1
Tigerbot Free-Space Optical
Final Report
Aaron Bennion
Seth Gibelyou
Tyler Bird
10 December 10, 2009
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Contents
Introduction .................................................................................................................................................. 3
Concept Generation and Characterization ................................................................................................... 4
Transmitter Design........................................................................................................................................ 5
Receiver Design ............................................................................................................................................. 6
Final Production ............................................................................................................................................ 8
Final Demonstration ................................................................................................................................... 10
Conclusion ................................................................................................................................................... 11
Suggestions for Future FSO Projects ........................................................................................................... 11
Appendix A: Tigerbot FSO Communication Link Specifications .................................................................. 12
Appendix B: Cost Analysis ........................................................................................................................... 13
Appendix C: Operating Instructions ............................................................................................................ 14
Appendix D: Functional Specification Document and BOFs ....................................................................... 17
Appendix E: Concept Generation and Selection ......................................................................................... 28
Appendix F: Project Plan ............................................................................................................................. 36
Appendix G: Test Plan ................................................................................................................................. 44
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Introduction
The free-space optical communications project was conceived to find an alternative to wired
communication links in situations where line-of-sight transmissions are possible. Some of the benefits
of an optical communications link are the ability to link data sinks to data sources without cumbersome
wires and cables which can be a safety hazard and also pose equipment mobility limitations. Team
Tigerbot was created to meet the challenge by building an optical communication link to connect the
digital audio output of a DVD player to the audio input for a digital audio receiver (also known as a
digital-to-audio converter or DAC). The system had to be able to transmit digital data in the form of light
pulses across a span of 5 to 20 feet and be able to reconvert the signal into digital electrical pulses that
are useable to the DAC. A block diagram of the system is shown in Figure 1.
Laser driver
circuitry
DVD
player
Digital to
audio
converter
Photodiode
driver
circuitry
Laser
Photodiode
receiver
Figure 1 – A block diagram of the complete free-space optical project. The red, dashed blocks indicate the portions of
the project that require design and assembly, while the other boxes indicate parts that are provided to the students.
The arrows indicate data path.
The purpose of this report is to showcase the various stages of the FSO development beginning with the
purpose, challenges, and importance of each individual component to the overall problem. The main
components consisted of the transmitter unit and the receiver unit. The design process was broken
down into four phases: concept generation and characterization phase, transmitter design phase,
receiver design phase, and the final production phase. This document will examine the work done in
each of these from the view point the goals for the phase, the challenges encountered and lessons
learned, and the final results.
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Concept Generation and Characterization
Background
The concept and generation phase of the project was where the project began. The problem of wireless
data transmission was posed and the limitations of the demonstration were given (i.e. we were told our
system had to be simple enough for a “dummy” to setup at both 5ft and 20ft). The team was given a
few components including op-amps, a laser diode and a photodiode to use in the solution of the
problem. This is where the fun began.
Goals
The main goal of the first phase was to find a design concept capable of meeting the challenge of
transmitting wireless optical signals. A secondary goal of this phase was to do a market analysis to try to
determine what other users might be interested in the product and to include as many customer needs
as were possible into our initial specifications.
Challenges
One of the first challenges posed by this stage of the project was to characterize all of the equipment
that we were given as well as what our system had to interface with. Equipment characterizations
included finding the output voltage and frequency of the DVD player (shown in Figure 2) and the input
voltage requirements of the digital-to-audio converter (shown in Figure 3 and Table 1).
Figure 2 - DVD player test setup and output waveform. Note that the output voltage swing is 1.1Vpp and that the frequency
is about 5MHz.
DVD player
voltage out
.56
.8
1.0
1.06
DAC input voltage
.56
.3
.12
.07
Potentiometer
Resistance (Ω)
.28
97
490
1034
Table 1 – Minimum usable DAC input voltages based on a voltage division of the
Figure 3 - Test setup for the DAC.
DVD signal using the potentiometer.
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Other characterizations included finding the operating thresholds, input and output currents of the laser
and photodiodes. All of the characterizations required for this stage of the project were included in the
Body of Facts document which is contained in Appendix D: Functional Specifications Document. These
characterizations gave us a technical starting point for our concept design.
As mentioned above, the other important aspect of concept design was customer input and intended
use. This phase included a customer needs analysis and concept generation sheet wherein various
needs were looked at and weighed in terms of their importance and feasibility. At this time we also
began looking for existing designs and trying to create our own that might be viable solutions to the
challenge.
Results
The culmination of all of this collecting of information was the creation of the Functional Specifications
Document (Attached as Appendix D). The functional specifications document combined our technical
limitations with the needs of our customer to create a list of specs that should be met to satisfy the
widest range of customers. This document was followed shortly by the Concept Generation and
Selection document (see Appendix E) wherein we used decision matrices to sort through the possible
solution designs we found. The end goal of the CG&S was to choose a transmitter design and a single
receiver design that balanced cost and size with simplicity and stability. These designs would become
the prototype of our proposed solution to the free-space optical problem.
Transmitter Design
Background
The purpose of the transmitter was to convert the digital voltage signals of the DVD player to optical
pulses via the laser diode. The specifications had been set and several possible solutions had been
found. The CG&S helped us decide which design might best meet our needs. The design we chose was
a simple BJT based current driver for the laser diode that was biased to operate the laser somewhere
between its threshold level and burnout. We chose the BJT because it was small, cheap and easy to use.
Our transmitter schematic is shown in Figure 4 below.
Goals
The main goal of the transmitter design phase was to
have a functional breadboard prototype and that we
could confirm with a meter and oscilloscope that the
diode was being driven within the correct limits. We
also needed to confirm that the transmitter was
capable of operating at the necessary frequency. This
was all due before the first design review.
Challenges
While the transmitter circuit was the simplest
component of our system to build, there were a few
important points for us to remember as we prototyped
it. Our first laser diode burnt out after a short period of
Figure 4 – Transmitter circuit schematic. The laser is
modeled as a forward biased diode in the BJT
collector.
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use. After adjusting the biases one of the first things that we tried to do was to protect the laser from
voltage spikes or circuit instabilities. The voltage control required adding a voltage regulator in series
with the power supply shown above. We used a basic 12V regulator to limit the +15V power supply that
we were provided with. We had to ensure that Rth was much less than (B+1)*RE for temperature
stability in our transmitter design. Once we had taken these precautions we were ready to proceed with
construction on the breadboard.
Results
Our breadboard circuit performed as we had hoped when we checked it with the o-scope and ammeter.
We were able to drive it as high as the function generator would go (15MHz) and could not see any
noticeable distortion of the signals within the driver circuitry. All of the building and testing was
completed before the first design review. We were also pleased that we didn’t burn out any more lasers
with our breadboard circuit.
Receiver Design
Background
The receiver was the second half of our solution. The receiver’s purpose was to convert the optical
pulses coming from the transmitter unit back into electrical pulses via the photodiode. The photodiode
output was not in itself capable of driving the DAC and so an amplifying circuit needed to be
incorporated into the receiver as well. We used the CG&S to take into account these technical
requirements and decided upon the receiver circuit shown in Figure 5 below.
Figure 5 - Receiver circuit schematic. Note that the photodiode is modeled as an alternating current source in parallel with a
small capacitance.
Goals
The goal of the receiver design phase was to build a working receiver which also meant that we would
have a functional system when the transmitter was included. By functional system we meant that the
system could take digital audio input from the DVD player, optically transmit it via our transmitter
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prototype, receive and convert it back to electrical signals via the receiver unit, and finally convert those
electrical signals to audio sounds using the DAC. Not only did the circuit have to function at the required
frequency, but it also had to function at 5ft and 20ft to meet the requirements of design review 2. This
particular design review also called for a circuit reduction plan of both the transmitter and receiver into
their final web-cam housings.
Challenges
This phase was definitely the most challenging phase as we worked to identify and calculate the
requirements of the receiver. The first major challenge was determining the anticipated input for the
photodiode. Working with optical power was a new experience for most of us and so we had to begin
with the characterizations of our laser. Knowing the optical power output and the spot size of the beam
allowed us to calculate the power density of the beam at various distances and thereby the power
received by the photodiode. (See Figure 6.) We had also characterized the photodiode output vs.
optical input early in the concept phase and thus had a starting place for our amplifying circuit
calculations.
Figure 6 - Optical power density measurements. Power density = optical power / (π x (.5 x beam diameter)2)
As can be seen from the schematic in Figure 5, the current from the photodiode is converted and
amplified into a voltage signal by the first op-amp. The second op-amp is mostly present to provide
additional gain while maintaining a sufficient bandwidth. This also helps with the stability of the circuit.
The power supply of the receiver had an important role in our receiver. In order to minimize the effects
of changing distances we gave the receiver sufficient gain to rail the op-amps at 20ft and 5ft, and
therefore the output signal would be the same. However a 15V rail was much higher than was needed
for the DAC so we needed to tone the input down. We learned that the op-amp corner frequency was
adversely affected when the power supply was lowered below ±5V, so we decided to use 5V voltage
regulators on our power supply. The remainder of the excess voltage was burned off using a voltage
divider at the output of the circuit.
Results
We were able to produce a prototype receiver that was capable of converting the optical signal into
musical satisfaction. The o-scope analysis confirmed our results and the resulting output waveforms for
transmission at 5ft and 20ft are shown below in Figures 7 and 8 respectively. We were also able to
measure our frequency response and found it to be at least 13MHz which was ample bandwidth for our
application.
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Figure 7 - Receiver output at 5ft transmission
Figure 8 - Receiver output at 20ft transmission
The other important result of this phase was our circuit reduction plan. This challenge was met
personally by Seth, who took it upon himself to learn Eagle PCB software. The result was a beautifully
laid out design for both the transmitter and the receiver circuits shown below (Figure 9) with our
trademark inscribed on them. All of this was completed and ready to present by the second design
review.
Figure 9 – Transmitter and receiver board designs created using Eagle PCB software.
Final Production
Background
The final phase of production is geared toward creating a user-friendly version of our solution to the FSO
problem. The original problem statement called for a system that fit within the provided web-cam
housings. Our circuit reduction plan was the bridge that we needed to cross the gap between these two
specs.
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Goals
The goal of the final production phase was to place our circuits within their final webcam housings and
create a set of instructions simple enough for a “dummy” to be able to use them. Thus a final system
with simple alignment properties and little or no adjustment required was what we were after. We also
needed to write a set of instructions that anyone could use to set up our system. The culmination of
the final production was to be a demonstration wherein our instructions and system would be set up by
the “dummy” and depending on how well it worked for them we would receive our grade accordingly.
Challenges
One important point consideration was the orientation of the circuit board within the housing. In
designing the circuit board schematic Seth planned ahead to make sure that the board would fit within
the required housing as well as making sure that it would place the laser/photodiode facing the front
and centered on the final product. He also made sure that the placement of the data and power
connections would allow for maximum motion and flexibility in the web-cam head. Once the cable
paths were chosen and the board design was confirmed to fit (he built it on a small piece of paper-board
first and placed the model inside the head) we were ready to mill the circuit. The circuit was milled in
the electric shop in the Clyde Building on copper board. All of the components were soldered on and
terminal ports were used for the photodiode and laser diode connections to make for easy trouble
shooting and repair.
We ran into a problem when we tried to attach a 5-pin DIN connector from an unknown source to our
power supply: the DIN connector had some sort of inverter built in, such that we had no positive voltage
to our circuits despite having a ±15V power supply. This didn’t affect our transmitter design, but would
be a problem for our receiver as we needed ±5V to power the op-amps. The solution to our problem
was to use two 78L05 5V regulators in the configuration shown in Figure 10.
Once we had implemented our voltage
correction we were ready to proceed
with the soldering of the components to
the board. Only once did we accidentally
solder the wrong lead of the laser to
ground, but that was found and fixed
when the diode wouldn’t turn on during
the first test.
The next step was to fully characterize
our system and write a spec sheet for it
and then to prepare a set of instructions
that included several easy to understand
pictures. Despite our best efforts we
Figure 10 – Solution to having only a negative power supply for the
receiver using two 5v regulators.
found it a challenge to keep the
instructions to anything less than 3
pages. We felt that they were reasonably clear and as concise as was needed for a “dummy”.
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Results
At the end of this phase we were in possession of a fully functional and fairly handsome free-space
optical communications link. (See Figures 11, 12 and 13 below.) The instructions were written and have
been attached to this document as Appendix C. We were then ready to demonstrate (or let someone
else demonstrate) our system.
Figure 11 – Final transmitter unit
Figure 12 – Final receiver unit
Figure 13 – Receiver unit in
operation.
Final Demonstration
Background
The final demonstration was a refereed competition between all four of the FSO teams to see whose
system was the most user-friendly. The race was to be done in two heats. The first heat involved a
“dummy” setting up the system at 5ft using the instructions we wrote. Teams were chosen at random
to determine order. The second heat was to be done in reverse order with the separation being 20ft
between receiver and transmitter. The official time keepers were our professors, and the official
“dummy” was decided to be the two 10-year-old sons of our professors. The team with the fastest time
was declared to be the most user-friendly and therefore the winner. Everyone was anxious about
putting their final grade in the hands of two 10-year-olds, but we were fairly confident in our design.
Challenge
Our team was to be third in the 5ft heat and second in the 20ft heat. Our first challenge came in the
form of an unexpected removal of the CD from the DVD player before our demo. This caused some
confusion in the operators, but it was remedied without costing too much time. The next challenge was
a little more painful, but brought some good lessons with it.
The team before ours had some troubles with their system and as a result the judges were directed to
adjust the focus of the laser lens in order to remedy the problem. We had not considered the fact that
an end user might know how to adjust the lens and therefore did little to prevent or direct it in our final
design. Our design relied heavily on an optimized beam spot that was made as large as possible for the
maximum distance. Unfortunately the operators misinterpreted the beam spot size and quickly
refocused the beam to a small spot, significantly increasing the alignment difficulty. We had to wait
until the penalty time expired before the problem was remedied and the focus adjusted. The lesson
that came with it was valuable: do not leave anything exposed that can adjust your system unless you
provide direction for how to properly use it.
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Result
Despite the setbacks mentioned above, the system worked as it was intended and the operators were
able to set it up with minimal assistance. It was decided that even two pages of instructions was too
much, especially since they included laser eye safety and electrical hazard warnings. A few pictures
could have been removed and a fair amount of text which would bring the final instructions to an
unintimidating single page document.
Conclusion
It is our conclusion that the Tigerbot free-space optical communications link was a viable and effective
solution to the problem posed for the FSO project. The design is simple, highly portable, and easy to
use. Based on the limitations posed by time, funds, and materials we felt that this system represents
the best efforts of the Tigerbot team. We also felt that it was a valuable learning experience in product
design and engineering. We also learned that the project management portion of the project was a
realistic introduction to the workplace environment. All in all, we would say the project was a success.
Suggestions for Future FSO Projects
After considering the question posed, we have come up with a few ideas that might provide for future
FSO projects. The first idea would be a simple extension of the current project only increasing the
distances and trying to find better/easier ways to align them. The second idea consists of multiple
receivers and a beam splitter which would allow two different locations/rooms to receive signals from
one optical source. This would require a little more equipment, but might make for more interesting
gain calculations. The last idea would be a battery powered laser-tag system. A laser with a rather wide
beam would be used in a hand-held “gun” and would be battery powered. The receiver would also be
battery powered but would consist of one or a few photodiodes placed on a harness with an amplifying
system that would light up LEDs as the target was “hit”. It might be too complex for one semester but
might work well for a year-long program like CapStone.
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Appendix A: Tigerbot FSO Communication Link Specifications
The Tigerbot FSO Comm Link is a simple, reliable and cost effective solution to wireless audio
transmission.
Features:
 The system consists of a transmitter and a receiver unit, each
with easy suction cup mounting.
 Highly portable, modular transmitter, receiver and power
supplies.
 Two 120V-to-±15V power supplies for standard wall outlets.
 Standard RCA cable data connection ports.
 High-speed data rate of greater than 6MHz.
 Simple to use, red laser makes for easy alignment.
Specification:
Receiver Output Voltage swing (5ft)
Receiver Output Voltage Swing (20ft)
Min Operating Distance
Max Operating Distance
Lower 3dB Frequency
Upper 3dB Frequency
Transmitter Power
Receiver Power
Optical Power Output
Eye Safety Rating/Warning
Max Elevation Change (deg)
Receiver Angular Response (deg.) @ 5ft
Transmitter Angular Response (deg.) @ 5ft
Transmitter Angular Response (deg.) @ 20ft
Receiver Gain
Value:
1.34 Vpp
1.26 Vpp
< 1ft
30 ft
5KHz (5 ft)
7.8KHz(20 ft)
8 MHz (5 ft)
6.7MHz(20 ft)
0.84 W
0.82 W
0.54mW (AVG)
Class IIIA (Output between 1.0 and 5.0 mW)
33.7
40.0
±0.35
±0.5
63 V/ma
Table 2 – Tigerbot FSO Comm Link specifications and operating characteristics
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Appendix B: Cost Analysis
The Tigerbot FSO link was designed around a tight budget and therefore the cost of the design solutions
was kept to a minimum. The major expenses within the system come not from components designed
into the system, but rather from outside components that the customer requested in the design. The
following table shows the cost break-down of the components in a single FSO system.
Transmitter
Receiver
Quantity
Price per
Price Total
Quantity
Price per
Price Total
Resistors
3
$
0.04
$
0.12
Resistors
4
$
0.04
$
0.16
Capacitor
2
$
0.20
$
0.40
Capacitor
3
$
0.20
$
0.60
12 V IC Regulator
1
$
0.41
$
0.41
5 V IC Regulator
2
$
0.47
$
0.94
2N3904 BJT
1
$
0.10
$
0.10
2
$
2.50
$
5.00
Laser Diode
1
$
7.20
$
7.20
LM 7171 OpAmp
Photo Diode
1
$
1.00
$
1.00
$
8.23
$
7.70
$
7.00
Sub-Total
External Parts
Costs:
Webcam Housing
1
$
7.00
$
7.00
Power Supply
1
$
33.00
$
33.00
$
48.23
Total
prices based on
electrical shop
Webcam
Housing
Power Supply
Grand Total
1
$
7.00
1
$ 33.00
$ 33.00
Total
$ 47.70
$ 95.93
Table 1 – Tigerbot FSO cost breakdown.
It is reasonable to assume that if the Tigerbot FSO were to be mass produced some of the costs would
decrease considerably. For example, the webcams which currently require disassembly before use
would likely be replaced by a custom made housing which would cost much several dollars less. The
power supplies are currently the most expensive unit, but the Tigerbot FSO only requires one-sided
voltage input due to the unique design of the receiver. The power supply could be replaced by a much
cheaper unit, thus saving several dollars. It is estimated that mass production accompanied with these
changes could yield a system cost of less than $50.
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Parts List and Identification
Appendix C: Operating
Instructions
Tigerbot Free-Space
Communications Link
Your FSO Communication Link should include
the following pieces of equipment:
1x Transmitter unit (identified by the RED
bands on the data cables)
Contents






Safety Warnings
Parts List and Identification
Receiver Setup
Transmitter Setup
System Alignment and Operation
Troubleshooting
RED band
1x Receiver unit (identified by the BLUE bands
on the data cables)
Safety Warnings
This device uses electrical power. Do
not dismantle. While most of the circuitry is
low voltage, opening up the system or power
supply could expose the user to dangerous
voltages that could cause injury.
This system includes a Class IIIA Laser
device which can be harmful to the eyes. Do
not look directly into the laser beam. Use
caution not to shine laser into the eyes of
others.
BLUE band
2x RCA data connection cables
2x Power supplies
If any of these parts are missing, please notify
the company as soon as possible.
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Receiver Setup
1) Find the receiver unit. Place the
receiver unit on top of your stereo
digital audio converter (the black box
that connects to the speakers).
2) Plug in one of the power supplies and
connect the other end to the white
power cord on the receiver unit.
Transmitter Setup
1) Find the transmitter unit. Place this
unit on or near the DVD player.
2) Plug in the second power supply and
connect the other end to the white
power cord on the transmitter unit.
3) Connect one end of an RCA data cable
to the black data cord on the receiver
unit.
3) Connect one end of the second RCA
data cable to the black data cord on the
transmitter unit.
4) Plug the other end into the “coax” or
“digital out” port on the DVD player.
4) Plug the other end into the coax1 port
on the digital audio converter (the black
box).
5) Make sure that the DVD player is
turned ON.
5) Make sure that the digital audio
converter is turned ON.
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System Alignment and
Operation
1) Make sure that all of the devices are
connected, have power, and are turned
ON.
2) Insert a music CD into the DVD player
and press PLAY.
3) Point the transmitter in the direction of
the receiver. You should see a bright
red spot in the general direction that
you are pointing. Carefully adjust the
transmitter so the spot is centered on
the receiver.
Troubleshooting
Problem: music won’t play…
Try this…






Make sure everything is plugged in and that
everything has power.
Make sure that the CD is playing.
Check the laser alignment and make sure
that the spot is centered on the receiver.
Move the receiver slightly to the left or
right.
Try turning the digital audio converter OFF
and then ON again.
Check with the Tigerbot team to see if the
laser is burned out.
Problem: the laser will not light up…
Try this…

4) You should be able to hear music as the
laser beam aligns with the receiver. If
you still do not hear music, but the
beam is centered on the receiver, try
moving the receiver slightly within the
beam.
5) Also try turning the power on the digital
audio converter (the black box) OFF and
then ON again.

Make sure everything has power and that
the transmitter is plugged in.
Check with Team Tigerbot to see if the laser
is burned out.
Problem: the music pops and crackles…
Try this…



Adjust the receiver slightly to one side
within the laser beam.
Turn the digital audio converter (the black
box) OFF and then ON again.
Move the transmitter a couple feet closer
and realign the system.
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Appendix D: Functional Specification Document and BOFs
Introduction
The free space optical communication project takes the digital output of a DVD player, converts that
signal to optical pulses which is then transmitted in free space up to at least 20 feet. The signal will then
be received, demodulated, sent to the digital to audio converter, and finally to the speaker output. The
system is required to be portable and must fit inside of a webcam. As the block diagram indicates, we
are responsible to design and implement the laser and photodiode driver circuitry.
Laser driver
circuitry
DVD
player
Digital to
audio
converter
Photodiode
driver
circuitry
Laser
Photodiode
receiver
Figure 1 – Block diagram of complete free-space optical project. Red, dashed blocks
indicate portion of project that require design and assembly. All other blocks are
provided to the students.
Customer Needs and Project Requirements
Customer Description
Our primary customers are the professors who will be using the actual project for future display in
electrical engineering demonstrations. By extension, interested students and other non-technical
people will use the basic functionality of our units as they play with the audio system turning the music
off and on by moving and realigning the laser. Our secondary customers are other interested
individuals, such as home audio listeners, PA system users, and musical concert technicians who might
be interested in a commercial version of the free-space optical link to save them the hassle and space
required by physical connections between their audio source and the speakers.
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Customer Responses and Interpretations
After reviewing the specifications of the primary users and asking for feedback from possible secondary
users we were able to create a matrix of the customer needs and as a team decide the relative
importance of each requirement to the design of the final project.
Table 1 – Customer Statements, Technical Interpretation and Relative Importance
#
Customer Statements
Interpretation
Importance
1 = vital
2 = important
3 = optional
1
“System must be easy to
System simply requires plugging into
setup.”
power supplies, audio source and sink,
1
and alignment.
2
“System must be easy to use.”
Once aligned and operating, system
needs no further adjustments.
1
3
“It must be able to work at
Circuitry must be capable of handling
both 5ft and 20ft with no
a range of optical power.
1
internal adjustments.”
4
“It would be useful to have in
Unit should be weatherproof.
outdoor applications like a
3
concert.”
5 “It would be nice if it worked in
Unit should have some robustness
smoke or rain or snow.”
against optical interference.
3
6 “I would like to be able to listen
The system must be able to sustain
to music using the system for
continuous transmission for the length
2
long periods of time.”
of an audio CD.
7
“It would be nice to have an
Unit should have an easy way to tell
alignment aide.”
whether or not the optical transmitter
2
and the receiver are aligned.
8
“The laser and receiver units
All laser and photodiode circuitry
should be small and portable,
should fit within a webcam, besides
1
preferably small enough to fit
the power supply.
within a web-cam.”
9 “It would be nice to be capable Transmitters could be interconnected
of transmitting to multiple
to the same audio source, or laser
3
receivers simultaneously, with
beam could be split and reflected to
only one base station”
other receivers.
10 “It needs to be able to connect
The system must have a minimum
1
to a DVD player digital output”
functional bit rate of at least 5KHz
The method of prioritizing needs comes from the nature of our customers. Our primary needs reflect
our primary customers: professors with requirements that directly influence our grade on the project.
Secondary users needs are very useful if we were to attempt a commercial production of the product,
but would also require more time and money than we have allotted to us at this time. Many of the
customers’ needs overlap however, such as the desires for simplicity of use, sustained operation, and
portable housing. Also, for the outdoor system, we considered that beyond minor bad weather, an
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audience would be likely to leave an outdoor arena before the weather was bad enough to severely
inhibit the functionality.
Specifications and Metrics
The metrics below were established in order to better measure the completion of meeting the
customers’ needs. They are a way of quantifying the achievement of our device in certain areas.
Therefore, at least one metric has been established for each need. The needs are again referenced by
their number and the metrics are also assigned a number for easy reference.
Table 2 – Free-Space Optical Requirement Metrics
Metric Need
#
#
1
2
3
4
5
6
7
8
1, 2
2
3
3
10
8
6
9
9
10
11
4
5
7
Metric
Number of user control interfaces
Time between necessary alignments
Minimum distance from transmitter to receiver
Maximum distance from transmitter to receiver
Minimum function bit rate
Volume of transmitter and receiver (separate)
Maximum transmitting time
Available power of transmitter/needed power for
receiver
Unit is water tight
Unit can handle interference from rain or fog
Unit has a visual alignment aide
Units
Marginal Ideal
Value Value
#
Min
Ft
Ft
MHz
in3
Min
Ratio
<2
>10
<5
>20
>5
<2.5
>10
>2
0
>480
<1
>30
>7.5
<1.7
>180
>4
Y/N
Y/N
Y/N
N
N
N
Y
Y
Y
Analysis
The most important metric would be metric 5. The primary use for our optical communication system is
the digital output of a DVD player, which operates at 5 MHz Our goal is to at least match this bit rate,
although if we can get our system to operate at higher frequencies we could possibly use it for other
applications. In order to give ourselves some lee-way we set our ideal value at 50% higher than the
anticipated need. The next primary requirements are metrics 3 and 4. Our primary customer will be
using our system for demonstrations in lecture hall or class room settings. Being able to operate in any
reasonable setup in that setting is critical to our success. Ideally the system could operate from a range
that extends both closer than the customer’s minimum request as well as farther away. We felt that it
would be ideal for the system to work at a minimum possible distance to a distance 50% greater than
the customer’s need, so that there was some head room for operation and customer flexibility.
Our secondary requirements are from metrics 1 and 6. In order to make the system as easy to use as
possible we would like to keep control interfaces and size to a minimum. Ideally that means 0 control
interfaces. Also we are limited on size if we are to fit the circuit in the customer’s webcam housing. The
volume of the housing is approximately 2.5 cubic inches; however it is not a simple cubic volume. We
decided that a circuit requiring 30% less space would give us a little more flexibility when trying to
mount it to the housing. This will help greatly in the setup for demonstrations, although they are not
20
critical for functionality. These will influence our primary customers’ satisfaction, and their measure of
our success, but only after our primary requirements have been met. The remaining metrics are tertiary
requirements that would be convenient, but have no effect on the primary measure of our success.
Ideal values for alignment and transmit time were based on anticipated user habits: 180 minutes of
continual use is enough to play several CDs, before allowing the system to cool down. Some of these,
like metrics 2, 8, or 10 will not be actively pursued, and will only be meet if their implementation is
simple and does not adversely affect any other metrics.
Summary
We believe that based on the feedback we have received from our customers and the scope of the
project that we have undertaken; we will be able to meet the requirements of all of the specifications
that we as a team have deemed necessary and even important to the project functionality. The
requirements for alignment transmitter power to receiver ratio and weather-proofing have are marginal
characteristics and therefore completion of the requirements if possible will slightly enhance the overall
design. The following pages indicate the proposed development plan and the associated characteristics
of our system.
21
Tigerbot Body of Facts
The following data is divided into the various functional blocks of our system, namely: the laser diode,
the photodiode, the DVD player and the audio receiver.
Laser Diode Specifications
The laser diode is a semiconductor laser with an approximate wavelength of 650 nm. The laser requires
a current source to drive it which must be less than 50mA (in practicality we don’t intend to drive the
laser beyond 30mA). It is assumed that the laser is capable of being driven to 5 MHz. Table 1 contains
that data points from our characterization of the laser diode. We used a laser driver and an optical
power meter to measure the optical power output of the laser at increasing levels of input current.
Figure 2 illustrates the laser’s input current vs. output power characteristics.
Laser 3 Characteristic Curve
Input Current (mA)
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Optical Power (mW)
0.0030
0.0035
0.0060
0.0080
0.0105
0.0150
0.0220
0.0900
0.6100
1.1000
1.7000
2.3000
2.8000
3.3000
3.9000
Table 1 - Laser input current vs. output optical power
22
4.5
Optical Power (mW)
4
3.5
3
2.5
2
1.5
1
0.5
0
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Input Current (mA)
Figure 1 - Laser diode operating characteristics
It can be seen that the laser threshold current is approximately 15mA. The minimum spots size
characteristics can be seen in Figure 1. The received power percentage can be seen from Table 2. The
beam divergence is calculated as:
Divergence = 2 x arc tan ((Df – Di)/2l)
Where Df is the final spot diameter, Di is the initial beam diameter and l is the length between the two.
Using the above formula and the values from Table 2, we find the beam divergence to be .038 degrees
at 5ft and .075 degrees at 20ft.
Laser Minimum Spot Size vs. Distance
Spot Size (mm)
2
3
4
6
Distance (ft)
5
10
15
20
Received Power Percentage
100%
44%
25%
11%
Table 2 - Photodiode received power percentages. (Note that the laser was driven with 29mA of input current.)
23
Below, in Figure 2, is a plot of the laser minimum spot size versus the distance from the laser, measure
in feet. It can be seen that as the distance from the laser increases the rate at which the beam spot
grows increases.
Spot Size in millimeters
7
6
5
4
3
2
1
0
5
10
15
20
Distance in feet
Figure 2 - Spot size characteristic plot
Figure 3 shows a simple diagram of how we measure the power density of the beam, which is necessary
for estimating the received power at the photodiode and for calculating the required gain of our
receiver stage.
Laser “high” output: 3.9mW
Laser “low” output: .61mW
Laser
d = 2mm at 5ft
d = 6mm at 20ft
d = 2mm at 5ft
Figure 3 - Laser divergence diagram
24
The power density is calculated using the following formula:
Power density = optical power/spot area = optical power/(π*(.5*diameter)2)
We decided that our circuit would work best if we were to focus the beam at 20ft so that the
maximum power is available at that distance. At 5ft the spot will be less focused but should
have ample power to drive the receiver. Tables 3 and 4 contain the data from the experiment.
Power density table
5ft -> Spot diameter = 2mm
20ft -> Spot diameter = 6mm
Low = .61mW
194.2 W/m2
21.6 W/m2
High = 3.9mW
1241.4 W/m2
137.9 W/m2
Table 3 - Power density for minimum spot sizes at 5 and 20 ft.
Power density with minimum
spot size left at 20ft
5ft -> Spot diameter = 4mm
20ft -> Spot diameter = 6mm
Low = .61mW
High = 3.9mW
48.5 W/m2
21.6 W/m2
310.4 W/m2
137.9 W/m2
Table 4 - Power density for beam focused at 20 ft.
Photodiode Specifications
The photodiode is a Honeywell SD3421 silicon photodiode. It has a 50% relative response to the laser
light of our transmitter. According to the documentation it has a rise time and a fall time of 15nS. Using
a test setup as shown in Figure 4 we determined the receiver current characteristics by shining the laser
at the diode from set distance with a fixed spot size. Table 5 contains the data points from the
characterization while Figure 5 shows the optical power input vs. current output curve.
Figure 4 - Photodiode test setup. Laser light was directed at the photodiode to produce a current.
25
Photo Detector Output
Optical Power
(mW)
0.0030
0.0035
0.0060
0.0080
0.0105
0.0150
0.0220
0.0900
0.6100
1.1000
1.7000
2.3000
2.8000
3.3000
3.9000
Output Current (µA)
0.3
0.5
0.8
1.3
1.8
2.9
4.4
10.4
131
287
555
683
800
847
Receiver vs. Spot Size is approximately 30-40%
Table 5 - Photodiode optical power input vs. current output
900
Output Current (μA)
800
700
600
500
400
300
200
100
0
Optical Power
Figure 5 - Photodiode operating characteristics
DVD Player Specifications
The signal source of our system comes from a commercial DVD player supplied by the university.
Because the end user will have no control over the output of the audio source it is important that we
26
characterize the system in order to build transmitter and receiver capable of operating on the signal.
The following values are the results of our test setup which is shown in Figures 6 and 7. Figures 8 and 9
give o-scope screen shots which display the actual voltage output from the DVD player from which the
data was gather.
Output voltage (unloaded) = 1.13Vpp
Rise-Time = 59nS
Output voltage (loaded) = 0.60Vpp
Bit rate ≈ 5MHz
Fall-time = 70nS
Pulse Shape = NRZ
Figure 7 Test setup for unloaded DVD
Figure 6 Test setup for loaded DVD
Figure 8 O-scope capture for unloaded DVD
Figure 9 O-scope capture for loaded DVD
Digital-to-Analog Converter Specifications
The digital-to-analog converter is a high quality stereo component provided by the university for this
project. It is capable of taking in a digital signal that includes audio information as well as error encoding
and converts it into an audio signal capable of driving a set of speakers. In order to find the minimum
operating voltages of the system we used a voltage divider in series with the DVD player digital output
and the DAC input to vary the voltage it received. This setup is shown in Figure 10. Table 6 shows the
characteristics of the system as we changed the input voltage until the fidelity of the audio signal was
lost.
27
Figure 10 - Test setup for DAC characterization
DVD player voltage out
DAC input voltage
.56
.8
1.0
1.06
-
.56
.3
.12
.07
-
Table 6 - DAC voltage characteristics
Potentiometer
Resistance (Ω)
.28
97
490
1034
-
DAC calculated
impedance (Ω)
58
66
73
77
28
Appendix E: Concept Generation and Selection
Introduction
The purpose of this document is to leave a paper trail of ideas and techniques that have been used in
the planning and design stages of our free-space optical communication project. Many of components
of our system are complete systems that simply require an interface. The instructors have also given us
very specific requirements for the unit housing and power supplies. There are still however parts of the
system that allow us a wide variety of options, all of which could meet the specifications. The two
subsystems for which alternatives are available are our laser driver circuitry and our photodiode
receiver circuitry. As a team we reviewed the designs used in previous projects that were successful and
added a few designs that we had found or created and were able to narrow them down based on
general considerations such as circuit size, speed and simplicity. This document should help to illustrate
the factors that led to our decision.
Body of Facts – Quick Review
The free-space optical project in essence requires converting a digital audio signal to laser light pulses,
receiving those pulses at a distance and then converting them back into a decipherable digital audio
signal. The major systems are shown in Figure 1.
DVD
Player
Laser
Driver
Circuit
LASER
Photodiode
Receiver
Driver
Circuitry
DAC
Figure 2 - Block diagram of FSO communication
In order to function properly the system must be able to translate the digital audio signal with fmin =
5MHz, and convert the loaded signal voltage of .56Vpp into a current source that swings from 17mA to
30mA at the same frequency. This current source will be used to drive the laser diode, which we
assume is capable of such frequencies.
The digital-to-audio converter is capable of converting a signal as small as .07Vpp depending on the
impedance of the stage preceding it. We assume that our photodiode driver circuit designs will have
impedance such that we can pass a signal as small as .1Vpp to the DAC and have it recover the original
signal.
In order to drive the DAC, we will need the receiver driver to amplify the photodiode output up to at
least .1Vpp. The laser diode has a lens that allows it to focus down to a minimum spot size that varies
slightly with distance. At 5ft the spot size is approximately 2mm in diameter which is the same size as
our photodiode sensor. Thus nearly 100% of the optical power falls on the sensor (not all of it is
absorbed because much of it is reflected off the surface and can be seen against relatively distant
objects). When the distance is increased to 20ft, the spot size is about 6mm and only 11% of the power
falls on the sensor. Therefore we need to create a receiver that will operate correctly with 10% or less
29
of the optical power. The photodiode has a threshold of 30μW. Our receiver circuit must be able to
amplify the output from 30μW to 3mW without exceeding a 1.0Vpp output swing. We also assume that
the photodiode is capable of greater than 5MHz frequency operation. Our amplifying circuitry must be
able to do the same.
The overall system must be able to fit within the web-cam housing provided by the professors and
therefore we are fairly limited on space. In order to meet that specification we focus on simplicity and
compactness. We assume that it will be possible to manufacture our chosen circuit on a board that will
fit within the housing.
Other considerations and assumptions are:
That a BJT will have a better frequency response than an Op-Amp and makes for a simpler circuit in
general for the transmitter (perhaps based upon our skill-set).
The receiver will be difficult to build without using two stages, and for a multi-stage system, an Op-Amp
is more convenient (again based upon our skill-set and advice given by the professors).
That using a PSPICE model will allow us not only to determine general behavior for our circuit but also to
see what modifications may help functionality before actually implementing the circuit.
Design Challenge 1
The transmitter circuitry is the first subsystem where we have a few options for designs. The following
figures are some of the alternatives that Tigerbot has considered as possible solutions to the challenge.
Figure 3 – Op-Amp Laser Driver Design (Free-Space Optical Link as a Model Undergraduate Design
Project, IEEE Vol. 50 No. 3)
30
Figure 4 - Tigerbot basic BJT design
Figure 5 - BJT Laser driver design (Free-Space Optical Link as a Model Undergraduate Design Project, IEEE
Vol. 50 No. 3)
Figures 2 through 4 represent the most common designs that we have found in previous FSO designs as
well as those designs most advocated by the professors for their proven functionality. In our efforts to
conserve time and avoid reinventing the wheel, we have chosen to focus on these tried and tested
designs.
Concept 1 is an op-amp design presented by doctors Selfridge, Schultz and Hawkins in a paper about
free-space optical links. Their design calls for a two-stage amplifier using the op-amps in a negative
feedback configuration. By making the driver a two stage system, the circuit can greatly enhance the
frequency response of the system and still have plenty of gain to work with in driving the laser diode.
Other key points are the capacitors located between the op-amps and the power supplies to help
protect against and voltage fluctuations in the power supplies.
31
Concept 2 is a classic BJT model following the design techniques learned in ECEn 313. The design
focuses on simplicity relying on the wide bandwidth and amplifying ability of the BJT. Key components
include the biasing resistors, which must be chosen just right so that the laser is driven within the active
region correctly, and also the power supplies because there is very little protection from voltage
fluctuations in this circuit. This circuit is ideally powered by batteries, but can be made to work from a
normal power supply.
Concept 3 represents a slightly more advanced version of a BJT laser driver found in the same article as
Concept 1. The major difference in this model with that in Concept 2 is that the design incorporates a
voltage controller chip that gives the added protection against voltage spikes. This system also uses
more resistors to bias the transistor than the previous concept. This design is geared more towards a
circuit that runs off a single power supply.
Evaluation and Proposed Solution
As a team we decided to compare each design against the following standards: cost, size, production
time, and reliability. The reliability metric will be used and weighted more heavily in the future as the
designs progress. Obviously we also need to know that each circuit is functional and meets the required
specs, but the goal here is to narrow down the type of circuit that we would like to implement.
Following that decision we will focus on modifying circuit values until it meets specifications or is
rejected, in which case we will move on to the next highest ranked concept design.
Our concept scoring consists of a preliminary metric using a standard: the BJT circuit proposed by Dr.
Selfridge, and comparing the other two designs against it. If the concept is better than our standard in
one area we give it a + and if it is worse we mark it with a -. Comparable areas will be marked with a 0.
At this point in time, our scoring is equally weighted among the categories.
Table 3 - Laser driver circuit comparison matrix
+ = Good
Selfridge BJT
Selfridge Op-Amp
Tigerbot BJT
0
0
0
0
0
0
-
0
+
0
0
0
-
0
+
0 = Neutral
- = Not Good
Cost (+/- $2 from
standard)
Size
Production Time /
Simplicity
Reliability
Totals
From our simple metric it can be seen that our preliminary designs do not differ much in terms of
production criteria. The main difference lies in the assumed circuit simplicity. Given the results from
the scoring matrix, our first choice for transmitter design will be the Tigerbot Basic design. It is
anticipated that we will first simulate the design using
PSPICE and should it fail to meet the technical specifications as a simulation, then we will of necessity
modify the circuit until it does or move on to the Selfridge BJT design.
32
Below is our weighted scoring matrix. The weights are from 2X and 3X as shown. We felt that size and
production time was most important with an equal weight of 3. Cost and reliability are still important
but have a weight of two just below size and production time.
Table 4 - Laser driving weighted scoring matrix
0 = terrible
1 = OK
2 = Great
3 = Awesome
Weight
Selfridge BJT
Selfridge Op-Amp
Tigerbot BJT
Cost (+/- $2
from
standard)
2X
2*2 = 4
1*2 = 2
2*2= 4
Size
3X
2*3 = 6
1*3 = 3
3*3 = 9
Production
Time /
Simplicity
3X
2*3 = 6
2*3 = 6
2*3 = 6
Reliability
2X
3*2 = 6
3*2 = 6
3*2 =6
22
17
25
Totals
From the table we see that the Tigerbot BJT design wins. These results help to ensure that our
comparison matrix was valid and that we can proceed forward with the Tigerbot transmitter.
Design Challenge 2
The receiver circuitry also requires choosing from among several alternatives. The figures that follow
are the designs that we have considered.
33
Figure 6 - Basic photodiode driver circuit 1 (from the SharpTM Reference Manual SMA99017)
Figure 7 - Basic photodiode driver circuit 2 (from the SharpTM Reference Manual SMA99017)
Figure 8 - Selfridge Op-Amp photodiode driver circuit (Free-Space Optical Link as a Model
Undergraduate Design Project, IEEE Vol. 50 No. 3)
Concept 1 is a very simple BJT circuit designed to amplify the current output from the photodiode. It
uses a simple voltage divider to reverse bias the diode and requires very little in terms of components
which save time and space. The drawbacks are that there is very little control over power fluctuations
34
and spikes and we have yet to determine if a single BJT is capable of the require amplification at an
operational frequency of 5MHz.
Concept 2 is a step up from concept 1 in terms of current and voltage control. There are a few more
resistors in place to help properly bias the circuit and a second BJT gives the system a higher gain
without having to sacrifice so much bandwidth. Again, however, the reliability of the circuit could be an
issue when combining imperfect discrete components.
Concept 3 is a tried and tested design used by several teams in the past. It uses Op-Amps which take up
more space and require larger interfaces, however they can be much more stable than a BJT. The design
features a two stage amplifier capable of high frequencies and large gains. In our metric this circuit will
be the standard.
Evaluation and Proposed Solution
Again we use the +/0/- scoring system to rate the designs and we will use concept 3 as our standard.
Table 5 - Photodiode driver design comparison matrix
+ = Good
Selfridge Op-Amp
Basic BJT 1
Basic BJT 2
0 = Neutral
- = Not Good
Cost (+/- $2 from
standard)
Size
Production Time /
Simplicity
Reliability
Totals
0
.5+
0
0
0
.5+
.5+
.5+
.5+
+
1+
.5+
0
Below, in Table 4 is our weighted scoring matrix for the receiver circuit. Again we used similar weighting
to the transmitters above.
35
Table 6 - Weighted matrix for the receiver designs
0 = terrible
1 = OK
2 = Great
3 = Awesome
Weight
Basic BJT 1
Basic BJT 2
Selfridge Op-Amp
Cost (+/- $2
from
standard)
2X
2*3 = 6
3*2 = 6
2*2= 4
Size
3X
2*3= 6
2*3 = 6
2*3 = 6
Production
Time /
Simplicity
3X
3*3 = 9
3*3 = 9
2*2 = 6
Reliability
2X
2*0 = 0
1*0 = 0
3*2 = 6
21
21
22
Totals
Again the table makes it easy to see which circuit we will pursue in our initial development: the Selfridge
design. Normally we would not give an added score to our standard in the matrix, however, the
Selfridge design has been advocated and used by nearly every team that has completed the FSO project.
In our opinion this is worth a higher score in the matrix. As with the transmitter design, the final result
will depend first upon the successful simulation and second upon actual functionality when building the
circuit.
Overall System Design
The remaining challenges that we foresee in the system design process are:
Whether or not to use a Proto-board or a PCB on our final design to meet the space, cost and
functionality requirements.
Where the power supply will enter the transmitter and receiver housing.
How the laser, the photodiode and the driver circuits will physically mount to the web-cam housing.
Possibly other un-anticipated design problems in system integration.
We feel that a large part of our decision in these matters is based upon actual empirical results from our
driver circuits and therefore cannot be accurately decided upon until we have a working transmitter and
receiver design. It is anticipated that we will return to these decision and again use a metric to decide
what the best choice will be.
Summary
As a team, the members of Tigerbot feel that we are using applicable criteria to determine the general
approach to solving the free-space optical problem. We intend to continue looking at viable options as
problems arise.
36
Appendix F: Project Plan
Introduction
The free-space optical communications project was conceived to find an alternative to wired
communication links in situations where line-of-sight transmissions are possible. Some of the benefits
of an optical communications link are the ability to link data sinks to data sources without cumbersome
wires and cables which can be a safety hazard and also pose equipment mobility limitations. Team
Tigerbot was created to meet the challenge by building an optical communication link to connect the
digital audio output of a DVD player to the audio input for a digital audio receiver (also known as a
digital-to-audio converter or DAC). The system must be able to transmit digital data in the form of light
pulses across a span of 5 to 20 feet and be able to reconvert the signal into digital electrical pulses that
are useable to the DAC. A block diagram of the system is shown in Figure 1. The purpose of this paper is
to outline the design and production process as well as to provide a timeline for the completion of the
various tasks and stages leading to a fully functional and marketable product.
Laser driver
circuitry
DVD
player
Digital to
audio
converter
Photodiode
driver
circuitry
Laser
Photodiode
receiver
Figure 9 – A block diagram of the complete free-space optical project. The red, dashed blocks indicate the portions of
the project that require design and assembly, while the other boxes indicate parts that are provided to the students.
The arrows indicate data path.
Scope
The scope of the paper is to lay out the time frames for phase completion and task execution within the
framework of scheduled design reviews. The report will touch on the resources provided to and
required of the team. This paper will also view the timeline from a broad overview of the process down
to the level of individual tasks and will explain some of the methods and calculations used to move the
project forward to production. For further information about the technical details of the project we
have included an appendix to this report that contains several of the project control documents, some
of which will be referenced in the text. Some references may also be made to documents located on
our web page at http://www.et.byu.edu/~thesaint/tigerbot/Control%20Documents/.
37
Resources
Most important to the successful completion of the project is an inventory of the resources available to
us throughout the design process. We have grouped our resources into three basic categories:
materials, people, and time.
Materials
Materials include all the physical components of the system. Our supervisors have provided us with the
data source (DVD player), data sink (DAC), and the final product housings (web cams) for our intended
project as well as some data cables. They have also provided us with some key components such as
power supplies, various op-amps, lasers and a photodiode to be used in the project implementation. All
other required materials will be obtained from the electric shop in the Clyde Building, electrical parts
vendors, or personal electrical supplies. While some funds are available to purchase parts from the
electric shop, the team members are responsible for the majority of the financial costs incurred
throughout the project.
People
Our human resources begin with the members of team Tigerbot. The members of Tigerbot are Seth
Gibelyou, Tyler Bird, and Aaron Bennion. The skills and abilities of each member were taken into
account as the team divided responsibilities and assignments as follows: Team Leader – Aaron Bennion,
Circuit Designer – Tyler Bird, Hardware Technician – Seth Gibelyou. The team leader is responsible for
most of the administrative, presentation and record keeping activities of the team as well as the
interaction between supervisors (professors) and the team. The Circuit Design Lead is in charge of the
design and simulation of the product components. The Hardware Technician is responsible for all of the
physical hardware as well as the actual assembly of the electronics. As a team, all members work
together to assist in each of these areas as needed. Weekly team meetings are held to coordinate
efforts as well as review the project status and work together on design challenges.
Human resources are also available outside of the team in the form of professors who understand the
technical challenges associated with the project. These professors double as our primary customers and
have set the key product requirements for us to meet. We also have secondary customers in the form
of acquaintances and associated who are interested in the project and give us feedback as to what they
would like to see in such a system.
Time
Time is the most precious resource for team Tigerbot. The customers have set the pace of the project
be scheduling 3 design reviews and a final presentation. The members of the team are all full-time
students working part-time jobs and therefore have limited time available to devote to the project each
week. Due to these limitations it is vital that the team plan effectively and make efficient use of time in
order to complete the project on schedule.
38
Timeline
The project timeline has basically been set by the customers who are the professors. The project has
been broken into 4 phases: 1) the Concept Generation and Design phase, 2) the Transmitter and Design
Review 1 phase, 3) the Receiver and Design Review 2 phase, and finally 4) the Final Product and
Presentation phase. The names suggest the broad scope of what needs to be covered within each phase
of development, however we have also broken down the requirements of the project into a list of tasks
which have been assigned a priority using MS Project. This list of tasks gives us specific direction for our
efforts and allows us to predict the time required to produce the product. The following sections layout
more specifically the steps within each phase of development.
Phase 1: Concept Generation and Design
The first stage of our project focused on characterizing the equipment that we were given in order to
better understand our design requirements. Referring back to figure 1, we needed to characterize all of
the black boxes because our communication device had to be able to interface with each one of those
blocks (except for the speakers). In order to focus on the Project Plan we will not focus on the test
methods here but the information obtained from our characterizations is contained within the Body of
Facts document in Appendix B. The characterizations included measuring the laser current vs. power
output curve, the photodiode input power vs. current output curve, the DVD player output voltage and
frequency, and the DAC minimum input voltage swings.
Having characterized the components provided to us, our next step was to begin generating concepts
for the transmitter and the receiver. This included doing a customer needs analysis as well as
researching existing methods for solving the problem of optical data transmission. Much of this
information is contained in the Functional Specs document attached as Appendix A. As a team we
narrowed down three possible designs each for the transmitter and receiver based on factors such as
simplicity, size, expected cost, anticipated stability and, probable production time. (See the website for
our Concept Generation and Selection document.) From these designs we were able to begin
simulations as well as begin to lay out tasks required for the remainder of the project. The culmination
of the Concept Generation phase was be the completion of the Functional Spec document and the
Project Plan in their preliminary forms. We have also prepared a Test Plan, for determining the
functionality of the key characteristics of the system, which has been updated and attached to this
document as Appendix C.
Phase 2: The Transmitter and Design Review 1
The transmitter design began with the simulation of the three designs chosen using P-SPICE. The goal
was to design a laser driver circuit that will run the laser above its threshold level but not as high as to
burn out the laser diode. The driver circuit also had to be capable of switching between high and low
outputs at or slightly above the frequency of the DVD player’s digital output signal. These system
characteristics can be found in Appendix B. Once we had simulated the various designs on P-SPICE and
39
incorporated these additional factors into our decision matrix we will proceeded with the design that
best met our needs. A schematic of our chosen transmitter circuit is shown in figure 2.
Figure 10 - Transmitter design schematic
Once we decided upon a transmitter design, the next step was to implement it on a breadboard and to
confirm functionality using a function generator and an oscilloscope. All this is documented in the
presentation for design review 1. (See website under Presentations: Tigerbot DR1.) The culmination of
this phase was design review 1 which was held on October 8, 2009. The tasks required to complete this
phase of the design are contained in Table 1 below.
1
2
3
4
5
6
7
8
9
Task
Days
Start
Finish
update project plan
0.16
10/8/2009
10/8/2009
Aaron
update CG&S
0.16
10/8/2009
10/8/2009
Aaron
Turn in documentation to professors for DR1
Simulate Selfridge BJT transmitter (upper 3-db,
current swing, DC bias)
Simulate Selfridge Op-amp transmitter (upper 3-db,
current swing, DC bias)
Simulate Tigerbot BJT transmitter (upper 3-db,
current swing, DC bias)
0.80
10/8/2009
10/8/2009
0.32
10/8/2009
10/8/2009
Tyler
0.32
10/8/2009
10/8/2009
Tyler
0.32
10/8/2009
10/8/2009
Tyler
Build promising design on breadboard
0.16
10/8/2009
10/8/2009
4,5,6
Seth
Troubleshoot transmitter design problems
Confirm transmitter function using o-scope (upper
3-db, current swings, DC bias)
3.20
10/8/2009
10/12/2009
7
Seth, Tyler
0.32
10/12/2009
10/12/2009
8
Seth, Tyler
Table 7 - Tasks required and team assignments for design review 1 (10/8/09).
Pre.
1,2
Team Member
Aaron
40
Phase 3: The Receiver and Design Review 2
The third phase of the project focused on the receiver and building a working prototype on a
breadboard. The receiver designs were simulated in P-SPICE and we had to make a decision as to which
receiver design we would build based on how well the design met the selection matrix that we had
chosen. The design had to take into account the current output of the photodiode based on our initial
characterizations and amplify the signal to a usable voltage swing that the DAC could interface with. A
lot more time was spent modifying values of components in our simulated circuits to make sure that the
simulation would provide a reasonable output based off of an anticipated input current. Once the
simulations fit our concept specifications we were able to build a circuit on a breadboard and begin
testing the circuit with the transmitter. Again, the specific test methods will not be mentioned here but
can be found in the attached Test Plan. Figure 3 is a schematic of the design that we chose for our
receiver circuit.
Figure 11 - Receiver design schematic
With the receiver prototype built, we were able to begin measuring the frequency response and
transmission distance limitations of our system. The goal of this phase was to get a receiver prototype
capable of transmitting audio at 5.1MHz over a distance of 1 to 30 feet. We set our requirements higher
than those of the customer to provide an error margin during actual operation. Our transmission
characteristics as well as our optical power transmission and density calculations were included in the
presentation for design review 2. (See the website under Presentations: Tigerbot DR2.)
The design review associated with this phase of the project also required that we complete a circuit
reduction plan. One of our team members (Seth Gibelyou) had access to Eagle PCB software and was
able to lay out our circuits on a board within the space limitations of the webcam. All of the tasks
needed to meet the second design review milestones are shown below in Table 2.
41
Task
Days
Start
Finish
Pre.
Team Member
Troubleshoot breadboard transmitter circuit
Confirm proper transmitter output using DVD
player and o-scope
Simulate BJT receiver design 1 (upper 3-db,
current swings, DC bias)
Simulate BJT receiver design 2 (upper 3-db,
current swings, DC bias)
Simulate Selfridge Op-Amp receiver design 1
(upper 3-db, current swings, DC bias)
Build promising receiver design on a
breadboard
Measure/calculate the power density of the
laser beam
calculate the minimum optical power
requirements/gain for the receiver
Find proper eye safety rating for the laser and
gather eye safety documentation
4.8
10/12/2009
10/15/2009
9
Seth
0.32
10/15/2009
10/15/2009
10
Seth
0.32
10/8/2009
10/8/2009
Aaron
0.32
10/8/2009
10/8/2009
Aaron
0.32
10/8/2009
10/8/2009
Tyler
0.16
10/8/2009
10/8/2009
0.48
10/8/2009
10/8/2009
0.32
10/8/2009
10/8/2009
16
Aaron
0.48
10/8/2009
10/8/2009
16
Aaron
4.8
10/8/2009
10/13/2009
15
Seth, Tyler
0.8
10/13/2009
10/13/2009
19
Seth
0.48
10/8/2009
10/8/2009
0.8
10/8/2009
10/8/2009
21
Seth
3.2
10/15/2009
10/19/2009
20,11
Seth, Tyler, Aaron
24
Troubleshoot receiver design on breadboard
Confirm receiver functionality using o-scope
and laser (DC bias, voltage swings, gain,
minimum detectable power, upper 3-db)
Order voltage regulator, spare op-amps,
diodes
Build power supply using components
provided
Trouble shoot system integration: transmitter
and receiver used with DVD and DAC
Confirm total system functionality using
breadboard circuits and power supplies.
1.6
10/19/2009
10/20/2009
23
Seth
25
Create circuit reduction plan
1.6
10/20/2009
10/21/2009
24
Seth, Tyler, Aaron
26
Turn in relevant documents before DR2
1.6
10/21/2009
10/22/2009
25
Aaron
27
Create presentation for DR2
1.6
10/22/2009
10/23/2009
26
Aaron, Tyler, Seth
10
11
12
13
14
15
16
17
18
19
20
21
22
23
12,13,14
Seth
Aaron, Seth
Aaron
Table 8 - Tasks required and team assignments for design review 2 (11/5/09).
Phase 4: The Final Product
The final phase of the design process is focused on fitting the reduced circuit into the final product
packaging. The product is characterized and instructions are written for the end-user. In the case of the
Tigerbot system, we began by implementing the circuit reduction plan and milling out the design on
copper-board in the department electric shop. The method is cheap and easy to use for production of
single units. The components were soldered to the board as well as the data and power connection
ports and the miniaturized circuit was mounted within the head of the web-cam. At this point the
testing both confirmed the functionality of the product but was also used in the final specifications to be
submitted with the final product.
A second major portion of phase 4 was the ease-of-use challenge. We had to write instructions for a
user with little or no technical experience to be able to connect and operate the FSO system. Using
these instructions and the FSO communication link hardware, two 10-year-olds set up the system at
different distances and were timed to measure the ease of set-up. How well the system performed in
the test determined the success level of our system.
42
Phase 4 is the culmination of the project. The final product will be presented to the customer along with
specification documentation, cost analysis, instructions, and a presentation about the project. The tasks
required during this phase of the project are show in Table 3. Also Figure 4 shows a flow chart of the
tasks from beginning to end with the critical path highlighted.
Task
Days
Start
Finish
Pre.
Team Member
14
10/20/2009
11/2/2009
24
Aaron
3.2
11/2/2009
11/4/2009
28
Seth, Tyler
1.6
11/4/2009
11/5/2009
29
Seth, Tyler, Aaron
31
Implement circuit reduction plan (order parts)
Implement circuit reduction plan (build final
circuit)
Implement circuit reduction plan (confirm
functionality using o-scope)
Implement circuit reduction plan (confirm
functionality of transmitter/receiver with DVD and
DAC)
3.2
11/5/2009
11/9/2009
30
Seth, Tyler, Aaron
32
Troubleshoot final circuits
8
11/9/2009
11/16/2009
31
Seth, Tyler, Aaron
33
Mount final circuits within webcam housing
0.8
11/16/2009
11/16/2009
32
Seth
34
0.8
11/16/2009
11/17/2009
33
Tyler
35
Attach power supplies to housing/circuit boards
Confirm final functionality against all relevant
specs using DVD, DAC, o-scope etc…
1.6
11/17/2009
11/18/2009
34
Seth, Tyler, Aaron
36
Write operating instructions
1.6
11/18/2009
11/19/2009
35
Aaron
37
3.2
11/19/2009
11/23/2009
36
Seth, Tyler, Aaron
38
Write final report
Complete preliminary idiot testing using idiot and
operating instructions
1.6
11/19/2009
11/20/2009
36
Seth, Tyler, Aaron
39
revise instructions
1.6
11/20/2009
11/23/2009
38
Aaron
40
3.2
11/20/2009
11/24/2009
38
Seth, Tyler
0.8
11/24/2009
11/24/2009
39,40
Seth, Tyler, Aaron
42
troubleshoot circuit problems found by idiot
Complete final idiot testing using idiot and
operating instructions
Submit all relevant documentation to professors
before final review
3.2
11/24/2009
11/26/2009
41
Aaron
43
Create presentation for final review
3.2
11/26/2009
11/30/2009
42
Seth, Tyler, Aaron
28
29
30
41
Table 9 - Tasks required and team assignments for final design review (12/10/09).
43
Figure 12 - Gantt chart with critical path highlighted in red for Tigerbot FSO design project.
Conclusion
As was mentioned earlier, the purpose of this report is to provide an outline of the free-space optical
project and the steps involved in moving it from a customer problem, to a design concept, to a prototype, to a final product capable of meeting the customer’s needs. As a team we feel confident that this
timeline is an accurate breakdown of the time and the steps required to complete the project and that it
lays out the steps required for our team to complete the project on time.
44
Appendix G: Test Plan
Test Plan Purpose
To lay out the methods and mechanisms used to confirm the functionality of the transmitter and
receiver blocks of the free space optical project as shown in Figure 1. Namely to determine whether or
not the transmitter and the receiver meet the functional specifications in three key areas: frequency
response, minimum transmission distance and maximum transmission distance.
Laser driver
circuitry
DVD
player
Digital to
audio
converter
Photodiode
driver
circuitry
Laser
Photodiode
receiver
Figure 1 – Block diagram of complete free-space optical project. Red, dashed blocks
indicate portion of project that require design and assembly. All other blocks are
provided to the students.
Test Plan Objectives
-
To confirm that the transmitter and receiver meet the frequency response requirements set
forth in the specifications in Table 1. Also to determine the maximum frequency response of
the transmitter and receiver circuits.
-
To confirm that the transmitter and receiver will meet the minimum transmission requirements
set forth in the specs in Table 1. Also to determine the minimum functional distance of
transmitter and receiver system.
-
To confirm that the transmitter and receiver will meet the maximum transmission requirements
set forth in the specs in Table 1. Also to determine the maximum functional distance of the
transmitter and receiver system.
45
Metric #
Need #
Metric
Units
Marginal
Value
Ideal Value
3
3
ft
<5
<1
4
3
ft
>20
>30
5
10
Minimum
distance from
transmitter to
receiver
Maximum
distance from
transmitter to
receiver
Minimum
functional bit
rate
MHz
>5.1
>7.5
Table 10 - Key functional specifications
Introduction and Analysis
This test plan is designed to validate the components of our free-space optical system in two key areas
broken up into three specs: transmission distances (minimum and maximum) and frequency response.
The necessity of these requirements is founded in the customer needs, outlined in our Functional Specs
Documents (http://www.et.byu.edu/~thesaint/tigerbot/Control%20Documents/FSD Rev1.docx).
The frequency response requirement is driven by the fact that our signal source, a DVD player, has a set
output frequency that was measured to be 5.1MHz and is not adjustable. Thus the free-space optical
system must be capable of operating at a frequency of 5.1MHz or greater. Table 1 shows our desire for
an error margin of 50% and therefore an ideal operating corner frequency of 7.5MHz.
The transmission distance limits of 5 to 20 feet were set by the customers themselves (i.e. professors
Hawkins and Schultz), for their own purposes, but likely reflect the customers’ intended uses. They are
the primary functional specs as expressed by the customer and therefore meeting these requirements is
paramount to a successful product. We chose to shorten the minimum operating ideal to less than 1
foot because we assumed the optical power transmitted would be ample for such a short distance and it
would increase the margins for meeting the customers’ needs. We extended the range for the ideal
maximum functional distance, again using a 50% margin for safety in meeting the customers’ needs.
Thus the ideal operating range is between 1 and 30 feet.
This test plan is organized into 2 sections, each defining the methods, mechanism and calculations that
will be used to measure key functional specs as outlined above. The specs will be covered in the
following order: frequency response and minimum and maximum operating distance.
46
Frequency Response Test Plan
I. Materials
- Transmitter and receiver circuitry/hardware as shown in Figures 2 and Figure 3 below.
- Power supply, oscilloscope, function generator, and associated electrical connectors.
Figure 2 – Transmitter schematic and values
Figure 3 – Receiver schematic and values (photodiode is shown as a current source in parallel with a capacitor)
47
II. Calculations and Expectations
The frequency response of the circuit depends heavily on the internal capacitances of the diodes, as well
as the gain-bandwidth of the op-amps. The transistor used in the transmitter has a current-gainbandwidth product of 300MHz, according to the data sheet. Frequency response is related to
bandwidth as follows:
frequency response = bandwidth = gain*bandwidth / gain
The gain of our transmitter stage is less than 15 and therefore should not be a major issue even for our
ideal value of 7.5MHz. The op-amps are model LM7171 and have a gain-bandwidth of 220MHz. The
gain of each stage does not exceed 10 which should result in a bandwidth of 20MHz or greater. The
internal capacitances of the diodes in our circuit are not expressed in any documentation that we could
find, and therefore we relied on SPICE simulations to estimate the frequency response of our receiver
circuitry using a model which replaces the diode with a current source and a small parallel capacitance
(around 1pF). The impedance of the capacitor is:
impedance = 1 / j2πC
At frequencies 5.1MHz and 20MHz the capacitor has an impedance of between 7Ω and 31Ω,
respectively. It is anticipated that this will be one of the limiting factors in the frequency response of the
FSO system.
The bandwidth is marked by the point at which the full-scale output of the device has attenuated by 3dB or has reached about 70.7% of its full-scale output. Thus our bandwidth will be determined by the
frequency at which the measured output is 70% of the full-scale output.
III. Methods
We break the frequency response testing into 3 parts. 1) Testing the transmitter, 2) testing the receiver
and 3) testing the two together.
1.) The transmitter. First we will set up our transmitter as shown in Figure 4, attaching the power
supply, oscilloscope and function generator as shown.
48
Figure 4 – Test setup for transmitter frequency response measurement
Our next step will be to input a signal of 100mV at a relatively low frequency (such as 50 or 100kHz) so
as to see the full-scale output signal as measured by the o-scope. Once this output voltage has been
recorded we increase the frequency until the output is .707 of the previously recorded value. This is our
transmitter bandwidth. Screen shots can be taken using the o-scope if needed.
2.) The receiver. First we will set up our transmitter as shown in Figure 5, attaching the power supply,
oscilloscope and function generator as shown. The receiver is somewhat more complicated and less
accurate because we are not able to use the actual diode at this stage of the test and have to rely on
approximations of the capacitive effects.
Figure 5 – Test setup for receiver frequency response measurement
49
Once the receiver has been connected, follow the same procedure as we did in step one with the
transmitter to determine the full-scale and 3-dB outputs and the associated frequency.
3.) The transmitter and receiver together. The test will begin by setting up the transmitter and the
receiver as shown in Figure 6, with the power supply, o-scope and signal generator attached as
depicted. It is also important to note that the laser beam must be aligned with the photodiode on the
receiver correctly in order to accurately measure the frequency response of the system. Alignment is
achieved by centering the laser beam on the photodiode using a secure mount or stand. In our test
case, the mount will be a breadboard.
Figure 6 – Block diagram test setup for FSO system frequency response measurement
We will then input an oscillating signal at a “low” frequency of 100kHz and 500mV into the transmitter.
The transmitter will convert the signal into laser light pulses which will are directed the receiver located
a distance of 20 feet away. Using the o-scope we will determine the full-scale voltage output. We will
then increase the frequency of the signal until the output reaches .707 of the full-scale output. This will
be recorded as our upper corner frequency. It is also important during this phase of testing to observe
the clarity and fidelity of the waveform that is passed through the system. We will input a square wave
to imitate the actual wave form of the DVD player and observe the output to be sure that it is at least
coherent as a square wave signal. Should we encounter any problems wherein the circuit does not
perform adequately, then we will modify the circuit until it at least meets the minimum specifications.
50
Minimum and Maximum Operating Distance Test Plan
I. Materials
- Transmitter and receiver circuitry/hardware as shown in Figures 2 and Figure 3 above.
- Power supply, oscilloscope, function generator, and associated electrical connectors.
- Measuring stick or tape.
II. Calculations and Expectations
The ability of the system to work at various ranges is dependent on three main variables: the output
power of the laser (controlled by the driver circuitry), the power density of the laser beam (controlled by
the focus length of the minimum beam spot), and the gain of the receiver (controlled by the resistors of
the receiver circuitry). The following calculations walk through how the variables are set and the
calculations needed to determine operating characteristics at a given distance.
The laser power output is simply a characteristic of the laser given any input current. The Tigerbot Body
of Facts (located at: http://www.et.byu.edu/~thesaint/tigerbot/Control%20Documents/FSD Rev1.docx)
contains the characterization curve of our laser. From the characterization we obtained values for the
threshold current, and the desired current swings. The values are located in Table 2.
Laser Characteristic
Value
Optical Output Power
Threshold Current
17mA
.09mW
Minimum Desired Input Current
19mA
.61mW
Maximum Desired Input Current
31mA
3.0mW
Table 11 - Measured laser diode characteristics for desired operating range
As a team we measured the minimum spots size for various distances and determined that the ideal
focal length for this application would be at 20ft. This maximizes the power density at the required
maximum distance which increases the ability of the system to detect the incoming signal. Figure 7
shows how we determined the spot size for the required specifications.
Laser “high” output: 3.9mW
Laser “low” output: .61mW
Laser
d = 4mm at 5ft
d = 6mm at 20ft
d = 2mm at 5ft
51
Figure 7 – Laser beam divergence for a minimum spot focus at 20 feet. Note that at 5ft the spot size
is 4mm. Although the divergence is not quite linear, it is close enough that we will approximate the
beam divergence as linear for distances between 1 and 30 feet.
We calculate the power density of the beam for a given distance as follows:
Power density = optical power/spot area = optical power/ (π*(.5*diameter)2 )
Thus we can calculate the power density of the laser. The calculated values for the required and ideal
distances are given in Table 3.
Power density with minimum
spot size left at 20ft
1ft -> Spot diameter approx =
3.5mm
5ft -> Spot diameter = 4mm
20ft -> Spot diameter = 6mm
30ft -> Spot diameter approx
= 7.5mm
Low = .61mW
High = 3.9mW
63.4 W/m2
405.4 W/m2
48.5 W/m2
21.6 W/m2
13.8 W/m2
310.4 W/m2
137.9 W/m2
88.3 W/m2
Table 12 – Power density values for a focus distance of 20ft
Next we must incorporate the characteristics of the photodiode. Again, the Body of Facts in the FSD
contains the characterization tables for the photodiode. The professors gave us an approximate size of
the photodiode receiver of 2mm. Using this value we are able to estimate the power received by the
photodiode, and combine it with the characterization curve for current output to determine what the
output of the photodiode will be.
power received = (power density) * (photodiode area)
Table 4 contains the expected current output for the various transmission distances.
Transmission
Distance
Power Received
“Low”
Current Out
“Low”
Power Received
“High”
Current Out
“High”
1ft
.20mW
36μA
1.27mW
363μA
5ft
.15mW
25μA
.98mW
252μA
20ft
.07mW
8μA
.43mW
89μA
30ft
.04mW
5μA
.28mW
55μA
52
Table 13 – Photodiode power received vs. current output for various transmission distances.
Now that we have calculated the expected current output of the photodiode, we are able to focus on
the receiver feedback resistor values to modify the gain such that the output signal falls between .5 and
1.0Vpp between the various distances. At present, the gain of our receiver is much too high which
means that we either have to attenuate its output or lower the gain. In either case, we have more than
enough to amplify the signal at 30 ft.
III. Methods
Now that we have determined all but the exact gain of the receiver, we are able to test the fidelity of
the transmitted signal at various distances. We will begin by measuring the minimum transmission
distance and then move to the maximum transmission distance.
1.) Minimum transmission distance. To begin we set up the transmitter and the receiver using the
power supplies, o-scope and function generator as shown in Figure 8. We will make d = 5 feet.
Figure 8 – Block diagram for the minimum transmission distance
We will input a square wave into the transmitter similar to the DVD player output (.9 to 1.0Vpp at
5.1MHz). We may have to attenuate the signal to get the output of the receiver to be between .5 and
1.3Vpp, but once we have adjusted the output we will not change it for the remainder of the test. We
will then move the transmitter closer to the receiver in 1 foot increments to determine whether the
signal will exceed 1.3Vpp or fall below .5Vpp. The point at which the output falls out of this range is the
minimum limit of our transmission system (in theory).
We will confirm our findings by connecting the entire system as shown in Figure 9, this time replacing
the function generator with the DVD player, and the oscilloscope with the digital-to-audio converter.
53
We will again start at a distance of d = 5 feet and gradually move the transmitter closer to the receiver
to confirm that the music continues to play through the distance measured previously. We also note
that we will not exceed the theoretical minimum distance so as not to overdrive the DAC.
Figure 9 – Block diagram for the minimum transmission distance
2.) Maximum transmission distance. We continue with the setup from step 1, only this time we begin
with the transmitter at a distance of 20ft. We will input the same square wave as before however we
still do not adjust the attenuation/gain from before so that we can determine how well the system
works at all distances without adjustment. We will gradually move the transmitter back in 1ft
increments until the output falls outside the range of .5-1.3Vpp. The distance will become our
theoretical maximum transmission distance.
Again we will confirm our findings by connecting the DVD player and the DAC as in Figure 9. This time
we do not need to worry about overdriving the system should we exceed the simulated maximum
distance and therefore can continue moving the transmitter back until either the music stops playing or
we are physically unable to move any further back.
Conclusion
At the conclusion of the tests outlined above we should have a good understanding as to whether or not
our free-space optical communication system meets or exceeds the most vital functional requirements.
We will know if it is capable of transmitting at or above the frequency of the DVD data, as well as
whether or not it can transmit usable data from 5ft to 20ft and beyond. These tests will help us
determine whether or not our system needs any adjustments. If the system meets the minimum specs,
we will also be able to decide whether or not we want to spend time to improve any of these functional
areas or focus on other aspects of the design.