GENERATING DC ELECTRIC POWER

FROM AMBIENT ELECTROMAGNETIC RADIATION

PRIORITY

[0001] This application claims priority to and incorporates by reference U.S. Patent Application No. 12/818,103 entitled "Generating DC Electric Power from Ambient Electromagnetic Radiation," filed on June 17, 2010. This application further claims priority to and incorporates by reference U.S.

Provisional Application No. 61/464,492 entitled "One or More Broadband or Multi-Resonant Antenna Integrated with SCARF Circuit ," filed on March 7, 201 1.

FIELD OF THE INVENTION

[0002] At least certain embodiments relate generally to the field of radio frequency engineering, and more particularly to harvesting RF energy from ambient electromagnetic sources and providing this energy as a power supply.

BACKGROUND OF THE INVENTION

[0003] RF energy harvesting devices of prior art systems are configured to collect RF and microwave signals. For example, rectifying antennas can be used to convert output from an antenna to

DC current. A broadband antenna connected directly to a rectifying circuit may also be used to harvest energy over a broad spectrum of EM frequencies. However, this approach suffers from interference problems since the captured signals can destructively interfere with each other, especially since this approach collects such a broad spectrum of frequencies, thus producing a noisy output that may negate achieving optimal AC to DC power conversion. For many years, RF energy harvesters have been utilized in RFID tags to provide small bursts of power to activate the tags for a short duration.

Recently, RF energy harvesting devices have been introduced commercially as stand-alone power sources. These RF energy harvesting power sources gather broadcasted, single-frequency signals from either a directed power source or a nearby RF or microwave communication source, such as a WiFi router. The power from these signals is inversely proportional to the distance squared. The power gain also falls off quicker for higher frequencies. Thus, a microwave source can only transfer a useful amount of power over a few meters.

SUMMARY

[0004] At least certain embodiments describe methods, apparatuses, and systems for converting energy from electro-magnetic (EM) radiation into electric power using a simultaneous collector of ambient radio frequencies (SCARF) circuit. In one embodiment this is done by capturing EM radiation from a plurality of ambient signals using an array of antennas where each signal has a resonant frequency and aggregating the ambient signals to generate an aggregated signal having a single frequency with greater AC power than the AC power of each of the plurality of ambient signals individually. The single frequency can be produced by either the sum of the resonant frequencies of the ambient signals or the difference between the resonant frequencies of the ambient signals. The aggregated signal is then converted into useable electric power using a rectifying circuit such that for every incremental increase in the AC power of the aggregated signal, there is a corresponding exponential increase in DC power at the output of the rectifying circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] For a better understanding of at least certain embodiments, reference will be made to the following description, which is to be read in conjunction with the accompanying drawings, wherein:

[0006] FIG. 1A depicts typical sources of ambient radio and microwave signals.

[0007] FIG. IB depicts a graph of the ambient electromagnetic power density at different frequencies in Mountain View, CA.

[0008] FIG. 2 depicts RF Energy Harvesting Device according to one illustrative embodiment.

[0009] FIG. 3A depicts RF Energy Harvesting Device according to one illustrative embodiment.

[0010] FIG. 3B depicts RF Energy Harvesting Device according to one illustrative embodiment.

[0011 ] FIG. 3C depicts a phase-lock loop according to one illustrative embodiment.

[0012] FIG. 4 depicts the Phase Shifter according to one illustrative embodiment.

[0013] FIG. 5 depicts the Phase Difference Detector according to one illustrative embodiment.

[0014] FIG. 6 depicts the RF Energy Harvesting System according to one illustrative embodiment.

[0015] FIG. 7A depicts the RF Energy Harvesting System according to one illustrative embodiment.

[0016] FIG. 7B depicts an embodiment of the energy harvesting system using a broadband or multi- resonant antenna. [0017] FIG. 7C depicts a detailed view of one embodiment of the circuit configuration of Figure 7B.

[0018] FIG. 7D depicts an embodiment of the energy harvesting system using multiple broadband or multi-resonant antennas.

[0019] FIG. 7E depicts a detailed view of one embodiment of the circuit configuration of Figure 7D.

[0020] FIG. 7F depicts an embodiment of the energy harvesting system using multiple levels of mixers.

[0021] FIG. 7G depicts an embodiment of the energy harvesting system using multiple levels of mixers.

[0022] FIG. 8 depicts method for RF Energy Harvesting according to one illustrative embodiment.

[0023] FIG. 9 depicts a graph of the exponential relationship between the input power to a rectifying circuit and the rectifying circuit's AC to DC power conversion according to one illustrative embodiment.

[0024] FIG. 10 depicts the projected DC power output from an RF Energy Harvesting Device implemented using the techniques described herein in comparison to the projected DC power output from prior art high-efficiency rectennas. DETAILED DESCRIPTION

[0025] Throughout the description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form to avoid obscuring the underlying principles of embodiments of the invention.

[0026] At least certain embodiments of the present invention describe methods, apparatuses, and systems for harvesting RF energy from ambient electromagnetic radiation, and converting that energy into useable electric power. Embodiments include simultaneously capturing electromagnetic radiation of multiple ambient RF or microwave signals using an array of antennas, each ambient signal having a different resonant frequency. The captured ambient signals are then combined to form an aggregated signal having a predetermined intermediate frequency and increased AC power. The aggregated signal is then converted into useable DC power using a rectifying circuit.

[0027] The techniques disclosed herein are accomplished using a simultaneous collector of ambient radio frequencies (SCARF) circuit coupled with the rectifying circuit such that, for every incremental increase in the AC power of the aggregated signal, there is a corresponding exponential increase in DC power at the output of the rectifying circuit. In one embodiment, the SCARF circuit includes an array of antennas, a mixer coupled with the array of antennas, and a filter coupled with the mixer. This SCARF circuit is coupled with a rectifying circuit such that the DC power at the output of the rectifying circuit has a peak AC to DC power conversion. [0028] An advantage of the embodiments disclosed herein is that electromagnetic radiation existing in the ambient environment can be harvested and combined to produce usable DC electric energy to power electrical devices. Radio and microwave frequencies are ubiquitous in our everyday life. Signals with these frequencies are emitted from multiple sources including: television & radio towers; cellular base stations; WiFi routers; and satellite radio. Only a tiny fraction of the signals broadcast from these sources actually reach their intended targets, while the rest are dissipated in space. In a sense, this is free energy that is constantly being wasted. The SCARF circuit harvests this energy and puts it to use. Figure 1A depicts the multiple sources of typical electromagnetic radiation that exists in the ambient environment. In the illustrated embodiment, RF energy harvester 100 is configured to receive at least some of the ambient electromagnetic signals from AM FM radio broadcasts, satellite TV and radio, HD radio broadcasts, cordless telephones, mobile phones and cellular base stations, and wifi routers; and convert them into usable DC electric energy to power electronic devices. RF energy harvester 100 can provide power to wireless sensor nodes, mobile phones, portable MP3 players, portable electronics, and RF ID tags among others. The RF energy harvester 100 can replace batteries in these devices thus eliminating the waste produced from expended batteries. These devices may only need the techniques disclosed herein to power the device for its lifetime. For electronic devices that are not constantly in use and require more power than the RF energy harvester 100 can provide, the output from the RF energy harvester 100 can be used to trickle charge a small rechargeable battery; and this battery can then be used to power these electronic devices. This is an ideal configuration for mobile phones and larger wireless sensor nodes. Trickle charging can extend battery lifetime and the time between charges. The advantage mat this RF energy harvester has over previous devices is that it can simultaneously collect multiple signals and aggregate the received AC power. This aggregated AC power is multiple times higher than would be collected from a single broadband antenna or from an antenna that collects only a single frequency signal.

[0029] The data shown in Figure IB demonstrates the presence of a broadly distributed source of ambient electromagnetic energy in typical populated environments that can be harvested to provide a long term power source for electronic devices. The RF energy harvesting device 100 utilizes an array of antennas to harvest these multiple RF and microwave frequency signals, and constructively combine them to produce a new signal with a higher AC power. The data shown in Figure IB shows a limited range of available electromagnetic sources, however the techniques introduced herein can utilize the entire electromagnetic spectrum. The array of antennas can be specifically designed to harvest signals having the greatest probability of being present in a particular ambient environment. Another advantage of the techniques introduced herein is that it is completely portable; and can be configured to be self-contained with minimal space requirements.

[0030] Figure 2 depicts an energy harvesting device according to one embodiment. In the illustrated embodiment, energy harvesting device 200 includes a SCARF circuit 250 coupled with a rectifying circuit 209. The SCARF circuit 250 includes an array of antennas including antenna 201 having a first resonant frequency and antenna 203 having a second resonant frequency different from the resonant frequency of antenna 201. These antennas are selected to provide a single predetermined intermediate frequency. One method for providing a single predetermined intermediate frequency is to couple the antenna through a filter (not shown) to pass only the predetermined frequency. Other embodiments allow only a predetermined frequency band to pass. These antennas can be connected to the SCARF circuit 250 using a feed circuit. A feed is an electrical connection from the antenna to the circuit. This electrical connection can be accomplished through a physical connection or by having the antenna in close proximity to a circuit input. These feeds each carry one or more frequency bands collected by the antenna. Antenna 201 and Antenna 203 are specifically selected to match the ambient frequencies with the highest power levels. For example, Antenna 201 can be designed with a resonant frequency of 1.9 GHz to match a Global System for Mobile Communications (GSM) transmission wavelength, while antenna 203 can be designed with a resonant frequency of 2.4 GHz to match a WiFi transmission wavelength.

[0031] SCARF circuit 250 further includes a mixer 205 coupled with each of antenna 201 and antenna 203. Mixer 205 is configured to aggregate the AC power of the resonant frequencies of antennas 201 and 203 and to output an aggregated signal having a single predetermined intermediate frequency having an amplitude that is either the sum of the resonant frequencies of antennas 201 and 203 or the difference between these resonant frequencies. As described herein, a mixer is defined as a non-linear circuit that accepts at its input two different frequencies and presents at its output a mixture of signals at several frequencies: the sum of the two frequencies; the difference of the two frequencies; both original input frequencies; and unwanted intermodulation products from the inputs.

[0032] SCARF circuit 250 also includes a filter 207 coupled at the output of mixer 205. The filter is configured to pass the intermediate frequency and to reject other frequencies. As used herein, the term filter is a device that removes from a signal some unwanted frequencies. As discussed above, SCARF circuit 250 is coupled with rectifying circuit 209. In at least certain embodiments, the rectifying circuit 209 is designed to match the intermediate frequency and to convert the aggregated signal into usable DC electric power. The rectifying circuit 209 receives the aggregated signal and outputs an output signal such that the DC power of the output signal has a peak AC to DC power conversion. The term rectifier describes a device that converts alternating current (AC) to direct current (DC), a process known as rectification. At low power levels, any increase in input power from the aggregated signal, produces exponentially greater AC to DC power conversion efficiencies. Rectifying circuits are well known in the field of RF engineering. Rectifying circuit 209 can be a half wave rectifier. A half-wave rectifier passes either the positive or negative half of an AC signal waveform. Half-wave rectification can be achieved with a single diode in a one phase supply. Rectifying circuit 209 can also be a full- wave rectifier. A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Alternatively, rectification circuit 209 can be a voltage multiplying rectifier. Cascaded stages of diodes and capacitors can be added to make a voltage multiplying rectifier. Due to the nonlinearity of the diodes used in the rectification circuit, the combined signals with higher voltage peaks produce exponentially greater DC currents than those from prior art systems. Device 200 further includes load resistance 211 at the output of rectifying circuit 209 that represents the electronic device that receives the DC power, which may include, for example, a wireless sensor node or a cellular phone.

[0033] The aggregated signal can either be converted to DC electricity directly using the rectifying circuit matched to the intermediate frequency of the aggregated signal as described in Figure 2, or the aggregated signal can be combined with another signal (aggregated or not) to produce a new signal with even greater AC power. Figure 3A depicts an RF energy harvesting device 300A according to one embodiment. In the illustrative embodiment, energy harvesting device 300A includes SCARF circuit 350 coupled with rectifying circuit 319. SCARF circuit 350 includes an additional mixer coupled with an additional filter, coupled with a voltage summer that combines the output from filter

313 with the output from filter 315. SCARF circuit 350 includes mixer 309 coupled with coupled with antenna 301 and antenna 303 and mixer 31 1 coupled with antenna 305 and 307. The additional antenna

305 and 307 are configured similarly to the antennas utilized in SCARF circuit 250 in Figure 2. Voltage summer 317 is coupled between filter 313 and filter 315 and the rectifying circuit 319. The voltage summer 3 17 has an output that is the sum of the output of the first filter 313 and the output of the second filter 315. An advantage of doing the additional aggregation is that the AC power output from the voltage summer 317 is higher when the signals from filter 313 and filter 315 constructively interfere.

[0034] Device 300A further includes load resistance 321 at the output of rectifying circuit 319 that represents the electronic device which will receive the DC power. The antennas 301-307 in the array are predetermined so that the intermediate frequencies exiting filter 313 and filter 315 are equal. For example, if antennas 301 and 303 are 1.9 GHz and 2.4 GHz, respectively, and antennas 305 and 307 are 100 MHz and 400 MHz, respectively, the predetermined intermediate frequency output from filters 313 and 315 will be selected to be 500MHz. In this example, the output intermediate frequency from filter 313 is the difference between the resonant frequencies from antennas 301 and 303, respectively; and the output intermediate frequency from filter 315 is the sum of the resonant frequencies from antennas 305 and 307, respectively.

[0035] A SCARF circuit 350 can further include a phase-lock loop (PLL) to ensure that the input signals into voltage summer 317 are in phase and are constructively interfering as depicted in Figure 3B, in order to increase the AC power input into the rectifying circuit 319. In Figure 3B, SCARF circuit 360 within RF energy harvesting device 300B includes a PLL 316 coupled between the outputs of filters 313 and 315 and the voltage summer 317. Figure 3C depicts an illustrative PLL according to one embodiment. In this illustrated embodiment, PLL 316 includes a phase detector circuit configured to detect a phase difference between the input from filter 313 and the input from filter 315. PLL 316 further includes voltage divider 332 coupled between the input from filter 313 and the first input of phase detector 331. Voltage divider 332 is configured to provide a low-amplitude version of the intermediate frequency input from filter 313 to an input of phase detector 331. PLL 316 further includes voltage divider 333 coupled between the input from filter 315 and the second input of phase detector 331 as shown. Voltage divider 333 is configured to provide a low-amplitude version of the intermediate frequency output from filter 315 to an input of phase detector 331. PLL 316 also includes a phase shifter 335 coupled between the input from filter 315 and the voltage divider 333. The phase shifter 335 has a control input 391 which is coupled with the output of the phase detector circuit 331. The phase shifter 335 is configured such that the phase difference between the frequency input from filter 313 and the frequency input from filter 315 is zero. The DC output of the phase detector 331 acts as a control input 391 to phase shifter 335.

[0036] Figure 4 depicts a phase shifter according to one embodiment. In the illustrated embodiment, phase shifter 400 includes a varactor 407 coupled between the control input 391 from Figure 3C and ground. As used herein, the term varactor is used to describe a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals. Phase shifter 400 further includes bias components 405 coupled between the control input 391 and the input from filter 315 from Figure 3C.

[0037] Figure 5 depicts a phase difference detector according to one embodiment. As used herein, the term phase difference detector refers to a frequency mixer circuit that generates a DC voltage signal representing the difference in phase between two signal inputs. In the illustrated embodiment, phase difference detector 500 includes a small-signal mixer 505 coupled with the output of voltage divider 332 and the output of voltage divider 333 of Figure 3C. The small-signal mixer 505 produces a DC output voltage which is proportional to the phase difference of the two input signals from voltage dividers 332 and 333. The small-signal mixer 505 also produces an output AC signal with a frequency that is double the frequency of the two input signals, which is filtered out using low pass filter 507 coupled with the output of small-signal mixer 505. Accordingly, the output of low-pass filter 507 is the DC output voltage with the AC signal filtered out. This signal is output to phase shifter 400 depicted in Figure 4.

[0038] As shown in Figure 6, the arrangement of the SCARF circuits depicted in Figures 3A-5 can be coupled together with a second SCARF circuit in order to increase the aggregated AC power, which in turn exponentially increases the AC to DC power conversion efficiency of the rectification circuit as depicted in Figure 9. In the illustrated embodiment, the output of SCARF Circuit 601 is coupled with the first input of a second-level PLL 605 and the output of SCARF Circuit 603 is coupled with the second input of second- level PLL 605. The output of second-level PLL 605 is connected to the input of a second-level voltage summer 607. The output of second-level voltage summer 607 is connected to the input of rectifying circuit 609, which is connected to load resistance 611.

[0039] Figure 7A depicts an energy harvesting system 700 according to one embodiment. As illustrated, this technique can be generalized to "N" pairs of SCARF circuits, each pair having an associated additional level of PLL and voltage summer circuits. In at least certain embodiments, for every pair of SCARF circuits coupled together in the system, an additional level of PLL and voltage summer circuits is added. As discussed above, the techniques introduced here are advantageous for harvesting RF energy from ambient electromagnetic radiation, and converting that energy into useable electric power. Embodiments include: simultaneously capturing electromagnetic radiation from multiple ambient RF or microwave signals using an array of antennas, each signal having a different resonant frequency; aggregating the multiple ambient RF or microwave signals, the aggregated signal having a predetermined intermediate frequency and increased AC power; and converting that aggregated signal into a useable DC power using a rectifying circuit. The techniques disclosed herein are accomplished using a simultaneous collector of ambient radio frequencies (SCARF) circuit coupled with a rectifying circuit such that for every incremental increase in the AC power of the aggregated signal, there is a corresponding exponential increase in AC to DC power conversion efficiency of the rectifying circuit.

[0040] In other embodiments, at least one broadband or multi-resonant antenna can be used and can be connected to the SCARF circuit as described above. A broadband antenna is defined as any antenna that functions satisfactorily over a wide range of frequencies. A multi-resonant antenna is defined as any antenna that has multiple resonances. A system using one broadband or multi-resonant antenna to feed all the inputs of the SCARF circuit is depicted in Figure 7B, which shows a broadband or multi- resonant antenna coupled with the SCARF circuit 701. Figure 7C shows a detailed view of one embodiment of the circuit configuration of Figure 7B. Both Figures 7B and 7C use an example variation of the SCARF circuit discussed above. These multiple resonances allow the antenna to capture signals from different frequency bands. As discussed above, at least one feed from each broadband or multi-resonant antenna connect to an input of the SCARF circuit. A feed is an electrical connection from the antenna to the circuit. This electrical connection can be accomplished through a physical connection or by having the antenna in close proximity to a circuit input. These feeds each carry one or more frequency bands collected by the antenna.

[0041 ] Each SCARF circuit has multiple inputs. Each input is designed to carry a single frequency band. This input may be connected to a filter to ensure that only the desired frequency band is passed. One or more broadband or multi-resonant antennas can be used to reduce the number of antennas that are connected to the SCARF circuit. A broadband antenna can capture a much greater bandwidth than a single resonant antenna and each multi-resonant antenna can capture multiple frequency bands. As shown in the diagram, multiple feeds (shown with bandpass filters) can be connected to the same broadband or multi-resonant antenna, each carrying a different frequency band. The differing frequency for each feed is accomplished through the use of filters or by choosing the location where the feed is connected to the antenna. The multiple feeds stemming from each antenna then become the inputs to the SCARF circuit. This reduces the number of antennas attached to the SCARF circuit. The system can also use a variant of broadband or multi-resonant antennas combined with single resonant antennas.

[0042] A system using multiple broadband or multi-resonant antennas to feed all the inputs of the SCARF circuit is depicted in Figure 7D. Figure 7E shows a detailed view of one embodiment of the circuit configuration of Figure 7D. Both Figures 7D and 7E use an example variation of the SCARF circuit discussed above. The system can also use a variant of multiple broadband or multi-resonant antennas combined with single resonant antennas, or any such combination of single resonant, multi- resonant and broadband antennas.

[0043] In addition, multiple levels of mixers can be used in the SCARF circuit to produce additional power output. The mixer depicted in Figure 2 shows a single level of mixer that may be used to output the sum or difference between the resonant frequencies of antennas 201 and 203. The SCARF circuit 250 uses mixers to combine different signals. The result of the mixing process produces several signals with different frequencies at the output of the mixers. The first signal produces a frequency equal to the difference between the two input signals (freql-freq2). This mixer also produces a signal that has a frequency that equals the sum of the two input signal frequencies (freql+freq2). The outputs from this mixer may be paired and passed through a second level of mixers which produce a signal that equals the difference of the two outputs. This is depicted in Figures 7F and 7G according to one embodiment. The resulting outputs from the second level mixers will all have the same frequency that can be combined and rectified into DC current. Like the first level mixers, the second level mixers also produce an output that equals the sum of the two input frequencies. These signals can again be paired and passed through a third level of mixers to produce even more power output. This process can be repeated until there are not enough output signals to produce a new SCARF circuit tree.

[0044] Figure 8 depicts a method of harvesting RF energy according to one embodiment. In the illustrated embodiment, method 800 begins at operation 801 where electromagnetic signals are captured using a SCARF device. As discussed above, the various antennas may be selected to provide an intermediate frequency to maximize the AC to DC power conversion. Method 800 continues at operation 803 where the captured signals are aggregated. In one embodiment, this may be

accomplished using a mixer as described with respect to Figure 3A. The aggregated signal is then input into a filter to pass a single intermediate frequency signal and reject other frequency signals (operation 805). The single intermediate frequency signal is then converted to useable electric power (operation 809), which can be used to charge an electronic device or battery. This concludes method 800.

[0045] Figure 9 depicts the efficiency of the rectifying circuit, showing a graph of the exponential relationship between the input power to a rectifying circuit and the rectifying circuit's AC to DC power conversion according to one illustrative embodiment. The efficiency of the rectifying circuit increases exponentially as the magnitude of the input AC voltage increases. And Figure 10 depicts the benefit of the RF energy harvesting described herein, showing the projected DC power output from an RF energy harvesting device implemented using the techniques described herein in comparison to the projected DC power output from prior art high-efficiency rectennas. In the illustrated embodiment, one set of points depicts a numerical approximation of the DC output power from high-efficiency rectennas. The graph illustrates that the DC output power from the rectennas linearly increases as the number of received RF signals increases. The other set of points in Figure 10 depicts the numerical approximation of the DC output from an RF Energy Harvester implemented according to the techniques described herein. The DC output power from method 800 increases exponentially as the number of received RF signals increases. This exponential increase in DC output power is due to the exponential increase in AC to DC power conversion efficiency of the rectifying circuit as depicted in Figure 9.

[0046] Throughout the foregoing description, for the purposes of explanation, numerous specific details were set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without some of these specific details. In addition, embodiments of the invention may include various operations as set forth above, or fewer operations or more operations, or operations in an order which is different from the order described herein. Accordingly, the scope and spirit of the invention should be judged in terms of the claims which follow as well as the legal equivalents thereof.