Sifting through the Airwaves: Efficient and Scalable Multiband RF

Report
Sifting through the Airwaves:
Efficient and Scalable Multiband RF Harvesting
Aaron N. Parks
Prof. Joshua R. Smith
Sensor Systems Laboratory
Dept. of Electrical Engineering
Dept. of Computer Science and Engineering
University of Washington
IEEE RFID 2014
Ambient RF harvesting power supplies
• Is ambient RF really a power solution?
– Not yet. Ambient energy availability is highly variable.
• Geographical distribution of ambient sources
• Multipath fading, occlusion, shielding, etc.
– Sensitivity of communications systems may always be
better than sensitivity of RF harvesting systems.
• Creates an inherent coverage issue for RF-powered devices
which target ambient communications signals.
– Intentionally “planted” energy can work well, but
“wild”, ambient RF will not be reliable with typical
harvesting techniques.
Rephrasing the question
• How can ambient RF be a power solution?
– Addressing availability and sensitivity limits:
• Existing work has focused on single band RF harvesting
• Single band availability is highly variable
• Single band power is not always sufficient for
harvesting, even when communication systems can
operate.
Combine power from multiple RF sources
Proposed method
• Typical RF harvesting systems seek to impedance
match one source antenna to one rectifier.
– A few systems have shown multiple antennas matched
to multiple rectifiers.
• Our system allows distribution of power from one
wideband source antenna to multiple rectifiers
– Splits incoming RF into several bands
– Matches each band separately to a unique rectifier
– Rectifier output power is combined at DC
Multiband harvester overview
Advantages of this method
• Allows a large number of bands to be well matched to
separate loads via multiple orthogonal current paths
• Impedance matching is simplified by the distribution of
the match across multiple bands
– We hypothesize that harvester impedance can be designed
to attain any desired value at an any (countable) number
of frequency points.
– This means the harvester and antenna can be well
matched at many frequency points.
• A single antenna means a single antenna port
– Manufacturability, cost
Multiband harvesting paradigms
• Two ways to approach multiband harvesting
1. Target several known bands
(targeted multiband harvesting)
• For instance, ISM bands could be targeted
• High efficiency can be achieved at each band
2. Target a large bandwidth
(divide and conquer)
• Target a large bandwidth with many closely-spaced
harvesting bands, with a constant ratio between band
frequencies (e.g., 400, 600, 900MHz)
• High efficiency will be hard to maintain across entire
spectrum because of interaction between bands
Detail: Front end topology
• Antenna is connected to a
common node (trunk)
• Bandpass filter isolates one
band from the others
– Constrained Q makes isolation
tough
• Separate matching network
for each band
• Separate rectifier for each
band
– 3-stage RF Dickson charge
pump
• Rectifiers are serially
connected
Detail: Power summation network
• When all bands are
excited, a simple serial
connection works
– What if some bands
aren’t excited?
– Dead weight!
• We reduce wasted
power with a network
of “shortcut” diodes
– Unexcited bands will
now be bypassed by
low threshold DC
diodes
Evaluating the power summation network
• A lumped element simulation of
an 8-band harvester with diode
models was done.
– Matched to a 50Ω multi-sine
source
– Two data sets collected
Sum output power
6mW
5mW
4mW
3mW
2mW
1mW
1
2
3
4
5
6
7
8
Number of excited bands (equal power per band)
• With diode summation network
• Without diode summation
network
• Most test cases benefit from the
summation network
• Shortcut diode leakage current is a
huge factor; higher leakage can
eliminate the benefit
Benefit of summation network
– Found benefit of the summation
network for every permutation of
excited/unexcited bands
150%
Median benefit of
summation network
over simple serial
connection
100%
50%
0%
−50%
1
2
3
4
5
6
7
8
Number of excited bands (equal power per band)
Two UHF multiband prototypes
• 2-band prototype
• 5-band prototype
back
front
2-band prototype characterization
• Bandpass filters were centered at
f1=539MHz, f2=915MHz
• Matching network tuned empirically for each rectifier,
assuming 50Ω source impedance
• Determined single-tone efficiency across frequency
– dB(S11) also recorded using a VNA
Single-tone excitation
Test power: -10dBm
Load resistance: 100kΩ
2-band prototype characterization
• Dual-tone excitation (at both design frequencies)
Dual-tone excitation
Test power: -10dBm
Load resistance: 100kΩ
• Interestingly, efficiency increases as second tone is
introduced!
– Perhaps due to increased diode conduction from bandto-band interaction
5-band prototype characterization
• Bands placed at 400, 600, 900, 1350MHz (ratio 1.5)
• Single-tone excitation: dB(S11) and efficiency
Single-tone excitation
Test power: -10dBm
Load resistance: 100kΩ
• Clearly, complexity has increased.
– Interaction between bands causes misleading S11 peaks
– Peak efficiencies are lower than 2-band. However, bands
are more closely placed.
– Fifth band efficiency is probably low due to poor RF diode
choice
Conclusions
• Conclusions drawn from this work
– It is possible to achieve decent efficiency at a number
of widely spaced frequencies
• Unknowns which deserve more attention
– Does efficiency necessarily scale as a function of
design variables such as:
• Number of bands
• Band spacing
– What about efficiency at intermediate frequencies?
Can a multiband harvester be a good wideband
harvester?
Acknowledgements
• Google Faculty Research Award
• NSF award number CNS 1305072
• Reviewers, and my lab members
• The RFID best paper committee!
Thank you!
Questions?
[email protected]
sensor.cs.washington.edu

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