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310 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 3, 2004

A PHEMT Frequency Doubling Active AntennaWith BPSK Modulation Capability

L. Cabria, J. A. García , Member, IEEE , E. Malaver, and A. Tazón , Member, IEEE

Abstract— This letter presents a novel frequency doubling activeantenna, based on a PHEMT device, with BPSK modulation capa-bility.A dedicated nonlinear transistorcharacterization revealstheexistence of two biasing regions, where the second harmonic couldbe generated with maximum level and phase opposition. Takingadvantage of this issue, a low frequency data signal applied to thegate terminal may be used to create a BPSK modulated signal, cen-tered at twice thecarrierfrequency. An adequate integration of thismodulator in a dual-frequency and dual-polarization slot coupledpatch, results in a compact and high performance solution.

Index Terms— Active integrated antennas, conversion gain, fre-quency doubler, high-electron mobility transistor (HEMT), radio

frequency identication (RFID).

I. INTRODUCTION

I N RECENT years, a tremendous interest has appeared ondeveloping compact and low cost transponders for modern

commercial wireless systems. That is the case, for instance,of radio frequency identication (RFID) applications, where a“tag” transponder is forced to answer with a modulated signalonce it is interrogated by a “reader” [ 1].

Active printed antennas [ 2] stand as a promising technique inthis eld, as they are able of combining the radiating propertiesof printed antennas with the signal processing capabilities of active devices. A wide variety of solutions may be found in theliterature, mainly in the way of simple backscatters with AM[3], SSB [4], or other modulation formats. Other topologies tryto avoid interference problems, generating the response signalfrequency apart from the interrogation. That is the case of thearchitecture in [ 5], where a wideband antenna is integrated witha subharmonic mixer.

In this letter, a novel frequencydoublingactive antenna is pro-posed. Based on the particular nonlinear behavior of a PHEMTdevice, as well as on the use of a dual-frequency dual-polar-ization slot coupled patch, a local data signal may be used tomodulate the second harmonic of an interrogating carrier in a

BPSK format. In Section II, results of an accurate nonlinear de-vice characterization areemployed to prove thepossibility of as-suring optimum frequency doubling operation with phase con-trol. Then, the integration of the active circuit in an appropriate

Manuscript received September 7, 2004; revised September 28, 2004. Thiswork was supported by the Spanish Ministry of Science and Technology(MCyT) under Projects TIC 2000-0401-P4-09 and TIC 2002-04084-C03-03,and by the European Commission through TARGET Network of Excellence.This work is also a result of the collaborating actions as part of UnidadAsociada CSIC(IFA)-University of Cantabria. The work of J. A. Garcia wassupported by the Ramón y Cajal Program from MCyT.

The authors are with the Department of Communication Engineering, Uni-versity of Cantabria, 39005 Santander, Spain (e-mail: jangel@ieee.org).

Digital Object Identier 10.1109/LAWP.2004.838821

printed radiator is described. Finally, measurement results arepresented to support the validity of the proposed approach.

II. OPTIMUM FET FREQUENCY DOUBLING OPERATIONWITH PHASE CONTROL

Frequency multipliers have been generally employed for thegeneration of high-frequency local oscillator signals. Some ar-chitectures, based on the use of eld effect transistors (FETs),have been proposed during the last years [ 6]. They offer a realalternative to diode or varactor solutions, providing conversion

gain as well as an improved efciency.A good FET multiplier design mainly relies on the proper

selection of the device biasing condition. Different works havedealt with this issue, suggesting some criteria for an optimumoperation in terms of the desired output harmonic. That is thecaseof [ 7], where the use of a piecewise linear transconductance(PLT) approximation was suggested. This simplied approachlost precision only when describing the working conditions forhigh-dc gate to source voltage values or small signal ex-citations. Taking into account that FET devices are mildly non-linear in nature, the inaccuracy of a PLT model in those condi-tions could be understood.

On the other hand, some modeling techniques have been pro-posed when interested in a precise distortion control [ 6], [8].They are based on the experimental extraction of the Taylor se-ries coefcients for the device main nonlinearity, and provideexcellent results as long as the device is kept in small-signalregime. In this section, the use of these coefcients will be gen-eralized to accurately predict the harmonic generation in anyoperating regime of an FET-based circuit.

When working on a certain load condition, the power seriesexpansion of the drain to source current nonlinearity with re-spect to the input voltage Ids(Vin) would result in (1)

(1)

where and are, respectively, the dc and ac componentsofVin, isthe dc component ofIds,while each representsthe th-degree coefcient in the Taylor series expansion,

.In Fig. 1, the experimentally extracted Ids(Vin) and

characteristics for a typical PHEMT device are shown.Under a low-level excitation, the device keeps working in the

neighborhood of the biasing point and a few termsin (1) are enough to describe the nonlinearity. For a large inputsignal, however, the device “moves” to zones quite apart fromthe quiescent point, and an impractical number of terms wouldbe required in the expansion.

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CABRIA et al. : A PHEMT FREQUENCY DOUBLING ACTIVE ANTENNA 311

Fig. 1. I d s ( V ) and G ( V ) characteristics along a 50 load line for atypical NE3210s01 PHEMT from NEC, working with V = 2 V .

Obtaining a general closed form expression for a certain fre-quency component in the current spectrum, as a function of theexcitation and the coef cients in (1), is a complex task. Whenexciting with one tone such expression may be derived.

It is widely known that the coef cients of a Chebyshev poly-nomial expansion for a nonlinearity may be directly relatedto the values of the output harmonics, if the nonlinear ele-ment is excited with a cosine function, [ 9].Using their properties [ 10], the Ids(Vin) Chebyshev coef cientsmay be described using the coef cients of a similar expansionfor any of its higher order derivatives. The phasor representing

the second order harmonic current, , can be then expressedin terms of the phasor representing the fundamental componentof the input voltage, , through the use of the dc, second,and fourth harmonic of the periodic waveform ( , ,and )

(2)

This expression was employed together with the extractedcharacteristic, to study the evolution of the generated

second harmonic current in terms of the dc value and theinput power, for the aforementioned device.

In Fig. 2, the calculated values are shown. It can be observedthat two regions with optimum frequency doubling operationexist, each with opposite phase.

Doublers are usually biased in region A, slightly belowpinch-off, where one-sided waveform clipping occurs. How-ever, a similar power level may also be obtained when applyinga high-dc value (region B). For such a value, the Ids(Vin)characteristic has reached saturation (see Fig. 1) and one-sidedclipping also appears.

Two speci c biasing points were selected in those regions,and . The calculated and mea-

sured conversion characteristics for a 50 lab model are thenshown in Fig. 3. The validity of our approach is con rmed witha minimum error in the prediction. Around ,both points would provide the same harmonic power level with

Fig. 2. Calculated second harmonic power evolution with V and Pin.

Fig. 3. (a) Conversion gain and (b) phase evolution in function of the inputpower for the two optimum bias voltages.

phase opposition. It could be expected, that switching the devicegate voltage with a low frequency data signal from one point toanother, would result in an optimum BPSK modulation of thesecond harmonic of the excitation frequency.

III. A CTIVE ANTENNA DESIGN

The BPSK modulating technique, proposed before, was im-plementedusing active antenna concepts.The printed antenna tobe employed had to be able to receive an interrogating signal andtransmit at double the frequency with a modulated data signal.Radiating structures with dual frequency capability could befound, either using the same or orthogonal polarizations. Takinginto account that the input and output ports of the doubler areconnected to different device terminals, gate and drain, respec-tively, the use of two orthogonal polarizations was preferable.

A rectangular patch [ 11 ], with two ports placed at perpen-dicular sides, was selected. The two operating frequencies 900and 1800 MHz are the resonating frequencies of the and

orthogonal modes. An aperture coupled topology was

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312 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 3, 2004

Fig. 4. Implementation details and schematic of the frequency doublerintegrated in a microstrip antenna. Dimensions are in mm.

preferred, based on previous experience with the integration of this kind of radiator and PHEMT circuit functions [ 12]. In orderto reduce the total size of the structure, it was designed to sit-uate the doubler circuit inside the surface of the patch, as can beobserved in Fig. 4.

The dimensions were rst selected to assure 50 matchingconditions. Some improvement in the conversion gain could beobtained when designing the antenna feeds to assure a better

match to the modulator input and output equivalent circuits. The900 MHz feed matching resulted however in a bandwidth re-duction, due to the high reactive nature of the PHEMT inputcircuit. Regarding the 1800-MHz feed, special attention shouldbe paid to the reactive part of the impedance, as it may reducethe phase shift between the two previously selected states (gatebias voltages). The dimensions in Fig. 4 correspond to the -nally selected 50 design. The active circuit and the feed lineswere printed on ARLON N25 substrate with a dielectric con-stant of 3.38 and thickness of 0.787 mm. The patch was builton an auxiliary layer using the same material, and was placedin an inverted position thanks to the use of nonconducting posts(not shown in the gure). In this way, the substrate below thepatch is air with the advantages in radiation related to its lowpermittivity.

Fig. 5. Scheme of transponder test setup.

IV. M EASUREMENT RESULTS

In order to characterize the integrated active radiating struc-ture, a speci c test setup was implemented in an anechoicchamber, reproducing a sort of RFID architecture (see Fig. 5).

A transceiver was employed, able to send the 900-MHz in-terrogating carrier with a high EIRP, as well as receiving anddemodulating the 1800 MHz BPSK response. To obtain theconversion gain of the tag patch, the Friis transmission formulacould be applied. However, having this antenna no circuitports, direct power measurements were impossible to do. In[5] the authors proposed the use of radar cross-section (RCS)measurement techniques for this kind of structures. They haveintroduced the conversion radar cross-section (CRCS) gurein the case of having not equal incident (interrogation) andscattered (response) frequencies

(3)

In (3), the tag is modeled with the gain of the receivingand the transmitting radiating structure , at 900 and

1800 MHz, respectively, together with the conversion gain of the frequency doubling modulator (Gdob) that connects both.An isotropic conversion gain, , couldbe de ned for the active doubling antenna following a similarapproach to [ 13]. It could be calculated from the receivedresponse power applying the radar equation

(4)

and are the gains of the cross polarized antennas em-ployed in the reader, at 900 and 1800 MHz, respectively.is the power in the RF source, determined by the requirementof having certain level range in the input of the doubler to as-sure an appropriate modulator performance. Using the optimalinput power, following (4), and biasing the transistor with thetwo selected gate voltages, the CRCS patterns were measured.As could be expected, the radiation patterns are quite similar atboth biasing points, seeFig. 6. In the broadsidedirection, a valuefor of 13 dB was obtained .

In order to verify the capability of the transponder antenna, apolar nonreturn-to-zero (NRZ) data signal was also used to biasthe gate of the transistor. In this case, the reader demodulatorwas connected to an oscilloscope to represent the recovered data

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CABRIA et al. : A PHEMT FREQUENCY DOUBLING ACTIVE ANTENNA 313

Fig. 6. Conversion radar cross-sections of the transponder forV =

0 0 : 3 6 V (“ ”) and V = 0 : 1 6 V (“0 ”), 0 dB reference area is0 1 7 d B c m .

Fig. 7. Original (dark line) and demodulated (gray line) data signals.

signal. In Fig. 7, the original and demodulated NRZ data signalsare shown.

Finally, some considerations about reliability are needed.

Working with large excitations, care has to be taken with theexcessive gate current, harmful to the long term reliability of the device. A gate current limiting resistor was added to controlthis issue, also leading to some reduction in the modulatedsignal sensitivity to variations in the device parameters withtime or temperature. Such variations are mainly manifested

as a residual ASK modulation, not critical in this application.The phase errors were kept below 2 .

V. C ONCLUSION

A novel PHEMT frequency doubling scheme with BPSKmodulation capability has been proposed. A dedicated non-linear device characterization, together with a simple analysisusing Chebyshev polynomials, has revealed the existence of twobiasing regions, where the second harmonic could be generatedwith maximum level and phase opposition. A low frequencydata signal applied to the gate terminal was used to create aBPSK response modulated signal, through a proper integrationof the active circuit in a dual-polarization and dual-frequencypatch. A good performance was obtained from the proposedarchitecture, with a 13-dB isotropic conversion gain.

ACKNOWLEDGMENT

The second author would like to thank Prof. J. C. Pedro of the University of Aveiro, Aveiro, Portugal, for his valuable com-ments about the Chebyshev polynomial representation of non-linear systems.

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