Abstract:
A passive radio frequency transponder (RF tag) having a diode rectifier receiver circuit outside the tag power rectification circuit, the tag power rectification circuit supplying power to the electronics of the RF tag. An additional innovative low current circuit protect the signal capacitor from overvoltage produced by the signal diode. An innovative circuit also clips the signal and sharpens it. An innovative low current circuit is used as a comparator to sharpen the signal pulses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a division of application Ser. No. 10/440,492 filed May 16, 2003, which claims the benefit of provisional U.S. application No. 60/430,553 filed Dec. 3, 2002, and claims the benefit of provisional U.S. application No. 60,385,528 filed Jun. 4, 2002. Said application Ser. No. 10/440,492 is a continuation-in-part of copending nonprovisional U.S. application Ser. No. 10/308,859 filed Dec. 3, 2002 now abandoned. Said application No. 10,308,859 is a continuation-in-part of U.S. application Ser. No. 10/162,418 filed Jun. 4, 2002, now abandoned, which is a continuation of application Ser. No. 09/426,235 filed Oct. 25, 1999, now U.S. Pat. No. 6,400,274 issued Jun. 4, 2002, which is a continuation of U.S. application Ser. No. 09/321,986 filed May 28, 1999. Said U.S. application Ser. No. 10/162,418 is also a continuation-in-part of U.S. application Ser. No. 09/227,768 filed Jan. 8, 1999, now U.S. Pat. No. 6,243,013 issued Jun. 5, 2001. Said application Ser. No. 09/321,986 is a continuation-in-part of application Ser. No. 08/733,684 filed Oct. 17, 1996 now U.S. Pat. No. 5,889,489 issued Mar. 30, 1989 which in turn is a continuation-in-part of application Ser. No. 08/521,898 filed Aug. 31, 1995, now U.S. Pat. No. 5,606,323 issued Feb. 25, 1997. Said U.S. application Ser. No. 10/162,418 is also a continuation-in-part of U.S. applications No. 09/114,037 filed Jul. 10, 1998, now abandoned, No. 09/195,733 filed Jan. 19, 1998, now abandoned, and No. 09/211,584 filed Dec. 14, 1998, which is a continuation of application Ser. No. 08/626,820 filed Apr. 3, 1996, now U.S. Pat. No. 5,850,181 issued Dec. 15, 1998. Said U.S. application Ser. No. 09/321,986 is also a continuation-in-part of application Ser. No. 09/263,057 filed Mar. 6, 1999, now abandoned, which claims the benefit of U.S. Provisional application No. 60/077,094 filed Mar. 6, 1998 and a continuation-in-part of application Ser. No. 09/266,973 filed Mar. 12, 1999, now abandoned, which claims the benefit of provisional application No. 60/077,872 filed Mar. 13, 1998. application Ser. No. 09/922,598 filed Dec. 29, 1998, U.S. Provisional application No. 60/070,347 01/02/98, U.S. Provisional application No. 60/385,528 filed Jun. 4, 2002, U.S. Provisional application No. 60/430,553 filed Dec. 3, 2002, and U.S. Pat. Nos. 6,400,274, 6,243,013, 6,028,564, 6,097,347, 5,808,550 and 5,606,23 are each incorporated herein by reference in its entirety. All of the above identified patents and patent applications are hereby incorporated herein by reference in their entirety including incorporated material. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The field of the invention is 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.  
         [0003]     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 of the objects, items, 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 (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 broadcast the information from the tag antenna. An active tag may also change the loading on the tag antenna for the purpose of transmitting information to the base station.  
         [0004]     The “rebroadcast” 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.  
         [0005]     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.  
         [0006]     A combination diode rectifier circuit and balanced modulator for modulating the antenna current at twice the carrier frequency was proposed by Gary T. Carroll in U.S. Pat. No. 4,724,427.  
         [0007]     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.  
         [0008]     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. The use of the prior art transistor switches to change the loading of the transponder antenna and to receive information, however, leads to increased cost in the use of a totally integrated system consisting of a single chip connected to an antenna. The transistor switch of the prior art must be fast enough and have low capacitance to work well contained on a chip in a reasonable time. Such transistors lead to increased costs in the chip manufacturing, as the entire chip must be made with the same technology and the entire chip does not need the speed of the one transistor element. The range of the communication distance from the base station to the tag is critical. This range is determined by the voltage built up by the antenna and rectifying circuits on the tag. Passive RF tags must do two things which are incompatible. First, there must be a steady supply voltage extracted from the modulated RF field to power the devices on the tag. Second, there must be a data signal recovered from the modulated RF field which has well defined zeros and ones for use by the tag digital electronics. If the signal is taken off from the voltage on the main power supply capacitor of the tag, the voltage swing must be low to provide good power for the electronics, and high to provide good signal.  
         [0009]     The information receiving sections of prior art RF tags draw down the main power supply capacitor which supplies power to the tag when no RF power is sent from the base station. This is wasteful of energy and useless, since there is no information to be received when the RF power is off.  
         [0010]     Prior art tags have modulating circuits and receiver circuits which reduce the voltage which can be produced by the rectifier circuits. Prior art tags have circuits which require relatively high current, which reduces the voltage built up by the antenna and rectifying circuits on the tag.  
       RELATED APPLICATIONS  
       [0011]     Copending patent applications assigned to the assignee of the present invention and hereby incorporated by reference, are: 
        Ser. No. ______ Sep. 9, 1994 entitled RF Group Select Protocol, by a et. al;     Ser. No. ______ Sep. 9, 1994 entitled Multiple Item RF ID protocol, by Chan et. al;     Ser. No. ______ Aug. 31, 1995 entitled Diode Modulator for RF Transponder by Friedman et al.;     application submitted Aug. 9, 1996 entitled RFID System with Broadcast Capability by Cesar et al.; and application submitted Jul. 29, 1996 entitled RFID transponder with Electronic Circuitry Enabling and Disabling Capability, by Heinrich et al.        
 
       OBJECTS OF THE INVENTION  
       [0016]     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 produce a communication system for communicating with the RF transponder of the present invention. It is a further object of the invention to produce a system for controlling the communication system using the present invention. It is a further object of the invention to produce a system for using and changing information received from the transponder of the present invention.  
       SUMMARY OF THE INVENTION  
       [0017]     The invention provides a diode receiver of a passive RF Transponder which is not part of the rectifier power supply circuit to measure the communication signal to the tag. The present invention has further advantages that the same rectification means used in the receiver circuit can also be used as a modulator. The present invention includes innovative protection means for protecting the receiver circuitry. The present invention includes innovative means for comparing a received analog signal with the moving average of the analog signal.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1 . A partial block diagram of the circuits of an RF tag.  
         [0019]      FIG. 2 . Voltage doubling power supply for tag shown with diode receiver outside tag power circuit and with resistor as current drain.  
         [0020]      FIG. 3 . Circuit for a preferred embodiment of the invention  
         [0021]      FIG. 4   a  RF power sent to the tag vs time  
         [0022]      FIG. 4   b . Voltage across power capacitor vs time.  
         [0023]      FIG. 4   c . Raw signal voltage VSIG across signal capacitor vs time.  
         [0024]      FIG. 4   d . A time expanded sketch of the signal voltage VSIG across resistor current drain  
         [0025]      FIG. 4   e . A time expanded sketch of the signal voltage VSIG across transistor current drain.  
         [0026]      FIG. 4   f . A time expanded sketch of the signal voltage VSIG with optional hysteresis circuit.  
         [0027]      FIG. 5 . A preferred embodiment of the invention.  
         [0028]      FIG. 6 . An alternative implementation of a moving average circuit.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     The invention is to use a diode arrangement separate from the rectification section supplying power to the chip as sketched in  FIG. 1 .  
         [0030]      FIG. 1 . is a block diagram of a tag antenna  70 , a tag rectification power supply  2 , a tag receiving section  4 , comprising an RF diode  40  and a tag signal capacitor  50 , a tag signal capacitor drain section  6 , and a gain section  80  for producing digital signals from the analog signal voltage VSIG produced across signal capacitor  50  by RF diode  40 . Optional VSIG averaging and compare circuit  7 , protection circuit  8 , and hysteresis circuit  9  are also shown. Additional tag electronic components and memory elements are not shown.  
         [0031]     The main power supply diodes  10  and  20  feed current to the main power supply capacitor  30  in block  2  in the voltage doubling scheme shown in  FIG. 2 . While a simple voltage rectification and doubling scheme is shown in the diagram, other full or partial wave rectification schemes as known in the art, and voltage doubling or other-voltage multiplication and addition schemes known in the art of power supplies, could be used as well. The raw power fed to capacitor  30  is conditioned by block  34  which has a voltage protection circuit and which supplies a steady and reliable chip power supply voltage VDD as output to run the tag electronics (The connections for powering the blocks are not shown). Several other voltage sources needed by the tag electronics may optionally be produced by the power supply  2 . These are shown in  FIG. 1  as VPMR and VNMR, and will be explained later.  
         [0032]     RF Diode  40  which is separate from the tag power rectification circuit  2  feeds current to charge signal capacitor  50 . The signal capacitor  50  charges up rapidly when the RF field (which is amplitude modulated) changes from the zero to the one state for on-off key signal modulation. (Amplitude modulation schemes where the RF carrier does not drop to zero are anticipated herein, but the examples given are for a 100% amplitude modulation of the carrier signal. Other modulation schemes such as phase and frequency modulation also anticipated.) The voltage VSIG which appears across the signal capacitor  50  is used to produce a digital signal. When the base station turns off the RF field in order to modulate the RF signal to send information to the tag, the charge stored in signal capacitor  50  is drained off by the signal capacitor current drain section  6 , which in the embodiment of  FIG. 2  comprises a resistor  60 . The signal capacitor  50  does not then further drain current from the capacitor  30  during the time where the RF power is turned off, and the capacitor  30  can power circuits such as a clock circuit for a longer time than if the receiver circuit were draining current. The RC time constant for draining signal capacitor  50  must be short compared to the pulse length of the pulse modulation of the RF. If the RC time constant is long, the waveform recovered from the field modulation will be badly distorted, making it difficult to generate the correct recovered bit pattern on the chip. If the resistor  60  is very large, there is not much current draw to load down the antenna  70 , but the time constant becomes long for a reasonable size signal capacitor  50 . If signal capacitor  50  is too small, it does not act as an RF short, and RF can leak into the gain circuit  80  and perturb the tag electronics with unpredictable results. Furthermore, with a small signal capacitor  50 , the circuit is more susceptible to noise and performance values vary strongly with parasitic capacitance changes. VSIG is sent to a gain circuit  80  via line  62  where it is turned into the clipped ones and zeros needed for the digital electronics circuits. Such methods of turning a modulated analog signal into a digital signal are well known to one skilled in the art. The resimicrosecond time constant and a reasonable size signal capacitor  50  is also very expensive to build on a chip, because large resistors take up a lot of chip area. In addition, the current drain through a resistor will have a strong dependence on VSIG.  
         [0033]     A more preferred embodiment of the invention is a novel circuit sketched in  FIG. 3 . In place of the resistor  60  used to drain down the signal capacitor  50  when the RF is shut off, the FET  120  is used. In contrast to the circuit of  FIG. 2 , a constant current is drawn through FET  120  independent of the voltage VSIG. The magnitude of the current through the n-FET  120  is determined by a voltage VNMR on line  132 . VNMR is derived from a constant current source (not shown) in the power supply  2 . Because the current is drawn from the signal capacitor  50  both when the RF field is on and when it is off, the amount of current drained by FET 120  must be chosen carefully. The current must be large enough for a quick discharge of signal capacitor  50  when the field turns off, yet small enough that the recharging of the power capacitor  30  is minimally affected when the field turns on. VNMR is produced by a well known technique of mirroring the current in a well known low current reference generator circuit. VNMR is very stable with respect to the circuit ground, and is relatively independent of the (possibly) fluctuating voltage VDD produced by the power rectification circuit of the tag. The current through n-FET  120  is thus determined by the current through another n-FET, the determination being made principally by the dimensional relationships of the two FETs.  
         [0034]     The modulated RF power sent to the tag is sketched in  FIG. 4   a , and the voltage VDD across the capacitor  30  is sketched in  FIG. 4   b . In the sample data pattern shown, data is Manchester encoded, meaning that the order of two half bits of different polarity determines the bit value being sent. Here, a half-bit  1  followed by a half-bit  0  denotes a 1 while a half-bit  0  followed by a half-bit  1  denotes a 0. The voltage across resistor  60  is the raw signal voltage VSIG which is sketched in  FIG. 4   c .  FIG. 4   d  shows a time expanded sketch of the signal voltage VSIG across resistor  60  of  FIG. 2 , while  FIG. 4   e  shows a time expanded sketch of the signal voltage VSIG when the n mirror FET  120  of  FIG. 3  is used  FIG. 4   f  is a time expanded sketch of the signal voltage VSIG when an optional hysteresis circuit  9 , discussed later, is used with the n mirror controlled FET of  FIG. 3 .  
         [0035]     The voltage VSIG is shown in  FIG. 4   d  as a function of time for a single pulse with expanded time scale for the circuit shown in  FIG. 2 . Note that the falling edge of the pulse is an exponential with a time constant given by RC, where R is the resistance of resistor  60  and C is the capacitance of signal capacitor  50 . In contrast, the voltage falls linearly with the circuit of  FIG. 3 , as sketched in curve  4   e . The time taken for VSIG to reach zero is determined by VNMR. The transistor current drain  120  takes up much less space on the chip than a resistor which would give an adequate RC time constant.  
         [0036]     An additional preferred embodiment of the invention is shown in  FIG. 5 . The voltage VSIG on signal capacitor  50  is averaged in an innovative way in block  7  over a time which may be comparable or may be short compared to an RF modulation frequency half cycle. The instantaneous voltage VSIG is then compared in block  7  to this moving average and when the instantaneous voltage VSIG drops to some threshold compared to the moving average, the gain circuit  80  drains the signal capacitor  50  much faster than the transistor current drain current drain  120  can to give a good falling edge to VSIG. The gain circuit  80  sends a spike voltage to transistor  590  in block  9  of  FIG. 5 . The novel method of taking the average sketched in  FIG. 5  is preferred to a standard averaging circuit using a capacitor and a resistor which is well known in the art for the same reasons that the circuit of  FIG. 3  is preferrepacitor circuit shown takes less chip area and is more controllable and less variable than the standard capacitor and resistor arrangement for averaging a voltage. The p-mirror set up shown in  FIG. 5  uses p-FET  510  and a current defined by VNMR thorough the n-FET  520  to define a voltage VPMR_LOCAL, which is defined with respect to VSIG. The p mirror circuit shown is well known in the art as a way of defining a voltage with respect to another voltage which is not at ground potential. The averaging capacitor  525  charges and discharges relatively slowly, so that VPMR_LOCAL is related to a moving average of VSIG. VPMR_LOCAL is used to compare the present value of VSIG with its moving average in a voltage comparator pull up pull down circuit, where p-FET  540  is controlled by VPMR_LOCAL to pass twice the current as transistor  510  in saturation mode, and n-FET  550  is controlled by VNMR to pass the same current as transistor  510  in saturation mode. When VSIG is steady, the voltage VPUPD is high, since transistor  540  can supply twice the current that transistor  550  needs. However, when VSIG drops a by a percentage, preferrably 25% and more preferably 10%, voltage VSIG minus voltage VPMR_LOCAL drops by a very much higher percentage, while VNMR is unchanged with respect to ground, and transistor  540  can not supply the current needed to transistor  550 , so VPUPD drops rapidly to a very low value. The voltage VPUPD is then used by the gain stage  80  as the signal voltage.  
         [0037]     An innovative optional protection circuit  8  is also shown in  FIG. 5  to protect the signal path electronics  50 ,  7 ,  80 , and  6 . Since the diode  40  rectifying the incoming RF from the antenna  70  is not part of the chip power supply, the voltage VSIG may build up to large values if the tag is close to the base station. The signal capacitor  50  would then be at risk unless some means of protecting it such as protection circuit  8  were implemented. Protection circuit  8  is innovative in that it requires less current and much less space on the chip than the conventional protection circuits used for the main power section included in block  2 . The voltages developed elsewhere on the chip can be used in a novel fashion to cut down the number of devices needed by the protection circuit. Block  2  provides a voltage VDD which powers the chip circuits, and VDD is regulated and limited Block  2  also contains a p mirror circuit which provides a voltage VPMR related to VDD and a n-mirror circuit which provides a voltage VNMR related to ground. VPMR controls the gate on the p mirror p-FET  560 . If VSIG is less than or equal to VDD, FET  560  demands ¼ the current of  570 ; thus, the voltage on “shunt”, the gate voltage of the high current FET  580 , is low and FET  580  is off. If VSIG&gt;VDD, by an amount on the order of tenths of a volt, (VSIG -VPMR) will be large enough to cause FET  560  to source more current than FET  570  can sink. Thus, the gate of the high current FET  580  will be pulled up, turning on FET  580  and acting to pull down VSIG, which will provide the desired protection. This protection circuit draws less than 50 nA when there is no overvoltage condition. The specific ratio of current demanded by FET  560  versus FET  570  when VSIG is less than or equal to VDD is not critical, but is preferably substantially less than 1. If the ratio is near or above 1, the shunt may turn on when VSIG is less than VDD, draining power from the field into the signal path unnecessarily.  
         [0038]     An optional feedback circuit is shown as block  9  in  FIG. 5  to provide a hysteresis in the signal measuring circuit. When the gain circuit  80  detects a falling edge on VSIG, circuit  80  provides a voltage pulse to a high current transistor  590  to short signal capacitor  50  to ground. The transistor  590  preferably can carry VSIG in this case is shown in  FIG. 4   f.    
         [0039]     The p-channel transistors  510 ,  540 , and  560  of  FIGS. 5 and 610  and  640  of  FIG. 6  may optionally have their n-wells connected to VSIG or VDD. There is less noise when the n-wells are connected to VDD. The n-channel transistors of  FIG. 5  have their bulk contact connected to ground.  
         [0040]      FIG. 6  shows an alternative implementation of a moving average circuit where the roles of the p-channel and n-channel devices have been reversed. Here, VPMR is used to generate VNMR_LOCAL for the moving average generation.  
         [0041]     While the particular circuits shown in  FIGS. 5 and 6  are preferred because they are particularly suited to low current operation and use the least number of devices and chip area possible, it is anticipated by the inventors that a number of equivalent circuits are possible which perform the same functions as the circuits shown. In particular, circuits where the roles of the p-channel devices and the n-channel devices are reversed are anticipated.