Abstract:
A tag operates in an environment where transmitted interrogators are not possible or desired. High-efficiency diode rectifiers and low-power oscillators are used in combination with an energy storage element and hysteretic switch. The combination of multi-diode rectifier, low power tunnel diode oscillator, energy storage element and hysteretic switch allow for nanowatt levels of received RF power to be stored to produce micro-watts of RF transmit power. The device trickle charges off of ambient RF radiation until enough energy is stored for transmission.

Description:
FIELD OF THE INVENTION 
     The invention relates to ambient radiation power scavenging, and more particularly, to a transponder using rectifiers and oscillators in combination with an energy storage element and switch. 
     BACKGROUND OF THE INVENTION 
     Existing transponder devices such as Radio Frequency IDentification (RFID) tags require either a self-contained power supply, RF energy transmitted from an interrogator, or both to operate. This typically requires close proximity to the interrogator or higher-power RF transmissions. In many environments this is neither feasible nor desirable. 
     While active RFID tags respond using energy derived from a battery on the tag, this limits tag life to the life of the battery or requires access to the tag to replace batteries as they drain. Tag access is not feasible or possible for many applications. Passive tags not require batteries, but do require RF interrogation signals of sufficient power and duration to enable them to generate the RF response signal. This limits the range over which tags can operate and can cause interference when interrogation signals are at higher powers. 
     Similarly, sensors that wirelessly communicate their data are limited in range and lifespan. These constraints eliminate applications requiring multiple, inaccessible RF transponder devices. 
     What is needed, therefore, are techniques for employing ambient radiation to power devices such as sensors and RFID transponders. 
     SUMMARY OF THE INVENTION 
     The invention solves the problem of operation of transponders including RFID tags in environments where transmitted interrogators are not possible or desired. This problem is solved using high-efficiency diode rectifiers and low-power oscillators in combination with an energy storage element (optionally with a charging path), and a hysteretic switch. 
     This combination of the multi-diode rectifier, low-power tunnel diode oscillator, energy storage element and hysteretic switch allow for nanowatt levels of received RF power to be stored to produce micro-watts of RF transmit power. The invention will “trickle” charge off of ambient RF radiation until enough energy is stored for transmission. In nonlimiting embodiments, the transponder can be used as a RFID tag or sensor. Nonlimiting examples of ambient energy sources include television and cell phone transmissions. 
     Embodiments provide a transponder apparatus operating from ambient radiation comprising a rectifier to at least partially rectify the ambient radiation; an energy storage element storing at least some energy of the rectified ambient radiation, a switch controlling output of at least some energy of the energy storage element, and an oscillator generating a signal for transmission. For other embodiments, the transponder is a Radio Frequency IDentification (RFID) tag; the transponder is a sensor; and the rectifier is a multi-diode rectifier. In some embodiments, the multi-diode rectifier comprises Schottky diodes, and the multi-diode rectifier comprises sixteen diodes. In yet other embodiments, the energy storage element comprises a low current charging path, and the energy storage element further comprises a low current charging path comprising a battery whereby a low duty cycle mode of operation is provided. For yet further embodiments, the switch is a hysteretic switch, the oscillator is a low-power oscillator, and the oscillator comprises a sensor. Other embodiments provide that the sensor is a capacitive sensor, and that the sensor is a Micro-Electro-Mechanical Systems (MEMS) capacitive sensor. For another embodiment, the oscillator comprises an inductor, a Micro-Electro-Mechanical Systems (MEMS) sensor, and a tunnel diode. 
     Additional embodiments provide a method for transponding using ambient radiation comprising receiving the ambient radiation, rectifying energy from the ambient radiation, storing the rectified energy from the received ambient radiation, switching power of the stored rectified energy, modulating the power, and transmitting the modulated power. More embodiments provide that the step of rectifying comprises full wave rectification, the step of switching comprises hysteretic switching, the step of modulating comprises a low power oscillator, and the step of oscillating comprises sensing a capacitance. 
     Yet an additional embodiment provides a Radio Frequency IDentification (RFID) tag sensor system operating from ambient radiation comprising a rectifier to at least partially rectify the ambient radiation, wherein the rectifier comprises sixteen Schottky diodes, an energy storage element storing at least some energy of the rectified ambient radiation, wherein the energy storage element comprises a low current charging path comprising a battery whereby a low duty cycle mode of operation is provided, a switch controlling output of at least some energy of the energy storage element, wherein the switch is an active hysteretic switch, and an oscillator generating a signal for transmission, wherein the oscillator comprises an inductor, a Micro-Electro-Mechanical Systems (MEMS) capacitive sensor, and a tunnel diode. 
     The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a transponder that operates off of ambient radiation configured in accordance with one embodiment of the present invention. 
         FIG. 2  is a block diagram of a transponder that operates off of ambient radiation comprising a hysteretic switch configured in accordance with one embodiment of the present invention. 
         FIG. 3  is a schematic of a sixteen diode rectifier configured in accordance with one embodiment of the present invention. 
         FIG. 4  presents rectifier test data for a sixteen diode rectifier configured in accordance with one embodiment of the present invention. 
         FIG. 5  is a graph showing the resultant curve representing the data of  FIG. 4  of input power versus output voltage configured in accordance with one embodiment of the present invention. 
         FIG. 6  depicts a Schottky diode configuration in accordance with one embodiment of the present invention. 
         FIG. 7  shows graphs of performance depicting voltage out for power in configured in accordance with one embodiment of the present invention. 
         FIG. 8  provides input match (S 11 ) for ports  2  and  3  configured in accordance with one embodiment of the present invention. 
         FIG. 9  presents data and a circuit model for a sixteen diode rectifier configured in accordance with one embodiment of the present invention. 
         FIG. 10  is a schematic diagram of a hysteretic switch configured in accordance with one embodiment of the present invention. 
         FIG. 11  provides data for a hysteretic switch configured in accordance with one embodiment of the present invention. 
         FIG. 12  is a flow chart depicting a method for a transponder using ambient radiation configured in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates embodiment  100  of an RFID transponder that operates off of ambient radiation. The antenna  105  receives RF power from the environment. The diode rectifier  110  is a high efficiency diode rectifier circuit that converts nano-watt levels of received RF power to milli-volt levels of DC voltage. The DC power is stored in the energy storage element  115 . Once the energy storage element  115  is charged to a pre-defined level, the hysteretic switch  120  will discharge the energy to the low power oscillator  125 . Low power oscillator  125  operates off of low voltage levels to produce micro-watts of transmit power. The microwatts of power generated from nano-watt levels of received RF energy is transmitted via antenna  130 . An optional low current (nanoamps) charging path and small battery may be added to the energy storage element  115 , allowing for a low duty cycle mode of operation. Battery power consumption is sufficiently low that the battery will last for essentially its shelf life. In embodiments, an optional capacitive sensor may be added to the low power oscillator  125  allowing for real-time change detection to take place. The capacitive sensor will shift its value of capacitance based on a sensed change in the environment in which the device is placed. This shift in capacitance will cause the transmitted signal to shift in frequency. A detected shift in transmit frequency can signify the user that a change has occurred. 
       FIG. 2  depicts embodiment  200  with receive antenna  205  operating at 831 MHz −28 to −38 dBm. For nonlimiting embodiments, 160 to 1600 nW activate the tag. Sixteen diode rectifier  210  has output  215  of 50 to 400 millivolts to energy storage element  220 . Energy storage element  220  comprises 1 to 10 microFarad capacitance (1.3 nJ to 80 nJ of stored energy), with E=½ (CV 2 ), taking 160 to 1,600 mS to charge and 40 to 800 mS to discharge. Energy storage element  220  output is provided to hysteretic switch  225  and nanowatt comparator  230 . Nonlimiting embodiment examples include a Maxim® MAX9027 at 1.8 volts at 450 nA. Maxim is a registered trademark of Maxim Integrated Products, Inc. Output of switch  225  is  235  50 to 200 millivolts. Switch  225  output  235  is applied to tunnel diode oscillator  240 . Tunnel diode oscillator  240  operates at 50 to 200 micro watts (65 millivolts at 1 milliamp, typical) and comprises inductor  245 , Micro-Electro-Mechanical Systems (MEMS) sensor  250 , and tunnel diode  255 . For embodiments, output to transmit antenna  260  is −26 dBm typical at 2 GHz. The LC tank circuit determines the transmit frequency. A nonlimiting embodiment example is a transmission of 50 microwatts every 5 mS or 200 microwatts every 20 mS (for a 10 nJ pulse). 
     Voltage Rectification 
     A two diode voltage doubler (a full wave rectifier) increases the voltage sensitivity of a Schottky diode detector when compared to a single diode detector circuit. The output voltage is essentially doubled. The shunt diode provides a DC offset equal to the peak voltage and the series diode acts as the detector. The Detector Input=Input Sine Wave+DC voltage. The Detector Output=Input Sine Wave Voltage, peak-to-peak (double that of a single diode detector). The diode impedance is added in parallel, resulting in a higher capacitive load and a lower series resistance. Embodiments include a sixteen diode rectifier. 
       FIG. 3  is a schematic of a sixteen diode rectifier  300  incorporated in embodiments. Sub-components comprise a first capacitor  305 , a first diode  310 , a second diode  315 , and a second capacitor  320  forming branches of the rectifier. 
       FIG. 4  presents rectifier test data  400  for a sixteen diode rectifier. Frequency is 831 MHz and data presented includes power in (Pin) in dBm, and the voltage out (Vout) in millivolts. For Pin from −50 to 0 dBm, Vout was 2 to 7,260 millivolts, respectively. 
       FIG. 5  is a graph  500  showing the resultant curve represented by the data of  FIG. 4  of input power versus output voltage. This is for a tuned circuit −20 dB return loss. 
       FIG. 6  depicts a Schottky diode configuration  600  for modeling. Considerations include package capacitance, leadframe inductance, and bondwire inductance. Modeling components include ports, inductances, capacitances and diodes. Ports include Port  1   605 , Port  2   610  and Port  3   615 . Inductance components include leadframe inductances L 1  0.4 nH  620 , L 2  0.4 nH  625  and L 6  0.4 nH  630 , and bondwire inductances L 3  0.7 nH  635  and L 4  0.7 nH  640 . Capacitances include coupling capacitance C 1  0.035 pF  645 , C 2  0.03 pF  650 , and C 3  0.03 pF  655 . Diodes include diode D 1   660  and diode D 2   665 . 
       FIG. 7  shows graphs of performance  700  depicting voltage out (mV) for power in (dBm). Datasheet values  705  compare with model results  710 . Modeling results are for 915 MHz. These conditions included a 3 microamp bias and an RL=100 k ohms. 
       FIG. 8  provides input match (S 11 )  800  where ports  2  and  3  of the model of  FIG. 6  are terminated in 47 pF capacitors. Shown are single diode pair input match versus frequency at −30 dBm  805  for m 7  at 450 MHz, with S(1,1)=1.0000/−3.703 and impedance of Z 0 *(0.062−j30.937) and m 6  at 6.000 GHz with S(1,1)=0.956/−68.054 and impedance of Z 0 *(0.072−j1.479) and single diode pair input match versus power level at 915 MHz  810  for m 6  with input power=20.000, S(1,1)=0.515/1.272 and impedance of Z 0 *(3.120−j0.097) and m 7  with input power=15.000, S(1,1)=1.000/−4.687 and impedance of Z 0 *(0.061−j24.437). An ADS HSMS-286C is representative but not limiting. 
       FIG. 9  includes data and circuit model  900  for a sixteen diode rectifier. Data graph  905  depicts point m 1  at a frequency of 865.0 MHz, dB(S(1,1))=8.308 and point m 5  at 860.6 MHz, dB(S(1,1))=6.443. Circuit  910  includes Port  1  with Z=50 ohms  915 , wire inductance L 2  of 0.987 nH  920 , wire inductance L 1  of 9.957 nH  925 . Capacitances include C 5  of 47 pF  930 , C 7  of 47 pF  935 , C 8  of 47 pF  940 , C 9  of 47 pF  945 , and C 11  of 47 pF  950 . Schottky diode models include X 1   955 , X 2   960 , and X 3   965 . 
     Hysteretic Switch 
       FIG. 10  is a schematic diagram  1000  of a hysteretic switch included in embodiments of the invention. Active switch  1000  comprises resistor R 1  of 10M, R 2  of 10M, R 3  of 200 k, R 4  of 10M, R 5  of 10M, R 6  of 2.2M, where R 1  serves as a trickle charge resister. Capacitors include C 1  of 10 μF and C 2  of 0.1 μF. Also included are nanopower comparator U 1 , which, in nonlimiting embodiments, is a MAX919EUK-T; and single pole, double throw (SPDT) switches S 1 B and S 2 B which, in nonlimiting embodiments, are ADG820 designation switches. 
       FIG. 11  provides data for an embodiment  1100  of a hysteretic switch. Active hysteresis switch waveform  1105  provides Cap and Switch out results for −2 to +2 seconds over −0.5 to +2.5 volts. Active hysteresis switch waveform  1110  provides Cap and Switch out results for −0.005 to +0.005 seconds over −0.5 to +2.5 volts. Active hysteresis switch positive edge switching time (50 Ohm load)  1115  provides Cap and Switch out results for −40 to +40 nanoseconds over −0.5 to +2.5 volts. Active hysteresis switch negative edge switching time (50 Ohm load)  1120  provides Cap and Switch out results for −40 to +40 nanoseconds over −0.5 to +2.5 volts. In embodiments, the hysteretic switch trickle charges batteries and or capacitors. Properties are derived from the input voltage level. Employing a hysteretic switch plus low-power components enables use of very small devices. 
     Method 
       FIG. 12  is a flow chart depicting a method  1200  for a transponder using ambient radiation. Ambient radiation is received  1205 , the energy from the ambient radiation is rectified  1210 , energy is stored  1215 , power is switched  1220 , power is modulated by an oscillator  1225 , and a signal is transmitted  1230 . 
     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.