Inductively coupled UHF RFID transponder for implanted medical devices 1.

Inductively coupled UHF RFID transponder

for implanted medical devices

1

광주 과기원

발표자 김광순

2

Contents Motivation Introduction Circuit Description DC supplying circuits Charging mode Communication mode

Power transfer method Electro Magnetic wave (antenna to antenna) Inductive coupling (inductor to inductor)

Comparison of Conventional Circuits Conclusion / Reference

Motivation

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Conventional RFID transponder• The conventional UHF RFID transponder size is too large(because of antenna)• The radiation of a antenna is injurious to a body.• Without the reader (external device), the transponder can’t be operated

Implanted medical device• Because the device is implanted in the body, the battery is necessary to operate the implanted medical device• The implanted device requires maintenance of the battery which requires regular surgery

Solution• If the inductor antenna is used at the UHF, the transponder size can be small. • Because the transponder battery is charged by the incident RF power , it can prevent regular surgery in the implanted medical device• The effect on a body is lower than EM wave method since the inductor antenna is operated by magnetic coupling.

Introduction

4

Fig.1 Total Block Diagram

Transponder Composition•External antenna

•Matching network

•Rectifier/Regulator/Switch

•Battery charging circuit

•Demodulator/Modulator/Oscillator

Reader Composition• Mixer, Power Amp

• Transmitter Antenna

• Receiver antenna

Integrated chip

Introduction

5

Test board•COB(chip on a board) packaging

Process / Layout• Samsung 0.18um process (MPW)

• Dimension: 810um X 400um

• A: rectifier B: regulator C: switch

• D: charging circuit, opamp, comparator

• E: oscillator F: demodulator G: modulator810um

400um A

B

C

D E

F

G

Fig.2 Layout Fig.3 Test board

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SPECIFICATION

Reader( mixer,

power amp )

Frequency 900 MHz (Carrier)

ASK data rate 160 kbps

Modulation depth 90 %

Duty cycle 70 %

RF power 0.5 W (27 dBm)

Transponder(integrated chip)

VDD 1.5 V 1.54 V

Charging Current 260 uA 260 uA

Charging Voltage 1.5 V 1.466 V

Modulator data rate 320 Kbps 320 Kbps

Table.1. Specification

Introduction

Circuit description (DC supplying)

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Rectifier• To convert the sinusoidal to DC power

• Simple structure

• Storage cap :

- C4 for prevent the voltage ripple

• Rectifier output :

- for DC supplying

• Demodulator inputFig.4 Rectifier Circuit

Fig.5 Rectifier operation theory

Voltage ripple


8

Regulator• To supply the constant voltage

• Power consumption: 23uW

• Regulated output voltage : 1.54V

Fig.6 Regulator circuit Fig.7 Gain and Phase of a regulator

5_

4 5regulaotr output ref

RV V

R R=

+50

1.5 1.0250 23

kV V

k k=

+

Phase margin 50 Kohm

23 Kohm1 V

•Gain : 3.3 dB

•Unit gain frequency : 35 MHz

•Phase margin : 78o ( stable )

-------- a

------ b


9

Regulator• Limiter : To prevent the damage of circuits at the high power

• Voltage reference : To supply constant reference voltage to a opamp

Fig.8 Limiter and Voltage reference Fig.9 Measured regulator output

1.72 V

1V

Vth 0.43 V

Vth 0.43 V

Vth 0.43 V

Vth 0.43 V

+

+

+1.5V

1.54VX

Y


10

Fig.10 Switch

Fig.12 Communication mode

Switch• To change a charging mode and a communication mode

Fig.11 Charging mode

0 1

Circuit description (charging mode)

11

Charging circuits• Power consumption: 69uW (charging circuit, opamp(A), comparator(B))

• Charging circuit conversion efficiency

: ( Ibattery/Iinput ) * 100 =

(195 uA / 241 uA ) *100 = 81 %

at VDD = 1.5V

Fig.14 Charging circuitFig.13 Block diagram for a charging circuit

1.1*I

I

100*I

195 uA

1.1*I

2.1*I

241 uA

1.5V

offon

------ c

z

Circuit description (charging mode)

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Charging circuit• Direct RF input power : 9.2 dBm (8.32mW, 900 MHz (CW))

• Constant current: 260uA , Charging Voltage :1.466 V

• Rechargeable battery for the measurement : 3.96 mF capacitor

• EOC(end of charging) voltage: 1.447V

• Total charging efficiency:

=( 0.39mW /8.32mW ) * 100

= 4.68 %

Charging power = 260uA * 1.5V

Fig.15 Measured charging profile

charging power

RF input power* 100 ------ d

Constant current region

28 s

1.466V

Circuit description (communication mode)

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Reader to Transponder (Downlink)

• Carrier frequency : 900 MHz

• Data rate:160 Kbps

• Modulation depth : 90 %

• Duty cycle : 70 %

• ASK modulation

• Digital encoding: Return to 1

- In order to transmit continuous

power to a transmitter

Fig.16 Return to 1 encoding Fig.17 Manchester encoding

Transponder to Reader (Uplink)

• Data rate:320 Kbps

• Duty cycle : 50 %

• Backscattering modulation

- EM wave method

• Load modulation

- Inductive coupling method

• Digital encoding: Manchester coding


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Demodulator• Minimum input power to operate a demodulator : 1.67dBm

• Low pass filter

- To terminate the high frequency component

• Power consumption: 54 uW

• Hysteresis comparator :

- Good for noise immunity

Fig.18 Demodulator input and averaging signal Fig.19 Demodulator circuit


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Backscattering modulator (EM wave method)• To transmit the data to a reader

• The change of a reflected power (backscattering modulation)

• The modulation due to the changed matching network

Fig.20 Backscattering modulation Fig.21 Measurement setup for backscattering modulation


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Load modulator (inductive coupling)• To transmit the data to a reader

• The change of a coupling coefficient

• The modulation due to the changed matching network

Fig.22 Load modulation Fig.23 Measurement setup for a load modulation


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Ring oscillator• 5-stage inverter ring oscillator

• Oscillation frequency: 210KHz

• To supply the clock

• Data rate of load modulator

• Power consumption: 63uW

Fig.24 Ring oscillator Fig.25 Measured oscillator output

Power transfer method

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Fig.26 Total Block Diagram

Power transfer method• Electromagnetic wave – antenna to antenna ( yagi-uda – dipole )

• Inductive coupling – inductor to inductor (1turn inductor – 2 turn inductor)

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Fig.27 EM wave experiment setup

Electro-magnetic wave • Distance of a reader and a transponder : 1m

• Transmitter antenna power : 27 dBm

Power transfer method (EM wave)

Transmitter antennaReceiver antenna

Transponder antenna

20

Fig.28 Yagi antenna

Yagi antenna (Reader)• 10 dB bandwidth : 170 MHz

• VSWR : 1.06 at 900 MHz

• Gain : 5.83 dBi

• 50 ohm Source

• Directional antenna

• Simulator : CST

Fig.29 Yagi antenna

Fig.30 Return loss


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Fig.31 Dipole antenna

Dipole antenna (transponder)• 10 dB bandwidth : 140 MHz

• VSWR : 1.16 at 900 MHz

• Gain : 1.88 dBi

• 50 ohm Source

• Simulator : CST

Fig.33 Return loss


Fig.32 Dipole antenna


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Measured charging mode• Reader output power : 27 dBm (900 MHz (CW))

• Constant current: 254uA , Charging Voltage :1.478 V – distance 0.8m


• EOC(end of charging) voltage: 1.445 V

Fig.35 Constant current for different distance

1.478V

27 s


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Measured demodulator output signal• Reader output power : 27 dBm

• The distance for measurement : 1 m

• Modulation : ASK (160 Kbps data rate)

• Encoding : return to 1

• Maximum comm. Distance : 1.8 m

Fig.37 Demodulator output data stream for arbitrary input data

01 0 1 1 0 0 0 1 0 01 1 10 1

01 0 1 1 0 0 0 1 0 01 1 10 1

Return to 1 encoding

Demodulator output

A

B

Fig.36 Block diagram for a communication

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Measured backscattering modulator• Input data rate : 320Kbps

• Encoding : Manchester encoding

• Difference of high and low (Fig.39 B):

5 mV

0 10 1 00 11 0 1 1 0 0 1 1 0

0 10 1 00 11 0 1 1 0 0 1 1 0

Manchester encoded data


Fig.39 Modulated signal by a transponder

Receiver antenna signal

A

B


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Fig.40 Inductive coupling measurement setup

Inductive coupling• Distance of a Reader and a transponder : 4 mm

• Power amp output power : 27 dBm

Power transfer method (inductive coupling)

Transmitter inductor

Receiver inductor

TransponderinductorTest board

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Fig.42 transmitter inductor smith chart

Transmitter inductor• SRF(self resonance frequency) : 1.3 GHz

• Inductance : 56 nH at 900 MHz

• Q Value : 90 at 900 MHz

Impedance matching• For maximum power transfer(50 ohm feed)

Fig.41 Reader inductor matching / Reader inductor Fig.43 Return loss

-29dB


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Fig.44 transponder inductor /conjugate matching network

Transponder inductor• SRF(self resonance frequency) : 1.39 GHz

• Inductance :32nH at 900 MHz

• Q Value : 83 at 900 MHz

• Inductor dimension : 6 mm X 5 mm

Impedance matching• Conjugate matching

Fig.46 Return loss

Fig.45 Transponder inductor smith chart



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Measured charging mode• Reader output power : 27 dBm (900 MHz (CW))

• Constant current: 268uA , Charging Voltage :1.457 V – distance 6.5mm


• EOC(end of charging) voltage: 1.44 V

Fig.48 Constant current for different distance

1.457V

26 s


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Measured demodulator signal• Reader output power : 27 dBm

• Modulation : ASK (amplitude shift keying)


• The distance for measurement : 4 mm

• Maximum comm. Distance : 6.5 mm

Fig.50 Demodulator output data stream for arbitrary input data

1 00 0 11 11 0 1 0 1 0 0 1 0

1 00 0 11 11 0 1 0 1 0 0 1 0

Return to 1 encoding

Demodulator output


A

B


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Measured load modulator• Input data rate : 320Kbps

• Manchester encoding

• The difference of high level and low level

: 4 mV

Fig.52 modulated signal by a transponder

00 1 00 01 1 0 0 1 0 1 1 1

00 1 00 01 1 0 0 1 0 1 1 1

Manchester encoding

Receiver inductor signal


A

B

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Table.2. Comparisons of conventional circuits

This work Conventional [1]IEEE 2007

This work Conventional[2]

JSSC 2006

Power transfer method

Inductive coupling EM wave

Frequency 900 MHz 4 MHz 900 MHz 950MHz

Operating VDD 1.54 4.1 1.54 1.5

Charging voltage, current

1.457 V260 uA

4.1 V1.5 mA

1.478 V260 uA

x

Charging efficiency

4.48 % 73 % 4.48 % x

communication uplink,downlink

x uplink,downlink

uplink,downlink

Maximum comm. distance

6.5 mm x 1.8 m 10 m

Antenna dimension

5mm X 6 mm-PCB (FR4)

9mm (diameter)-Coil with ferrite core

Dipole antenna- 160 mm

Dipole antenna- 160 mm

Comparisons of conventional circuits

Conclusion

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Implanted medical device - Because the inductor antenna is used at the UHF, the transponder size can be small.

(inductor dimension : 5mm X 6 mm)

- The transponder battery is charged by the incident RF power , it can prevent

regular surgery in the implanted medical device.

(Charging voltage =1.5V , charging current =260uA,

power consumption of charging mode = 92 uW)

- Due to the wireless communication, the reader receives the information from a body easily.

(Communication mode power consumption = 160 uW, communication distance = 6.5 mm)

- The effect on a body is lower than EM wave method since the inductor antenna is operated by magnetic coupling. (Inductive coupling)

Reference

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[1] Toshiyuki Umeda, “A 950-MHz Rectifier Circuit for Sensor Network Tags With 10-m Distance”, IEEE J. Solid-State Circuits, vol. 41, No. 1, pp.35-41, Jan. 2006 .

[2] Pengfei Li, “A Wireless power Interface for Rechargeable Battery Operated Medical Implants”, IEEE Transactions on Circuits and Systems II: Express Briefs 54 (10), pp. 912-916 , Oct. 2007.

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Q & A

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Rectifier• Rectifier sensitivity

• Rectifier output voltage for the frequency

• Rectifier efficiency : 5.58 % (Load 10Kohm, Input power 3dBm)

Fig.1 Measured rectifier sensitivity Fig.2 rectifier output voltage for different frequency

load 10Kohm

3 dBm

1.05 V

APPENDIX 1

APPENDIX 2

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Fig.3 Charging Profile

Battery Charging Profile• Constant Current (fast charging)

• Constant Voltage (slow charging)

• End of Charging

Silver Zinc rechargeable battery• High Energy Density

- 40% more than lithium-ion

- Increase run time

- Smaller size

• Nominal Voltage = 1.5V

• Safety (No lithium)

37

Characteristic of charging circuit• Direct RF input power

• The variety of Charging current :

Due to the varying incident power

• Minimum charging current : 180 uA

Fig.16 Measured Charging current for different input power

APPENDIX3

Fig.17 Measured EOC voltage for different input power

APPENDIX 4

38Fig.4 Regulator schematic

Regulator schematic

APPENDIX 5

39Fig.5 Demodulator schematic

Demodulator schematic

APPENDIX 6

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Fig.7 Modulated input signal

Fig.6 Demodulator input and Averaging signal Fig.8 Demodulator output

Simulation result• Communication mode

APPENDIX 7

41

Demodulator• Direct input power : 6 dBm

• Modulation : ASK (amplitude shift keying)


• Arbitrary transmitter data stream

• Demodulator output data stream

• Sensitivity : 1.67 dBm

Fig.23 Demodulator output data stream for arbitrary input data stream

01 0 1 1 0 11 0 0 0 1 1 0 1 0

01 0 1 1 0 11 0 0 0 1 1 0 1 0


APPENDIX 8

42Fig.9 Smith chart for a conjugate matching

Conjugate matching network

Fig.10 Matching network

43

Fig.11 Available power measurement method

Available power• Measurement of Rectifier output voltage

- Load 10 Kohm

• Antenna input power: 27 dBm (500 mW)

• Available power

: 625uW at the distance 1.05 m

Fig.12 Available power for different distance

APPENDIX 9

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Inductive coupling• Measurement of rectifier output voltage

- Load 10 Kohm

• Reader output power: 27 dBm

• 19mm X 19mm at the distance 9.4 mm

- Area of the voltage variance 0.625 mW ~ 0.225 mW

Fig.14 Available power for different distanceFig.13 Available power measurement method

APPENDIX 10

APPENDIX 11

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Modulator spectrum for arbitrary input data with Manchester encoding

Fig.15 Backscattering modulator spectrum for arbitrary input data (Manchester encoding)

Fig.16 Load modulator spectrum for arbitrary input data (Manchester encoding)

APPENDIX 12

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Fig.17 Backscattering modulator spectrum for 320 KHz NRZ encoding

Fig.18 Load modulator spectrum for arbitrary 320 KHz NRZ encoding

Modulator spectrum for 320 KHz with NRZ encoding

Inductively coupled UHF RFID transponder for implanted medical devices 1.

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