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Additive Phase Noise in Linear and High-EfficiencyX-Band Power Amplifiers

Jason Breitbarth, Srdjan Pajic, Narisi Wang and Zoya PopovicSchool of Electrical and Computer Engineering

University of Colorado, Boulder, COEmail: jason.breitbarth@colorado.edu

Abstract- This paper compares additive (residual) phasenoise of linear and highly saturated high-efficiency MESFETand HBT 10 GHz power amplifiers (PA). A custom discrim-inator measurement system is developed to characterize thePAs and exhibits a phase noise floor of -164 dBc/Hz at 10 kHzoffset for a 10GHz fundamental. The additive phase noisemeasurements show a phase noise of -120dBc@1 kHz forthe class-E MESFET PA, a roughly 10-dB increase whencompared to the class-A PA with the same device. The HBTamplifier exhibited nearly the same phase noise in class Eand linear class A.

I. INTRODUCTION

In a variety of applications which require stable fre-quency and good selectivity, such as Doppler radar andphase-modulated communications, phase noise of the pri-mary oscillator is critical [1], [2]. Oscillator noise ishowever only one element in the overall system phasenoise. The analog front end chain almost without exceptioncontains amplifiers, which also add noise close to thecarrier. This noise component is often referred to asadditive or residual phase noise and is typically dominatedby the flicker (1/f) noise of the device [3].

Additive phase noise has been characterized in mi-crowave linear amplifiers [4]-[7]. However, there has beenlimited characterization of additive phase noise in saturatedpower amplifiers. In [8], the phase noise of a 50% efficientsaturated 4.6-GHz MESFET PA is shown to have an 11 dBincrease in phase noise compared to the same circuitin class-A bias. High-efficiency amplifiers operating inswitched mode have lately been studied extensively inthe microwave frequency range [9], as well as at lowerRF frequencies [10]. These PAs have been proposed forradar and high-power communications transmitters [11],in which case the additive phase noise is a significantparameter of interest.

Measurements from [12] show a significant S21 phasechange across bias conditions. The nonlinearity of capac-itance to drain voltage may affect phase noise as AM-PMconversion within the amplifier in large signal condition.The goal of this work is to quantify and compare theadditive phase noise of high-efficiency and linear PAsimplemented with both FET and HBT devices at X-band.

100 pF

2.490- 1.56c

8.2pF 138

0.19- 0O73

0~ ~ ~~. IIIF ..(a) (b)

126°- 0 -6.94

^2 | 4.25 l: e

F 0.9331.21

(c) (d)

Fig. 1. Layout and photograph of the (a-b) GaAs MESFET and (c-d) InP DHBT class-E PA. The PAs are fabricated on 0.635-mm thickRogers TMM6 substrates with er=6. The 50Q microstrip lines in thematching networks are 0.93mm wide. High impedance bias lines areused for providing gate and drain supply voltages for the MESFET PA.Single layer mm-wave capacitors are used for power supply bypassingand AC coupling. The HBT PA is biased with external bias Tees.

II. POWER AMPLIFIER DESIGN

For additive phase noise comparison, both class-A andhigh-efficiency saturated PAs are designed with SkyworksGaAs MESFETs and Northrop Grumman InP DHBTs.The class-A PAs are standard conjugate match designs.The high-efficiency amplifiers are designed to operate inswitched class-E mode, following simplified ideal-switchtheory backed by load-pull measurements [13]. In theideal switch model, the transistor is treated like an idealswitch with a shunt capacitance. For design, the outputcapacitance of the device is found from the s-parametersat the appropriate bias point. In practice, the switch isdriven with a 50% duty cycle. The peak voltage and currentstresses on the device are 3.6VDD and 2.71DD, requiringa power backoff around 1.5 dB. Since the device has tobe a good switch, the operating frequency limit for class

0-7803-9542-5/06/$20.00 ©2006 IEEE

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E is several times smaller than fT, and depends on thetransistor output capacitance COUT and maximal currentrating, IMAX. The output impedance presented to thedevice, assuming an ideal switch, a high-Q output circuitassumption, a 50% duty cycle and soft turn-on voltagewaveforms [10] is given by

ZE =0.28

US COUT (1)

This impedance is designed at the fundamental switch-ing frequency and implemented with transmission linesat 10GHz. The matching network also presents a highimpedance (open) at the second harmonic. In practice, thedevice does not have enough gain at the third harmonic tojustify a more complicated harmonic tuning circuit. Afterdesigning the output impedance, a load and source pull isperformed in the neighborhood to optimize the efficiency,gain and power for a realistic non-ideal switching device.The class-E topology is not as sensitive to circuit variationsas some other tuned amplifier topologies and lends itselfwell to the described design approach, with demonstratedpower added efficiencies (PAE) of 60% to 70% in [11],[14], [15].

For the MESFET and HBT transistors used for the PAsin this work, the output capacitances and fTrf,max are(0.11 pF, 30/60 GHz) and (0.185 pF, 80/150 GHz). The cor-responding optimal source and load impedances for classE operation are found to be (9.1-j2.4 Q, 35.9+j37 Q) and(15. 1-jO.5 Q, 15.8+j18.2 Q). The measured performancefor the PAs from Fig. 1 is given in Tables I and II.

III. PHASE NOISE MEASUREMENT SYSTEM

In order to measure phase noise to a suitably lowlevel, a custom 10-GHz source is designed as shownin Fig. 2. A 100 MHz crystal oscillator is multipliedusing a LPN7100 comb generator from Picosecond PulseLabs (PSPL). The 2 GHz harmonic is bandpass filtered tocreate an intermediate frequency and amplified by a HittiteHMC479 SiGe amplifier. A second comb generator fromPSPL, model LPN7103, yields harmonics past 10GHz.The 10GHz harmonic is bandpass filtered and amplifiedfirst by a Hittite HMC462 broadband LNA (NF=2.5 dB)and then by a HP8449B power amplifier (Pout=18dBm).The phase noise of the 1OGHz oscillator is measured

using a very low-loss (14.6dB) 125-ns delay-line discrim-inator. The delay-line measurement offers a very goodnoise floor far from the carrier but the measurement islimited in sensitivity close to the carrier. The measuredphase noise is shown in Fig. 3 and compared with aHP83620A synthesizer at 1OGHz. At lOOkHz offset, thephase noise floor of the multiplied oscillator is measuredto be -123dBc/Hz.

The residual phase noise of the amplifier under test(DUT) is measured using the phase discriminator tech-

Wenzel Hittite Picosecond 2GHz Hittite

100MHz 12dB HMC479 LPN7100 2X HMC479Sprinter Agilent Hittite 1 OGHz Picosecond

18dBrn

8449B HMC462 LPN7103

Fig. 2. Schematic representation of the IOGHz source designed using aIOOMHz low phase noise source and frequency multiplication with combgenerators.

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mcoU,).0za,)

mUl)

co

HP83620A Synthesizer

:1OGHz Multiplied Source

-100

-120

-140

-160_101

,Delay Linte Noise Floor,102 103 104

Frequency Offset (Hz)105 106

Fig. 3. Phase noise measurement of the 1OGHz source using a delay linemeasurement at 1OGHz. For comparison, the phase noise of an AgilentHP83620A is shown. Below lOkHz, the phase noise of the precisionreference is below the noise floor of the measurement.

nique shown in Fig. 4. The sensitivity of the system isdetermined by the phase noise of the oscillator and LO-RFand LO-IF isolation of the mixer. The Marki MicrowaveM8-0420 7dBm mixer exhibits a LO-RF and LO-IF iso-lation of 44 dB and 32 dB at 10 GHz. The measured noisefloor is shown in Figs. 5 and 6 at -164 dBc/Hz at 100 kHzoffset.

/ Nf\ Narda Phase Shifter/1OGHz 1OdB

8dBn

7dBrMarki !/

1 -04200</2dBmDU~~~~~~~~~~~~~~~~~~~~~~~~~~,

* LNA> *. ADC

Fig. 4. Schematic of the discriminator residual phase noise measurementsystem. The two branches have noise common from the oscillator. Anyadditional noise added by the DUT above what is characterized withoutit in place is due to the DUT.

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TABLE ICOMPARED CHARACTERISTICS OF 10-GHz MESFET AND HBT CLASS-A HYBRID PAs.

VDC-OUT [V] VDC-IN IDC-OUT [mA] PldB [dBm] G [dB]GaAs MESFET 5 -0.64 70 19 10

InP DHBT 5 0.72 40 21.9 10.5

DC Operating points for Collector and Drain (Output) and Base and Gate (Input) with associated current consumption, output power and gain. TheGaAs MESFET and InP DHBT devices have fTs of 30GHz and 80GHz, respectively.

TABLE IICOMPARED CHARACTERISTICS OF 10-GHz MESFET AND HBT CLASS-E HYBRID PAs.

VDC-OUT [V] VDC-IN 1DC-OUT [mA] POUT [dBm] G [dB] rI[%] PIN [dB] A[[°] CL [dB]GaAs MESFET -1.55 4.2 4 20.7 8.3 70 13 53 1.5

InP DHBT 0.35 4.35 0 21 9.8 69 12 105 3.3

VDC-OUT and IDC OUT-drain (collector) DC voltage and current, VDC-IN-gate (base) DC voltage, POUT-nominal output power, G-powergain, q7-drain (collector) efficiency, PIN-input return loss, AO-relative phase deviation at nominal POUT, CD-compression level at nominal outputpower.

IV. ADDITIVE PHASE NOISE MEASUREMENTCOMPARISON

The residual phase noise measurements were carried outfor both class-E and linear class-A amplifiers. Two com-pression levels were measured for class-A to understandphase noise behavior as the device compresses.

The MESFET amplifier exhibits a significant changebetween class-E and linear class-A modes as shown inFig. 5. While consistently following a 1/f behavior, thenoise level increases by 15dB between classes A and E.The class-A PA in I-dB compression has a noise pedestalincrease of 2-7 dB between 100 Hz and 10kHz.

These results differ slightly from those discussed in [8],where the phase noise offsets less than -1OOdBc/Hz con-verged for the class-E and class-A 5-GHz PAs, while themeasurements presented here show the same increase inphase noise from 10 Hz to 1 MHz. The difference betweenthe results in this work and that in [8] is possibly due tothe fact that the 50-% efficient PA in [8] is likely operatingin class AB and has different AM-PM conversion, asdiscussed in the next section.The HBT amplifier phase noise measurement results are

shown in Fig. 6. A large noise pedestal exists between10 Hz and 10 kHz for both classes of HBT amplifiers. Theclass-A and class-E PAs have a relatively large amount ofnoise at small offsets, but above 10kHz offset, the noisedrops to levels below -160dBc/Hz.

V. DISCUSSION

In order to help explain the phase-noise measurementof the HBT and MESFET PAs, the AM-PM conversionis measured at different bias points in class-E and shownin Fig. 7. In this measurement the drain (collector) biasvoltage is changed and the S21 phase measured on anetwork analyzer. The results are normalized to 5 V drainbias. This data shows significant AM-PM conversion under

-80

-90

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mU/)

cnco

Class-E MESFET

103 104Frequency Offset (Hz)

Fig. 5. Class A vs. Class E residual phase noise measurement of the10-GHz MESFET PAs. Here it can be seen the phase noise of the class-E PA is degraded by abotu 15dB from the class-A version. The class-Aamplifier is shown in OdB and 1dB compression with a slight degredationof phase noise at 1dB in compression.

large signal conditions. Any voltage noise on the drain orcollector bias will result in phase noise at the output, asdiscussed in [5]-[7].The results in Fig. 7 are consistent with the measured

phase noise. The MESFET amplifier has a significant AM-PM increase between small-signal operation (PIN=3 dBm)and high compression (PIN= 12.4 dBm). Consistently, inFig. 5, we see a broad phase-noise increase between linearclass-A and highly-saturated class-E, as would be expectedwith a higher AM-PM conversion.The HBT amplifiers in Fig. 6 also show consistent

results with the AM-PM measurements. The AM-PM ratioof the HBT amplifiers is greater than that of the MESFETand virtually the same for low and high compressionconditions. This may cause the increase in phase noiseof the HBT over the MESFET.

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1874

N -I

m-

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Phase Noise: Floor: :: :

102 103 104Frequency Offset (Hz)

106

Fig. 6. Class A vs. Class E residual phase noise measurement of the10-GHz HBT PAs. Here it can be seen the phase noise of the class-E HBT is not significantly different from teh class A HBT for eitherOdB or 1dB compression levels. The HBT is shown here to not followconventional 1/f behavior of 10dB/decade increase in phase noise as thefourier frequency becomes smaller.

5MESFET: PIN=3dBm MESFET: PIN=12dBm

L~~~~~I// HT:N1B

(Da)(D 5

-a5

-10C/)0) ~~~~~HBT: PI =6dBmn

15n HBT: gP lldBm

(D. -20-

(Dcc _25

-301 1.5 2 2.5 3 3.5 4 4.5 5

Drain Voltage (V)

Fig. 7. AM to PM conversion for the drain/collector bias voltage tooutput phase shift (in degrees). The MESFET has a marked differencein AM-PM conversion V/degree between class-A and class-E operation.The HBT amplifiers both exhibit nearly the same AM-PM conversion fordifferent modes of operation. The MESFET always has lower AM-PMconversion. These measurements are consistent with the residual phasenoise measured for the MESFET and HBT class-A and class-E amplifiers.

The noise spectra seen in the HBT phase-noise plotsseem to be related to the noise of the power supply thatis used in the measurement, further concluding that moreupconversion of DC bias noise is taking place in the HBTdevices. It would be interesting to examine bias-noisereduction methods [16] for class-E PAs, which points totradoffs in upconverted noise and gain and PAE.

thank Dr. Paul Watson, WPAFB, and Wendy Lee, NGST,for donation of InP DHBT devices used in this work.

REFERENCES

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[2] A. Diet, C. Berland, M. Villegas, and G. Baudoin, "Eer architecturespecifications for ofdm transmitter using a class e amplifier," IEEEMicrowave Wireless Compon. Lett., vol. 15, no. 8, pp. 389-391,Aug. 2004.

[3] C. E. Laboratories, "An-1026 1/f noise characteristics influencingphase noise," Santa Clara, CA, 2003.

[4] F. G. Ascarrunz, E. S. Ferre, and F. L. Walls, "Investigations ofam and pm noise in x-band devices," in 1993 IEEE InternationalFrequency Control Symposium, pp. 303-311.

[5] 0. Llopis, J. B. Juraver, B. Tamen, F. Danneville, M. Chaubet,A. Cappy, and J. Graffeuil, "Nonlinear noise modeling of a phemtdevice through residual phase noise and low frequency noisemeasurements," in IEEE MTT-S Digest, 2001, paper WE2C-5, pp.831-834.

[6] M. Rudolph and P. Heymann, "The influence of microwave two-port noise on residual phase noise in gaas-hbts," in 34th EuropeanMicrowave Conference - Amsterdam, 2004, pp. 945-948.

[7] P. A. Dallas and J. K. A. Everard, "Measurement of the crosscorrelation between baseband and transposed flicker noises in agaas mesfet," in IEEE MTT-S Digest, 1990, paper NN-3, pp. 1261-1264.

[8] S. Romisch and M. Weiss, "Experimental investigation of phasenoise in high-efficiency class-e amplifiers," IEEE InternationalUltrasonics, Ferroelectrics, and Frequency Control Joint 50th An-niversary Conference, pp. 718-724.

[9] T. B. Mader, E. Bryerton, M. Markovi, M. Forman, and Z. Popovic,"Switched-mode high-efficiency microwave power amplifiers in afree-space power-combiner array," IEEE Trans. Microwave TheoryTech., vol. 46, no. 10, pp. 1391-1389, Oct. 1998.

[10] F. Raab, "Idealized operation of the class e tuned power amplifier,"IEEE Trans. Circuits Syst., vol. 24, no. 12, pp. 725-735, Dec. 1977.

[11] T. Quach, P. Watson, W. Okamura, E. Kaneshiro, A. Gutierrez-Aitken, T. Block, J. Eldredge, T. Jenkins, L. Kehias, A. Oki,D. Sawdai, R. Welch, and R. Worley, "Ultrahigh-efficiency poweramplifier for space radar applications," IEEE J. Solid-State Circuits,vol. 37, no. 9, pp. 1126-1134, 2002.

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[13] S. Pajic, N. Wang, P. Watson, T. Quach, and Z. Popovic, "X-bandtwo-stage high-efficiency switched-mode power amplifiers," IEEETrans. Microwave Theory Tech., vol. 53, no. 9, 2005.

[14] M. Weiss and Z. Popovic, "A 10 ghz high-efficiency active antenna,"in 1999 Microwave Symposium Digest, 1999, pp. 663-666.

[15] P. Watson, R. Neidhard, L. Kehias, R. Welch, T. Quach, R. Worley,M. Pacer, R. Pappaterra, R. Schweller, and T. Jenkins, "Ultra-highefficiency operation based on an alternative class-e mode," in 22ndGaAs IC Symposium, 2000, pp. 53-56.

[16] E. S. Ferre-Pikal, "Reduction of phase noise in linear hbt amplifiersusing low-frequency active feedback," IEEE Trans. Circuits Syst.,vol. 51, no. 8, pp. 1417-1421, Aug. 2004.

ACKNOWLEDGMENT

This work is supported by a DARPA Intelligent RFFront Ends (IRFFE) program, grant N00014-02-1-0501and by Picosecond Pulse Labs, Boulder, CO. The authors

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