IEEE SENSORS JOURNAL, VOL. 18, NO. 11, JUNE 1, 2018 4503

An X-Band Doppler Motion SensorWith a Two-Ports Oscillator

Young-Gi Kim, Hyoun-Kyou Dam, Jin-Woo Lee, Jung-Hyung Bae, Jae-Yeon Hwang, Hyun-Jin Kim,Patrick Roblin, Senior Member, IEEE, and Chang-Woo Kim, Member, IEEE

Abstract— In this paper, we propose an X-band Dopplermotion sensor based on a high-efficiency dual-output oscillatorcircuit integrated with a common resonator and a passive mixer.The oscillator provides two output signals with different powerlevels. The oscillator and mixer circuits are implemented ina 0.5 µm AlGaAs/InGaAs depletion-mode pseudomorphic highelectronic mobility transistor process. The oscillator shows a highoutput power of 21.15 dBm at 11.2 GHz and a peak efficiencyof 41.1 % at 10.7 GHz. For a 10.3 GHz oscillation, a phase noiseof −132 dBc/Hz is obtained at a 10 MHz offset. The outputpower centered at 10.58 GHz is 11.67 dBm at one port and9.12 dBm at the other port, and a phase noise of −105.8 dBc/Hzat 1 MHz offset when providing a 2.55 V supply with 28 mAconsumption for practical Doppler motion sensor applicationthat detects human motion at a distance of 13 m. A detectionrange of 30 m has been proven for human motion. This paperdemonstrates the feasibility of a compact high efficiency Dopplermotion sensor without a bulky power divider or amplifier bymonolithic combining the proposed oscillator and mixer circuit.

Index Terms— Doppler radar, sensor systems, microwave singleintegrated circuits (MMICs).

I. INTRODUCTION

DOPPLER theory detecting the frequency shift in areflected wave signal is applied to detect tiny body

movements without any sensors attached to the body. Basedon the Doppler theory, microwave Doppler radars monitoringof biomedical sign signals, such as respiratory, cardiac, arterialmovements, and sleeping status have been demonstrated withcommercially available devices since the late 1980s [1]–[3].However, most of these radar systems are bulky, consume sig-nificant power and are not suitable for portable systems. Today,

Manuscript received November 3, 2017; revised December 25, 2017 andFebruary 20, 2018; accepted March 22, 2018. Date of publicationApril 2, 2018; date of current version May 9, 2018. The work ofC.-W. Kim was supported by the Basic Science Research Program throughthe National Research Foundation of Korea funded by the Ministry ofEducation under Grant NRF-2015R1D1A1 A02061041. The associate editorcoordinating the review of this paper and approving it for publication wasDr. Piotr J. Samczynski. (Corresponding author: Young-Gi Kim.)

Y.-G. Kim is with the Department of Information and Commu-nication, Anyang University, Anyang 430-714, South Korea (e-mail:kimyg@anyang.ac).

H.-K. Dam is with Wistek, Gyeonggi-do 435-845, South Korea.J.-W. Lee, J.-H. Bae, and H.-J. Kim are with the RFDNC Co., Ltd., Seoul,

South Korea (e-mail: svengalideck@naver.com; mmicjhbae@gmail.com;hjkim@rfdnc.com).

J.-Y. Hwang is with Microinfinity Company, Ltd., Suwon 443-270, SouthKorea.

P. Roblin is with the Department of Electrical and Computer Engi-neering, Ohio State University, Columbus, OH 43210 USA (e-mail:roblin.1@osu.edu).

C.-W. Kim is with the Department of Electronic Engineering, Kyung HeeUniversity, Seoul 17104, South Korea (e-mail: cwkim@khu.ac.kr).

Digital Object Identifier 10.1109/JSEN.2018.2821762

especially in space-constrained portable battery-operated radarsystems, there is a growing demand for circuits that meetlow power and low chip area requirements. In some systems,circuit space and power savings can be important design goalsrather than very high performance unless performance loss isexcessive

There has been much research and development ofmicrowave single integrated circuits (MMICs) based onCMOS technology for Doppler sensors as a preparation forincreasing demands [4], [5]. The design approach of con-ventional Doppler RF transceivers requires multiple RF sub-blocks such as power dividers, phase shifters, and baluns.However, these sub-blocks contain large size and significantinsertion loss to supply RF signals to the Tx and Rx blocks

This paper presents a pseudo-high electron mobility tran-sistor (PHEMT) integrated Doppler sensor composed of anoscillator with two output ports and a passive mixer. One out-put port of the oscillator is connected to the transmit antennaand the other port is connected to the local oscillator (LO)port of the mixer to reduce the strain on the die area and bulkpower splitter or balun.

The sensor IC was fabricated in a 0.5 μm AlGaAs/InGaAsdepletion-mode PHEMT process. The overall performance ofthe sensor will be described.

II. DOPPLER SENSORS

Fig. 1 shows a simplified schematic explanation for theprinciple of a Doppler sensor. The equation for Dopplerfrequency shift, fd , of object moving with speed of ν withrespect to a stationary transmitter is

fd = fν

c(cos θin + cos θout) (1)

where f is the transmitted signal, θin is the angle betweenthe moving object velocity and the incoming wave, θout

is the angle between the moving object velocity and theoutgoing wave, and c is the speed of the wave. Typically θin

is approximately equal to θout for mono-static measurementand (1) reduces to [6]:

fd = 2 fν

c(cos θin) (2)

Fig. 2 depicts a comparison of the conventional Dopplersensor and presented Doppler sensor. In the conventionalDoppler sensors power splitters, baluns or phase shiftersare used to provide high frequency signal generated froma one output port oscillator to both a transmitting antennaas a RF signal and down converting mixer as a LO signal.

1558-1748 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

4504 IEEE SENSORS JOURNAL, VOL. 18, NO. 11, JUNE 1, 2018

Fig. 1. Schematic explanation for the principle of Doppler sensor.

Fig. 2. Comparison of Doppler sensor topology. (a) Presented Doppler sensor.(b) Conventional Doppler sensor.

The circuitry of splitters, baluns or phase shifters occupy a lotof die area since they need transmission lines of quarter wavelength or matching networks, which are big burdens againstintegration of the circuits. We present a compact sensor byremoving the power splitter using a two ports oscillator shownin Fig. 2 (a) as compared with the conventional circuit shownin Fig. 2 (b).

A previous Doppler sensor chip driven by an oscillatorhaving one output port has three buffer amplifiers and occupiesa large chip area [4].

System-on-chip based Doppler sensors in [7] and [8] useda bulky power divider to drive the mixer and send RF signalsto the transmit antenna.

Differential oscillators or push-push oscillators can be usedwithout a power splitter. However, the output is not sufficientto pump the mixer. Therefore, a buffer amplifier or a poweramplifier circuit is required [9], [10].

This paper presents a two output port oscillator circuit shar-ing a resonator to provide RF signal power to the transmittingantenna at one port and to drive the mixer from the other

Fig. 3. Two output-port oscillator with a common resonator.

port without any power splitter in the Doppler motion sensorapplication.

III. CIRCUIT DESIGN

Two negative resistance generating circuits are connected toa common resonator for the proposed two port oscillator asshown in Fig.3.

The resonator consists of a planar inductor Lr and a parasiticcapacitor Cr. The gate feedback inductors Lg1 and Lg2 driveQ1 and Q2 to induce the negative resistance. The sourceresistors Rb1 and Rb2 provide self-biasing and temperaturestabilization. Cb1 and Cb2 are by-pass capacitors. L1 and L2are matching inductors. Rl1 and Rl2 are load resistances. Thetotal large signal admittance exhibited by the negative resis-tance circuits and resonator is zero for steady-state oscillationsaccording to following balance condition

Gresonator = − (Gnegat ive1 + Gnegat ive2

)(3)

Bresonator = − (Bnegat ive1 + Bnegat ive2

)(4)

where Gresonator , Gnegat ive1, and Gnegat ive2 represent thelarge-signal conductances of the resonator, negative resistancegenerating circuit 1, and negative resistance generating circuit2, respectively. Bresonator , Bnegat ive1, and Bnegat ive2 representthe large-signal susceptances of the resonator, the negativeresistance generating circuit 1, and the negative resistancegenerating circuit 2, respectively.

The oscillation power is shared with the two active devicesin the negative-resistance generating circuits. Therefore thetotal oscillator output power obtained is larger than is possiblefrom an oscillator with a single active device without inflictingany damage to the active devices. The negative resistancepart is connected with reduced resistance by parallel loadresistances and active devices. Real part of the resonatorimpedance can be kept as low as 1.6 ohm for a high Q and ahigh efficiency [11]. Each active device is optimized to have4 fingered 75 um gate under the condition (3) and (4) for ahigh efficiency and a high power oscillation.

The output signal frequencies at each port are locked,regardless of the circuit parameter differences, i.e. impedancesof loads, transistor sizes, and micro-strip lines geometry,between the negative-resistance generating circuits.

KIM et al.: X-BAND DOPPLER MOTION SENSOR WITH A TWO-PORTS OSCILLATOR 4505

Fig. 4. Passive resistive mixer.

Fig. 5. Microphotograph of the oscillator and mixer.

The circuit layout was designed so as to provide a minimumdistance between the active devices and to have a ground viaand the resonator shared by both of the negative resistancegenerating circuits to minimize the phase difference.

Different power levels from each output port were synthe-sized with a careful layout assisted by EM simulations. Theoutput power difference between the ports is caused by asym-metry of layout and bond wire length. All the interferencesfrom each part are included in the repeated EM simulationsand layout modifications.

Fig.4 depicts a simplified circuit of the designed mixer. TwoHEMTs with 10 um gate and one HEMT with two fingered25 um gates are connected in parallel for the passive resistivemixer. A shunt 803 ohm resistor is connected to the gate toavoid gate floating. Drain, source and gate are connected toIF, RF and LO respectively

IV. EXPERIMENTAL RESULTS

The proposed monolithic oscillator and a passive resistivemixer circuits based on a 0.5 μm AlGaAs/InGaAs HEMTprocess are fabricated on the same die. Fig. 5 shows thefabricated die, where the circuits inside the dotted polygonspresent the oscillator and mixer. The chip sizes are 0.26 mm2

for the oscillator and 0.09 mm2 for the mixer.Fig. 6 shows the output spectrum at one of the output

port in the circuit. The oscillator provides a maximum powerof a 21.15 dBm at 9.16 V supply with 47 mA currentwhen accounting for a connector loss of 0.12 dB. The centerfrequency is 11.24 GHz with a free running phase noiseof 118 dBc/Hz at 10 MHz offset . The second harmonic issuppressed by 33.12 dB. The other port shows an output powerof 18.31 dBm. When the oscillator is locked by a phase lockingloop (PLL) and the supply voltage drops to 2.45 V for low-voltage phase performance, as shown in Figure 7, phase noiseimproves to −132 dBc/Hz at 10 MHz offset.

The measured and simulated oscillation output powers, effi-ciencies, 2nd harmonic suppressions, frequencies, and phasenoises with supply voltages are plotted in Fig. 8. The lowersupply voltage forces the output power level, second harmonic

Fig. 6. Measured output spectrum from one output port.

Fig. 7. Phase noise with PLL locking in low voltage measurement.

Fig. 8. Measured and simulated(∗) oscillator characteristics.

suppression and oscillation frequency shift to lower values.The oscillator exhibits a high efficiency of 41.14 % for a 3 Vsupply. The total output power is 15.53 dBm with a phasenoise of −106.8 dBc at 1 MHz offset and −22.55 dB 2nd

harmonic suppression at this point.Table I gives a summary of the most significant measured

results compared with other high power Si or GaAs based

4506 IEEE SENSORS JOURNAL, VOL. 18, NO. 11, JUNE 1, 2018

Fig. 9. Measured RF and LO return losses of mixer.

Fig. 10. Measured conversion gains versus applied LO powers with IFfrequency variation of the mixer.

oscillators that have been reported. The proposed oscilla-tor exhibits the highest power and peak efficiency in sili-con or GaAs processes above the X frequency band comparedto the previously reported results.

The supply voltage is reduced to 2.55 V with 28 mA forpractical Doppler motion sensor applications subject to regula-tion. Then the oscillator provides output powers of 11.67 dBmat one port and 9.12 dBm at the other port. The centerfrequency is 10.58 GHz with a phase noise of −105.8 dBc/Hzat 1 MHz offset.

The return losses of RF and LO ports for the mixer aremeasured and plotted in Fig. 9. The return losses of both portsare maintained less than −10 dB from 10.4 GHz to 10.7 GHz.

The conversion gains for various LO powers are plottedin Fig. 10. The converted If frequencies are changed from4 KHz to 500 KHz in the figure. The optimum LO powerfor the highest conversion gain is 4 dBm for 4 kHz of Iffrequency. The optimum power increases as the IF frequencyis changed to higher frequency up to 100 kHz. The conversiongain reaches maximum value of −10.87 dB when LO poweris 9 dBm with 100 kHz of the IF frequency.

The passive resistive mixer with a −11.8 dB conversiongain and a 2.3 dBm 1 dB compression point on the same

Fig. 11. Measuring setup to assess the Doppler motion sensor.

Fig. 12. Photo of the Doppler motion sensor RF transceiver.

Fig. 13. Signal converted from a moving motion close to the Doppler motionsensor.

die, a planar transmitting antenna, a receiving antenna withan antenna gain of 5.1 dBi, an active low-pass filter with an85.7 dB gain and a 96 Hz unity-gain frequency are designedand connected to the oscillator for Doppler motion sensor testas shown in Fig. 11. The transmitting antenna is connectedto the high power output port and the LO port of the mixeris internally connected to the other port of the oscillator.A photograph of the RF transceiver with the transmitter andreceiver antennas of the sensor is shown in Fig. 12.

Fig. 13 shows the resulting down-converted wave stimulatedby very slow moving motions of a metal plate located 0.5 mto 1 m away from the antenna. In this figure the low frequencypulse corresponds to a slower motion and the high frequencyto a faster motion. The signal converted from a very slowhuman motion 13m away from the sensor can be detectedwith a current of 28 mA at 2.55 V supply voltage.

KIM et al.: X-BAND DOPPLER MOTION SENSOR WITH A TWO-PORTS OSCILLATOR 4507

TABLE I

PERFORMANCE COMPARISON OF REPORTED SILICON OR GaAs BASED INTEGRATED POWER OSCILLATORS

Fig. 14. Time domain waveforms in 100 ms reading converted at 30 m dis-tance from the Doppler motion sensor (a) with human motion and (b) withoutmotion.

The maximum detection range extends to 30 m by changingthe supply to 5 V at 35 mA. Fig. 14 (a) shows the signalwith a human motion from a 30 m distance. Fig. 14 (b)shows the signal without any motion at the same distance. Theactive low pass filter shows peak gain at a few Hz. The peakoccurs when the Doppler shifted frequency, converted fromthe moving velocity, is matched to the filter’s peak frequency.Comparing the waveform to the waveform in (b), we can seethat there are several smaller pulses in addition to the peakpulse in Figure 14 (a). The peak corresponds to the matchedfrequency and the small peaks match weakly.

V. CONCLUSION

This paper proposes a two-port oscillator based on a com-mon resonator and demonstrates the feasibility of an X-bandDoppler motion sensor composed of this oscillator. The pro-posed oscillator provides different high power RF outputs attwo output ports.

The proposed oscillator exhibits the best power and peakefficiency among all the reported integrated oscillators insilicon or GaAs based integrated oscillators for frequencieshigher than X band.

The proposed circuit simplifies the transmitter by elim-inating the disadvantages of bulky power dividers, lownoise amplifiers or power amplifiers in conventional Dopplerdetectors.

VI. ACKNOWLEDGMENT

The authors would like to acknowledge the contributions ofDr. Jae-Jin Lee and Min–Gun Kim in the development of thelow pass filter, and are grateful to the IC Design EducationCenter for providing access to the CAD tools.

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Young-Gi Kim was born in Seoul, South Korea. Hereceived the B.S. and M.S. degrees in electronicsengineering from Hanyang University, in 1983 and1984, respectively, and the Ph.D. degree from theUniversity of Texas at Arlington in 1993.

From 1986 to 1997, he was with Korea TelecomResearch Laboratory, where he was engaged in longdistance optical fiber communication and monolithicmicrowave integrated circuits for wireless applica-tion. In 1996, he moved to Anyang University, wherehe is currently a Professor with the Department

of Data Communication Engineering. He was a Visiting Scholar with theDepartment of Electrical and Computer Engineering, Ohio State University,Columbus, OH, USA. His research interests include monolithic microwaveintegrated circuits and sensors.

Hyoun-Kyou Dam was born in Seoul, South Korea.He received the B.S. and M.S. degrees in electronicsengineering from Anyang University, in 2013 and2015, respectively.

Since joining Wistek in 2016, he has been workingon synthetic aperture radar, signal seeker, LNA,block up converter, and beacon receiver for militaryand satellite applications.

Jin-Woo Lee was born in Changwon, South Korea.He received the B.S. degree in information andcommunication engineering from Anyang Universityin 2017.

Since joining RFDNC Co., Ltd., in 2017, he hasbeen working on application engineering related toRF sensors and RF measurements using program-ming and microcontrollers.

Jung-Hyung Bae was born in Seoul, South Korea.He received the B.S. and M.S. degrees in infor-mation, communication engineering from AnyangUniversity, in 2002 and 2004, respectively.

He is an RFIC/MMIC design engineer with exper-tise in product development, wireless communi-cation systems, microwave sensors and hardware.He has been a key player in MMIC and microwavesensor network systems for the past 15 years at aprivate microwave components companies. He hasextensive technical reports and product documenta-

tion experience. In 2014, he moved to RFDNC and worked as a Vice President.

Jae-Yeon Hwang was born in Seoul, South Korea.He received the B.S. and M.S. degrees in infor-mation, communication engineering from AnyangUniversity, in 2012 and 2014, respectively.

Since joining Microinfinity Co., Ltd., in 2014, hehas been working on a RF front end in navigationsystem in MEMS-based tactical-grade inertial mea-surement unit for an inertial navigation system forsmart guided missiles.

Hyun-Jin Kim received the B.S., M.S., and Ph.D.degrees in electronics engineering from KwangwoonUniversity, Seoul, South Korea, in 2000, 2002, and2007, respectively.

Since creation of Radio Frequency Design andCreative Ltd. (RFDNC), Seoul, South Korea,in 2013, he has been President of the company. Hisinterests include monolithic microwave integratedcircuits and radar systems.

Patrick Roblin (M’85–SM’14) received theMaitrise de Physics degree from Louis PasteurUniversity, Strasbourg, France, in 1980, and theM.S. and D.Sc. degrees in electrical engineeringfrom Washington University, St. Louis, MO, USA,in 1982 and 1984, respectively.

In 1984, he joined the Department of ElectricalEngineering, Ohio State University (OSU),Columbus, OH, USA, as an Assistant Professor,where he is currently a Professor. He is thelead author of the two textbooks High-Speed

Heterostructure and Devices (Cambridge Univ. Press, 2002) and NonlinearRF Circuits and Nonlinear Vector Network Analyzers (Cambridge Univ.Press, 2011). He is the Founder of the Non-Linear RF Research Laboratory,OSU. He has developed two educational RF/microwave laboratories andassociated curriculum for training both undergraduate and graduate students.His current research interests include the measurement, modeling, design,and linearization of nonlinear RF devices and circuits such as oscillators,mixers, and power amplifiers.

Chang-Woo Kim (M’17) was born in Seoul, SouthKorea, in 1961. He received the B.S. and M.S.degrees in electronic engineering from HanyangUniversity, Seoul, in 1984 and 1986, respectively,and the Ph.D. degree in electronic engineering fromShizuoka University, Hamamatsu, Japan, in 1992.

From 1992 to 1996, he was with the Microelec-tronics Laboratories, NEC Corporation, Tsukuba,Japan, where he worked on the high-frequency andhigh-power heterojunction devices and ICs used formobile and satellite communication applications.

Since 1996, he has been with the Department of Electronic Engineering,Kyung Hee University, Gyeonggi-do, South Korea, where he is a Professor.From 2004 to 2005, he was a Visiting Professor of the Radio CommunicationLaboratory, University of Cincinnati, OH, USA. His research interests includemicrowave/mm-wave solid-state device modeling, MCIC and MMIC design,and RF characterization. Dr. Kim is a member of IEEK, KIEE, and KEES.