

gives us a value around 10pF. A capacitor was substituted in and the ringing was avoided. It may

be mentioned that the better the pulse shape the more photons can be counted because the width

is smaller.

6.0 Final Scope of Work Statement

The motor was not operational at the time of this report although much work went in to

finding the appropriate BLDC motor controller for it. Motor controllers from several companies

that matched the voltage and current specifications of the motor were found. The model with the

least cost was purchased from Luminary Micro. Upon receipt the motor controller was tested for

connectivity since it operated over a LAN or in a standalone mode. It did not function correctly

Figure 12. Measured Pulses After Correction

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and upon several calls to the manufacturer, it was determined that the firmware was outdated.

About 15 hours was wasted in troubleshooting the motor controller, after that it was decided that

it was not worth the cost of spending more time to get Luminary Micros product to work and it

was sent back. To avoid further problems such as this Applimotion, the manufacturer of the

motor, was contacted for advice on a motor controller that would drive the motor. A controller

developed by Applimotion was recommended but did not fit in the budget. The same controller

was found at the Space Dynamics Laboratories (SDL) and borrowed for testing purposes. The

results are listed in table 2. One of the main goals was to measure the power output and the

loading torque.

Table 2. Motor Measurements

Motor Measurements Volts 8 12 Amps 0.15 0.26 Watts 1.2 3.12 RPM ~2 ~50

Torque 0.156 0.213

Work on the preamplifier was completed and validated as shown in figure 14. In this

figure is shown the output from the preamplifier (blue) and the final resulting output from the

second stage amplifier (yellow). The peaks detected by the oscilloscope are also shown, the gain

of the second stage is 2.5V/.08V which is equal to a gain of 31.

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Figure 13. Measured Pulses of Amplifier System

The comparator was tested and the results are shown in Figure 14. The yellow pulse is an

incoming pulse after the preamplifier and the blue pulse is after the comparator stage. This pulse

is then sent into an input port of the MSP430 Experimenter board. The board was tested using

the button to simulate an incoming pulse and example code is shown in Figure 14.

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Figure 14. Comparator Output to Digital System

6.1 Lessons learned and suggestions for future activities

There are many lessons that were learned that can be applied to future designs, especially

since a fabricated board will be designed for the system in the future. The location of the input

terminal to the PMT's high voltage amplifier needs to be as close to the amplifier as possible, and

the ground terminal also needs to be connected to the amplifier. Long wires carrying the initial

pulse signal can generate noise as well as change the input impedance of the coaxial transmission

line.

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7.0 Other Issues

Although the amplifier design functions fully, a better understanding of it could be made.

The capacitor value to dampen out the ringing in the input line was selected based on

compensating the amplifier. A better solution would be to model the amplifier and find the poles

and zeros of the system to ensure system instabilities such as ringing do not occur. This could

also be done with an end to end simulation in Pspice with a model of the transmission line. This

may not be possible if more information from Hamamatsu could not be obtained about the

impedance of the high voltage amplifier. In this case the capacitor values could be hand tuned to

find the best pulse 'shape'.

Another issue would be verification, although simple verification of the amplifier stages

has been obtained, more can be done. The TDS7704B oscilloscope has 4 input ports and can be

used as a DSO. A probe could be placed at each of the amplifier stages, after the comparator and

before the digital counting system. The pulses could then be recorded at each stage through the

system. This data could then be imported into Matlab and a script similar to the peak detection

script to validate the data. If a pulse from a photon was detected on the input but no digital pulse

was seen on the exit of the system then it would be known that the system was not functioning

correctly.

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8.0 Project Budget and Costs

The project had a budget of $1200. Cost estimation was fairly straightforward since the

most of the photometer components were fairly inexpensive. Chips were acquired Texas

Instruments which helped stay under the budget cap of $1200. Time spent on the project was

300 hours, which at a students wage is around $3000 for the development costs.

Table 3. Budget and Component List

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9.0 Project Management Summary

Time was a limiting factor in the design, as manpower was limited. Mechanical design

work also affected the design as the motor and the PMT had to be selected before and integrated

into the mechanical design. The electrical portion of the project was done in about the space of

three months.

Figure 15. Project Schedule

10.0 Conclusion

Integrating the electrical components of a photometer was not extremely difficult but

there are a few items that need to be worked on. Although all not all of the ideas were

implemented all of the design goals were reached at the time of writing except for the motor

controller, which a schematic has been included for future work. The prototyping of the

amplifier was completed and verified to some extent. More testing and verification can be done,

but this can happen during the optical calibration and testing. Prototype analog hardware was

developed. Although the design could potentially be used for testing it would be recommended

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that the schematic be used for a PCB design. The project went well with the exception of timing

and the design goals were met.

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REFERENCES [1] Stening, R. J.: 2003, ‘SPACEWEATHER IN THE EQUATORIAL IONOSPHERE’, J. Space Science Reviews. 107: 263–271.

[2] “Photon Counting Using Photomultiplier Tubes” Hamamatsu Photonics http://sales.hamamatsu.com/assets/applications/ETD/PhotonCounting_TPHO9001E04.pdf

[3] “Op Amps For Everyone Design Guide.” Texas Instruments. http://focus.ti.com/lit/an/slod006b/slod006b.pdf

[4] “Compensate Transimpedance Amplifiers Intuitively.” Texas Instruments. http://focus.ti.com/lit/an/sboa055a/sboa055a.pdf

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Appendices

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Figure 16. Photon Counting System Schematic

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Figure 17. Motor Controller Schematic

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Figure 18. Photometer Assembled

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Figure 19. Photometer Optical-Mechanical System

Frangibolt A

ctuator

Slide C

over

Motor H

ousing

UV

Filter M

irrors

PhotoM

ulitplierT

ube

Rotating

Tube

Electronics (underside)

FOV

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Figure 20. Code Example for MSP430 Photon Counter Interface

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Table 4. Field of View

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Table 5. Orbital Parameter Spreadsheet

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Table 6. Dynamic Range Spreadsheet

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Figure 21. M-Code for Measurement Script

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Figure 22. Test Setup for Measuring Pulses

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Table 7. Motor Parameter Spreadsheet