Executive Summary
Transmitter Module
DVD Characterization
Laser Characterization
Transmitter Circuit
Receiver Module
Commercial Receiver Characterization
Photodetector Characterization
Laser Beam Characterization
Receiver Circuit
Full System
Component Packaging
System Characterization
System Performance
Cost Summary
Lessons Learned
Unexpected Problems
Future Projects
Appendix I: Function Specifications Documents
Appendix II: Concept Generation and Selection Document
Appendix III: Schedule
Appendix IV: Instruction Manual
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1

Executive Summary
The purpose of this report is to demonstrate our design of a digital audio system that uses light to transmit the signal across 5 to 20 feet. The free space optical communication system, see figures 1&2, transmits audio data over a distance using light rather than an electrical signal. The system consists of two parts: a laser transmitter and an optical receiver. The laser transmitter takes the digital bit stream out of a DVD player, and produces matching optical pulses. The receiver then translates these optical pulses back to an electrical bit stream that can be read by an audio receiver. The system produces high-fidelity-audio transmission at DVD bit rates. To accomplish this, the device operates at high frequency.
During the semester we took this design problem from concept to a final product. Within this report we will show what design decisions were made and our reasons for making those decisions. This document contains all of the product design and specification reports that were part of the design process. We will show the results of each decision and make reference to the appendix where more detail is provided.
This report concludes with a demonstration of our finished product. We show that we were successful ate completing the design objectives. The device conveniently fits inside two webcam packages, and has enough dynamic range to operate between 5 and 20 feet without any adjustment to the electronics. We also include a section detailing the lessons that we learned and suggestions for this project in future semesters.
Figure 1: Completed system
2

Figure 1: Block diagram of system
The transmitter module will take the digital bit stream out of a DVD player, and produce matching optical pulses. In order to ensure proper functionality, we have characterized all the elements that will affect its operation. These elements are seen in Figure 3. DVD player produces the digital bit stream that is the input to the receiver circuit; the photodetector converts the optical signal produced by the laser into an electrical signal; the pre-amplifier increases the signal amplitude, effectively reconstructing the digital bit stream produced by the DVD player; the commercial home theater converts the digital signal into an analog signal.
Laser Beam
DVD Player
Laser
Transmitter Circuit
Transmitter Module
Figure 3: Block diagram of elements that comprise the transmitter module or affect its operation.
3

DVD Characterizations
The first step that we took in designing our transmitter module was to characterize the output of the DVD player. From the block diagram (Figure 2) we see that it is one of the four key elements of the overall system. This being the case, we spent quality time finding the characterizations of this subsystem. By having done this, we now have good insight into the design of our transmitter, which is the second subsystem in series of the overall system.
Using an O-scope, we measured the DVD digital output. This is the output that the transmitter will connect to. In Figure 4, the waveform is displayed. From here we measured the max voltage and min voltage. We found them to be 940 mV and 0mV respectively; roughly 1V peak to peak. We found no DC offset in the signal. This is good since a DC offset would affect the biasing of the transmitter. We then measured the bit rate. We found this by measuring the time for one period in our waveform. We next took the inverse of the period to find the frequency. This we found to be roughly 5.5Mbps. So this told us that our transmitter must have a frequency response above 5.5MHz. The goal of Digi-Light was to have a frequency response above 8 MHz to allow for some buffer. The rise and fall time was also measured. They we found to be 42ns and 66ns respectively. Lastly, the output impedance of the DVD player plays a role as the generator resistance to our transmitter. This greatly affects the transmitter, so a well characterized output impedance was incorporated into the design. The resistance was found by to be 58 Ω and was established by voltage division with the following method.
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑤𝑖𝑡ℎ 100 Ω load
𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑤𝑖𝑡ℎ 𝑜𝑝𝑒𝑛
=
100
100+𝑅 𝑔
𝑅 𝑔
= 58 Ω
Figure 4 displays the waveform a digital signal from the DVD’s digital out port. The vertical axis is voltage, with 200mV per division and the horizontal axis is time, with 200 ns per division.
It show a signal with 1V peak to peak and a period of roughly 200 ns.
4

Volts
Seconds
Figure 4: DVD digital output
Laser Characterization
Knowing the details of the laser, used in this product, was vital in designing and implementing a well performing product. Details such as current vs. power were accounted for in the design. We are operating in the linear region of the laser; reducing ringing, and giving fast swings in power for the receiver to interpolate as our digital signal. This region is from 13mA to
27mA. Refer to Figure 5 for the graph of the laser’s current vs. power plot to note that our desired modulation falls within the linear region, and is well below the recommended max current of 50mA.
5

4000
3500
3000
2500
2000
1500
1000
500
0
0 5 10 15 20 25 30
Current (mA)
Figure 5: Plot of power vs. current showing the linear region of the laser. This region is from 10mA to 30mA.
Transmitter Circuit
Taking into account the fact that we want to operate in the linear region, we want the laser current to modulate from 13 mA to 27 mA. The following is the derivation of our circuit; explaining why we chose the values that are used.
Figure 6: Small signal model
𝟏𝟒𝒎𝑨 =
𝑽
𝑹𝑬
𝑹
𝟒
=
. 𝟖𝟐𝟓 × 𝑹
𝟒
𝑹
𝟒
+ 𝟏. 𝟕𝟑
𝑹
𝟒
𝑹
𝟒
= 𝟓𝟕 Ω
6

Using small signal model, see Figure 6, we calculated 𝑅
4 a 14mA swing.
so that the DVD signal creates
Figure 7: Transmitter Schematic
𝑹
𝟐
= 𝟏𝟎 × 𝑹
𝟒
𝑰
𝑩
= 𝐈
𝐄
𝑹
𝟏
− 𝛂 × 𝐈
𝑰
𝟏
𝐄
𝑰
= 𝟐𝟎𝐦 − 𝟐𝟎𝐦 ×. 𝟗𝟗𝟑𝟏 = 𝟏𝟑𝟖µ𝐀
𝟏
= 𝟏𝟑𝟖µ𝐀 + 𝑰
= 𝟏𝟑𝟖µ𝐀 +
𝟐
(𝟐𝟎𝒎 × 𝑹
𝟒
+. 𝟕)
=
(𝟏𝟎 × 𝑹
𝟒
𝟏𝟓 − (𝟐𝟎𝒎 × 𝑹
𝟒
)
+. 𝟕)
𝟏𝟑𝟖µ𝐀 + (𝟐𝟎𝒎 × 𝑹
𝟒
+. 𝟕)/(𝟏𝟎 × 𝑹
𝟒
)
With 𝑅
4
set by the small signal, we are able to calculate the bias for the BJT. We set the
DC current through 𝑅
4 to 20mA so that the 14mA swing will result in a range of 13mA to 27mA through 𝑅
4
and the laser, see Figure 8, which also shows a simulation of the circuit design that confirms our calculations.
7

Current
Time
Figure 8: Transient Response of transmitter
Next we verified that our circuit had a frequency response capable of operating at the
DVD bit rate of 5.5MHz. Refer to Figure 9 for the Pspice simulation of the frequency response.
It can be seen that the circuit is capable of functioning well above the actual bit rate of the DVD player. Because of the capacitor that isolates DC input from the BJT DC offset to the DVD player, we see that our circuit will not function at low frequencies. dB
Frequency
Figure 9: Frequency Response of Transmitter showing good functionality at 5.5 MHz
8

Lastly, to test that our built circuit was consistent with our simulation of it in Pspice, we measured the voltage drop through 𝑅
4
.
Connecting the DVD digital-audio output to the input of the transmitter then produces an optical signal that matches the shape of the DVD’s output. As seen by the resulting current through 𝑅
4
.
Figure 10 is the waveform that resulted.
798𝑚𝑉
𝑇ℎ𝑒 𝑝𝑒𝑎𝑘 𝑡𝑜 𝑝𝑒𝑎𝑘 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑠 798𝑚𝑉. = 14𝑚𝐴
57 Ω
Volts
Seconds
Figure 10: Voltage drop across the emitter resistor
The receiver circuit needed to be capable of correctly reconstructing the optical pulses received from the transmitter to the same electrical bit stream produced by the DVD player.
Since the receiver circuit had to have a dynamic working range of 5 ft and 20 ft, we have characterized all the elements that will affect its operation. These elements are seen in Figure 10.
The laser beam is the input to the receiver circuit; the photodetector converts the optical signal produced by the laser into an electrical signal; the pre-amplifier increases the signal amplitude, effectively reconstructing the digital bit stream produced by the DVD player; the commercial home theater converts the digital signal into an analog signal.
9

Laser Beam
Photodetector am
Pre-Amplifier
Receiver Circuit
Commercial home theater amplifier
Figure 11: Block diagram of elements that comprise the receiver circuit or affect its operation.
Commercial Receiver Characterization
The commercial home theater amplifier (commercial receiver) receives the digital signal from the receiver circuit and converts it to an analog signal with which it drives the speakers.
The receiver circuit needs to output a signal that the commercial receiver will be able to interpret correctly. If the signal has too much noise, is too distorted or too attenuated, the commercial receiver will be unable correctly perform analog to digital conversion. The commercial receiver was characterized through a series of tests that included: attenuation of the signal, distortion of the signal, and inverting the signal. Measurements were recorded using a digital oscilloscope and recorded on Table 1. The characterization shows that the commercial receiver is quite versatile in interpreting correctly distorted and attenuated signals.
Parameter
Square wave voltage Pp
Value
Min Max
Time constant threshold (τ)
Input impedance
200 mV 1 V
430 ns
85 Ω
Reversible polarity -
Table 1 : Commercial home theater amplifier input characteristics.
Photodetector Characterization
The receiver circuit uses a photodetector as a way to convert the optical signal back into an electrical signal. It is important to know how the photodetector behaves in determining how it will be used in the receiver circuit. The photodetector used in the receiver circuit is SD-3421-2.
Information about its electrical and optical characteristics was obtained from a manufacturer’s datasheet. This data is recorded in Table 2 and provides insight to the functionality of the photodetector. Since the reverse breakdown voltage occurs at 75 V, we bias the photodetector directly with the 15 V dc power supply. By directing our laser at the photodetector, we measured the power received by the photodetector and the corresponding amount of current the photodetector produced. The results of this test were recorded in Figure 12. This test confirmed that the photodetector has a responsivity of .4 A/W.
10

Parameter Value
Reverse breakdown voltage 75 V
Responsivity 0.4 A/W
Diameter 2 mm
Table 2: SD-3421-2 photodetector characteristics.
Measured Photodetector Current vs Optical Power
200
180
160
140
120
100
80
60
40
20
0
0 50 100 150 200 250 300 350 400 450 500
Power (uW)
Figure 12: Measured photodetector current produced as a function of power received by the photodetector.
Specific measurements of the power received and corresponding current produced from the photodetector are recorded in Table 3. This data gives information on the amount of current that was needed to be amplified in the receiver circuit pre-amplifier. We realized that the receiver had to work at both 5 ft and 20 ft, both of which distances produce different amounts of current.
Distance
20 ft
5 ft
Power received by photodetector (P received
)
320 µW
595 µW
Current
128 µA
238 µA
Table 3: Estimated amount of optical power received by photodetector at given distance and corresponding current produced.
Laser Beam Characterization
The laser used in the transmitter creates optical pulses that represent the digital signal produced by the DVD player. As the laser beam leaves the laser it immediately begins to diverge.
Table 4 shows the corresponding spot size at the end of the laser beam for the distances of 20 ft and 5 ft. The power that reaches the photodetector at these distances will be a fraction of the power emitted by the laser. Knowing the approximate amount of power received by photodetector was crucial in determining the amount of gain needed for the pre-amplifier.
11

Distance
20 ft
5 ft
Spot Size (diameter)
5 mm
3.6 mm
Table 4: Measured diameter of circular area at the end of laser beam for given beam length (distance).
The following is a derivation of amount of power received at the photodetector as a function of optical power emitted by the laser.
𝑃 𝑙𝑎𝑠𝑒𝑟
= 𝑃𝑜𝑤𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑙𝑎𝑠𝑒𝑟
𝑃 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑
= 𝑃𝑜𝑤𝑒𝑟 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑 𝑏𝑦 𝑝ℎ𝑜𝑡𝑜𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟
𝐴 𝑙𝑏
= 𝐿𝑎𝑠𝑒𝑟 𝑏𝑒𝑎𝑚 𝑠𝑝𝑜𝑡 𝑠𝑖𝑧𝑒 𝑎𝑟𝑒𝑎
𝐴 𝑝𝑑
= 𝑝ℎ𝑜𝑡𝑜𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟 𝑎𝑟𝑒𝑎
𝑃 𝑙𝑎𝑠𝑒𝑟
𝐴
(
𝐴 𝑙𝑏 𝑝𝑑
) = 𝑃 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑
From the above derivation, we determine the resulting power ratio of distances at 5 ft and 20 ft.
(
𝑃 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑
𝑃 𝑟𝑒𝑐𝑒𝑖𝑣𝑒𝑑
𝑎𝑡 20 𝑓𝑡
𝑎𝑡 5 𝑓𝑡
) =
320
595
= 0.53
The equation reveals that the amount of current produced by the photodetector at 20 ft is about half of what is produced at 5 ft.
Receiver Circuit
We tested the characteristics of our photodiode for recreating the DVD output waveform across different loads. We first applied a 1 V sinusoidal voltage to the transmitter and placed the photodetector at a distance of 5 ft from the laser. With this configuration, the photodetector produced 88 μA. Next, we used the circuit in Figure 13 to select an appropriate load resistor that would give us good signal quality and sufficient voltage input to the receiver amplifier. Figure 14 shows the peak to peak voltages across the load resistor with the transmitter located at 5 and 20 feet. The red line in Figure 14 represents maximum load resistance with acceptable signal quality. From this analysis we determined that a load resistance of 330 ohms would give adequate voltage and good dynamic range, as well as high signal quality. Figure 15 is our receiver schematic. Since R3 was chosen to be 330 ohms the resulting input to the amplifier was roughly 30mV. This voltage is caused by the current allowed to flow through the reverse biased photodiode. Since we needed an output voltage of approximately 1 V peak to peak, we arbitrarily chose R1 to be 1k ohm, which required R2 to be 33 K ohms. With this configuration, we resulted in a receiver gain of 11.3k V/A. The equations below show how we arrived at this conclusion.
12

𝑉𝑖𝑛
1
= 𝐼𝑝(𝑅3)
𝑉𝑜𝑢𝑡
1
𝑉𝑖𝑛
1
= 1 +
𝑅
2
𝑅
1
𝐼 𝑝
= µA ; 𝑉𝑜𝑢𝑡
1
𝑉𝑜𝑢𝑡
𝐼 𝑝
1
= 1 V
= 11.36 𝑘 (
𝑉
𝐴
)
𝑉𝑜𝑢𝑡
1
𝑉𝑖𝑛
1
= 34
Figure 13: Response test circuit
13

Signal Quality Threshold
Figure 14: Demonstrating max load to reverse biased photodiode with acceptable signal quality
Figure 15: Receiver schematic demonstrating single stage non-inverting amplifier
14

Frequency Response of Receiver
Figure 16: Simulated frequency response of receiver amplifier demonstrating a cutoff frequency of 36 MHz
The frequency response of the receiver circuit was simulated and the cutoff frequency was found to be 36 MHz. Figure 16 shows the plot of the simulated response. Since the frequency response is much higher than the required 5.5 MHz for the DVD player out, the circuit should function and we will begin testing the system.
After completing the receiver circuit prototype, there was some concern as to whether the system could be improved with the use of a feedback capacitor. The feedback capacitor would serve as an active low pass filter that would cause the upper corner frequency to roll off at lower frequencies. This would theoretically make the receiver amplifier more stable. With our noninverting amplifier configuration, the equation for determining the upper corner frequency with the feedback capacitor is described below.
𝑓 𝑐
=
1
2𝜋𝐶 𝑓
𝑅 𝑓
Since the feedback resistor is 33k Ω and the desired upper corner frequency would be about 8 –
10 MHz, the feedback capacitor would need to be about 0.45 pF. Capacitance values this small are generally not manufactured and capacitance fluctuations of this magnitude are most likely inherent in the circuit. Since our prototype exceeded expectations and worked perfectly at a range of 5 to 20 ft, we chose not to add a feedback capacitor or alter the amplifier resistor values so that one could be used into our final design.
15

After completing the transmitter and receiver, the entire system was built and put together.
We used an O-scope to verify that our overall system functioned as engineered. We were expecting to measure an output signal of 1 V peak-to-peak upon receiving a modulated light signal from the transmitter, at 5 feet. Figure 17 demonstrates this waveform. Some ripple can be seen on the waveform; however, it does not appear that it affects the performance of the full system. This was determined after testing the system with DVD input and commercial receiver output at a range of 5 to 20 ft. The system was clearly able to reproduce a high fidelity signal from which a DAC can drive audio.
Voltage
Time
Figure 17: Output waveform of the amplifier at 5 feet
We realized that how we packaged the transmitter and receiver, would affect many of the specifications that were desired from the Functional Specifications Document (refer to Table 3 in the FSD found in the appendix). Keeping such things in mind as; time in learning the operation of device, fits in packaging correctly (doesn’t have protruding parts) and number of interconnect cables per device, we came up with the following. The transmitter as well as the receiver were to have the interconnect cable exiting the webcam from the back. This method maximized the amount the direction the beam could be pointed and received. We also tested to make sure that the cables wouldn’t cause any torque on the setup, causing the beam to drift over time. Next, we incorporated the +- 15 v and ground wires into one single cable, in order to facilitate the connection to the power supply. We used RCA cables to connect to the DVD player and
16

commercial receiver. Since the transmitter and receiver need interconnect cables that go to two different locations we decided to have two cables exiting the webcams. The components used to implement the systems were of small magnitude. There was little trouble inserting them into the cavity of the webcam. Heat shrink was placed on all conductors, to reduce the risk of short circuits. Lastly, wire ties were placed around the exiting cables, on the interior of both webcams.
This was done to provide protection from cables being accidently ripped out of their respective connections.
Figure 18 is of our transmitter which shows sufficient size to fit in the webcam. Figure
19 had more components, whence filled up more volume, but due to the shaping of the PCB material, it fit within the packaging.
Figure 18: Actual transmitter in webcam package demonstrating sufficient size
Figure 19: Left-photodiode side of circuit. Right-Op amp side
17

Tolerances
Max pitch was determined to be 63 degrees. This measurement was determined by raising the receiver above the plane of the transmitter until audio transmission ceased. The horizontal distance from the transmitter to the receiver and also the vertical height of the receiver was used to calculate the pitch angle. The following equation was used: tan −1 𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙 ℎ𝑖𝑔ℎ𝑡
( ℎ𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑖𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
)
Alignment sensitivity was determined by placing the transmitter and receiver directly across from each other at distances of 5 and 20 ft. We measured the angle between the laser or photodetector normal and the incident at the point where the -3dB voltages output signal occurred. Tables 5 and 6 demonstrate the 3dB angle points for the
Movement of Laser normal
5 ft
20 ft
Left and Right Top
.14º
.07º
.05º .02º
Table 5: The 3dB angle points of transmitter.
Bottom
.1º
.04º
Movement of Photodetector normal Left and Right Top
5 ft
20 ft
18.6 º
15.4º
Table 6: The 3dB angle points of receiver.
Bottom
The functional distance of the system was measured by placing the transmitter and receiver modules at various distances apart from each other and by inspection verified the audio quality. The maximum distance apart was determined to be 60 ft. The system also continued to function with no gap between modules, making the minimum distance 0 ft.
Power consumption
Power consumption was measured by connecting an amp meter in line with the power cable for both the transmitter and receiver modules. The power supplies for each module produce
+-15 V. Table 7 shows the power consumption of each module. Based on the information from the table, the total power consumption of the system is between 658 mW and 1022 mW for distance range of 5 ft to 20 ft.
Transmitter
Receiver at 5 ft
Current from +15Vdc supply
24.5 mA
35 mA
Current from -15Vdc supply
0
8.7 mA
Power
367.5 mW
655 mW
Receiver at 20 ft 10.7 mA 8.7 mA
Table 7: Power consumption of transmitter and receiver modules.
291 mW
18

Frequency Response
Using a sinusoidal voltage with 1 Volt peak-to-peak and frequency of 5.5 MHz as the input to the transmitter and an oscilloscope to measure the receiving signal, the lower corner frequency is 30 kHz and the upper corner frequency is 17.86 MHz.
Laser Classification
Connecting a DVD player to transmitter module, the transmitter laser power was measured using an optical power meter. The laser produced 1.69 mW. Optical power between 1 and 5 mW falls in the Class 3R laser classification.
Based on our defined customer needs as outlined in Appendix I: Functional Specification
Document (FSD), we developed metrics and measurements to establish a way of evaluating our system performance. We established goals at the beginning of the project and compared our actual system performance to those goals. Table 8 shows the comparison of the predetermined performance goals to our actual system performance. The FSD explains the metrics and measurements used for the comparison.
Metric Measurement Goal Actual System
Performance
10 17.86 1 Capable of operating at 5.5 MHz Upper corner frequency
(MHz)
2 Operates on 120 VAC at 60 Hz Input circuit voltage (DC voltage)
3 Time in learning the operation of device
4 Fits in packaging correctly
Setup time for new user
(minutes)
Length of protrusions
(inches)
+-15
3 none
+-15
5.5 tx = 3/8 rx = 0
5 Dynamic operating range
6 The receiver should amplify the signal back up to its original value
Maximum and minimum working distance (ft)
Signal Vpp
5-20
1
0-60
5ft < 1
20ft > .3
7 Max angle of incidence at receiver
8 Transmitter BJT current
Degrees mAmperes
0-20
13-27
0-18.6
13-27
9 Number of interconnect cables per device
Cables 2 2
Table 8: Comparison of actual system performance to pre-established performance goals.
19

As seen in Table 8, our system met or exceeded nearly all of the pre-established performance goals. Not only is our system capable at operating at 5.5 MHz, the upper corner frequency implies that it would possibly function at bit rates that are higher in magnitude. Both the transmitter and receiver circuits operate at the voltages supplied by the power supplies. We did not use any voltage regulators but managed to keep our circuits as simple as possible.
Unfortunately, we did not meet the goal for setup time for new users. We believe that three minutes may have been overly ambitious since no project achieved that setup time. Although our goal for packaging was determined set at having no protrusions from the webcam packaging, we decided that a small amount for the transmitter laser would be beneficial. The laser seemed to be the component that was most susceptible to malfunction. If the laser were to malfunction, we would be able to replace it without taking apart the packaging. The laser also had an adjustable lens that could be controlled if it protruded slightly from the webcam. For these reasons, we chose to have it protrude slightly.
The actual dynamic operating range of system exceeded our goal by triple the amount.
The signal quality at 60 ft is just as good as it is between 5 and 20 ft. However, due to the The
We successfully replicated the output signal Vpp. The max angle of incidence at the receiver was limited by the placement of our photodiode. The photodiode is mounted in the interior of the webcam packing as seen in Figure 12. Luckily, our actual system performance still came close to the desired angle. The transmitter BJT current goals were based on measurements from the laser characterization. Figure 4 shows the linear operation region of the laser. As seen from the graph the linear region extend from about 10-30 mA. Assuming that each laser will be similar but also giving us some buffer, we established the goal to be 13-27 mA. Our actual system performance is right exactly what our goal was. Again relating to the packaging, we wanted it to be as aesthetically pleasing as possible. We chose to have only two cables extending from each model, and successfully achieved this goal.
The total cost of the receiver and transmitter is summarized in Tables 9 and 10. The main expense for each module was the power supply, which contributed to 68 percent of the total cost of each module. The other components were relatively inexpensive in comparison. Total system cost came to $94.00.
Component Cost
BJT N2 4124
Laser
Resistors (3)
Capacitor
Power Supply
Webcam Packaging
Miscellaneous (wire, heat shrink, wire tie)
Total
Table 9: Transmitter cost summary.
$0.10
$7.20
$0.12
$0.20
$32.00
$7.00
$0.50
$47.12
20

Component
LT1226 Op-amp
Photodetector
Resistors (3)
Capacitors (2)
Power Supply
Webcam Packaging
Miscellaneous (wire, heat shrink, PCB material)
Total
Table 9: Transmitter cost summary.
Cost
$5.86
$1.00
$0.12
$0.40
$32.00
$7.00
$0.50
$46.88
As the semester progressed we had to complete the Milestones by certain due dates. This requirement forced us to make good use of our time. By using the Project schedule we were able to plan what tasks had to be completed so that we could move forward and meet our deadlines.
We learned that good time management was the best relief from stress. As we completed each milestone and stayed on schedule we were confident that we would be able to complete the project.
We learned that by working together we could complete more in less time, and create a better final product. At times when a single person was solving a given problem it was common for another team member to see the solution. By working together and communicating as we developed our ideas we drew from the knowledge and expertise of each individual.
At the onset of the semester we dreamed of systems to optimize the functionality of the system that was complex. As we focused on the needs of our customer we realized that overcomplicating things would only make our task more difficult, and in the long run raise the price of the final product. In a real world situation this would be valuable to be sure that the demands of the customer were met, and that in the end we produce an appropriate product. It would be valuable to inform the customer of our recommendations, but in the end the customer is king.
One unexpected problem that we found in our final design was that if the digital audio signal was put on the input of the transmitter, while the transmitter was powered, the laser very occasionally burned out. Through trouble shooting, we found that rarely a spike in voltage well above 1 V peak-to-peak would be introduced on the input line while connecting and disconnecting the input signal. We resolved this issue by putting in the operating instructions a correct order of procedure for setting up the device. This problem only very rarely occurred, but to be safe the manual clearly demonstrates correct instructions.
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Another problem we came across during the design process was found in our original receiver circuit. We had an inverting-stage amplifier that would simply not amplify any signal, not even from a signal generator. We switched to a non-inverting stage and the problem went away. We feel that the issue was an inherent problem in the op-amp that we were using.
We appreciate the work that goes into planning and executing these senior projects. As we contemplated on our experience and tried to develop ideas for future semesters the difficulty of this planning was apparent. One of the aspects of our project that we felt was valuable was that we were able to take a concept from design to finished project within a single semester.
This gave us the feeling of accomplishment that is lacking in some of the ongoing research type projects. We also liked that the project required a large variety of knowledge and learning.
Topics that we were not as familiar with, such as optics, lasers and photodiodes required new research and study. Other topics such as BJT’s, operational amplifiers gave us the opportunity to better understand the principles that we learned in previous semesters with a real world design.
We were only able to come up with a few suggestions for this project in future semesters. One idea was to transmit data that required a higher bit rate, such as blue ray audio. Another idea was to use a laser to measure distance and or temperature.
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Introduction
Product Description
Block Diagram
Project Requirements
Point Evaluation of Customer Needs
Customer Needs
Analysis of Customer Needs
Project Specifications
Metrics
Analysis
Summary
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This document is the Functional Specification Document for Digi-Light, Fall, 2009. Our project is Free Space Optical Communications System. Our team members consist of McKay
Killpack, Spencer Chadderdon, and Darren Turetzky.
Our project is to design an optical transmitter and receiver that are capable of transmitting and receiving data from a DVD player. The device will work at high speeds that will produce high fidelity audio transmission at DVD bit rates. The device will fit inside two webcam packages, and have enough dynamic range to operate between 5 and 20 feet without any adjustment to the electronics.
In this document, the needs of our customers will be presented, evaluated, and then translated into goal specifications for our product. Analysis will then follow that which has been presented by the customer needs and metrics. We expect to meet the above description and layout the accompanying tolerances for those different aspects that it will meet. We also expect to fully develop and have a functioning product by the end of the semester to present to our customer.
DVD Player
Transmitter
Light
Interconnect
Figure 1: Block Diagram showing full system. cables
Receiver
Speakers
DAC
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In Table 1 and Table 2, we have mapped point values to each of the customer needs.
This is done so that the needs can be categorized and more easily evaluated for their importance in the project. The customer needs are listed by number for easy reference later in the document.
# Description
1 This feature is not important and would provide small added value to the product
2 This feature is somewhat useful, but would not influence my choice
3 The feature is highly desirable but I would consider a product without it
4 The feature is critical, I would not consider a product without it
#
1
Customer Needs
Fits into provided package
2
Easy to operate
3
Mounts to table
Interpretation of Needs
Transmitter and receiver need to fit into two webcam packaging
Able to align the transmitter with the receiver easily without too tight of tolerances
Make sure that cables do not interfere with built in capabilities of product
Imp.
3
4
4
4 Sounds great
Aesthetically pleasing
5
Produces high fidelity audio transmission
Only two cables extending from each webcam package
4
3
6
Adequate documentation
7
Dynamic operating range
8
Portable system
9 Reliable
10
Quick set up time with provided instructions
Detailed instructions so that the customer can duplicate our design if needs be.
Functions at 5 and 20 feet with no adjustments to electronic components
Plugs into any outlet and converts from 120 V AC to
15 V +-DC
The lasers and amplifiers are resilient to failure
Operating instructions must be clear and concise for general public
2
3
4
3
2
Our customer has laid out what they require of the product. We have analyzed their needs and have labeled them according to the customer’s importance. Some of the needs are quite specific, with details already presented to us. Other needs are somewhat vague, and are left up to our discretion to fulfill the customer’s needs. The needs that are labeled with an importance of 4 are our highest priority in our design of the product. They are such that if not met the product will render useless for the customer. So again these needs must be met. The rest
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of the needs were labeled with an importance of 3 or 2. The reason being is because some of them, if not met 100%, would still be considered a product; but would degrade however the significance of our product.
The customer needs with an importance of 3 or less, will be discussed now. Need 1 requires that the product fits into the provided package. This was interpreted as meaning the transmitter and receiver must fit into two webcam packaging. This will help the product be portable as well as be aesthetically pleasing. Also the webcams will help in aligning the laser with the photo detector for the transmitting and receiving stages of the design. Need 5 was aesthetically pleasing. This was interpreted as meaning only two cables extending from each webcam package. The two cables should provide the power and the interconnection to either the
DVD player of the audio receiver. The length of the cables should be long enough to be able to place our transmitter and receiver at convenient locations, but not be too long and get in the way of things. Need 6 stated, “Adequate documentation”. We interpreted that as providing detailed instructions so that the customer can duplicate our design if needs be. During our group meeting we will be documenting what we discuss as well as any problems or design modifications that we have. This will not only help our customers throughout the procedure but will help us be more productive as a group. Need 7 was having a dynamic operating range. We were told that it should function at 5 and 20 feet with no adjustments to electronic components. We put this as having an importance of 3. The customer ideally desirers the product to work at 20, but if it is just shy of that it will reflect on our compensation. Need 9 was reliability. The customer wants the lasers and amplifiers to be resilient to failure. This might turn out to be a challenge. Our tactic is to be sure to allow for tolerance in our amplifying circuits. From our preliminary efforts we have found the lasers to be susceptible to burning out. We need to design our circuits so that we do not drive our components too hard. Customer need 10 was quick set up time with provided instructions. This means that the operating instructions must be clear and concise for the general public. We will be evaluated on how quickly someone, unfamiliar to the product, can set it up and get it working. In order for us to do this we will test out how volunteers can assemble our product and revise our instructions until we get our desired assemble time. All of these needs are important and will be weighed accordingly in our design process.
The following needs are considered to be the most pertinent and will be discussed here.
Customer need number 2 is ease of operation. The ability to align the transmitter with the receiver easily, without too tight of tolerances, is essential. Otherwise the product is useless if one is constantly fidgeting with the equipment to get it to play the desired music. The use of lenses at the receiving end might prove to be useful and will be assessed during production.
Need 8 requires our system to be a portable system. We decided that to best accomplish this need, without resulting to battery pack, is for the product to be compatible with 120 VAC @ 60
Hz. We chose this metric due to the fact that the equipment that our product with interface with also runs off of the same power source. Customer need number 3 is for the product to mount to the table. We must make sure that the interconnect cables do not interfere with built in capabilities. We interpreted this because of the fact that any oscillations in the equipment would severely degrade from the performance and reliability of the system. Mounting to a table would provide a stable foundation to assure stable transmissions. Lastly, custom need 4 is that it has to sounds great and produces high fidelity audio transmission. This is by far the most important of
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all the customer needs and most of the subsequent needs are there to contribute to this one need.
Our team is dedicated to delivering to the customer a quality, reliable and great sounding product.
We have developed metrics that are seen in Table 3 that allow us to better measure the completion of meeting the customer needs. They are a way of quantifying the achievement of our device in certain areas. The needs are again referenced by their number and the metrics are also assigned a number for easy reference.
Metric Need Metric
# #
1 4, 9 Capable of operating at 5.5 MHz
2 2,8,10 Operates on 120 VAC at 60 Hz
3 2,10 Time in learning the operation of device
Fits in packaging correctly (doesn’t have
4 8 protruding parts)
Units Marginal Ideal
Hz
Value
5.5MHz
Value
8MHz
Volts +-5<V<+-15 =+-15
Minutes
Inches
<5
<=2
<3
<=0
5 7 Dynamic operating range
6 4,9
7
The receiver should amplify the signal back up to its original value
9 Max angle of incidence at receiver
Feet
Volts degree
8 4,9 Transmitter BJT current
9 5 Number of interconnect cables per device mA
Unitless
5<d<20 0<d<25
.7<V<1.3 =1
15<a<20 >20
0<mA<30 13<mA<27
=4 =2
Table 3: Metric system for measuring completion of customer needs.
Metric 1- Capable of operating at 5.5 MHz
Upon measuring the output rate in cycles per second from the DVD player, we found that it was functioning at around 5.5MHz. So our marginal value, in order to operate, will have to at least function at that rate. We will shoot for a value of 8 MHz for safety. Our system will have to duplicate that same signal; however, we will be converting an electrical signal to oscillating light intensity.
Metric 2 – Operates on 120 VAC at 60 Hz
Since all of the interfacing equipment will operate off of this type of power source we decided that it would be convenient to have our product compatible with the same power. We will be using transformers to then convert to +- 15 VDC so that we can power our op-amps and other circuitry.
Metric 3 - Time in learning the operation of device
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We will be competing against other teams who are also working on designing projects that have the same functionality that our product will have. Our goal is to have the fastest setup time compared to the other teams. We feel if someone can correctly interface our product with the needed equipment in less than 3 minutes that we will be competitive. We must continue to revise our set of instructions until we feel they are simple enough and still convey the needed information for fast assembly.
Metric 4 - Fits into packaging correctly
We feel that if the circuitry and all components fit into the provided packaging, the more aesthetically pleasing the final modules will be. If components are protruding from the packaging, it will might possible cause complications relating to metric 3. Basically if our circuits fit inside the webcam casings, we fill have fulfilled the customers need.
Metric 5- Dynamic operating range
If our product doesn’t work at 20 feet, this will greatly undermine our competitiveness with the other teams. Our goal will to have the product function at 25 feet to give us some edge room. The customer needs are important to us and we will set our goal at 25 feet.
Metric 6- The receiver should amplify the signal back up to its original value
We need to have a signal comparable to the output of the DVD player. We measured the peak to peak voltage to be 1 volt. Through a series of amplifying stages we have a goal of reconstructing the original signal back to its original form. If our signal is plus or minus .3 Volts from the ideal value of 1we will have unpredictable responses.
Metric 7- Max angle of incidence at receiver
We hypothesize that our receiver current will diminish by the cosine of the incidence angle. Our receiver must be designed to tolerate these changes in power at the photo detector.
Metric 8- Transmitter BJT current
Our current vs. power plot shows that we get our most abrupt change in power between
13 and 27mA. Having our current within this region will ensure stability in our transmitter.
Assuming all the lasers we will use have similar characteristics, we will design our transmitter to oscillate between such values.
Metric 9- Number of interconnect cables
There are four possible cables that may extend from our receiver and transmitter. They correspond to positive and negative voltages, ground, and digital signal. We are confident that we will be able to separate these signals into two cables. One will provide power to our modules and the other will carry the digital signal.
The Digi-Light team is fully aware of its customer needs. We expect to have a great sounding, reliable product for the customer by the end of Fall semester 2009. We feel that this
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aspiration is quite within reach and we are up for the task of fulfilling all the needs and metrics here presented. We see no future glitches or major obstacles in our design. This is our goal— to make a product that will meet and exceed the needs of our customers by following the points we have outline here.
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Introduction
Purpose
Overview
Decision Process
Body of Facts
Specification Definitions
Concept Direction
Concept Selection
Chosen Concepts
Non-Critical Concepts
Summary
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Purpose: The purpose of this document is to outline the processes and decisions we
(Digi-Light) made as a team; in developing our final project in the free space optical communication system. In this document we will list our facts and assumptions that were critical in the design of our transmitter and receiver components. We will list and define the concepts of the design and the different options available for the design of the communication system. We will then show the process of ranking the options for a concept decision, as well as how we came to the conclusion that we did.
Overview: The free space optical communication system is a process of transmitting and receiving audio data across an area using light rather than an electrical signal. To demonstrate the functionality of the system, data from a DVD player will be transmitted and received through an air medium. The system will consist of two parts: a laser transmitter and an optical receiver. The laser transmitter will take the digital bit stream out of a DVD player, and produce matching optical pulses. The receiver will translate these optical pulses back to an electrical bit stream that can be read by an audio receiver. The system will produce high fidelity audio transmission at
DVD bit rates.
System Block Diagram
Figure 1 shows the basic structure of the free space optical communication system. The only components needing to be designed are the transmitter and receiver.
Figure 2
Decision Process: Our project involves the design and construction of several components. In the concept definitions section we will identify concepts and definitions that will be considered for being implemented in the final project. The concept direction section identifies which of the concepts previously discussed are critical to the success of the project. These
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critical concepts are evaluated in the concept scoring and selection sections. After the designs have been evaluated, the chosen concept is discussed. Non-critical concepts are highlighted briefly and will not be focused on until a specific design need requires that we give them more attention.
Figure 2 contains the facts and assumptions and is the basis for our design concepts. We have kept in mind what the customer needs are and have added what assumptions we have made in developing this product. It also provides evidence that we have taken into account all the customer constraints in coming up with alternatives.
#
1
Facts
Fits into provided package
Assumptions
Transmitter and receiver need to fit into two webcam packaging
Will be easy to operate
2
3
5
6
7
8
Mounts to table
4 Sounds great
Aesthetically pleasing
Adequate documentation
Dynamic operating range
Portable system
9 Reliable
10
Quick setup time with provided instructions
Figure 3
Able to align the transmitter with the receiver easily without too tight of tolerances
Cables do not interfere with built in capabilities of product
Produces high fidelity audio transmission
Only two cables extending from each webcam package
Detailed instructions so that the customer can duplicate our design if needs be.
Functions at 5 and 20 feet with no adjustments to electronic components
Plugs into any outlet and converts from 120 V AC to 15 V +-DC
The lasers and amplifiers are resilient to failure
Operating instructions will be clear and concise for general public
Fact 1 is that the product fits into provided package. We assume that the electronics needed to implement the design will be of a size such that they will allow the transmitter and receiver to be contained within two webcams. We have also assumed that the webcams will have the ability to be opened, and that the cameras within can be easily removed from the package. We have assumed that we will have the ability to reseal the webcam after connecting cables and mounting the hardware. These assumptions are because we have examined the webcam and have made measurements that lead us to such a conclusion. We have no doubt to be able to do this.
Fact 2 is, will be easy to operate. We assume that our transmitter will be able to be aligned with the receiver easily without too tight of tolerances. From analysis of the beam divergence of the laser in our transmitter, we have found that we can have some variance in alignment angle. Although we have found this to be only a few degrees, that this is very doable
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by the fact that the beam will be visible to the naked eye and that the installer can make the necessary adjustments in order for correct alignment.
Fact 3 is mounts to table. By this we have assumed that the necessary interconnect cables will not interfere with the mounting of the table. From preplanning, we have come up with a path to route these cables to minimize any hindrance of the overall system. We plan to have the cables exit the webcam directly in the rear. We also assume that by doing so that the integrity of the package will not be compromised.
Fact 4 is sounds great. Since we are in the process of building our product, we can only assume that it will produce high-fidelity-audio transmission. However, from our analysis and characterizing of the DVD player and audio stereo amplifier, we can justify this assumption of high-fidelity-audio transmission. It is only a matter of actually building and testing.
Fact 5 is esthetically pleasing. We have assumed that we will only need to interface two interconnect cables. These cables will provide the essential TX and RX audio signals and also provide power for operation. This is the assumption made.
Fact 6 is adequate documentation. The customer may ask for detailed instructions so that he/she can duplicate our design if needs be. This could be handy in finding replacement parts for this product as well as providing the technical background behind our product. We have assumed this to be the case and will include this documentation with the final product.
Fact 7 is dynamic operating range. This fact, along with others, comes directly from the customer needs. It required that it functions at 5 and 20 feet with no adjustments to electronic components. All our preliminary work leads us to believe that our product will meet this requirement. We feel this way because our derived solutions can and have been verified in
Pspice; a program that accurately simulates schematics.
Fact 8 is portable system. Plugs into any outlet and converts from 120 V AC to 15 V +-
DC. We assume that anyone wanting to use the product will have ready access to AC power.
Fact 9 is reliable. We interpret this to mean the lasers and amplifiers are resilient to failure. The most sensitive part is the laser. We have found that the linear region of the laser is between a current of 10 mA to 30 mA. We have assumed that by restricting our transmitter to operate in this range that the most delicate aspect will have an extended life span.
Fact 10 is quick setup time with provided instructions. We assume that by having setup trials of the product using test subjects, we can critique our instructions so that they are clear and concise as possible. This will result in fast setup times.
Figure 3 provides a set of specifications, jointly agreed upon by Digi-Light, on what will be built. It documents the proposed solution to the customer need
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Power
Transmitter
Receiver
Specification
External Aspects
Definition

Use components that can tolerate 15 V

Step down voltage with voltage regulators

Use voltage dividers to subdivide voltages
 Vector Board method
 Dead Bug method

Transimpedance Amplifier (TIA)

Amplification using BJT

Amplification using single Op-amp

Use existing switch to power on unit

Adjustable lens on outside of receiving webcam
Figure 4
Power
We will be using transformers to convert 120 VAC to +-15 VDC. This is the case since our electronics will need to first operate off of DC current. Different electronics will need to have different voltages. We will use voltage divider methods to reach such values in voltage.
To ensure that these values of voltage remain as constant as possible, large capacitors will be placed in parallel with the initial DC voltage. This will help eliminate fluctuations due to the AC power source.
Transmitter
Since using a breadboard introduces stray capacitances, we have decided that either using a vector board method or a dead bug method will greatly reduce this concern. In fact either of these methods would work well, but we have decided to use the vector board. It will organize the components in a single plane, facilitating in the replacements of parts.
Receiver
The transimpedance amplifier of TIA would provide a receiver in the fact that it takes a input current and produces an output voltage. This is very similar to our system. We have a photodiode that produces a current. This current could then be amplified to produce our desired output voltage to then me feed to the audio stereo. We also have examined the possibility of using a single BJT for amplification. If we reverse bias our photo detector, we could use a provided voltage source provide large voltages to be dropped across an input resistor and then use a BJT to amplify the remaining small amount. Lastly, we have considered using single stage
Op-amp. Our team has decided to use the transimpedance amplifier. This is because it uses two stages for amplifying. This reduces ringing and preserves our gain bandwidth, so we can easily operate at 5.5 MHz.
External Aspects
We might use an existing switch located on the webcams to turn on and off the transmitter and receiver. This would reduce any sudden impulses that might cause damage to some sensitive components. Considered, is the option of placing lenses to help focus the laser beam.
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The design of the communication system is centered on virtually two components. The functionality of these two components is critical for the success of our communication system.
The table below describes these components.
System Need System Concept Description
Laser Driving
Signal
Reconstruction
Transmitter
Receiver
Produces a current driving laser pulse that maintains signal appearance and is capable of creating high power
(lower milliwatt) light pulses
Produces current pulses that can be amplified and reconstructed into digital signals that are capable of high fidelity audio
Figure 5
Proposed Block Diagram of Transmitter and Receiver Components
The block diagram below shows the substructure of the system components that need to be designed and constructed. The interconnecting lines show the signals that need to be obtained and considered when designing the components. The goal is to reconstruct the DVD player signal as closely as possible after it has been carried through the air in the form of light pulses emitted from a laser.
Laser Driver
DVD Player
Digital Signal
Laser
Laser Driving
Current Pulses
Photodetector
Light Pulses
Signal Amplifier
Current Pulses Reconstructed
Digital Signal
The designs for the system components have been evaluated in areas that best meet the customer expectations of the system.
Concept Scoring
Transmitter Design:
The criteria for our weighting are based on effectiveness of the following design attributes:
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1.
Difficulty: How long would it take to implement this design feature?
2.
Size: Would the solution fit in the packaging appropriately?
3.
Signal Appearance: Does the design create a functioning signal?
4.
Current Range: Will the signal drive the laser in the correct current region?
5.
Cost: Does the cost of the design meet a low budget solution?
Concept Scoring: Transmitter
Ease
Size
Signal Appearance
Current Range
Cost
Sum/Rank
20
15
35
5
5
5
5
2
5
25
5
4
4
3
5
3
1
100 470 405 260
3
1
3
The signal appearance is weighed heavily since it is crucial to good performance. This entails the integrity of our sent. Ease also is heavily weighed. We only have a certain amount of time to get this done so we want the design to not be too cumbersome. The cost is not significant since the components used are not expensive. We are confined to a fixed package size so that
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will have relevancy to the design and therefore, a weight of 15. From the concept scoring it is clear that the digital switch vector board is the path to choose.
Receiver Design:
The criteria for our weighting are based on effectiveness of the following design attributes:
1.
Ease: How long would it take to implement this design feature?
2.
Size: Would the solution fit in the packaging appropriately?
3.
Signal Appearance: Does the design create a functioning signal?
4.
Alignment Sensitivity: How easy will it be to align the laser with the photodetector?
5.
Output Voltage: Will the output signal have a voltage range that will work with the commercial receiver?
6.
Cost: Does the cost of the design meet a low budget solution?
Concept Scoring: Receiver
Ease
Size
Signal Appearance
Alignment Sensitivity
Cost
Output Voltage
Sum/Rank
15
10
30
15
5
25
2
3
4
5
2
5
4
5
1
2
5
2
100 390 245
4
4
2
3
3
4
320
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The signal appearance is weighed heavily since it is crucial to good performance. This entails the integrity of received signal. Ease also is heavily weighed. We only have a certain amount of time to get this done so we want the design to not be too cumbersome. The cost is not significant since the components used are not expensive. We are confined to a fixed package size so that will have relevancy to the design and therefore, a weight of 10. Alignment sensitivity has to do with how efficient it can amplify. If it has a better ability to amplify weaker signals it will help meet many customer needs; this is why it has a weighted the most. Output voltage has to do with the reconstruction of our original signal. We would like our final signal to resemble the original as close as possible. From the concept scoring it is clear that the transimpedance amplifier is the path to choose.
The final design will involve a transmitter that uses a digital switch using the vector board technique and a receiver using a transimpedance amplifier. We believe this design will be the most effective in completing the project based on the customer needs and expectations. These design concepts were chosen because they scored the highest in the concept selection section.
They have trade-off with other design concepts but we believe they will be the best design concept for the present time.
The power structure and external alignment concepts are important design characteristics but are not critical. The power structure deals with voltage requirements that tie into the design the transmitter and receiver components but since we’d like them to work at a dynamic range, this issue become less critical. The power supplies are already provided which makes it less critical for us to focus on this concept. The external alignment involves placing lenses outside of the packaging. We will only address with design concept if the need for more optical power arises in the design of the other more critical components.
This document serves to record the process by which we select the most appropriate design that fulfills the customer needs for the project of the free space optical communication system. We believe that we have made significant progress in considering the design that will most meet our demands. We have generated several options and considered several that seem most feasible. We have confirmed our initial assumption of the design process that will be the most effective.
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