Abstract:
An RF transponder having a plurality of antennas is disclosed, where the voltages produced by the plurality of antennas are cascaded to produce a tag voltage higher than that produced by a single antenna.

Description:
FIELD OF THE INVENTION 
     The field of the invention is the field of Radio Frequency (RF) transponders (RF Tags) which receive RF electromagnetic radiation from a base station and send information to the base station by modulating the load of an RF antenna. 
     BACKGROUND OF THE INVENTION 
     RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the objects, items, animals, and people. It has been known since the early 60&#39;s in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a “base station” at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current led to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna could be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a “passive” tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics, but not to broadcast the information from the tag antenna. An active tag also changes the loading on the tag antenna for the purpose of transmitting information to the base station. 
     The “rebroadcast” or “reflection” of the incoming RF energy at the carrier frequency is conventionally called “back scattering”, even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed “back” to the transmitting antenna. 
     In the 70&#39;s, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor. A transistor switch also changed the loading of the transponder in U.S. Pat. No. 4,786,907 by A. Koelle. 
     Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station. 
     The continuing march of semiconductor technology to smaller, faster, and less power hungry has allowed enormous increases of function and enormous drop of cost of such tags. Presently available research and development technology will also allow new function and different products in communications technology. 
     One fundamental problem with passive tags is that the range is limited by the voltage picked up by the tag antenna and rectified by the tag power conditioning circuits. The voltage must be high enough to run the tag electronics, and the voltage is generally the limiting factor in determining the distance from the base station antenna at which the tags may be used. Even active tags having a battery to run the tag electronics are limited in the voltage picked up by the tag antenna. 
     RELATED PATENTS AND APPLICATIONS 
     Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,521,601; 5,528,222; 5,538,803; 5,550,547; 5,552,778; 5,554,974; 5,563,583; 5,565,847; 5,606,323; 5,635,693; 5,673,037; 5,680,106;5,682,143; 5,729,201; 5,729,697; 5,736,929; 5,739,754; 5,767,789; 5,777,561; 5,786,626; 5,812,065; and 5,821,859. U.S. Patent applications assigned to the assignee of the present invention include: application No. 08/626,820, filed: Apr. 3, 1996, entitled “Method of Transporting RF Power to Energize Radio Frequency Transponders”, by Heinrich, Zai, et al. (now U.S. Pat. No. 5,850,181, issued Dec. 15, 1998); application No. 08/694,606 filed Aug. 9, 1996 entitled RFID System with Write Broadcast Capability by Cesar et al.; application Ser. No. 08/681,741 filed Jul. 29, 1996 entitled RFID Transponder with Electronic Circuitry Enabling and Disabling Capability, by Heinrich, Goldman et al.; application Ser. No. 09/153,617 filed Sep. 12, 1998, entitled RFID Interrogator Signal Processing System for Reading Moving Transponder, by Zai et al; U.S. application filed Nov. 13, 1998 by Duan et al. entitled “RF Identification Transponder having a spiral antenna; U.S. application filed Nov. 13, 1998 by Duan et al. entitled “RF Identification Transponder having a helical antenna; U.S. application filed Nov. 13, 1998 by Duan et al. entitled “RF Identification Transponder employing patch antenna; and U.S. application filed Nov. 13, 1998 by Kodukula et al. entitled “RF Identification Transponder having a spiral antenna; and U.S. application filed Nov. 13, 1998 by Duan et al. entitled “Distributed Impedance matching circuit for high reflection coefficient load. The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to produce an RF transponder comprising circuits which can be made at low cost. It is a further object of the invention to produce an RF transponder which can be used at high frequencies. It is a further object of the invention to produce an RF transponder with maximum range. It is a further object of the invention to produce an RF transponder with circuits which require very little current. It is a further object of the invention to produce an electronic chip for an RF transponder which can be produced simply with standard semiconductor manufacturing techniques. It is a further object of the invention to provide an RF transponder having a novel multiple antenna power and information receiving circuitry for maximum range. 
     SUMMARY OF THE INVENTION 
     The present invention is a system, apparatus and method to use multiple antennas to receive RF power and signals from a base station, where circuitry is provided which will cascade the DC voltages produced by rectifying the signals at each of the multiple antennas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sketch of a circuit diagram for receiving power from two tag antennas. 
     FIG. 2 shows a sketch of an RF tag having two dipole antennas. 
     FIG. 3 shows a sketch of an RF tag having two patch antennas. 
     FIG. 4 shows a sketch of a system for communicating power and information between a base station and an RF tag 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a sketch of a circuit diagram for receiving power from two tag antennas  102  and  104 . Tag antennas  102  and  104  are sketched here as dipole antennas, but they could be any one or any combination of dipole, patch, loop, or slot antennas as are known in the art. Tag antennas  102  and  104  could also be any one of the above and/or any one or any combination of DOG, HOG, or SOG antennas which are described in great detail in: U.S. application filed Nov. 13, 1998 by Duan et al. entitled “RF Identification Transponder having a spiral antenna; U.S. application filed Nov. 13, 1998 by Duan et al. entitled “RF Identification Transponder having a helical antenna; U.S. application filed Nov. 13, 1998 by Duan et al. entitled “RF Identification Transponder employing patch antenna; and U.S. application filed Nov. 13, 1998 by Kodukula et al. entitled “RF Identification Transponder having a spiral antenna; and U.S. application filed Nov. 13, 1998 by Duan et al. entitled “Distributed Impedance matching circuit for high reflection coefficient load”, which are included by reference in this application. Of particular importance to the invention are the innovative impedance matching schemes noted in the above identified applications. Tag antennas  102  and  104  are shown in FIG. 1 providing power and information to voltage doubling circuits  106  and  108  respectively. Voltage doubling circuits  106  and  108  are well known voltage doubling circuits for providing power to passive RF tags, and could be replaced with single rectification circuits or higher order cascade voltage multiplication circuits as known in the art. RF diodes  112  and  114  act together to charge capacitor  116  to a voltage twice the voltage available with a single RF diode. Elements  118 ,  120  and  122  perform a similar function in circuit  130 . Circuits  106  and  108  charge capacitor  124  which may act as a power supply for tag electronics  130 , or may act as a modulated signal receiver for tag electronics in the case that the tag electronics  128  have a battery power supply as in the case of an active RF Tag. Power or signals are fed into the tag electronic section over lines  126  and  128  respectively. Line  128  is shown as the conventional ground. 
     The tag electronics may also receive modulated signals by many other means than by measuring voltage across capacitor  124 . For example, signals may be recovered by measuring the voltage across capacitors  116  or  122 , or receiving circuits may be added as shown in U.S. patent application Ser. No. 08/733,684 which are completely apart from the power receiving circuits  106  and  108 . 
     Modulation of the antenna reflectance characteristics to send signals from the tag to the base station may be performed by modifying the bias conditions on one or more of diodes  112 ,  114 ,  118 ,  120 , or by shorting out capacitors  116 ,  122 , and/or  124  under control of the tag electronic circuitry. (Circuits not shown). 
     FIG. 2 shows a sketch of an RF tag having two dipole antennas  102  and  104  connected to a semiconductor chip  202  which contains the tag electronic circuitry and the RF diodes and capacitors shown in FIG.  1 . The dipole antennas  102  and  104  may be close together and closely coupled as shown in FIG. 2, or they may be widely separated (by more than the wavelength of the RF radiation which they are receiving) and hence not closely coupled. 
     FIG. 3 shows a sketch of an RF tag having two patch antennas  302  and  304  feeding power or information to a semiconductor chip  202  through impedance matching sections  306  and  308 . Impedance matching strips  306  and  308  may be short so that patch antennas  302  and  304  are closely coupled, or may be long so that they are not closely coupled. Patch antennas  302  and  304  are made from electrically conducting material such as metal or conducting polymer material, and are generally connected to a dielectric material  310  which separated patches  302  and  304  from a conducting ground plane  312 . 
     From FIGS. 2 and 3, it is easy to see that one patch antenna may be combined with one dipole antenna to produce more voltage than a single antenna. Note also that patch antennas  302  and  304  could both be circular polarized antennas of opposite handedness, so that at least some voltage would be generated by the combination, where a single antenna might not receive any power or information. In the same way, dipole antennas  102  and  104  could be constructed so that they are mutually perpendicular, instead of parallel as shown. In this case, linearly polarized RF radiation would give at least some power to the combination of the two antennas. 
     FIG. 4 shows a sketch of a base station  410  connected to a computer  420  which is used to send and receive RF signals  430  through antenna  440  to and from an RF tag  450  having two antennas  460  and  470 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise then as specifically described.