2.45 GHz Low Power Rectenna Design for Wireless Sensor & RFID

Applications

Ph.D. Candidate: Yunlei Li

Advisor: Jin Liu

9/10/03

Outline

• Introduction

• Rectifier

• Antenna

• System

• Conclusion

Radio Frequency Spectrum

Frequency Wavelength Band designation

Wireless sensor & RFID applications

30-300 kHz 10-1km LF (low frequency) LF RFID: Passive IC tag

inductive coupling

300-3000 kHz 10-1km MF (medium frequency)

MF RFID: Passive IC tag

inductive coupling

3-30 MHz 100-10m HF (high frequency)

HF RFI D: Passive IC tag

Inductive coupling

(6.78 MHz, 13.56 MHz, 27.125 MHz)

30-300 MHz 10-1m VHF (very high frequency)

Wireless sensor & Active RFID

300-3000 MHz 1m-10cm UHF (ultra high frequency)

Wireless sensor & Active RFID: transceivers

(315, 433, 868, 915, 2450 MHz)

3-30 GHz 10cm-1cm SHF (super high frequency)

5.8 GHz Active RFID

Beamed microwave power transmission

Electromagnetic Power Transmission

• RF Power launched through electromagnetic waves by an antenna

= c/f Near field: the area from the antenna to the point where the

electromagnetic field forms at a distance of D </2 Far field: The area after the point at which the

electromagnetic wave has fully formed and separated from

the antenna at a distance of D </2

RF Power Transmission:near field

• Passive RFID tag• Inductive Coupling

(transformer effect)• Energy in magnetic field

strength• Coil antennae• Reader-> transponder

Power & data• Transponder-> reader Data

back by load modulation

RF Power Transmission:far field

• Friis Transmission Equation

Pr=PtGtGr2/(4R)2

Pr: Receiveded power

Pt:Transimitted power Gt:Transimitter antenna gain

Gr: Receiver antenna gain

R: Transmission distance

10

100

1,000

10,000

100,000

1,000,000

0 5 10 15 20 25 30

Transmission Distance (m)

Re

ce

ive

d p

ow

er

lev

el (

uw

)

Calculated received powerAssuming Gt=20dB, Gr=10dB

Rectenna: RF to DC Conversion• Rectenna element=

Rectifier+Antenna– Frequency reflecting plane

– Dipole or patch antenna

– Microwave low pass filter

– Schottky barrier diode

– Low pass filter passing DC

– Load resistor

• Applications– Wireless power

transmission between space and earth

high power Rectenna array

– Wireless sensor (GAP4S) & long range RFID

Low power Rectenna used to

convert RF power to DC to

charge a battery or big Cap

• Performance Goal-high efficiency

Overall efficiency o=DC output power/incident RF power

>85% (high power & optimized load)

Conversion efficiency c=DC output power/(incident RF power-reflected RF power)

>90% (high power & optimized load)

Rectifier/RF Detector

• Single diode • Voltage doubler

Schottky Diode• Equivalent circuit of a

Schottky diode

Rj=0.026/IT, IT=Is+Ib

Is=diode saturation current, a

function of barrier height

Ib=external applied bias current

Cj=diode junction capacitance

Lp, Cp=Parasitic inductor & Cap

Rs=Parasitic resistance representing losses

Voltage sensitivity of a diode in mV/W

2=0.52/(IT(1+2Cj2RsRj)(1+Rj/RL))

• N-typeLow Rs

External bias (High barrier, low Is)

High flicker noise

• P-typeHigh Rs

Zero bias (low barrier, high Is

Low flicker noise

Low pass filter for better efficiency

Microstrip Patch Antennas

Radiation performance of single layer patch

Microstrip Patch Array

Hybrid-Ring Coupler• Hybrid ring coupler to split powers from the input to two outputs• Power split ratio

• Note: there is an upper limit on line impedance of about 150 for many microstrip transmission lines

)/1(

2log20

122

1

ZZZ

ZP

Gain of Microstrip Patch Array

• The Maximum gain of a microstrip phase 2nx2m arrayGdB=10log(4A/2)-(D1+D2)/2

A=D1*D2

• D1=effective width of the uniformly spaced array

• D2=effective height of the uniformly spaced array =attenuation in dB per unit length of a 50 ohm transmission

line being used in the monolithic feed [A typical value of is 0.4dB/ft for a 50 ohm microstrip line on 1/32th-in (0.794mm) Teflon fiberglass at 2.2 GHz

System Design

• Monolithic Integration of rectenna (antenna array with rectifier) with RF detector

• Impedance matching of patch antenna (or antenna array) to the input of the rectifier using corporate feed network

• Ring coupler to split power from antenna to rectenna and demodulator separately to maintain 8dB power split ratio

• Use single diode rectifier to maximize efficiency of the Rectenna• Use voltage doubler detector to maximize its voltage for better

demodulation• Ring coupler isolate the Rectenna & Detector and allow

separate impedance matching network design

Conclusion

• A 2.4GHz low power Rectenna & detector will be designed and simulated with ADS

• The system will be monolithically integrated onto a single circuit board

• A high gain patch antenna array boosts the power level at the input of the Rectenna for better power conversion efficiency

• A hybrid ring coupler is used to divide the power between Rectenna and detector

• The system implements a key RF front end for GAP4S wireless sensor system