TEST PLAN
FREE SPACE OPTICAL TX/RX DEVICE
BRUCE BLACK, JUSTIN BENNETT, JOSEPH SHIM
CONTENTS
Introduction ...................................................................................................................................................................3
Power Over Distance .................................................................................................................................................3
Frequency Response ..................................................................................................................................................6
Ease of Setup .............................................................................................................................................................6
Conclusion .................................................................................................................................................................7
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INTRODUCTION
This document represents the test plan that BJB Systems LLC made to test critical areas of the design of our free
space optical transmitter and receiver. These tests will be used to confirm that our design meets the critical
specifications. These tests will contain the test plan for the following three critical areas:
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Transmitted power over distance (between 5 and 20 feet)
Frequency response of the system (Up to 5MHz)
Ease of Setup
The focus of this is document is to communicate the test plan of these three areas which we will discuss
in detail.
POWER OVER DISTANCE
Background: The power transmitted over distance corresponds to how much optical power is received at the
detector at various distances from the transmitter. This fraction of power relates to the diameter of the beam at
the detector, which is derived from the divergence of the transmitted beam over distance. Since our detector is
2mm in diameter we can use equations to calculate how much optical power we receive. When the laser is
considered to be fully off, 15mA of current flows through it. When the laser is considered fully on, 35mA flows.
Therefore, the average transmitted power output of our laser diode (assuming a %50 average duty cycle signal
over time) is equivalent to 25mA of current flowing continuously. When we measured the optical output power of
the laser with the current supply and optical power meter, we obtained an optical power reading of 3.07mW at
25mA of current through the laser.
Thus, the fraction of power received at the detector with radius 𝑟, over distance 𝐿 with beam divergence 𝜃 is:
𝑃𝑟𝑒𝑐 = 𝑃𝑡𝑟𝑎𝑛 ∙
(1𝑚𝑚)2
𝐴𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟
𝜋 ∙ 𝑟2
≈ 𝑃𝑡𝑟𝑎𝑛 ∙
=
3.07𝑚𝑊
∙
𝐴𝑏𝑒𝑎𝑚
𝜋 ∙ (𝐿 ∙ 𝜃)2
(𝐿 ∙ 𝜃)2
𝜃 varies based on how the lens of the laser is adjusted. We hope to produce the least divergent beam possible,
and at 20 ft (6.096 m), we have calculated this value from the following formula:
𝜃 = tan−1
𝑅
5 𝑚𝑚
5 𝑚𝑚
= tan−1
≈
= 0.047°
𝐿
6.096 𝑚 6.096 𝑚
𝐿 varies between 5 and 20 feet, which corresponds to variations between 1.524 and 6.096 meters. We will use
these values in the calculations to determine what should be the strength of our signal at the distances specified.
At 5 ft (1.524 m), our received power is calculated as:
𝑃𝑟𝑒𝑐 = 𝑃𝑡𝑟𝑎𝑛 ∙
(1𝑚𝑚)2
𝐴𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟
𝜋 ∙ 𝑟2
≈ 𝑃𝑡𝑟𝑎𝑛 ∙
=
3.07𝑚𝑊
∙
𝜋 2 ≈ 2.0 𝑚𝑊
𝐴𝑏𝑒𝑎𝑚
𝜋 ∙ (𝐿 ∙ 𝜃)2
(1.524 𝑚 ∙ 0.047° ∙
)
180°
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Similarly, at a distance of 20 ft (6.096 m), the expected power received at the detector from the laser beam is:
𝑃𝑟𝑒𝑐 = 𝑃𝑡𝑟𝑎𝑛 ∙
(1𝑚𝑚)2
𝐴𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟
𝜋 ∙ 𝑟2
≈ 𝑃𝑡𝑟𝑎𝑛 ∙
=
3.07𝑚𝑊
∙
𝜋 2 ≈ 120 𝜇𝑊
𝐴𝑏𝑒𝑎𝑚
𝜋 ∙ (𝐿 ∙ 𝜃)2
(6.096 𝑚 ∙ 0.047° ∙
)
180°
We can now use these maximum and minimum values of received optical power to calculate how our photodiode
responds to the incoming varying laser beam. By using the value of responsivity of the photodiode that we derived
from its performance earlier on, we can determine the current through the photodiode. Our photodiode
responsivity value is 4 A/W. Therefore, the current through our photodiode with responsivity 𝜌 and received
optical power is:
𝐼𝑝ℎ𝑚𝑎𝑥 = 𝜌 ∙ 𝑃𝑟𝑒𝑐 = 4
𝐼𝑝ℎ𝑚𝑖𝑛 = 𝜌 ∙ 𝑃𝑟𝑒𝑐 = 4
𝐴
∙ 2.0𝑚𝑊 = 8𝑚𝐴
𝑊
𝐴
∙ 120𝜇𝑊 = 0.4𝑚𝐴
𝑊
Our receiver circuit design is simply a non-inverting 100 V/V photodiode preamplifier with a 50-ohm input
impedance on each amplifier terminal, so the output voltage may be expressed by the following formula:
𝑉𝑜𝑢𝑡 = 100 ∙ 𝐼𝑝ℎ ∙ 50
For corresponding values of 𝐼𝑝ℎ𝑚𝑎𝑥 and 𝐼𝑝ℎ𝑚𝑖𝑛 , we have the following expected voltages at the output of the
preamplifier for the two distances:
𝑉𝑜𝑢𝑡𝑚𝑎𝑥 = 100 ∙ 8𝑚𝐴 ∙ 50 = 4𝑉
𝑉𝑜𝑢𝑡𝑚𝑖𝑛 = 100 ∙ 0.4𝑚𝐴 ∙ 50 = 200𝑚𝑉
Clearly, the challenge of our circuit is going to be whether it will not cause the signal at the home theater receiver
to be too great or small depending on the maximum and minimum acceptable received optical power. With our
amplifier, we have designed it around a photodiode preamplifier circuit from the datasheet for the LM1226
amplifier, with an overall gain of 100 V/V. The home theater receiver is expecting a voltage greater than
200𝑚𝑉𝑝𝑘−𝑝𝑘 (as was determined previously by our attenuation measurements of the electronics). Therefore, our
receiver circuit must not produce a signal less than this or the hometheater receiver will not be able to reliably
detect it. We have chosen to measure the output voltage of the receiver at the line driver, to establish that we are
above the 200𝑚𝑉𝑝𝑘−𝑝𝑘 requirement for the home theater receiver. All of the previous calculations indicate that
we are in the ball park for these measurements.
List of Equipment
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Transmitter
Reciever
Power Supplies
Oscilloscope
Function generator
Tape measure or ruler
DVD Player/Home theater receiver/speakers
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Test Description
1.
2.
3.
4.
5.
6.
Start by connecting a function generator and power supply to the transmitter. Set the function generator
to square wave, 5.6 Mhz, with 75-ohm impedance. This matched impedance will reduce reflections on
the coax line from the function generator to the transmitter.
Similarly, connect a power supply and oscilloscope to the receiver. Probe the output of the op-amp stage
with the oscilloscope and remember to set the probe to 10X if appropriately using a 10X probe.
Measure the distance between the receiver and transmitter to be 20 feet. Use the oscilloscope
“measure” function to verify that the output of the receiver is above 200𝑚𝑉𝑝𝑘−𝑝𝑘 .
Vary the frequency dial on the function generator to note any impedance mismatches in the circuit, and
attempt to correct any mismatched impedances if necessary.
Connect th DVD player at the transmitter, and Dolby 5.1 decoder receiver to the optical receiver. Verify
that DVD audio can be heard through the system (continuously!)
Repeat the test over a distance of slightly less than feet feet to verify that the same signal is not
overdriving the home theater receiver coax input.
Expected Results
We expect to be able to quantify the relationship of received power over distance by using the method outlined
above. We have already determined through brief experimentation with our prototypes that it is possible to
reproduce the signal at the receiver with enough headroom to be able to operate the system with a seperation
between the transmitter and receiver in excess of 20 feet. Therefore, we fully expect to be able to produce signals
at the output of the receiver in excess of 200𝑚𝑉𝑝𝑘−𝑝𝑘 .
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FREQUENCY RESPONSE
The frequency response of our system is cricital to its performance. If the corner frequency is too close to the
required frequency to be able to pass, then we will likely experience attenuation of our signal. However, with
modern digital sampling oscilloscopes, measuring the frequency response of a system has become a trivial task.
List of Equipment

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
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
Transmitter
Reciever
Power Supplies
Oscilloscope
Function generator
Test Description
The simplest way to test the bandwidth of the system is to measure the magnitude of a sinusoid over a varying
frequency. By determining when the magnitude in voltage of the transmitted sinusoid at the output attenuates to
-3dB, or 70.7% of the original value, an upper corner frequency is located.
1.
2.
3.
4.
Start by connecting a function generator and power supply to the transmitter. Set the function generator
to sine wave, 100KHz, with 75-ohm impedance. This matched impedance will reduce reflections on the
coax line from the function generator to the transmitter.
Similarly, connect a power supply and oscilloscope to the receiver. Probe the output of the op-amp stage
with the oscilloscope and remember to set the probe to 10X if appropriately using a 10X probe.
Measure the distance between the receiver and transmitter to be 20 feet. Use the oscilloscope
“measure” function to obtain the magnitude of the incident waveform.
Increase the frequency on the function generator until the magnitude on the oscilloscope reads 70.7% of
the original value at 100kHz. This is the corner freqency of the system.
Expected Results
It is expected that as the input frequency increases the rise and fall times will increase. At some upper limit the
signal will become too distorted to pass a square wave signal and retain any detectible transitions from high to
low. However, we expect that the circuit will perform under the given specifications, given that our circuit has
already successfully transmitted continuous DVD quality audio. This test is merely to determine where that upper
corner frequency is, and how close we came to hitting our mark.
EASE OF SETUP
Simplicity implies robustness of the design. Another test we plan on doing is having some children set up the
transmitter and receiver. The way we plan on quantifying the data is the time it takes to set up.
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This is important since we are designing it for a standard consumer and if children can set it up easily and without
breaking it then it proves the robustness of the design. Also it may show some places where we may need to
improve.
List of Equipment
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Transmitter
Reciever
Power Supply
DVD player
Speakers
DVD
Test Description
The test is to supply a list of simple instructions and the equipment to chilldren and have them set it up without
help. We will time how long it takes and record any confusion that they may have in setting up the transmitter
and reciever. The timer ends when the sound from the DVD player makes it to the speakers.
Expected Results
We expect the children will be able to set up the devices with ease. We also expect the process to take 10 minutes
or less.
CONCLUSION
The various tests we plan to perform will help to assure that our design meets specifications. It will also identify if
we need to make any additional improvements. By the successful completion of these tests, our product will be
ready for any additional performance tests and finalizations of the product.
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