LINEAR WIDEBAND VHF/UHF QUAD LINC TRANSMITTER SYSTEM

Gamal Hegazi, Thanh Chu, Scott Heibel, Jake Jordan and Haluk SasmazerRockwell Collins

Cedar Rapids, IA 52498

ABSTRACT

This paper presents the design, build and measurements ofa wideband quadrature out-phased transmitter systembased on Linear Amplification with Non-linearComponents (LINC) that covers the 30 to 450 MHzVHFIUHF band with 185 Watts output power The key inthis transmitter design is the decomposition of thebaseband signal into in-phase (I) and quadrature (Q)components that get digitally processed and up-converted,resulting in two constant-envelope signals each that can beamplified efficiently with non-linear class-D amplifiers.After amplification, the output signals get combined toreconstruct the amplified, original up-converted, non-constant envelope signal. The result is improved linearityof the transmitter system and reduced sensitivity tocomponents mismatch as opposed to conventional LINCtechnique. This transmitter system has potential to enableJoint Tactical Radio System (JTRS) to use highestthroughput "non-constant envelope" waveforms such asOrthogonal Frequency Division Multiplexing (OFDM) andQuadrature Amplitude Modulation (QAM) to provide thewarfighters access to higher data rates and greatercommunication range. The superior linearity andefficiency of the transmitter system is demonstrated in thispaper for various complex varying envelope signals suchas AM, 8-PSK, QAM and OFDM, and is compared withtraditional LINC systems as well as legacy backed-offclass ABpower amplifiers.

INTRODUCTION

Highly linear transmission of quadrature-modulatedsignals is required in a number of modern communicationsystems. Among these are spectrally efficient systemssuch as VHF Digital Link (VDL) Modes 2 and 3 andorthogonal frequency division multiplex (OFDM) systems.However, highly linear performance is difficult to realizewhile simultaneously achieving high power efficiency. Anumber of approaches exist to solve this problem. Onesuch approach is linear amplification using non-linearcomponents (LINC) [1-2]. LINC transmission works byseparately amplifying two constant envelope signalcomponents using highly non-linear but very powerefficient amplifiers as shown in Fig. 1. The amplifiedcomponents are then combined at high power levels,resulting in the desired linearly amplified signal. Whilethis technique does eliminate the need for highly linear

amplifier components, it introduces a new set of potentialproblems.The main problem with the LINC approach in Fig. 1 is

the fact that the constant envelope components containconsiderable energy outside of the occupied bandwidth ofthe signal. This causes the combined output signal tocontain spectral re-growth which is adverse to the purposeof creating a spectrally efficient communication system.To solve this problem a quadrature outphasing technique

was proposed in [3] which first separates the in-phase andquadrature components of the complex baseband signal asshown in Fig. 2. Both the in-phase and quadraturecomponents now become real and can be decomposed intoconstant envelope signals. The resulting constant envelopesignals of the real I and Q components are spectrally well-contained and the output combined signal

ConstantEnvelope ~~~~~No-linear

Componen --. Upconverter amplifier , LinearlyAmplified RF

Baseband Signasignal Constant

Envelope nvelopeDecomposition

Constant amplifier m

EnvelopeComponent

Figure 1. Regular LINC architecture.

Constant No -linearEnvelope g Up converter amplifier

I-phase ComponentI hsBaseband RE SignalSignal Constant

Envelope +Decomposition

Constant No linear LinearlyEnvelope Up converter amplifier AmplifiedComponent RE Signal

Figure 2. Quadrature LINC architecture.

*A portion of this work was performed under Cooperative Agreement ONR N00421-06-2-0001 with the Office of Naval Research (ONR).

1-4244-1513-06/07/$25.00 ©2007 IEEE

1

does not suffer from the spectral re-growth problem of theregular LINC architecture of Fig. 1.

MHz. The push-pull version of the class-D poweramplifier delivers twice the power with similar poweradded efficiency.

CLASSDAMPLIFIER DESIGNClass-D single-ended power and efficiency

At the heart of the LINC system is the efficient class-Dpower amplifier module. Special attention was paid to thedesign of the class-D amplifier to ensure high efficiencyover the whole bandwidth. The schematic of the push-pullversion of the power amplifier is shown in Fig. 3. Anotherversion of the amplifier was designed using a single-endedapproach. As can be seen in Fig. 3, the amplifier consistsof a pulse shaping comparator feeding a wideband pre-driver. The pre-driver comparator chip shapes the inputsinusoidal signal turning it into an ideal square pulse with50% duty cycle that is used to drive the driver circuit. Thepre-driver in turn feeds the class-D driver power amplifier.The class-D driver amplifier provides an almost idealsquare wave with enough voltage swing to drive the finalclass-D power stage.

0

.24-

w

100

90

80

70

60

50

40

30

20

10

0 100 200 300

Frequency MHz

- Efficiency- Powe 5

400 500

Figure 4. Measured results of the class-D single-endedpower amplifier.

The final stage transistors were GaN devices fromEudyna, and they were modeled as switch-based devicesfollowing the method described in [4]. The advantage ofthis switch-based model is guaranteed convergence andconsistently accurate non-linear simulations.

EXPERIMENTAL SETUPAND TESTING

Figure 3. Push-pull class D power amplifier schematic.

The driver circuit provides the floating point needed atthe node connecting the two final transistors together.When the upper transistor is ON the current flows throughthe load encountering only the small resistance of thetransistor, and when the lower transistor is ON the loaddumps its current into that transistor with very littleresistance in series. The resulting waveforms driving thefinal stage are close to ideal square pulses that maintain thehigh efficiency class D operation desired.At the output of the power amplifier is a bank of

harmonic filters to meet the harmonic power levelspecifications.The measured results of the single-ended class-D power

amplifier is shown in Fig. 4. The amplifier delivers morethan 60 Watts of output power with an average poweradded efficiency of 60% over the band from 30 to 450

The LINC architecture was implemented using theaforementioned class-D power amplifier module. Twosuch modules were used to implement a regular LINCarchitecture as shown in Fig. 5, while four modules wereused to implement the quad LINC architecture as shown inFig. 6.

Vc 1 adjustabl5 V

Figure 5. Regular LINC architecture implementation.

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woo-,*o -+- - --

efficiency was about the same 54%. The efficiency of thefinal stage only was 55%.

Table 1. Measured results for a single tone signal.

Figure 6. Quad LINC architecture implementation.

A photo of the experimental setup is shown in Fig. 7 forthe Quad LINC architecture. In that photo the four class-Damplifiers are shown on the left feeding the harmonic filterbanks in the middle, and then the outputs are combinedwith two-stage Wilkinson combiners. The regular LINCimplementation utilizes only two amplifiers and one stageof Wilkinson power combining.

Figure 7. Photo of the Quad LINC experimental setup.

Several waveforms were tested with the above setupstarting from a single tone signal to more complexvarying-envelope signals. The constant envelope signalsfeeding the class-D amplifiers were generated using twodual-channel arbitrary waveform generators utilizingMATLAB files that were written for each specificwaveform. The measured results of the single tone signalat 100 MHz are shown in Table 1 below. For the QuadLINC case, the output power was 185 Watts with a totalefficiency of about 54%. The final stage efficiency wasalmost 58%. For the regular LINC case, the output powerwas 87 Watts, which is half the Quad LINC power asexpected since only two amplifiers are used, and the

Freq (MHz)Total Po (W)Eff(%) Final Stage OnlyTotal eff(%)

Regular LIN - SIN WAFreq (MHz)Total Po (W)Eff(%) Final Stage OnlyTotal eff(%)

10018557.89%53.63%

1008755.00%53.5%

The single tone signal represents a constant envelopesignal and is used as the baseline to compare power andefficiency of other complex waveforms with time varyingenvelopes such as 2-tone, 8-PSK, QAM and OFDM.

The two-tone signal was generated from the arbitrarywaveform generators as four constant-envelope signalsfeeding the class-D amplifiers for the Quad LINC case,and two constant-envelope signals for the regular LINCcase. The reference input and output resulting signals areshown in Fig. 8 and Fig. 9 for the regular LINC and QuadLINC cases, respectively.

Regular LINC Two-Tone

50

460

20

999 99.9 99.98 --- 001-0.2--0.0-100-

-10 Input ref \ a LINC.-20

-4

-60-99.95 99.97 99.98 100.00 100.02 100.03 100.(

Figure 8. Two tone testing for regular LING.

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QuadLINC Two-Tonesignals a the outputs of the class-D power amplifiers. Theoutput power for Quad LINC was 22 Watts with a power-added efficiency of 7%, while for regular LINC the outputpower was 15 Watts with 8% efficiency.

Reg LINC - VDL

Input red.

99.95 99.97 99.98

30

20

10

0

-10

-20

-30

-40

-50

100.00 100.02 100.03 100.(

Figure 9. Two-tone testing for Quad LINC.

I-nput re.-

99.95 99.97 99.98 100.00 100.02 100.03 100.(As seen in figures 8 and 9 the inter-modulation productsare better than 45 dBc in both cases indicating excellentlinearity. However, since Wilkinson combiners were usedto combine the non-coherent signals at the outputs of theclass-D amplifiers, both the power and power-added-efficiency drop in this case. The resulting output power

was 37 Watts with efficiency of 11% for Quad LINC, and33 Watts with 20% efficiency for regular LINC. Theabove results indicate that for an AM type signal theregular LINC would be the architecture of choice since theefficiency is better than Quad LINC, while no linearityadvantage is seen for the Quad LINC architecture over thatof regular LINC.

Next, the VDL signal, which is a 8-PSK waveform, was

tested with the same technique using the arbitrarywaveform generators. In this case, the Quad LINC had an

obvious linearity advantage over that of regular LINC as

can be seen in the measured results of Fig. 1O and Fig. 1.

The spectral re-growth in Fig. has improved by at least20 dB close to the signal in the Quad LINC case,

indicating that in this case the Quad LINC architecture isadvantageous to the regular LINC as expected since thiswaveform is a complex one and results in a spreadspectrum in the regular LINC constant envelope signalswhich is much better controlled for Quad LINC because ofthe I and Q decomposition as explained in [3]. Both thepower and efficiency drop in this case as well because ofthe Wilkinson power combining of the non-coherent

Figure 1O. VDL testing for regular LINC.

Quad LINC - VDL

403020 e10

Input ref. O

-10-

-20-30-Pw-40 -

-50 L

-60

99.95 99.97 99.98 100.00 100.02 100.03 100.05

Figure 1. VDL testing for Quad LINC.

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50403020100

-10-20-30-40-50-60

The next signal tested was an Orthogonal FrequencyDivision Multiplexed (OFDM) signal that was composedof 128 QAM signals. The constant envelope signals weregenerated as before using MATLAB files and arbitrarywaveform generators. The measurement results are shownin Fig. 12 and Fig. 13 below.

Rpgular LINC'- OFDMlr Output (B) and Inpujt (R) Sper:trurri

g1)

0

99 100 101 1[2 103 104 10_Frequlency (MlHz)

This case also shows the linearity advantage of the QuadLINC architecture over regular LINC as demonstrated inthe less spectral re-growth at the output signal close to thecarrier. However, since the peak-to-average power of thiswaveform is high (above 10 dB), the power and efficiencysuffer even more than the previous signals due to the non-coherent combining of the class-D power amplifier outputsusing Wilkinson combiners. The output power in theQuad LINC case was 5 Watts with a power-addedefficiency of 1.5%, while for the regular LINC case theoutput power was 5 Watts with an efficiency of also 1.5%.This indicates that for high peak-to-average ratiowaveforms an alternative combining technique must beused that will allow the amplifiers to modulate one anotherrather than using combiners with high isolation as in thiscase. However, that would have a negative impact onlinearity that will need to be corrected with somelinearization technique such as pre-distortion. Analternative approach would be to utilize a power recyclingtechnique where the non-coherent power gets rectified anddumped back into the power supply rather than gettingwasted as heat in the combiner isolation resistor [2].

The above results can be summarized in Fig. 14 and Fig.15 for the Quad LINC and regular LINC casesrespectively.

Figure 12. OFDM testing for regular LINC.Output power and efficiency vs. Peak-to-average

power

QuadLire OFDMI Input (F.) anid Output (B) Spectrum

99 100 01 102Frequency (MlHz)

200

1801

o 160

* 140W 120 -

100-

S 80-

° 60-

Q. 40-

O 20-

0

- - Efficiency

0 2 4 6 8 10 12

Peak-to-average power (dB)

Figure 14. Output power vs. peak-to-average power ratiofor Quad LINC

Figure 13. OFDM testing for Quad LINC.

Figure 14 shows that in the Quad LINC case, the outputpower starts at 175 Watts for a non-varying envelope

5

El

- - - ==!!W

signal represented by a single tone signal, with anefficiency of 55%, but both power and efficiency drop asthe peak-to-average power ratio increases as in the case ofthe various varying-envelope complex signals tested.In the regular LINC case, as shown in Figure 15, theoutput power starts at 90 Watts for a non-varying envelopesignal such as the single tone signal tested, with anefficiency of 55%, but again both power and efficiencydrop as the peak-to-average power ratio increases as in thecase of the various varying-envelope complex signalstested.

Output power and efficiency vs. Peak-to-averagepower

_O

0-

a)

.2

0.0-

Q-

100

90

80

70

60

50

40

- - . Efficiency

The results of this paper show the linearity advantage ofthe Quad LINC architecture over the regular LINCarchitecture for complex varying-envelope waveforms.Both architectures suffered from dropped output powerand power-added-efficiency for high peak-to-averagecomplex signals, although the linearity was maintained.This indicates that for high peak-to-average ratiowaveforms the LINC architecture needs adaptive biastechnique that could maintain high efficiency as theenvelope level drops to near zero. In addition to adaptivebias, it is suggested to recycle the non-coherent powerback to the power supply instead of wasting it as heat inthe isolation resistor of the Wilkinson combiner as wasdescribed in [2]. Implementation of the above twotechniques should maintain high efficiency for our highlinearity LINC transmitter architecture even for large peak-to-average ratio waveforms.

ACKNOWLEDGEMENTN*IN

30 N A portion of this work was performed under Cooperative

10 l Agreement N00421-06-0001 with the Office ofNavalo 2 Research (ONR). The authors wish to acknowledge the

0 2 4 6 8 10 12 assistance of Mr. Matt Durkin and Dr. Chris HicksPeak-to-average power (dB) throughout the course of this work.

Figure 15. Output power vs. peak-to-average power ratiofor regular LINC

CONCLUSION

In this paper we discussed the design and measurements ofa wideband linear LINC architecture that was implementedwith highly efficient class-D power amplifiers both in theregular LINC as well as Quad LINC architectures.

REFERENCES

[1] Chireix, H., "High Power Outphasing Modulation,"Proc IRE, vol. 23, no. 11, pp. 1370-1392, November1935.

[2] X. Zhang, L. E. Larson and P. Asbeck, "Design ofLinear RF Outphasing Power Amplifiers", BostonArtech House, 2003.

[3] G. Hegazi, T. Chu and R. Groshong, "Improved LINCPower Transmission Using Quadrature OutphasingTechnique", Digest of IEEE-MTT-S Symposium,2005, Long Beach CA.

[4] R. Negra, T. Chu, M. Helaoui, S. Boumaiza, G.Hegazi, and F. Ghannouchi, "Switch-Based GaNHEMT Model Suitable for Highly-Efficient RF PowerAmplifier Design", Digest of IEEE-MTT-SSymposium, 2007, Honolulu, HI.

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