Abstract:
A modulated backscatter radio frequency identification device includes a diode detector configured to selectively modulate a reply signal onto an incoming continuous wave; communications circuitry configured to provide a modulation control signal to the diode detector, the diode detector being configured to modulate the reply signal in response to be modulation control signal; and circuitry configured to increase impedance change at the diode detector which would otherwise not occur because the diode detector rectifies the incoming continuous wave while modulating the reply signal, whereby reducing the rectified signal increases modulation depth by removing the reverse bias effects on impedance changes. Methods of improving depth of modulation in a modulated backscatter radio frequency identification device are also provided.

Description:
GOVERNMENT RIGHTS STATEMENT 
   This invention was made with Government support under Contract DE-AC0676RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 

   TECHNICAL FIELD 
   The invention relates to wireless communications systems, radio frequency identification devices, wireless communications methods, and radio frequency identification device communications methods. 
   BACKGROUND OF THE INVENTION 
   Remote wireless communications may be implemented using radio frequency (RF) technology. Exemplary applications utilizing RF technology include identification applications including, for example, locating, identifying, and tracking of objects. Radio frequency identification device (RFID) systems have been developed to facilitate identification operations. For example, one device may be arranged to output and receive radio frequency communications and one or more remotely located devices may be configured to communicate with the one device using radio frequency communications. The remotely located device(s) may be referred to as a tag, while the other device may be referred to as a reader. Some advantages of radio frequency communications of exemplary radio frequency identification device systems include an ability to communicate without contact or line-of-sight, at relatively fast speeds, and with robust communication channels. 
   The assignee of the present invention develops RFID backscatter tags that can be read at extremely long ranges. Various designs are disclosed in patent documents listed below. Some prior RFID tag front end designs have been a compromise between tag receive sensitivity and the quality of backscatter modulation the tag was able to produce due to the choice of the front end component values. For superior tag receive sensitivity, the tag has not been able to produce full depth modulation in some designs. When the tag cannot produce full depth modulation for the length of its entire response to the reader, the tag cannot be read at the maximum distance that would be possible if it did produce full depth modulation for the length of its response. It would be desirable to find a solution to this problem. 
   SUMMARY OF THE INVENTION 
   Aspects of the invention provide the addition of a single component to front end circuitry of a radio frequency identification device, which enables full depth modulation for the entire length of the tags&#39; backscattered response message back to the reader. 
   Other aspects of the invention provide a modulated backscatter radio frequency identification device comprising a diode detector configured to selectively modulate a reply signal onto an incoming continuous wave; communications circuitry configured to provide a modulation control signal to the diode detector, the diode detector being configured to modulate the reply signal in response to be modulation control signal; and circuitry configured to increase impedance change at the diode detector. The detector diode impedance change is reduced when the diode detector rectifies the incoming continuous wave while modulating the reply signal, whereby reducing the rectified signal increases impedance change by removing the reverse bias effects. 
   Yet other aspects of the invention provide a method of improving depth of modulation in a modulated backscatter radio frequency identification device including a diode detector configured to selectively modulate a reply signal onto an incoming continuous wave and communications circuitry configured to provide a modulation control signal to the diode detector, the diode detector being configured to modulate the reply signal in response to the modulation control signal, the method comprising increasing impedance change at the diode detector. The depth of modulation is a function of the impedance change whereby increasing the diode detector&#39;s impedance change also increases the depth of modulation and the range at which the tag&#39;s response can be read. This would otherwise not occur because the diode detector rectifies the incoming continuous wave while modulating the reply signal, whereby reducing the rectified signal increases modulation depth by removing the reverse bias effects on impedance changes. 
   Still other aspects of the invention provide a modulated backscatter radio frequency identification device comprising an antenna; a diode detector coupled to the antenna, for use in receiving radio frequency data from a reader and in replying to the interrogator by modulating a reply signal onto an incoming continuous wave from the reader, the diode detector having an output and an input; communications circuitry including a processor having a digital input and having a modulation control output configured to provide a modulation control signal to the diode detector, the diode detector being configured to modulate the reply signal in response to be modulation control signal; front end circuitry coupled between the diode detector and the communications circuitry, the front end circuitry including a comparator having an output coupled to the digital input of the processor, having a positive input coupled to the output of the diode detector, and having a negative input, the front end circuitry further including a voltage divider having a first resistor coupled between the positive input and the negative input and having a second resistor coupled between the negative input and ground, the front end circuitry further including a capacitor coupled between the negative input and ground, the front end circuitry further including a resistor coupled between the positive input and ground, and the front end circuitry further including circuitry configured to selectively short the capacitor. 
   Further aspects of the invention provide a modulated backscatter radio frequency identification device comprising an antenna; a diode detector coupled to the antenna, for use in receiving radio frequency data from a reader and in replying to the interrogator by modulating a reply signal onto an incoming continuous wave from the reader, the diode detector having an output and an input; communications circuitry including a processor having a digital input and having a modulation control output configured to provide a modulation control signal to the diode detector, the diode detector being configured to modulate the reply signal in response to be modulation control signal; front end circuitry coupled between the diode detector and the communications circuitry, the front end circuitry including circuitry configured to reject spurious radio frequency signals having an output coupled to the digital input of the processor, having a first input coupled to the output of the diode detector, and having a second input, the front end circuitry further including a voltage divider having a first resistor coupled between the first input and the second input and having a second resistor coupled between the second input and ground, the front end circuitry further including a capacitor coupled between the second input and ground, the front end circuitry further including a resistor coupled between the first input and ground, and the front end circuitry further including a transistor having a control electrode coupled to the modulation control output, having a first power electrode coupled to the second input, and having a second power electrode coupled to ground. 
   Still further aspects of the invention provide a modulated backscatter radio frequency identification device comprising an antenna; a diode detector coupled to the antenna, for use in receiving radio frequency data from a reader and in replying to the interrogator by modulating a reply signal onto an incoming continuous wave from the reader, the diode detector having an output and an input; communications circuitry including a processor having a digital input and having a modulation control output configured to provide a modulation control signal to the diode detector, the diode detector being configured to modulate the reply signal in response to be modulation control signal; front end circuitry coupled between the diode detector and the communications circuitry, the front end circuitry including a comparator having an output coupled to the digital input of the processor, having a positive input coupled to the output of the diode detector, and having a negative input, the front end circuitry further including a voltage divider having a first resistor coupled between the positive input and the negative input and having a second resistor coupled between the negative input and ground, the front end circuitry further including a capacitor coupled between the negative input and ground, the front end circuitry further including a resistor coupled between the first input and ground, and the front end circuitry further including a transistor having a control electrode coupled to the modulation control output, having a first power electrode coupled to the second input, and having a second power electrode coupled to ground. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
       FIG. 1  is a block diagram of an exemplary wireless communication system. 
       FIG. 2  is a block diagram of components of an exemplary wireless communication device of the system. 
       FIGS. 3A and 3B  are to be assembled together.  FIGS. 3A and 3B  provide a circuit schematic representation of components depicted in  FIG. 2 , in accordance with various embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Attention is directed to the following commonly assigned applications, which are incorporated herein by reference: U.S. patent application Ser. No. 10/263,826, filed Oct. 2, 2002, Publication No. 2004-0066752, entitled “Radio Frequency Identification Device Communications Systems, Wireless Communication Devices, Wireless Communication Systems, Backscatter Communication Methods, Radio Frequency Identification Device Communication Methods, and a Radio Frequency Identification Device” by inventors Michael A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 10/263,809, filed Oct. 2, 2002, Publication No. 2004-0198222, entitled “Method of Simultaneously Reading Multiple Radio Frequency Tags, RF Tag, and RF Reader”, by inventors Emre Ertin, Richard M. Pratt, Michael A. Hughes, Kevin L. Priddy, and Wayne M. Lechelt; U.S. patent application Ser. No. 10/263,873, filed Oct. 2, 2002, Publication No. 2004-0066279, entitled “RFID System and Method Including Tag ID Compression”, by inventors Michael A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 10/264,078, filed Oct. 2, 2002, Publication No. 2004-0066281, entitled “System and Method to Identify Multiple RFID Tags”, by inventors Michael A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 10/263,940, filed Oct. 2, 2002, Publication No. 2004-0198233, entitled “Radio Frequency Identification Devices, Backscatter Communication Device Wake-Up Methods, Communication Device Wake-Up Methods and A Radio Frequency Identification Device Wake-Up Method”, by inventors Richard Pratt and Michael A. Hughes; U.S. patent application Ser. No. 10/263,997 filed Oct. 2, 2002, Publication No. 2004-0070500, entitled “Wireless Communication Systems, Radio Frequency Identification Devices, Methods of Enhancing a Communications Range of Radio Frequency Identification Device, and Wireless Communication Methods”, by inventors Richard Pratt and Steven B. Thompson; U.S. patent application Ser. No. 10/263,670, filed Oct. 2, 2002, Publication No. 2004-0067764, entitled “Wireless Communications Devices, Methods of Processing a Wireless Communication Signal, Wireless Communication Synchronization Methods and a Radio Frequency Identification Device Communication Method”, by inventors Richard M. Pratt and Steven B. Thompson; U.S. patent application Ser. No. 10/263,656, filed Oct. 2, 2002, Publication No. 2004-0066280, entitled “Wireless Communications Systems, Radio Frequency Identification Devices, Wireless Communications Methods, and Radio Frequency Identification Device Communications Methods”, by inventors Richard Pratt and Steven B. Thompson; U.S. patent application Ser. No. 10/263,635, filed Oct. 4, 2002, Publication No. 2004-0066278, entitled “A Challenge-Based Tag Authentication Model”, by inventors Michael A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 10/269,756, filed Oct. 10, 2002, Publication No. 2004-0203478, entitled RFID Receiver Apparatus and Method, by inventor Jeffrey Wayne Scott; U.S. patent application Ser. No. 09/589,001, filed Jun. 6, 2000, entitled “Remote Communication System and Method”, by inventors R. W. Gilbert, G. A. Anderson, K. D. Steele, and C. L. Carrender; U.S. patent application Ser. No. 09/802,408; filed Mar. 9, 2001, entitled “Multi-Level RF Identification System”; by inventors R. W. Gilbert, G. A. Anderson, and K. D. Steele, now U.S. Pat. No. 6,765,476; U.S. patent application Ser. No. 09/833,465, Publication No. 2002-0149468, filed Apr. 11, 2001, entitled “System and Method for Controlling Remote Device”, by inventors C. L. Carrender, R. W. Gilbert, J. W. Scott, and D. Clark; U.S. patent application Ser. No. 09/588,997, filed Jun. 6, 2000, entitled “Phase Modulation in RF Tag”, by inventors R. W. Gilbert and C. L. Carrender; U.S. patent application Ser. No. 09/589,000, filed Jun. 6, 2000, entitled “Multi-Frequency Communication System and Method”, by inventors R. W. Gilbert and C. L. Carrender, now U.S. Pat. No. 6,745,008; U.S. patent application Ser. No. 09/588,998; filed Jun. 6, 2000, entitled “Distance/Ranging by Determination of RF Phase Delta”, by inventor C. L. Carrender; U.S. patent application Ser. No. 09/797,539, filed Feb. 28, 2001, entitled “Antenna Matching Circuit”, by inventor C. L. Carrender, now U.S. Pat. No. 6,738,025; U.S. patent application Ser. No. 09/833,391, filed Apr. 11, 2001, Publication No. 2002-0149484 A1, entitled “Frequency Hopping RFID Reader”, by inventor C. L. Carrender. 
   Referring to  FIG. 1 , an exemplary wireless communication system  10  is depicted. The exemplary system  10  includes a first communication device  12  and a plurality of second communication devices  14 . First and second communication devices  12 ,  14  are arranged to implement wireless communications  16  in the depicted exemplary embodiment. Possible wireless communications  16  include first wireless signals  18  communicated from first communication device  12  and second communication signals  20  communicated from respective second communication devices  14 . 
   System  10  is provided to illustrate exemplary structural and method aspects of the present invention. In the illustrated embodiment, system  10  is implemented as a radio frequency identification device (RFID) communications system. For example, in such an arrangement, first communication device  12  may be implemented as a reader or interrogator, and second communication devices  14  may be implemented as transponders, such as RFID tags in some configurations, wireless signals  18  may be referred to as forward link wireless signals and wireless signals  20  may be referred to as return link wireless signals communicated responsive to forward link wireless signals  18 . Exemplary wireless communications  16  include electromagnetic signals, such as radio frequency signals. 
   Referring to  FIG. 2 , an exemplary arrangement of one of second communication devices  14  is shown. The exemplary configuration of device  14  includes antennas  30  and  31 , communication circuitry  32 , front end circuitry  34 , and energy source  36 . 
   Energy source  36  may comprise any of a plurality of possible configurations corresponding to the implementation of communication device  14 . Communication device  14  may be implemented in passive, semi-passive or active configurations in exemplary arrangements. 
   In semi-passive implementations, energy source  36  may comprise a battery utilized to provide electrical energy to communication circuitry  32  to implement processing of wireless signals  18  while electromagnetic energy received within device  14  is utilized to generate wireless signals  20 . 
   For passive implementations of device  14 , received electromagnetic energy is utilized to provide operational electrical energy to components of device  14  as well as provide radio frequency energy for communicating wireless signals  20 . In such an implementation, energy source  36  may comprise a power antenna (not shown) and discrete components arranged to convert received electromagnetic energy into usable operational electrical energy. 
   It may be desired to conserve electrical energy of a battery (if utilized) in order to extend the useful, operational life of the battery. In one embodiment, communication circuitry  32  is arranged to operate in a plurality of operational modes, including at least first, second and third different operational modes in one embodiment. Individual ones of the operational modes have different power requirements and consume electrical energy at different rates. Exemplary operational modes are described in a U.S. patent application Ser. No. 10/263,940, entitled “Radio Frequency Identification Devices, Backscatter Communication Device Wake-up Methods, Communication Device Wake-up Methods and A Radio Frequency Identification Device Wake-up Method,” naming Richard Pratt and Mike Hughes as inventors, incorporated herein by reference. 
   Antennas  30  and  31  are arranged to receive electromagnetic energy including signals  18  and to output electromagnetic energy including signals  20 . In alternative embodiments, as shown in the above-incorporated application Ser. No. 10/263,940, a single antenna is employed instead of two antennas. An additional antenna (not shown) may be provided in passive applications to provide operational energy. 
   Communication circuitry  32  includes a processor  38  according to at least one configuration. An exemplary processor  38  is shown in  FIGS. 3A and 3B  and may be implemented as a model number MSP430F1121 or MSP430F1121A available from Texas Instruments, Inc. Descriptions of operation of this processor and pin descriptions can be found on Texas Instrument&#39;s website. Other processor selections or configurations are possible. 
   Processor  38  of communication circuitry  32  is configured to execute instructions to control communication operations of device  14 . For example, processor  38  of communication circuitry  32  is arranged to process received wireless signals  18  and to control communication of outputted wireless signals  20 . In one arrangement, processor  38  is configured to control antenna  30  to generate wireless signals  20  using backscatter modulation communication techniques. Communication circuitry  32  may control outputting of wireless signals  20  using backscatter modulation according to at least one radio frequency identification device communication protocol. 
   For example, communication circuitry  32  controls electrical characteristics of antennas  30  and  31  according to backscatter embodiments. In some embodiments, the processor  38  provides a modulation signal to alter electrical characteristics of one of the antennas  30 ,  31  wherein electromagnetic energy is selectively reflected by the antenna. One of the antennas  30 ,  31  reflects electromagnetic energy to create wireless signals  20 , according to some exemplary backscatter implementations. 
   The modulated signal may be encoded with information to be communicated from device  14  to device  12  (e.g. to a reader). Exemplary information includes identification information, such as a unique serial number which identifies the communicating device  14 , or any other desired information to be communicated. According to some exemplary arrangements, communication devices  12 ,  14  are configured to communicate wireless signals  18 ,  20  using on/off key (OOK) modulation, such as a FM 0  or FM 1  encoding scheme. Other types of modulation or schemes may be utilized to communicate information between devices  12 ,  14 . 
   Communication circuitry  32  arranged to implement RFID communications may be referred to as radio frequency identification device communication circuitry. Communication circuitry  32  may be operable to control communication of wireless signals  20  responsive to processing of one or more commands embodied in wireless signal  18 . 
   Processing of received signals  18  may include extracting an identifier from the wireless signals  18  (e.g., an identifier of the communicating device  12  and\or identifying device  14 ) and also include processing of commands within signals  18 . Responsive to processing, device  14  may selectively output or communicate wireless signals  20  including identification information or other desired requested information from first communication device  12 . 
   Initially, device  12  may output one of signals  18  defining a universal wake-up signal. Such a signal may comprise, for example, a 4 kHz modulated signal. Devices  14  monitor for the reception of the 4 kHz modulated signal wake-up and to enter different operational modes wherein signals  18  may be processed and signals  20  may be communicated. 
   As illustrated in the exemplary configuration shown in  FIGS. 3A and 3B , a 32 kHz crystal may be coupled with pins  5  and  6  of processor  38 . Processor  38  may utilize an internal clock divisor to select and provide reference signals of any of multiple possible frequencies. For example, processor  38  may divide by 8 to provide the 4 kHz modulation signal. 
   Referring to  FIGS. 3A and 3B , exemplary circuitry of communication device  14  is shown. The depicted circuitry of  FIGS. 3A and 3B  illustrates exemplary configurations of antennas  30  and  31 , communication circuitry  32 , processor  38  and front end circuitry  34 . Energy source  36  (not shown in  FIGS. 3A and 3B ) may be coupled with the illustrated VCC terminals and AGND terminals. The depicted exemplary circuitry of  FIGS. 3A and 3B  is provided to illustrate possible methodologies and structures which may be utilized to implement aspects of the present invention. Other alternative arrangements and methods are possible. 
   Radio frequency energy is received via antennas  30 ,  31  and detector diodes  54 ,  56 ,  58 , and  60  coupled with respective antennas  30 ,  31 . The diodes rectify incoming RF. The electrical energy applied to a comparator  40  corresponds to the modulation of the signals  18  provided by the first device  12 . Comparator  40  operates to reject spurious signals and trigger wake-up functionality described in the U.S. patent application incorporated by reference above (Ser. No. 10/263,940). 
   The front end circuitry  34  includes comparator  40 . The comparator  40  requires a predetermined minimum voltage difference between pin  3  (non-inverting input) and pin  4  (inverting input) to change state. In the illustrated embodiment, the comparator  40  needs to see more than a 5 mV difference between pins  3  and  4  to change state; however, alternative values are possible. The DC voltage on pin  3  of the comparator  40  varies depending on distance between the device  14  and the device  12  because an ON-OFF key (OOK) method of communication is used in the illustrated embodiment, and because the diodes  54 ,  56 ,  58 , and  60  act as voltage rectifiers. The output of the comparator  40  is a digital signal coupled to digital I/O pin  10  of the processor  38 . 
   The front end circuitry  34  includes a voltage divider defined by resistors R 1  and R 3 . The voltage divider (R 3 /R 1 ) on pin  4  helps to reduce the effects of the DC voltage variations on pin  3  by biasing pin  4  at one-half the DC voltage on pin  3 . 
   The front end circuitry  34  also includes a capacitor C 1 . The capacitor C 1  helps to average out the instantaneous voltage changes (noise). This also explains poor tag performance at very close ranges since the voltage difference between pins  3  and  4  gets big enough to make it hard to detect the OOK modulation. 
   The front end circuitry  34  also includes a resistor R 17 . The resistor R 17  sets the load on the detector diode output. The resistor values shown have been selected to optimize the load on the detector diode output; however, other values are possible 
   The value of resistor R 1  affects range versus battery life. In the illustrated embodiment, the resistor R 1  has been set at 2.0M. Other values, such as 10M, are possible. In this case when not communicating, the DC voltage on pin  4  would be, for example, 83% of pin  3 , therefore, very minor RF signals may be able to cause the comparator  40  to change state. A change in state of the comparator causes the processor  38  to wake-up, and process the incoming signal. 
   The output of the comparator  40  follows the voltage difference between pins  3  and  4  (its output goes from ground to Vcc) whenever the voltage difference exceeds, for example, ˜5 mV. The output of the comparator  40  is a digital value, and is coupled to digital I/O pin  10  of the processor  38 . 
   Operation of the processor  38 , in accordance with some embodiments, will now be described. In some embodiments, the processor  38  has a sleep mode and an awake mode. 
   In these embodiments, when the comparator  40  is inactive, the processor  38  is asleep. 
   When the comparator  40  changes state, the processor  40  reacts to the state change by interrupt. In some embodiments, on a predetermined comparator transition, e.g., the first transition, the processor  38  wakes up to full speed. 
   The processor  38  begins to measure the period associated with the incoming synchronizing pulses. The period is measured using counters internal to the processor  38  and the I/O pin  10  interrupt. In some embodiments, if the synchronizing data rate does not meet prescribed period limits, the processor  38  goes back to sleep. 
   In some embodiments, if the synchronizing data rate does meet prescribed period limits:
         (a) The processor  38  continues to sample the incoming data using the measured period and, if there is an error (e.g., no data), the processor goes to sleep;   (b) If no error, in some embodiments, the CRC bits are tested. If there is an error in the CRC bits, the processor  38  goes to sleep;   (c) If no error in the CRC bits, the command portion of the data from the reader is executed;   (d) The device  14  responds to the command; and   (e) The processor  38  goes to sleep.       

   Some prior RFID tag front end designs have been a compromise between tag receive sensitivity and the quality of backscatter modulation that the tag was able to produce due to the choice of the front end component values. For superior tag receive sensitivity, the tag has not been able to produce full depth modulation in some designs. When the tag cannot produce full depth modulation for the length of its entire response to the reader, the tag cannot be read at the maximum distance that would be possible if it did produce full depth modulation for the length of its response. 
   Some embodiments of the invention provide the addition of a component to the front end circuitry  34  of an existing design of device  14 , which enables full depth modulation for the entire length of the device&#39;s backscattered response message back to the device  12 . 
   More particularly, the inclusion of a transistor  42  in the front end circuitry  34  makes full depth modulation realizable in the device  14 . While other embodiments are possible, in the illustrated embodiment, the transistor N Channel Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET) such as the Vishay Siliconix TNO200T/TS. 
   Drain  44  of the transistor  42  is coupled to negative (inverting) input  46  of the front end comparator  40 . Source  48  of the transistor  42  is coupled to circuit ground AGND of the device  14 , in the illustrated embodiment. Low or negative voltages could be employed, instead of ground, in alternative embodiments. Gate  50  of the transistor  42  is coupled to a modulation control line  52  of the processor  38 . When the device  14  modulates, the transistor  42  keeps the negative input  46  of the comparator  40  at relative ground during the positive alternation of the modulation waveform instead of allowing capacitor C 1  to gradually charge from the modulation. The charging of capacitor C 1  is the action that raises the bottom or reduces the depth of modulation. The transistor  42  holds this line at relative ground during the positive of the modulation cycle, for the entire length of the modulation cycle, allowing for the largest possible modulation transitions from the device  14 . The larger the amplitude of the modulation transitions, the longer the range from which the device  14  can be successfully read by the device  12 . 
   The capacitor Cl is shorted whenever the line  52  labeled ACLK is HIGH. If the transistor  42  were not installed in the circuit, the voltage on capacitor C 1  would come to a quiescent level above ground due the action of the detector diodes  54 ,  56 ,  58 , and  60 . The device  12  is constantly transmitting RF when the device  14  is trying to communicate back to the device  12 . 
   Consider only the antenna and detector diodes while the device  12  (e.g., a reader) is transmitting a continuous wave and the device  14  (e.g., a tag) is communicating back to the device  12 . The tag communicates by forward-biasing and then removing the forward bias on the detector diodes  54 ,  56 ,  58 , and  60 , but the diodes are still rectifying the incoming RF. So the detector diode output continues to increase in voltage as data is transmitted. The depth of modulation is a function of the change in voltage applied to the antenna or antennas  30 ,  31  through the detector diodes  54 ,  56 ,  58 , and  60 . By keeping the voltage on capacitor C 1  as low as possible, maximum modulation depth is achieved. 
   Laboratory tests have shown that employing this transistor on the device  14  improves the backscatter performance of the device  14  by a minimum of 3 dBm. All backscatter message lengths benefit from this modification, but the improvements are much more apparent with longer backscatter responses which are required by some applications. Without this modification, it would be difficult to read longer backscatter messages from the device  14  at anything other than minimal ranges. 
   This modification has immediate applications on, for example, semi-passive modulated backscatter RFID tags to optimize the depth of their backscatter modulation. 
   In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.