10/02/2010

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ALMA MATER STUDIORUM – UNIVERSITA’ DI BOLOGNA

Antennas and circuits for RF energy scavenging

Prof. Alessandra Costanzo (Alessandra.Costanzo@unibo.it)

Co-authors: Francesco Donzelli, Diego Masotti, Matteo Fabiani and Aldo Romani

Doctorate course on:Devices, circuits and systems for energy efficiency

Bologn a, 5 febbraio 2010

Ultra-low energy requirements

Pervasive ICT systems needs “stand-alone”operations, absence of battery maintainancesuch as:

•Ambiental sensors•Wearable devices•Implantable medical systems•Solutions required for “energy autonomoussystems”•“RF energy harvesting/scavenging”•“RFwireless power transmission”

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Energy-Harvesting

Environment as a source of highly-available

low-density energy

Mechanical

Thermal

Electromagnetic

Solar

etc.

em radiation

thermal gradientssolar radiation

mechanical vibrations

Why Harvesting Energy?

Growing integration of electronics into human lives

and environments Paradigm of pervasivity

Micro-/Nano-electronics

allow for ultra-low power

designs

Sustainability and

energy autonomy

Longer-lifes

Avoid bulky batteries

Unprecedented applications

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Electronic systems with minumum active cycle

Extended Ultra-Low-Power standby modeMinimum active duty cycleInterrupt driven performance on-demand

RF energy harvesting: subjects described

• General block description of a “RECTENNA”

(RECTifiying antENNA)

• Problems to be solved depending on applications:

• Wireless power transmission

• RF power harvesting

• Electromagnetic techniques to effectively quantify

available RF power

• Rectifier circuit selection

• EM nonlinear co-design of the RECTENNA

• Design example

• State of the art overview

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RF non-linear receiving circuits

CasoCaso 22:: HARVESTING RF

ENERGY

UNKNOWNUNKNOWN• Frequency source

• Source Intensity

• Polarization

• Direction of arrival

• Antennas requirements:

• Wideband/multiband

• Low directivity

• Circularly polarized

Case Case 11: WIRELESS

POWER TRANSMISSION

KNOWNKNOWN• Frequency source

• Source Intensity

• Polarization

• Direction of arrival

Antennas requirements:

• Single frequency

• High directivity

• Linearly polarized

Antenna selection based on different requirements:

Block

diagram

•• DesignDesign specificationspecification: deliver the maximum DC power to

the load given the antenna RF received power.

• Rectenna efficiency:

•• RectennaRectenna designdesign problemsproblems toto bebe solvedsolved::1. quantify RF power incident on the rectenna

2. need for “electromagnetic” characterization of the

antenna to provide meaningful received power

evaluation

3. Rectifier topology and devices selection

4. Nonlinear analysis and design of the entire system to

quantify RF/DC conversion operation

)η(PP

Pη RF

RF

DCDCFR ==−

RECTENNA” (RECTifiying antENNA)

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1 -Quantify RF power incident on the rectenna

•• NEARNEAR--FIELDFIELD FREQUENCYFREQUENCYDEPENDENTDEPENDENT BEHAVIOURBEHAVIOUR

• provide effective input

impedance to the

rectifier YA(ω)

• far from ideal 50 Ω

impedance-30

-25

-20

-15

-10

-5

0

0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

Frequency (GHz)

Return Loss (dB)

numerical modelling measurements

ElectromagneticElectromagnetic SimulationSimulation of the antenna layout

•• FARFAR--FIELDFIELD FREQUENCYFREQUENCYDEPENDENTDEPENDENT BEHAVIOURBEHAVIOUR

• provide the effective received

EM field EA (ω, θ,φ) to receive

the RF energy

Rogorous evaluation of PRF

• The incident power density

(W/cm2) may NOT be

received at all by the rectenna

depending on:

θ

φ

ψψ

ψ

ψ

(θ , φ )

E

E

E

E

E

E : incident electric field

E : electric field radiated

EE

EIncident RF source 1

Incident RF source k

Incident RF source n

RECTENNA

inc

inc

inc

inc1

k

k

k k

n

k

k

n

n

n

k

k

1

inc

inc

D D

A

D

D

AA

A

x

y

z

θ,φ EA

of RF source k

by the antenna

: polarization angle of

: incidence direction of

φ

φ

φ

θ

θ

θ

1.Carrier frequency

2.antenna effective area

3.Direction of arrival

4.Polarization

5.Link budget

• To accurate predict rectenna PRF (POWER AVAILABLE

FOR RECTIFICATION)

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11 -EuMC02 Sept 29, 2009

“Electromagnetic conversion efficiency”

From the power density in the harvester location

Non-realistic prediction of the available power to the rectifier ),(A),,r(p),,r(P effinc ϕθϕθ=ϕθ

η

ϕθ=ϕθ

2

2),,r(

),,r(p incE

AV

DCEM

DCRF

P

P=η −

By combining the antenna performance, computed by EM simulation, with the reciprocity theorem, the actual available RF power at the rectifier input port Pav may be derived

Does not account for near-field frequency behavior of the antennapolarization mismatch between the antenna and the incoming field.

The circuit-antenna assembly is then optimized as a whole simultaneously accounting for its radiation and nonlinear behavior

New figure of merit:electromagnetic-conversion efficiency

( )RFAAinc ;, ωφθE

Plane wave incidentfrom the ambient

I. Antenna in receiving mode

φA

θA

Receiver equivalent excitation( )

A

AincRF

RY

U

rj,,J

+

⋅

η

λ=ωφϑ

1

2 EE

II. Antenna in transmitting

modeFrom the Reciprocity

Theorem:φA’

θA’

( )RFAAA ;',' ωφθE

1 –Equivalent circuit representation of the incident RF power

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Rigorous circuit model of the receiving antenna

θ

φ

ψψ

ψ

ψ

(θ , φ )

E

E

E

E

E

E : incident electric field

E : electric field radiated

EE

EIncident RF source 1

Incident RF source k

Incident RF source n

RECTENNA

inc

inc

inc

inc1

k

k

k k

n

k

k

n

n

n

k

k

1

inc

inc

D D

A

D

D

AA

A

x

y

z

θ,φ EA

of RF source k

by the antenna

: polarization angle of

: incidence direction of

φ

φ

φ

θ

θ

θ

JA(ω, θ,φ) YA(ω)

3 - Rectifier topology and devices selection

1)(eII DαV

SD −=

SPICE MODEL

Main parameters

influencing efficiency

Low-threashold SCHOTTKY diodes: low

available RF power requre special care on

device selection and rectifier topology

• high saturation current IS• Low junction capacitance CJ0

• Low ideality factor n

• low threashold voltage Vth

ID

nkT

qSoluzione:

VD

kyworks Single Diode SMS7630-001

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2.2. twotwo stage full stage full wavewave rectifierrectifier forfor ultra-low RF available power (in the

µW range):

CascadeCascade ofof N N stagesstages ((voltagevoltage mutipliermutiplier):):• Usually adopted in passive RFID when the

transmitted power is significant and known

• Outcomes:

• losse increases with the number of

stages

• Solution not suitable for ultra-low power

available

vin(t)

t

Si opta per nuove

soluzioni progettuali

Rectifes the positive half wave

Rectifier topology: choices

Rectifes the negative half wave

Rectifier efficiency depends on available power

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PACKAGED diode and parasitic effect

Stima della capacità parassita del package

PM

J

R

J0V C

V

V1

CC +

+

=

0.15nH0.18pF

At microwave frequencies the package effects need be accounted for

• Impiego del modello del varactor (per VR =

15V)[Rigorous Modeling of Packaged Schottky Diodesby the Nonlinear Lumped Network (NL2N)–FDTD Approach, Emili et. AI

EEE-MTT2000]

Induttanza parassita

fornita dal costruttore

Induttanza parassita

Capacità parassita

Equivalent circuit of the diode and its package

Circuito

equivalente

del package

Designing the rectenna as a whole

Rectifier to maximize RF-DC conversion efficiency

full-wave peak-to-peak Schottky-diode RF-DC power converter

low threshold voltage in order to guarantee turn-on at µW power level

Need for high diode areas and saturation currents:

lowest zero-bias junction capacitance (Cj0), + highest saturation current (IS)

A diode package model has also been included in the rectenna design.

Cp

Cp

The load

Designed to harvest the maximum DC power

The RF source

Extremely variable range

Internal impedance variable with frequency

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Rectenna non-linear design

• Design based on harmonic balance of the entire circuit

• Design specification on optimum efficiency

L

O

A

D

antenna EM-

based equivalent

circuit

antenna-rectifier

matching network

capacitance design:

discharge period much

longer than the RF

optimum

load

)η(PP

Pη RF

RF

DCDCFR ==− electromagnetic conversion efficiency

CONFRONTO CON LO STATO DELL’ARTE

Rettificatori ad alta efficienza (oltre 80%) sono

stati realizzati negli ultimi anni, in contesti di

• Antenne o schiere fortemente direttive(G > 11dB)• Alte potenze disponibili (Pr ~ 100 mW)

Strassner, Chang:

“Highly efficient C-

Band Polarized

Antenna”

Per potenze disponibili più basse ed antenne poco performanti, le efficienze sono generalmente più contenute (40÷50%)

Akkermans, Van

Beurden: “Analytical

Models for low-

power rectenna

design”

Hagerty, Popovic: “Experimental and

Theoretical characterization of a broadband

arbitrarly polarized rectenna”

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Case 1: harvesting from known sources

FirstFirst exampleexample: harvesting from

GSM1800 sources with known location

• Peack power of a GSM terminal= 2W

• Unica componente di attenuazione

(condizione free loss)

• Harvester distance = circa 1m

• Conversion efficiency in the 1.92:1.98

GHz band

• Substrate Rogers DuroidRT5870• εr = 2.33, Tanδ = 0.0012, h = 17.5 µm•Gain ~ 12dB

Harvesting from known sources: design results

0

10

20

30

40

50

60

70

80

90

100

1.92 1.94 1.96 1.98

Eff

icie

ncy

(%

)

Frequency (GHz)

0

20

40

60

80

100

1.92 1.94 1.96 1.98

Eff

icie

ncy

(%

)

Frequency (GHz)

STARING POINT with an input

reference impedance of 50 Ω

PRF= 1.38 mW

Broadband efficiency 7 uncorrelated nonlinear

regimes

% harmonics

Design variables:

Matching network

Rectifier capacitances

Load

Design specs: PDC > 0.6 mW,

RL < -10 dB

FINAL POINT (accounting for the

dispersive behaviour of the antenna

impedance)

0102030405060708090

100

0 2 4 6 8 10 12 14

Eff

icie

ncy

(%

)

Pin_(RF) (mW)

f0 = 1.95GHz

Starting point

Final point

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Matching network

rectifier

Proptotype realization

Lab test: un GSM terminal in “calling” state

DC rectifier

output

voltage

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Typical measurement round

Incident field monitored by a

commercial envelope field probe

• Estimated RF input voltage

source

0

0.05

0.1

0.15

0.2

0.25

0.3

1 m 90 cm 80 cm 70 cm 60 cm 50 cm

RF

Voltage

[V]

Cordless phone GSM Motorola

0

1

2

3

4

5

6

7

8

1m 90 cm 80 cm 70 cm 60 cm 50 cm

|E| [V/m]

Cordless Phone GSM Motorola

Equivalent RF voltage source

Measurement results

PHONE CALL - GSM

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 5 10 15 20 25

Time (ms)

DC

Volt

ag

e o

n m

atc

hed

Lo

ad

(V)

1 90 cm

80 cm

70 cm

60 cm

50 cm

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Modelling and measurements

PHONE CALL

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2.0 4.0 6.0 8.0 10.0 12.0Time

Volt

ag

e o

n m

atc

hed

Lo

ad

(V

) MHB Simulations

Measurements

28 -EuMC02 Sept 29, 2009

x

zy

Efficient harvesting from 3 common transmitting standards (GSM 900, GSM 1800 and Wi-Fi)

Wearable implementation

inner patch+

two rings +

near-field coupling(frequency dependent)

Two sets of three slots arranged orthogonally Excited by two feed lines, departing from a 90-degree broad band power divider

3 resonant structures

Circular polarization

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29 -EuMC02 Sept 29, 2009

0.9 GHz

1.75 GHz

2.4 GHz

Isolated antenna

Radiating antenna

R. Vullers,“Energy harvesting for Autonomous Sensor

systems”, Holst center 2009

Idea: to simultaneously extract power from all most common bands

Compact layout wearable!

0

200

400

600

-35 -30 -25 -20 -15 -10 -5 0

Pav [dBm]

Vd

c [m

V] 0.9GHz

0

200

400

600

-35 -30 -25 -20 -15 -10 -5 0

Pav [dBm]

Vd

c [m

V]

2.45GHz

Gap-coupled multiband resonator

VDC vs

RF Input

power

Multiresonator Aperture Coupled Antenna

single-frequency

30 -EuMC02 Sept 29, 2009

0.9 GHz

1.75 GHz

2.4 GHz

Isolated antenna

Radiating antenna

Gap-coupled multiband resonator

Polyurethane

foam

Taconic RF-60

Ground and

aperture slot

(t=35µm)

Microstrip feedline

(t=35µm)

Radiator Patch (t=35µm)

H1 = 4 mm

0.635 mm

Neltec

Plexiglass

Working principle

Cross section: layers dimensions and materials

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31 -EuMC02 Sept 29, 2009

Antenna + matching networknear-field and far-field behaviour

-30

-25

-20

-15

-10

-5

0

0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

Frequency (GHz)

Retu

rn Loss (dB)

numerical modelling measurements

φ

θ

0.9 GHz

1.75 GHz2.4 GHz

Radiation pattern in

the broadside direction (θ = 0°)

32 -EuMC02 Sept 29, 2009

Simulation results in terms of circular polarization

phi = 0° @ 2.4 GHz

-50

-40

-30

-20

-10

0

10

20

30

-180 -120 -60 0 60 120 180

theta (°)

dB

V/m

vertical

horizontal

phi = 0° @ 0.9 GHz

-50

-40

-30

-20

-10

0

10

20

30

-180 -120 -60 0 60 120 180

theta (°)

dB

V/m

vertical

horizontal

phi = 0° @ 1.76 GHz

-50

-40

-30

-20

-10

0

10

20

30

-180 -120 -60 0 60 120 180

theta (°)

dB

V/m

vertical

horizontal

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33 -EuMC02 Sept 29, 2009

Rectenna simlulation results

0

10

20

30

40

50

60

70

0 2000 4000 6000 8000

Input power [µW]

Co

nv

ersi

on

eff

icie

ncy

(orig

inal)

[%

] Effic@0.9GHz Effic@1.76GHz

Effic@2.45GHz

0

200

400

600

800

1000

0 500 1000 1500RF-Received Power [µµµµW]

DC

Po

wer

[ µµ µµW

]

2.4GHz 2.45GHz 2.48GHz

0 25

Incident power density [µµµµW/cm^2]5 201510

0

200

400

600

800

1000

0 500 1000 1500RF-Received Power [µµµµW]

DC

Po

wer

[ µµ µµW

]

0.89GHz 0.9GHz 0.915GHz

0 1 2 5Incident power density [µµµµW/cm^2]

3 4

0

200

400

600

800

1000

0 500 1000 1500RF-Received Power [µµµµW]

DC

Po

wer

[µµ µµ

W]

1.73GHz 1.76GHz 1.785GHz

0 10 30Incident power density [µµµµW/cm^2]

20

34 -EuMC02 Sept 29, 2009

Simultaneous harvesting from the three sources

Three-tone analysis of the rectenna performance

0

50

100

150

DC

Po

wer

[µµ µµ

W]

P 0.9GHz: 93 WRF µP 1.8GHz WRF µ: 40.2

P 2.4GHz: WRF µ68.7

P 0.9GHz: 93 WRF µ

P 1.8GHz WRF µ: 101

P 2.4GHz: WRF µ68.7

P 0.9GHz: 37.4 WRF µ

P 1.8GHz WRF µ: 40.2

P 2.4GHz: 27.3 WRF µ

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35 -EuMC02 Sept 29, 2009

Rectenna layout on conventional substrates

Rectifier layerAntenna layer

14 cm

36 -EuMC02 Sept 29, 2009

Predicted and measured DC output voltage

2450-MHz incident wave @ 0.5-meter

for varying direction of

incidence

0

100

200

300

0 0.4 0.8 1.2 1.6 2

Incident power density (uW/cm^2)

DC output (mV)

measurement @Ψ = 90°

measurement @Ψ = 45°

measurement @ Ψ = 0°

Model @ Ψ = 90°

0

200

400

600

0 0.4 0.8 1.2 1.6 2

Incident power density (uW/cm^2)

DC outp

ut (m

V)

Measured

Predicted

900-MHz incident

wave

@ 1-meter

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Towards wearable implementations

Performance during bending

must be preserved. In our design

at 0.9 and 2.45 Ghz it remains unchanged

Electro-textiles: generally

created by incorporating

conductive threads into fabrics

by means of weaving and

knitting

A challenging technology conversion, which

implies:

oAn overview of suitable electrotextile materials

oA detailed investigation about antenna

performance when bent on curved surfaces

The multi-resonator powers a wristwatchharvesting from a cell phone at 40 cm apart

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39 -EuMC02 Sept 29, 2009

Wearable implementation

Pile

Kapton

Pile fabrics

Ground and aperture

slot

(t=35µm)

Microstrip feedline (t=35µm)

Patch – Zeven(t=254µm)

Metallic Shield (Zeven) (t=35µm)

H2 = 10 mm

H1 = 4 mm

Height substrate = 0.635 mm

Wearable rectenna

cross section

40 -EuMC02 Sept 29, 2009

Modelling the effects of antenna bending

Bending in the presence of human body (εr= 57)

• The antenna is bent around cylinder with diameter of 15 cmthat is typical for rib cage

Frequency [GHz]

||Einc|| [V/m] (original

technology)

||Einc|| [V/m] (wearable-flat)

0.90.9 4.744.74 4.14.1

1.761.76 5.105.10 4.84.8

2.452.45 6.906.90 4.324.32

Bending in the absence of human body

Frequency [GHz]

||Einc|| [V/m] (original

technology)

||Einc|| [V/m] (wearable)

0.90.9 4.744.74 4.64.6

1.761.76 5.105.10 6.376.37

2.452.45 6.906.90 4.784.78

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41 -EuMC02 Sept 29, 2009

Power management

• The best rectennaperformance depends on optimum loading conditions

• Need for an “optimum resistor emulator”

• Design of a custom power management system and storage

PRF

PRECT_OUT PRECT_OUT

MAX PHARVEST

42 -EuMC02 Sept 29, 2009

Output rectenna characteristics

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43 -EuMC02 Sept 29, 2009

Optimum rectenna load

• a good trade-off for the rectenna load generally occurs in the vicinity of the halved open-circuit DC output voltage

PRF=1

00uW

PRF=1

00uW

44 -EuMC02 Sept 29, 2009

The switch-based converter for dynamically keeping the rectified voltage VRECT around half of its open-circuit value

matchingnetwork

CRECT1

CRECT2

L C

CHOLD C

HOLD +

-

ENABLE

φ

φ

φ

φ

SAMPLEHALF

controlunit

>

ENABLE

HALF

SAMPLE

D1

D2

D3

SW1

SW2

SW3

SW4

rectifier boost converter storageantenna

control

VRECT

VHARVEST

VW

The converter operates in two phases. The converter operates in two phases. 1. When VRECT > VW, Φ is high and an increasing current flows from the

rectenna output through L, SW3 and SW4 while VRECT starts decreasing.

The output capacitor is disconnected.

2. When VRECT < VW, Φ is low and SW1-SW2 are closed. A decreasing

current flows through L, SW1, SW2, D3 onto C, so that VHARVEST rises.

The energy initially stored in L is thus transferred to C. Meanwhile, the

rectenna is disconnected from the converter and VRECT rises because of

the received RF power.

The voltage VW should be periodically updated after many commutations,

(e.g. at the beginning of each time slot of GSM900 standard.)

The boost converter consists of:

• two pairs of synchronous switches

(SW1-SW2, SW3-SW4)

•

•an inductor L, a diode D3

•a storage capacitor C.

•a simple control circuit is used for

driving the switches.

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45 -EuMC02 Sept 29, 2009

Rectenna + converter time-domain analysis

0

200

400

600

800

1000

1200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60 70

Cu

rren

t (u

A)

Vo

lta

ge (V

)

time(us)

VRECT VHARVEST IL

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Vo

lta

ge (V

)

time (us)

V_harvest

• Time-domain analysis results of the system excited by a 100 mW source at 900 MHz process and the related switching phases.

• The time constants governing the converter operation are mainly related to the oscillating circuits, that are built up during operation, and to the drive strength of the rectenna.

• As far as power consumption is concerned, special care needs to be put in sizing parameters such as L, C and the hysteresis ∆V.

storage process

transient

steady-state

46 -EuMC02 Sept 29, 2009

Conclusions

An integrated procedure for the design of RF harvester is introduced

New figure of merit: the “electromagnetic conversion efficiency” Simultaneusly accounts for

near-field and far-field EM behaviour nonlinear circuit performance ennvironmental effects

A multiresonator rectenna is designed and tested A possible wearable implementation is studied both in flat and

bend configuration the presence of human body is accounted for

Acknowledgment: this work was supported by Eurotech Group as part of the research project on “Self-Powered Portable and Wireless Electronic

Systems” carried out jointly with the University of Bologna.

10/02/2010

24

Challenges RF harvesting researches world-wide

Potenze elevate, ma ottenute in Potenze elevate, ma ottenute in

condizioni non realistichecondizioni non realistiche.Antenna collocata nelle vicinanze di un sistema di

ripetitori radio (campo EM fuori normativa)

Budget di potenza e hardware ambigui Budget di potenza e hardware ambigui Densità di potenza RF incidente a qualche m

dall’hot-spot non compatibile con la potenza

richiesta per la carica del 30% di un Blackberry in

90’. Uso di batterie speciali.

Intel activity

• The researchers powered a wall-mounted

household weather station with an LCD screen

using a TV antenna pointed at a local TV

station.

• The Intel researchers aimed a TV antenna at a

TV station 4 km away, and harvested enough

energy to power a mini weather meter.

• The technology used for this technique is an

extension of that used in off-the-shelf RFID

tags in which the tag reader supplies power to

the otherwise unpowered ID tag.

• With gadgets becoming more and more

energy efficient, the idea of using RF energy

harvesting to power them is closer to

reality...though it will be quite awhile before we

have gadgets on the market powered through

this method.

Details about the experiment:

• The Intel Researchers set up a TV antenna on a balcony with line of sight to the KING-TV tower 4.1 km

away.

• The TV station broadcasts on channel 48 between 674 and 680 MHz with an effective radiation power

(ERP) of 960 Kw.

• The TV antenna used was a UHF log periodic with 5 dBi gain connected to a 4 stage charge pump power

harvesting circuit of the same design as that found in an RFID tag.

• Across an 8 KOhm load the team measured 0.7V, corresponding to 60 microwatts of power harvested.

• That was enough to drive a thermometer/hygrometer and its LCD display, which is normally powered, by

a 1.5 volt AAA battery