1. 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

2. 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.

3. 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

4. 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

5. Multiband harvester overview

6. 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

7. Multiband harvesting paradigms
Two ways to approach multiband harvesting
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

8. 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

9. 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

10. 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
With diode summation network
Without diode summation network
Found benefit of the summation network for every permutation of excited/unexcited bands
Most test cases benefit from the summation network
Shortcut diode leakage current is a huge factor; higher leakage can eliminate the benefit
Median benefit of summation network over simple serial connection

11. Two UHF multiband prototypes
back
front

12. 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

13. 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 band-to-band interaction

14. 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

15. 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?

16. Acknowledgements Google Faculty Research Award
NSF award number CNS
Reviewers, and my lab members
The RFID best paper committee!

17. Thank you! Questions?
sensor.cs.washington.edu