RADIO FREQUENCY OVER OPTICAL FIBER DESIGN AND IMPLEMENTATION FOR THE EXAVOLT ANTENNA

James Lamar Bynes IIIDepartment of Physics

University of Hawai‘i at MānoaHonolulu, 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 INTRODUCTION1.1 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.

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 10-meters high will be mounted on the outer membrane

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Figure 1 EVA full-scale model of SPB.

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 AFBR-2310Z 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

transmitted power is expressed 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-to-voltage. The voltage was then read to an on-board dsPIC microcontroller (MCU) through an external analog-to-

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Figure 2 Avago fiber pair. Fiber Receiver (left), fiber transmitter (right)

Figure 4 Internal circuitry of Avago AFBR-1310Z fiber transmitter.

Figure 3 Equation used for transmitted power.

Figure 5 Transimpedance current to voltage circuit (left), mosfet voltage to current circuit (right).

digital converter (ADC) chip. The MCU also provides a current to the FLD using a digital-to-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.

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

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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.

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 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 RESULTS3.1 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

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Figure 7 Test setup showing fabricated board within temperature chamber.

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, 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.

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Figure 10 Transmitted power vs temperature showing intervals of stable transmitted power.

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.

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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 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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