RADIO FREQUENCY OVER OPTICAL FIBER DESIGN AND IMPLEMENTATION
FOR THE EXAVOLT ANTENNA
James Lamar Bynes III
Department of Physics
University of Hawai‘i at Mānoa
Honolulu, HI 96822
ABSTRACT
The ExaVolt Antenna (EVA) is a planned ultra-high energy (UHE) particle observatory
under development for NASA's suborbital super-pressure balloon program in Antarctica. EVA
will use an antenna array to capture UHE events from deep space then transfer this information
to a payload. In order to reduce weight and mitigate signal attenuation, RF signals will be
transmitted across the balloon over an RF over optical fiber link to the payload, in place of
traditional coaxial cables. A fiber transmitter and receiver pair is evaluated within this report in
order to determine whether or not it will be reliable for the crucial mission in which it will
partake within EVA. The design and implementation of three tests boards allow careful
evaluation of the fiber transmitter and receiver pair. It was concluded that with careful
microwave circuit design and modulation of the Fabry-Perot laser used within the transmitter
will be sufficient enough to send data within EVA to a payload. This project is currently under
review for NASA’s suborbital super-pressure balloon program and also complies with NASA
objective 1.6 with an overall goal of understanding the distance sources of UHE particles.
1 INTRODUCTION
1.1 Motivation
EVA is a planned NASA balloon-borne particle observatory capable of measuring the
absolute flux levels and energy spectral characteristics of the UHE cosmogenic neutrino flux [1].
UHE neutrinos contain energies in the Exavolt range (1018 eV or higher) and can propagate
through vast galactic distances without attenuation. Studying the universe by looking at these
particles will open the door to understanding the behavior of distant sources, allowing
breakthroughs currently not possible by particle accelerators, such as: discovering the origins of
the universe, as well using this knowledge to further our understanding beyond the Standard
Model of particle physics. Two methods of detection
by EVA are of interest: the Askaryan effect due to
UHE neutrino interaction with the Antarctic ice, and
gyrosynchrotron emission due to UHE cosmic rays
interacting with the atmosphere [2]. Both methods
portray information with radio waves, thus an
antenna array will be the implemented on EVA.
Figure 1 EVA full-scale model of SPB.
EVA will employ a suborbital super-pressure
balloon (SPB) 115-meters in diameter, which is
currently under consideration for a NASA SPB
mission in Antarctica [3]. An RF reflective layer 10meters high will be mounted on the outer membrane
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of EVA and positioned in such a way which will allow a synoptic view of the Antarctic ice sheet
during its flight. An inner feed antenna, supported by tendons hanging from the inside of the
balloon, shall act as a focal point for the outer RF reflective band. Figure 1 shows a full-scale
model of the SPB with the RF reflective band and an inner feed antenna array. Power will be
provided by strategically placed photo-voltaic panels on the balloon.
1.2 Radio Frequency over Optical Fiber
In order to transfer the RF signals to a payload, the use of standard coaxial cables cannot
be used due to weight constraints. Instead, a network of internal RF over optical fiber (RFoF)
links shall be implemented. The Avago AFBR-1310Z fiber transmitter and AFBR-2310Z fiber
receiver from Avago Technologies were chosen for evaluation for possible use on EVA [4][5].
The Avago fiber pair was chosen because it incorporates a linear wide bandwidth InGaAsAl/InP
Fabry-Perot laser diode (FLD) and a floating monitor photodiode (MPD) for closed loop
operation, and it is also able to operate in a non-stable temperature environment while
maintaining optimal performance. A number of channels utilizing the RFoF system will be
implemented within the balloon, thus creating strict power and weight constraints.
2 SETUP & METHODS
2.1 Design & Fabrication of Evaluation Boards
This project began with just the bare components of
the Avago AFBR-1310Z fiber transmitter and AFBR2310Z fiber receiver shown in Figure 2. The FLD accepts a
laser bias in milliamps which is then used to maintain a
steady optical power output of a signal containing data
which is modulated over the fiber pigtail, where the
Figure 3 Equation used for
transmitted power.
Figure 2 Avago fiber pair. Fiber Receiver
transmitted power is expressed
(left), fiber transmitter (right)
in dB (Figure 3). The MPD
which is adjacent to the FLD outputs a current which is proportional
to the optical power of the FLD. Figure 4 shows the schematic
diagram of the internal circuitry of the transmitter.
The first step in characterizing the Avago fiber
optic pair was to design and fabricate boards that will be
suited for such tasks. The required circuit boards were
designed with Mentor Graphics PADs. Three boards were
fabricated: A controller, a transmitter, and a receiver. The
controller board contains all the necessary components
which monitored the current in milliamps from the MPD as
well as providing the laser bias for the FLD. In order to
determine the current from the MPD, a transimpedance
amplifier circuit allowed conversion from current-tovoltage. The voltage was then read to an on-board dsPIC Figure 4 Internal circuitry of Avago AFBR1310Z fiber transmitter.
microcontroller (MCU) through an external analog-todigital converter (ADC) chip. The MCU also provides a current to the FLD using a digital-to2
analog (DAC) chip then through a voltage-to-current converter circuit. The MCU communicated
with both chips via the SPI protocol where all embedded software was written in C. Figure 5
shows how these circuits were designed before they were finalized for fabrication of the
controller board.
The controller
board
connects
directly
to
the
transmitter board in
order to provide a
direct link for the
laser bias as well as
the
MPD.
In
designing
the
transmitter board,
Mentor
Graphics
Hyperlynx was used in order to properly match the transmission line at 50Ω. This is necessary to
mitigate reflections from the injected RF signal and deliver the optimal signal power to the
transmitter board. The receiver board did not have any special connections besides an RF output
connector for the signal. Overall, the boards allowed for RF data to be sent over the fiber pigtail
while providing total control in modulation of the FLD.
Figure 5 Transimpedance current to voltage circuit (left), mosfet voltage to current circuit
(right).
2.2.1 Preliminary Testing of Evaluation Boards
Once fabricated, a preliminary test was performed using a Network Analyzer to verify the
matching of the 50Ω transmission lines. This test insured that further data which was determined
from this point on was valid by obtaining S-parameter plots for S(2,1), S(2,2), and S(1,1). Next,
the input and output RF power of the transmission lines was measured for swept frequencies
from 100MHz to 6GHz. With these measurements, a gain (dB) vs. frequency (Hz) bode plot was
created in order to supplement the data obtained from the Network Analyzer. This was done for
three different temperatures: 25°C, 50°C, and 75°C. As per the datasheet, the gain temperature
dependence of the Avago fiber optical pair should vary no more than ±2dB from room
temperature to 85°C.
2.2.2 Secondary Tests of Obtaining Lookup Table for FLD
The next sets of tests determine how the transmitted power can be stable as a function of
the laser bias and temperature. In a normal fiber optic system, a thermoelectric cooler is usually
used to keep the temperature of the FLD stable. This way, the only fluctuation which is observed
in the transmitted and optical powers comes from drifts seen in the laser bias. Because of the
power budget in EVA as well as the number of potential fiber links for the entire system, a
thermoelectric cooler for each fiber link would not be realizable. Therefore, the RFoF system
must compensate for the fluctuation of temperature and how it will affect the transmitted power
of the FLD. With these measurements, a look up table (LUT) can be implemented by the
microcontroller. A temperature sensor provides input data, and a corresponding laser bias output
controls the FLD so that a steady power transmission in maintained over the fiber pigtail.
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Figure 6 Fabricated controller board (left) and fiber transmitter/receiver pair with fiber pigtail (right).
The test setup includes the following:
 Agilent E4432B 250MHz-6GHz sine wave generator.
 Tektronix TDS6804B 8GHz digital storage oscilloscope.
 Micro Climate -70°C to 175°C temperature chamber.
 The fabricated controller, transceiver, and receiver boards (Figure 6).
 Other various RF equipment.
Figure 7 shows the test setup used for
most of the conducted tests. Because the
output power of the fiber optic system varies
greatly over temperature, all tests were
conducted in a temperature controlled
environment. A 500MHz signal at 0dBm
(224mV) was injected into the system where
the data was then transmitted over the fiber
pigtail, this data was then analyzed using an
oscilloscope. Both the transmitter and
receiver boards were positioned inside the
temperature controlled environment, where
Figure 7 Test setup showing fabricated board within
temperature chamber.
the controller board remained at room
temperature during all the tests. Two tests were conducted in order to obtain this data. The first
set of data was obtained by conducting tests over a range of temperatures and laser bias currents.
The laser bias was varied from 50mA to 85mA while keeping the temperature constant, this was
then repeated at intervals of 5 degrees Celcius from 25°C to 85°C, all parameters were recorded
including the MPD current (mA) and the transmitted power (dB) of the signal. All obtained data
was able to provide insightful graphs which describe the relationship of temperature and the
transmitted power due to the laser bias. These graphs are shown and explained in the results
section.
3 RESULTS
3.1 Results from Preliminary Tests
According to the S-parameters on the left of Figure 6, the S(1,1) plot for the transmitter
shows that an unstable amount of return loss occurs. This plot does not drop below -10dB until
about the 3.5GHz range, where the power then dips to -25dB at 3.88GHz, which is desirable. It is
expected that the amount of return loss will drop at higher frequencies since FR4 was used as a
substrate for the boards. This is due to the fact that FR4 absorbs power at higher frequencies. The
S(2,2) plot seems to be experiencing the same behavior except there is no large dip at 3.88GHz,
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still the FR4 substrate absorbs the signal at higher frequencies. The S(2,1) plot starts at around 6dB then drops to about -16dB at 6GHz. Overall a loss of at least 10dB is expected due to the
fact that a 10dB attenuator was used on the fiber pigtail.
Figure 8 S-parameters plot for fiber optic system (left). Bode plot of same fiber optic system (right).
The right of Figure 8 shows the bode plot for the frequency test. Each shade denotes the
same test but with different temperatures. The test that is done at 75°C has lower overall power
than the other two tests. This is due to the fact that the corresponding laser bias at that
temperature is too low. The output power at 100MHz is already at approximately 7.5dB loss at
the lower temperatures, this is due to the fact of the 10dB attenuator in the fiber pigtail. Finally a
3dB cutoff is experienced very early in the plot. According to the datasheet, the transmitter and
receiver pair should not experience this 3dB cutoff before 5.5GHz. There are several reasons
why the power begins to roll off earlier than expected in the frequency domain. The main
reasons are shown in the S-Parameters plot, where non-ideal matching creates excessive
reflection within the circuit, as well as the decision to use FR4 substrate which attenuates higher
frequencies as was previously discussed.
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3.2 Results from FLD Stabilization (LUT)
Figure 9 Transmitted power vs laser bias, temperature range from 20°C to 75°C.
Figure 9 shows the graph obtained relating the transmitted power according to the
provided laser bias. Each line in this Figure corresponds to a change in temperature of 5 degrees
Celsius, with 20°C above and 75°C below. The laser bias was swept from 45mA to 85mA in
5mA increments, these numbers were chosen because any laser bias before 45mA would result
in no transmitted power, and 100mA is the absolute maximum allowed current through the FLD
per the datasheet. Figure 9 shows that as laser bias increases, the transmitted power initially
increases until the power peaks, and then begins to roll off, where the peak and roll-off points are
functions of temperature. This confirms a non-linear relationship between the laser bias and
transmitted power. For example, the transmitted power at 75°C will always been less than the
transmitted power at 20°C, which suggests that the transmitted power may only be stable for
certain intervals of temperature.
Figure 10 Transmitted power vs temperature showing intervals of stable transmitted power.
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Figure 10 displays the intervals in which the transmitted power may be stable at different
intervals of temperature. Due to variances in manufacturing and other real-world effects, the
results obtained for the final system will vary, but for the most part, it will use the same method
described here. For the temperature intervals of 20°C - 32°C, 33°C - 46°C, 47°C - 68°C, and
69°C - 75°C, the laser bias will be adjusted using a LUT, as described in Section 2.2.2, so that a
steady transmitted power is obtained throughout the fiber optic system.
Figure 11 Look up table from 20C to 29C (left), stable transmitted power as a result of using the lookup table (right).
Figure 11 shows a LUT (left) for the interval from 20°C to about 29°C, the corresponding
graph of the transmitted power is shown to the right which shows that it could be stable. Further
calibration must be done in order to provide a flatter transmitted power, although small
fluctuations shouldn’t matter much in the final system. During the production of each fiber optic
link for the final system, an automated test will be in place in order to determine a LUT per fiber
optic pair; this is due to the fact that not all pairs will have the exact same characteristics. In
order to transition from one temperature interval to the next, software PID controller will be
implemented to ensure that there will be no unexpected spikes and ensure smooth transition from
each interval.
4 CONCLUSION
The design and fabrication of the fiber transmitter, receiver, and controller boards
allowed further evaluation of the decision to implement the Avago fiber transceiver pair for the
design of EVA. Due to non-linearity of the transmitted power with respect to the laser bias and
temperature, a standard transfer function was non-realizable. Although a LUT may be used for
the final system, the question remains whether or not it would be practical to control and
maintain a steady transmitted power over each fiber optic system via software. Also, much effort
will be needed in order to design an automated system that will allow the calibration of multiple
fiber optic links and their corresponding LUTs for different temperature intervals. Careful
consideration will determine whether or not the Avago transmitter/receiver pair could be used for
EVA.
ACKNOWLEDGEMENTS
I would like to thank Dr. Gary Varner and Dr. Peter Gorham for giving me this
opportunity to conduct this project in evaluating the Avago fiber optic pair for a potentially
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upcoming NASA sponsored mission. I would also like to thank Hawai‘i Space Grant Consortium
for their continued support in allowing this opportunity. I would also like to thank my colleagues
Khanh Le and Steven Ewers for their help in previous semesters on mentoring the microwave
circuit design.
REFERENCES
[1] P. W. Gorham, et al., “The ExaVolt Antenna: A Large-Aperture, Balloon-embedded Antenna
for Ultra-high Energy Particle Detection”, University of Hawai‘i , 9 Aug 2011
[2] D. Saltzberg, P. Gorham, D. Walz, et al., “Observation of the Askaryan Effect: Coherent
Microwave Cherenkov Emission from Charge Asymmetry in High Energy Particle
Cascades,” Phys. Rev. Lett., 86, 2802 (2001)
[3] H. M. Cathey, “The NASA Super Pressure Balloon. A Path to Flight,” Advances in Space
Research Vol. 44, Issue 1, 1 July 2009, pages 23-38
[4] Avago Technologies, “AFBR-1310Z/AFBR-1310xZ Fiber Optic Transmitter for Multi GHz
Analog Links Data Sheet.” (2013) http://www.avagotech.com/docs/AV02-3184EN
[5] Avago Technologies, “AFBR-2310Z Fiber Optic Receiver for Multi GHz Analog Links Data
Sheet”, (2011) http://www.avagotech.com/docs/AV02-3183EN
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