Stony Brook University
Department of Electrical and Computer Engineering
ESE 363: Optical Fiber Communications
Spring 2011
Project: A Full Duplex Fiber Optic Digital Voice Link
Instructor: Professor Harbans Dhadwal
Report authors: Tak-Fung Yip & Jyotsna Jeyapaul
Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
Abstract
There is an ever growing demand for an increase in the speed of communication. As technology
progresses to send and receive information faster, fiber optics are leading the way. Fiber optic
technology has evolved since its conception.
In its earliest form in the late 19th and early 20th century, light was sent through glass beams to
see through the body cavity. Optical rays were then used in the 'photophone' by Alexander
Graham Bell to transmit voice signals. One of the more known uses of optical fiber
communications in today’s technology is in 3G and 4G networks of cellphone service providers.
This has led to greatly reducing the time needed to send and receive various forms of
communication including emails, texts, images, etc.
For our project, the concept was to input a sound signal into our circuit and recover it with
minimum loss. The input signal is be received after being passed through an optical fiber and a
series of analog-digital and digital-analog conversions.
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
Table of Contents
Abstract ....................................................................................................... 2
Section 1
Introduction
1.1
Goals and Impacts ....................................................................................... 4
Section 2
Background
2.1
Project Survey and Planning ...........................................................................
Section 3
System Design
3.1
Design Constraints .........................................................................................
3.2
Design Considered .........................................................................................
3.3
Final Design ....................................................................................................
Section 4
Results and Discussions
4.1
Multi-Disciplinary Issue ..................................................................................
4.2
Professional Ethnical Issues ...........................................................................
4.3
Impact of Project on Society or Contemporary Issues ..................................
Section 5
Summary and Conclusion
5.1
Summary and Conclusion ..............................................................................
Acknowledgements .......................................................................................
References .....................................................................................................
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Section 1
Spring 2011
Introduction
1.1 Goals and Impacts
The purpose of our project is to demonstrate that a sound signal can be sent into a circuit
containing an optical fiber wire which then converts the signal from its analog form to a digital
signal that can be detected at the output. This signal can then be recovered by sending it
through a digital analog convertor.
Section 2
Background
2.1 Project Survey and Planning
Section 3
System Design
3.1 Design Constraints
Technical
-The slow slew rate from Op-Amps with gain was an obstacle in this
-Noise caused by the master clock
Financial
Most components used in the construction of this project were readily made available by the
laboratory that we worked in. The only investment was in the purchase of the breadboard and
the use of headphones and speakers. (I’m not sure of the cost of the breadboard and other
items we used)
Ethical
This project has no known ethical constraints.
Social
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
There are no known social constraints of this project.
Environmental
We tried carrying out our preliminary testing process in an environment with minimal
background noise to make sure that we could easily hear for a signal. This was also required for
us to clarify the improvement in our signal output.
Section 3
System Design
3.2 Design Considered
We began by considering a sample of the microphone frequency response.
Frequencies from 200Hz to 16kHz were measured with the speaker. Frequencies ranging from
35 Hz to 190 Hz are measured with the subwoofer. All the sound loudness is seen to be
identical by calibration with the sound level meter (90 dBA). This is demonstrated in the figure
below:
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-10
dB
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-35
Section 3
Frequency
System Design
3.3 Final Design
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
We improved upon our initial microphone frequency response through the use of adding
certain parameters. We input our sound wave in the form of a sine wave, made sound level
meter reference dBC = 88 +/- 0.1% and had the oscilloscope set to acquiring an average
waveform. The frequency response is seen as below:
Frequency Response of HP-257
Headset Microphone
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30
dB
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Frequency
The amplitude response is as shown below:
Amplitude(mV)
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For our circuit, we had to make use of a second order Sallen-Key low pass filter circuit to
attenuate frequencies that could possibly damage the system. To establish a cut off frequency
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
we introduced a LF 356N operational amplifier. At a low input frequency of 1Hz, the unity gain
was almost 1. When the input amplitude was 1.94V, the output amplitude was 1.88V. As the
frequency was increased to 3.5 kHz, the gain decreased. At 1.85V, the expected output
amplitude was 1.31V while the measured was 1.28V. Initially, a very low output was registered.
We speculated some possible reasons for this. The input impedance may have been very large
or that the circuit required us to introduce a buffer.
We tried the same second order low pass filter circuit using UA741C operational-amplifier with
a double power supply. The circuit design is as shown below:
Where, R1 = R2 = 10kΩ, C1=6.8nF, C2=3.3nF
The chosen high cut off frequency was 4kHz while the measured high cut off frequency was
3.5kHz.
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
We found the attenuation, Av = Vout /Vin =513mV/ 525mV = 0.977
In the figure below, we used Vcc= +- 5V and an offset input voltage= 0V.
The first time that we tested the Sallen Key Low Pass filter, we were using a Vcc+ of 5V and a
ground for Vcc-. The 2V DC offset was applied within the input signal. From the output on the
oscilloscope, the signal was seen to be slightly clipped.
This was due to the fact that the op amp required a double power supply but we used a single
supply. When we modified the supply to double and tracked it, our output was in phase, in
unity gain and without clipping. Tracked means to use the Track key in the power supply, the
positive and negative voltages are matched. Essentially, the tracking feature simplifies the
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
simultaneous power supply sequencing. We repeated the circuit again with C1= 5nF and C2=
2,2nF. The high cut off frequency was measured at 5.1kHz.
I didn’t include the responsivity because I was not sure of what to include
The next important part of our circuit was the setting up our LED driver circuit. As part of our
choice, we tested 2N2222A and 2N3904. Both had very slow rise times (micro-seconds) and
high phase shift between the input and output. Thus, we decided to use a MOSFET rather than
a BJT since BJTs have a lower slew rate in comparison. The BJTs that we tested had a higher
distortion at the output compared to a MOSFET using the same high frequency. The MOSFET
has a faster response compared to BJT. The LED driver circuit is as shown:
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
However, a trade-off situation had arisen since more current means more power and that in
turn yields more distortion. We realized that we needed a low input current of about a few mA
for MOSFETS and BJTs. The range of the collector current is roughly about 20mA. The input
resistor is a low 1kOhm resistor to block out the smaller currents while the load resistor is used
in the overall current control of the circuit acting like a current controlled voltage source.
The MOSFET testing was done for different scenarios. The eventual choice was of the 2n7000
MOSFET whose sample response with noise looked as follows:
The top curve represents the output at the transmitter while the bottom curve is the output at
the receiver. The distortion was overcome by adding a coupling capacitor. The figure below
shows the same output without the distortion.
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
The top waveform is the output across the load resistor while the bottom waveform is the input
across the input resistor. The conditions to achieve the waveform were: Vin=5Vpp, Voff= 2.5V,
Vdd= 9V, Fin= 512 kHz, R(load)= 62 ohm, Rin=100 ohm. The optical power is 15.51uW. The
transmitted power is -18.07dBm
The final design schematic that we used for our project is as seen on the attached sheet. (we
can attach the schematic sheet at the end)
Our final circuit is shown below:
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
Below is a sample of the signals displayed when our circuit was connected. The yellow line
indicates the PCM in while the green line indicates the PCM out of the audio code chip.
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Section 4
Spring 2011
Results and Discussions
4.1 Multi-Disciplinary Issues
Working as a team gave us an insight into working in a real life situation. The project required
us to apply our skills in research and analysis to determine our circuit components and how
they will help us achieve our objectives. We were also required to design our project based on
the given guidelines, to ensure that we achieve our objectives.
The other challenge was to work together as an effective team. From the start, we recognized
each other’s strengths and worked towards complementing our work styles.
Section 4
Results and Discussions
4.2 Professional Issues
Section 4
Results and Discussions
4.3 Impact of Project on Society or Contemporary Issues
Section 5
Summary and Conclusions
5.1 Summary and Conclusions
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Tak-Fung Yip, Jyotsna Jeyapaul
A Fiber Optic Digital Voice Link
Spring 2011
Acknowledgements
We would like to acknowledge Professor Harbans Dhadwal for his guidance through this
project.
We would also like to extend our gratitude to Tony Olivo for his help in the lab.
References
[1]
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