experimental evaluation of an x-band gan high efficiency

Trans. JSASS Aerospace Tech. JapanVol. 12, No. ists29, pp. Pj_31-Pj_37, 2014

Original Paper

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Experimental Evaluation of an X-band GaN High Efficiency Onboard SSPA for Deep Space Missions

By Yuta KOBAYASHI1), Atsushi TOMIKI1), Shinichiro NARITA1) and Shigeo KAWASAKI1)

1)Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan

(Received June 20th, 2013)

This research proposes an X-band high efficiency onboard SSPA (solid-state power amplifier) for deep space missions by focusing on GaN (gallium nitride) HEMT (high electron mobility transistor) whose remarkable material properties, such as high thermal conductivity, wide band gap, and high breakdown voltage, are suitable for high power and high efficiency applications. Developing a high efficiency onboard SSPA is one of the great issues when we consider some missions toward Mars, Jupiter, and much farther planets because of the requirements of both ultra-long distance communication and low power consumption. As a first step toward developing a SSPA for deep space, a breadboard model is fabricated based on preliminary design. It consists of a buffer amplifier, a driver amplifier unit, a high power amplifier unit, an automatic level control unit, a variable attenuator, DC/DC convertors, and an over current protection unit. Here, GaN HEMT is used in both driver amplifier and high power amplifier units. RF (radio frequency) characteristics of these amplifier units are evaluated in experiments. The driver amplifier unit achieves output power of 31.5 dBm with power gain of 33.5 dB and less than -26 dBc of IM3 (third order intermodulation distortion) at P1dB (1dB compression point) at 8.425 GHz. Moreover, the maximum efficiency is up to 35.2%. On the other hand, the high power amplifier unit achieves 42.3 dBm of output power with 46.1% of PAE (power added efficiency) at P3dB (3dB compression point) at 8.40 GHz. In addition, the integrated GaN SSPA bread board model achieves the maximum output power of 41.9 dBm and the maximum total efficiency of 31.0% at 8.40 GHz. At least more than 5% total efficiency improvement can be seen compared to the previous onboard SSPAs. Moreover, space applicability of GaAs (gallium arsenide) MMIC (monolithic microwave integrated circuit), GaN HEMT and DC/DC convertor that are expected to be used in the SSPA are confirmed in total ionizing dose test.

Key Words: SSPA, Satellite Communication, Deep Space Mission, GaN, X-band

Nomenclature

Gain : Gain [dB] Id : Drain current [A] IM3 : Third order intermodulation distortion [dBc]PAE : Power added efficiency [%] Pin: : RF input power [dBm] Pout : RF output power [dBm] TID : Total ionizing dose [krad] Vd : Drain voltage [V] Vg : Gate voltage [V]

1. Introduction

One of the most indispensable impacts on onboard power consumption has been generally caused by a transmission power amplifier among the other satellite bus subsystems. Especially, in deep space missions, it is quite difficult to reduce onboard transmission radio frequency (RF) power due to ultra-long distance communication between the earth stations and satellites. Therefore, developing a high efficiency onboard power amplifier is one of the great issues when we consider some missions whose destination is far away from both the sun and the earth, such as a planet beyond Jupiter because the requirements of ultra-long distance

communication and low power consumption must be realized simultaneously.

With respect to onboard high power amplifiers, travelling-wave tube amplifier (TWTA) and solid-state power amplifier (SSPA) have been mainly used. In general, TWTA uses a vacuum tube called traveling-wave tube and its component total efficiency that is computed by dividing RF output power by total power consumption including power supply unit or control unit is up to 50 to 60%. In contrast, SSPA usually uses a Gallium arsenide (GaAs) based field-effect transistor (FET) and its total efficiency is around 20 to 30%. Here, the power that cannot relate to amplification cannot help becoming heat. We have to manage this heat to be radiated to outside of a spacecraft. Thus, TWTA has been mainly used as a high power amplifier that deals with more than 20W output power1). However, regarding mechanical environment tolerance, lifetime, occupied area (footprint), and discharge risk, SSPA is superior to TWTA because TWTA uses a fragile sculpted-glass tube and needs high voltage (kV) operation2). Thus, high efficiency SSPA has been strongly required3).

Here, the total efficiency of an onboard high power amplifier strongly depends on its operation point, in other words, linear or nonlinear operation region. In general, the more complicated the modulation scheme becomes, the lower the efficiency becomes because we have to adopt wide band and linear (large back off) operation point in order to keep

Copyright© 2014 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.

Trans. JSASS Aerospace Tech. Japan Vol. 12, No. ists29 (2014)

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signal qualities. However, regarding deep space missions, the allocated frequency band is so narrow that we do not need to consider the wide band operation of amplifiers. In addition, amplitude modulation scheme cannot be adopted in deep space missions because the distance between a satellite and the earth is too long to measure weak signals from satellites in the presence of noise. Therefore, in deep space missions, input signal of onboard amplifiers is narrow band and low peak to average power ratio (PAPR)4). In other words, amplifiers can be operated at nonlinear (high-efficiency) operation point without any excess back off. However, SSPAs for only deep space missions have never developed so far since the requirements of band width, output power, and operation frequency are included in those for near earth missions, and space environmental requirements for deep space are generally less severe than those for near earth. Thus, it can be said that the efficiency has been compromised in deep space missions by substituting SSPAs for near earth missions.

From these backgrounds, this research proposes an X-band high efficiency onboard SSPA for deep space missions by focusing on high-efficiency operable device (gallium nitride: GaN) and high-efficiency operation point without considering any excess back off. This paper shows experimental evaluations of a bread board model (BBM) of X-band SSPA and results of space environmental testing.

2. GaN SSPA

In this research, GaN high-electron mobility transistor (HEMT) is used as amplification device since its remarkable material properties, such as high thermal conductivity, wide band gap, and high breakdown voltage are suitable for high power and high efficiency applications5,6). In addition, GaN is considered having strong space-tolerant characteristics, such as the operability under high temperature or radiation condition7-9).

Figure 1 indicates block diagram of an X-band GaN SSPA system. Proposed system consists of a buffer amplifier, a driver amplifier unit, a high power amplifier unit, a directional coupler (CPL), an isolator (ISO), an automatic level control (ALC) circuit, a variable attenuator (VAR ATT), a high/low switch, an on/off switch, DC/DC convertors, an over current detection and protection circuit, and a sequence circuit. RF signal from a transponder is input to the driver amplifier unit through the VAR ATT and the buffer amplifier first. After that, amplified RF signal is input to the high power amplifier unit to achieve desired output power level. For achieving stable output characteristics, the ALC uses output power from the high power amplifier unit to control input power of the driver amplifier unit by adjusting the VAR ATT. Moreover, output power level is also selected by adjusting the VAR ATT according to the high/low switch. With respect to direct current (DC), the total SSPA system can be turned on and off by the on/off switch. Unstable DC power from satellites’ primary battery is converted into stable secondary power for the amplifier units by the DC/DC convertors. In case of over

current, the detection and protection circuits are inserted between the DC/DC convertors and the sequence circuit. Since normally on type transistors are used for both driver amplifier and high power amplifier units, the sequence circuit that can appropriately supply drain and gate voltages step by step is adopted.

Overview of proposed SSPA and comparisons between previously-used X-band onboard SSPAs and the target specifications of proposed SSPA are shown in Fig. 2 and table 1 respectively3,10-12). As shown in table 1, proposed SSPA has the possibility to achieve more than 10% higher efficiency than previous onboard SSPAs since we focus on GaN instead of GaAs and consider only deep space applications. Here, the

Fig. 1. Block diagram of X-band SSPA using GaN.

Fig. 2. Overview of X-band SSPA using GaN.

Table 1. Comparison of X-band onboard SSPAs. PLANE

T-C GOSAT ALOS-2

SPOT-5 MER This work(target)

Final stage device

GaAs GaAs GaAs GaAs GaN

Outputpower [W]

10 20 22 17 20

Size [mm] 271*150*80

313*147*142

271*160*70

174*134*47

150*120*75

Weight [kg]

1.9 2.05 1.5 1.37 1.5

Powerconsumption [W]

44.5 98 91 66 55

Totalefficiency [%]

22.5 20.4 24.2 25.8 36.4

Y. KOBAYASHI et al.: Experimental Evaluation of an X-band GaN High Efficiency Onboard SSPA for Deep Space Missions

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(a) Appearance

(b) Schematic

Fig. 3. Driver amplifier.

Fig. 4. Measured S-parameters of driver amplifier.

GaN HEMTGaAs MMIC

ISO

Vd

Vg

RFoutRFin ISO

Vg

Vd

S21

S22

S11

target specifications are calculated based on preliminary design results3). In this preliminary design, the efficiency of the driver amplifier unit is expected to be larger than 20%, that of the high power amplifier unit is larger than 55%. In addition, the conversion efficiency of the DC/DC convertors needs to be larger than 84%. Generally, in considering the total efficiency of an SSPA component, the most significant points are the efficiency of a final-stage high power amplifier and DC/DC convertors since these circuits deal with high power and they easily affect the total efficiency. Thus, this research must pay much attention to developing these circuits.

3. Experimental Evaluations of GaN SSPA

3.1. Driver amplifier Driver amplifier consists of GaAs monolithic microwave

integrated circuit (MMIC) amplifier (EMM5078ZV) and GaN on SiC HEMT amplifier (TGF2023-01) whose gate length is 0.25 um, the unit gate width is 125 um, the number of gate fingers is 10 and gate periphery is 1.25 mm13) for the realization of small size and high efficiency. In addition, isolators are inserted between these amplifiers in order to

prevent coupling between different semiconductors (GaAsand GaN). Figure 3 shows the appearance and schematic of this amplifier and the size is 100*35*20 mm. Measured small signal S-parameters of the driver amplifier are shown in Fig. 4. Figure 4 indicates that measured S11 and S22 are smaller than -10 dB and S21 is larger than 33 dB at the frequency between 8.20 and 8.60 GHz. In addition, input-output characteristics at 8.30, 8.425, and 8.55 GHz are shown in Fig. 5. It is confirmed that P1dB of each operation frequency are 30.6 dBm (8.30 GHz), 31.5 dBm (8.425 GHz), and 30.6 dBm (8.55 GHz) and the maximum power added efficiency (PAE) is 31.1% (8.30 GHz), 35.2% (8.425 GHz), and 31.6% (8.55 GHz). Moreover, Fig. 6 indicates 3rd order intermodulation distortion where f1 is 8.425 GHz and f2 is 8.435 GHz. In Fig. 6, 2f2-f1 is named as “High,” and 2f1-f2 is named as “Low”. It is confirmed that IM3 at P1dB point is -26.6 dBc (High) and -26.0 dBc (Low). From these results, proposed driver amplifier achieves small size and high efficiency without losing high linearity compared to previous driver amplifiers using only GaAs. 3.2. High power amplifier

High power amplifier is a single-stage amplifier that uses GaN on SiC HEMT (TGF2023-05) whose gate length is 0.25 um, the unit gate width is 125 um, the number of gate fingers is 40 and gate periphery is 5 mm13) and the substrate of AR1000 for the realization of small size and high efficiency.

Fig. 5. Input-output characteristics of driver amplifier.

Fig. 6. IM3 of driver amplifier.

PAE

Gain

Pout

8.30 GHz8.425 GHz8.55 GHz


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Figure 7 shows the appearance and schematic of the high

power amplifier and the size is 50*60*20 mm. Measured small signal S-parameters of the high power amplifier are shown in Fig. 8. Figure 8 indicates that measured S11 and S22 are smaller than -8.5 dB and S21 is larger than 10 dB at the frequency between 8.35 and 8.75 GHz. In addition, input-output characteristics at 8.40, 8.425, and 8.45 GHz are shown in Fig. 9. It is confirmed that P3dB of each operation frequency are 42.3 dBm (8.40 GHz), 42.1 dBm (8.425 GHz), and 42.0 dBm (8.45 GHz) and the maximum PAE is 47.3% (8.40 GHz), 45.1% (8.425 GHz), and 46.6% (8.45 GHz). From these results, although there still remains about 10% efficiency improvement for achieving high efficiency SSPA, it is confirmed that using GaN on SiC HEMT is feasible for small size and high efficiency high power amplifier. 3.3. GaN SSPA BBM In addition to the driver amplifier and the high power amplifier shown in the previous sections, we have integrated VAR ATT, buffer amplifier, CPL, and ISO as a BBM of GaN SSPA. Figure 10 shows the overview of GaN SSPA BBM. Since this is the first step to integrate the SSPA total system, ALC, control unit, and power supply unit are not included. Input-output characteristics of GaN SSPA BBM at 8.40, 8.425, and 8.45 GHz are shown in Fig. 11. In this measurement, the loss of both CPL and ISO are included as shown in Fig. 10. In Fig. 11, there are two types of efficiency: drain efficiency ( )and SSPA total efficiency. With respect to the former, drain efficiency shows the efficiency of only RF unit of GaN SSPA

(a) Appearance

(b) Schematic

Fig. 7. High power amplifier.

Fig. 8. Measured S-parameters of high power amplifier.

Fig. 9. Input-output characteristics of high power amplifier.

GaN HEMTVg

RFoutRFin

Vd

PAEGain

Pout

8.40 GHz8.425 GHz8.45 GHz

Fig. 10. GaN SSPA BBM.

Fig. 11. Input-output characteristics of GaN SSPA BBM.

Pout

Gain

SSPA totalefficiency

8.40 GHz8.425 GHz8.45 GHz


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as shown in Fig. 10. Here, PAE and drain efficiency are almost the same in case the input power of SSPA is quite low compared to the output power. Regarding the latter, SSPA total efficiency is calculated by dividing RF output power by SSPA total power consumption including control unit and conversion efficiency of power supply unit (DC/DC convertors) as well as RF unit. In this calculation, the estimated power consumption of control unit and power supply unit based on preliminary design result 3) is used as a substitute for measurement data. It is confirmed that the maximum output power of each operation frequency is 41.9 dBm (8.40 GHz), 41.7 dBm (8.425 GHz), and 41.8 dBm (8.45 GHz) and the maximum drain efficiency is 37.7% (8.40 GHz), 36.5% (8.425 GHz), and 37.5% (8.45 GHz). In addition, the maximum SSPA total efficiency is 31.0% (8.40 GHz), 30.1% (8.425 GHz), and 30.8% (8.45 GHz). In comparison with table 1, it still needs work to achieve our target value since the high power amplifier cannot realize its desired characteristics so far. However, it can be seen that even though additional improvements are required, the achievement to date indicates at least more than 5% total efficiency is improved compared to the previous onboard SSPAs shown in table 1.

Fig. 12. TID test setup.

Fig. 13. Measurement results of converted voltage and current of DC/DC convertors during TID test.

DUT(DC/DC)

DUT(AMP)

Vd

Vdc/dc

Vg

Current meter

SG

PSRF SWDriver AMP

CPL ATT

PC

Load

CPL

Current meter

Current meter

Voltage meter

Irradiationchamber

BRU48 12S2.5A(A)BRU48 12S2.5A(B)BRU48 15S2A(A)BRU48 15S2A(B)

(a) EMM5078ZV

(b) TGF2023-01

(c) TGF2023-05 Fig. 14. Measurement results of input power, output power, gain, PAE and drain current during TID test with CW signal at 8.425 GHz.


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4. Space Environmental Testing

Total ionizing dose (TID) test using 60Co is adopted as a space environmental testing since wide band gap characteristics of GaN are considered having a strong tolerance to a single event phenomena compared to Si or GaAs. GaAs MMIC amplifier (EMM5078ZV), GaN on SiC HEMT amplifiers (TGF2023-01, TGF2023-05), and DC/DC convertors (BRU48-12S2.5A, BRU48-15S2A) are devices under tested (DUT) in this time. Figure 12 shows the configuration of TID test. As shown in Fig. 12, DUT and some kinds of measurement instruments are inside of an irradiation chamber. The instruments that are not expected to be exposed in spite of being inside of the chamber (dotted line region) are protected by lead blocks. Here, each amplifier is set on a heat sink, and then 16-hour continuous wave (CW) operation is conducted at practical input power level during TID test. Total radiation dose of 40 krad and exposure dose rate of 2.5 krad/h are adopted in this test. In TID test, 8.425 GHz signal is supplied to DUT by signal generator (SG), then input and output powers are monitored by power sensor (PS) as well as both drain and gate currents are monitored by current meters. In contrast, regarding DC/DC convertor, 48 V DC power is supplied to DUT by stabilized power supply, then voltage meter and current meter monitor converted voltage and current. Here, each DC/DC convertor is tested in two different ways so that the exposed surface becomes different. It is called (A) that the surface where the connectors are is exposed. In contrast, we name that the opposite surface is exposed as (B). Measurement results of converted voltage and current of DC/DC convertors during TID test are shown in Fig. 13. Figure 13 indicates that the voltage and current of BRU48-15S2A(A) are decreasing after TID reaches 31.5 krad. However, it is obvious that these DC/DC convertors can tolerate up to 30 krad in spite of output voltage value or exposed surfaces. Here, standard requirements of TID for each component in science satellite or space probe missions are generally 20 krad. In contrast, measurement results of input power, output power, gain, PAE, and drain current of EMM5078ZV, TGF2023-01 and TGF2023-05 during TID test with CW signal at 8.425 GHz are shown in Fig. 14. In addition, comparison between before and after TID test in respect to input-output characteristics of EMM5078ZV, TGF2023-01, and TGF2023-05 at 8.4 GHz are shown in Fig. 15. From both Figs.14 and 15, it can be seen that there is no specific degradation during TID test in these amplifiers even though amplifiers operate at their practical (nonlinear) input power level. Thus, with respect to radiation hardiness, it is confirmed that DUT in this TID test have enough tolerance to be used in space.

5. Conclusion

An X-band small-size and high-efficiency onboard SSPA is proposed in this paper for the future deep space missions. As a first step, a breadboard model is fabricated based on

(a) EMM5078ZV

(b) TGF2023-01

(c) TGF2023-05 Fig. 15. Comparison between before and after TID test in respect to input-output characteristics at 8.4 GHz.

Before radiation testAfter radiation test

PAE

Gain

Pout


PAEGain

Pout


PAEGain

Pout


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preliminary design. By focusing on GaN on SiC HEMT that has wide band gap, high breakdown voltage, and high thermal conductivity properties, driver amplifier unit realizes small size of 100*35*20 mm, and achieves output power of 31.5 dBm with power gain of 33.5 dB and less than -26 dBc of IM3 at P1dB point at 8.425 GHz. Moreover, the maximum efficiency is up to 35.2%. On the other hand, high power amplifier unit that is also using GaN on SiC HEMT realizes the size of 50*60*20 mm, and achieves 42.3 dBm of output power with 46.1% of PAE at P3dB point at 8.40 GHz. In addition, the integrated GaN SSPA bread board model achieves the maximum output power of 41.9 dBm and the maximum total efficiency of 31.0% at 8.40 GHz. At least more than 5% total efficiency improvement can be seen compared to the previous onboard SSPAs. Moreover, space tolerance of GaAs MMIC, GaN on SiC HEMT and DC/DC convertors that are expected to be used in the SSPA are confirmed in total ionizing dose test. From these results, although, there still remains requirement to make higher-efficiency power amplifier for realizing the high-efficiency SSPA, it is confirmed that using GaN on SiC HEMT is feasible for small size and high efficiency SSPA.

Acknowledgments

The authors would like to express deep gratitude to M. Aoki and S. Tsuchiya of DST, S. Furuta, Y. Moriguchi, and M. Ono of NEC Network and Sensor Systems, Ltd, and M. Ito, T. Yamashita, O. Shigeta, and H. Nunomura of AI Electronics Ltd. for their great support in progressing the preliminary design, trial production and evaluation of GaN SSPA. And the authors wish to acknowledge the assistance and support of ISAS Kawasaki Research Laboratory.

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experimental evaluation of an x-band gan high efficiency

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