Patent Publication Number: US-6909326-B2

Title: Amplifier modulation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. Ser. No. 09/978,329, filed Oct. 15, 2001 now U.S. Pat. No. 6,737,973, the disclosure of which is herein incorporated by reference. 

   TECHNICAL FIELD 
   The invention relates to radio-frequency systems, and more particularly, to amplifier modulation in radio-frequency systems. 
   BACKGROUND 
   Radio-Frequency Identification (RFID) technology has become widely used in virtually every industry, including transportation, manufacturing, waste management, postal tracking, airline baggage reconciliation, and highway toll management. One common use of RFID technology is in an Electronic Article Surveillance (EAS) system that is to protect against shoplifting or otherwise unauthorized removal of an article. In particular, an EAS system may be used to detect the presence of EAS markers (tags) that pass through an energizing field. Retail outlets, libraries, video stores and the like make use of RFID technology in conjunction with EAS systems to assist in asset management, organization, and tracking of inventory. 
   A typical RFID system includes RFID tags, an RFID reader, and a computing device. The RFID reader includes a transmitter that may provide energy or information to the tags, and a receiver to receive identity and other information from the tags. The computing device processes the information obtained by the RFID reader. In general, the information received from the tags is specific to the particular application, but often provides identification for an item to which the tag is fixed, which may be a manufactured item, a vehicle, an animal or individual, or virtually any other tangible article. Additional data may also be provided for the article. The tag may be used during a manufacturing process, for example, to indicate a paint color of an automobile chassis during manufacturing or other useful information. 
   The transmitter outputs RF signals that create an energizing field, from which the tags receive power, allowing the tags to return an RF signal carrying the information. The tags communicate using a pre-defined protocol, allowing the RFID reader to receive information from multiple tags in parallel, or essentially simultaneously. The computing device serves as an information management system by receiving the information from the RFID reader, and performing some action, such as updating a database or sounding an alarm. In addition, the computing device serves as a mechanism for programming data into the tags via the transmitter. 
   To transfer data, the transmitter and the tags modulate a carrier wave according to various modulation techniques, including amplitude modulation (AM), phase modulation (PM), frequency modulation (FM), frequency shift keying (FSK), pulse position modulation (PPM), pulse duration modulation (PDM) and continuous wave (CW) modulation. In particular, the transmitter makes use of an amplifier, typically a Class-A or a Class-A/B amplifier, to drive an antenna with a modulated output signal. These amplifiers may require significant power to communicate with the tags. An amplifier may require, for example, 10 watts of power to produce an RF signal having a single watt of power. In other words, a conventional reader may dissipate over 9 watts of power to produce a single watt of output, resulting in approximately 10% efficiency. The heat dissipation requirements and power consumption of such an amplifier are not well suited for a number of applications, including those that require a low-cost, hand-held RF reader. Consequently, conventional hand-held readers may have smaller power outputs, such as 100 milliwatts, but have limited communication ranges and similar power inefficiencies. 
   SUMMARY 
   In general, the invention is directed to an efficient amplifier for use in radio-frequency identification (RFID) applications. In particular, the invention provides a highly efficient amplifier that requires little power, yet has significant modulation bandwidth to achieve high data communication rates. The amplifier incorporates many elements of a Class-E amplifier, yet overcomes bandwidth and other limitations typically inherent in such an amplifier. 
   In one embodiment, the invention is directed to an apparatus for producing an amplitude modulated RF signal to communicate with an RFID tag. The apparatus makes use of a class E amplifier that includes a first transistor. A second transistor is used to connect a current path in parallel to the first transistor. The current in this path may be limited by a series resistor or other means. A controller selectively controls the first and second transistors to achieve 100% amplitude modulation at a high modulation bandwidth. 
   In another embodiment, the invention is directed to an apparatus for producing an amplitude modulated RF signal having less than 100% amplitude modulation, such as 10% amplitude modulation. The apparatus comprises a class E amplifier having a first transistor and an inductor coupling the first transistor to a supply voltage via a first resistor. A second transistor is connected in parallel to the first resistor. A controller is coupled to the first and second transistors. By activating and deactivating the second transistor, the controller varies the supply voltage and causes amplitude modulation of the produced RF signal. 
   In another embodiment, the invention is directed to a radio-frequency identification (RFID) reader that comprises an amplifier that produces an amplitude modulated signal. The amplifier includes an inductor coupling a first transistor and a shunt capacitor to a power supply via a first resistor. A second transistor within the amplifier is used to connect a current path in parallel to the first transistor. A third transistor is connected in parallel to the first resistor. A controller selectively controls the first, second and third transistors. The RFID reader includes an antenna to receive the amplitude modulated signal and output an RF communication. 
   In another embodiment, the invention is directed to a method of generating an amplitude modulated signal. A first transistor of a class E amplifier is modulated at a frequency for a first period of time. When modulating the first transistor, a second transistor connected in parallel to the first transistor is deactivated. The first transistor and the second transistor are then simultaneously deactivated and activated, respectively, for a second period of time. 
   The invention provides many advantages. Unlike conventional Class-E amplifiers that are limited to relatively narrow modulation bandwidth, the inventive amplifier described herein is able to achieve substantially increased data transmission rates. In particular, the large inductance of a conventional Class-E amplifier resists rapid amplitude modulation of the current passing through it, thus rapid amplitude modulation of the RF energy produced by the conventional Class-E amplifier is resisted. By utilizing a second transistor in parallel with the first transistor, and selectively activating and deactivating the transistors, the current of the inventive amplifier does not decay and rebuild during modulation, as with conventional Class-E amplifiers, but rather, the current level remains relatively constant. In addition, the inventive amplifier requires less power than other amplifiers typically used to achieve higher bandwidth. Accordingly, the invention provides reduced heat dissipation requirements, thereby reducing the need for costly heat sinks and batteries. Accordingly, the amplifier may be used in a fully portable, hand-held RFID reader that can conform to RFID standards requiring a higher modulation frequency. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a block diagram illustrating an example Radio-Frequency Identification (RFID) system. 
       FIG. 2  is a block diagram illustrating an example RFID reader. 
       FIG. 3  is a schematic diagram illustrating an example amplifier for use within the RFID reader. 
       FIG. 4  is a graph illustrating an exemplary amplitude modulated signal. 
       FIG. 5  is a schematic diagram illustrating another example amplifier for use within the RFID reader. 
       FIG. 6  is a graph illustrating another amplitude-modulated signal. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram illustrating an example Radio-Frequency Identification (RFID) system  2 . RFID system  2  includes an RFID station (“station”)  4  that interacts with tag  14  via radio frequency signals  16 . In particular, station  4  includes RFID reader (“reader”)  12  that provides energy to tag  14 , and receives information from tag  14 , by producing and receiving RF communications  16  via antenna  15 . Reader  12  and tag  14  communicate using RF communications  16  that are amplitude modulated according to a defined protocol, such as the ISO/IEC 14443 and ISO/IEC 15693 standards specify. 
   Station  4  provides a workstation or other computing environment for processing the information received from tag  14 , and for providing tag programming information to reader  12 . Station  4  includes, for example, processor  8  that communicates with reader  12  via communication link  13 . Reader  12  may be internal to station  4 , as illustrated, or may be external or even a hand-held, portable reader. Accordingly, link  13  may be an RS-232 serial communication link, a wireless link, or other suitable connection for exchanging information with reader  12 . 
   Processor  8  maintains data  11  that represents a compilation of the various articles, tags and associated information. An operator interacts with station  4  via input device  10  and display  6 . Processor  8  receives the input from the operator and, in response, updates data  11 . The operator may provide, for example, identity or other information describing an article to which tag  14  is affixed. Data  11  may contain, for example, a bar code database storing bar code information for the articles. Processor  8  communicates the information to reader  12  via link  13 , which outputs appropriate RF signals  16  to program tag  14 . Processor  8  may store data  11  on any suitable computer-readable medium, such as random access memory (RAM), non-volatile memory, a magnetic medium, and the like. 
   As described in detail below, reader  12  incorporates a highly efficient amplifier that requires little power, yet achieves significant data rate. In particular, the amplifier incorporates many elements of a Class-E amplifier, yet overcomes the modulation bandwidth limitations inherent in such an amplifier. Accordingly, RFID reader  12  can conform to RFID standards requiring a wider modulation bandwidth. The efficiency and reduced power requirements of the amplifier allow reader  12  to achieve reduced size and weight. Consequently, reader  12  may be readily incorporated into a desktop workstation or a portable hand-held computing device, such as personal data assistant (PDA) or the like. 
     FIG. 2  is a block diagram illustrating an example embodiment for reader  12 . Communication interface  22  receives programming information from station  4  via link  13 , and forwards the information to controller  24 . During transmission, controller  24  directs amplifier  28  via control lines  25  to efficiently produce an amplitude-modulated signal  27  in accordance with a modulation scheme, such as 100% amplitude modulation. Amplifier  28  incorporates many elements of a Class-E amplifier, yet overcomes the modulation bandwidth limitations inherent in such an amplifier. 
   Coupler  30  receives signal  27  and provides signal  27  to antenna  15  for transmission as an RF communication. Coupler  30  also provides receiver  32  with a signal  29  representative of an inbound RF communication received via antenna  15 . Receiver  32  extracts digital information from signal  29 , typically by demodulating the signal, and forwards the information to controller  24  for communication to station  4  via communication interface  22 . 
     FIG. 3  is a schematic diagram illustrating an example amplifier  28  that efficiently produces a 100% amplitude modulated signal for transmission by antenna  15  (FIG.  2 ). Amplifier  28  includes a number of components that are arranged as a typical Class-E amplifier, outlined in  FIG. 3  by dotted lines  55 . In particular, inductor  40  acts as a current source for amplifier  28 , and is coupled to a supply voltage  34 . Amplifier transistor  50  connects inductor  40  to ground, and may be a metal oxide semiconductor field-effect (MOSFET) transistor having a drain connected to inductor  40  and a source to ground. 
   Controller  24  is connected to a gate of amplifier transistor  50 , which is operated as a switch in response to a control signal. In particular, application of a positive bias voltage to the gate of amplifier transistor  50  causes a large current to flow from supply voltage  34 , through inductor  40  to ground. Shunt capacitor  52  holds the voltage on the drain of amplifier transistor  50  during on-to-off switch transition, thereby avoiding switching losses. Resistor  53  represents the load of amplifier  28 , i.e., antenna  15  (FIG.  2 ). Capacitor  42 , inductor  48  and resistor  53  are designed such that the drain voltage of amplifier transistor  50  falls back to zero prior to the off-to-on switch transition, again avoiding switching losses. 
   In addition to these components, amplifier  28  includes bias transistor  39  coupled to resistor  41 , and connected in parallel to amplifier transistor  50  and shunt capacitor  52 . As described in detail below, bias transistor  39  provides a second path for supply current from inductor  40  when amplifier transistor  50  is open. In particular, a drain of bias transistor  39  is coupled to inductor  40  via resistor  41 . A source of bias transistor  39  is connected directly to ground. 
   Controller  24  selectively activates transistors  39  and  50  to cause amplifier  28  to produce an output signal in which an envelope for the signal is 100% amplitude modulated. In particular, controller  24  switches the amplifier transistor  50  at a high-frequency, such as 13.56 MHz, while holding open bias transistor  39 . Switching amplifier transistor  50  causes energy to be stored within inductor  40 , and current to periodically flow through amplifier transistor  50 . As a result, amplifier  28  produces an output signal having an envelope of 100% amplitude. 
   After switching amplifier transistor  50  for a period of time, controller  24  then simultaneously deactivates amplifier transistor  50 , and activates bias transistor  39  for a second period of time. During this period, current flows from inductor  40  to ground via resistor  41  and bias transistor  39 , reducing the envelope of the output signal to substantially 0%. In this manner, controller  24  maintains current flow through inductor  40  and prevents the energy stored within inductor  40  from decaying. 
   When controller  24  initiates switching of transistor  50  during subsequent modulation cycles, current through inductor  40  is not needed to re-energize inductor  40 , since the stored energy in inductor  40  was maintained, thus allowing for a shorter rise time from 0% amplitude to 100% amplitude. Consequently, amplifier  28  can achieve increased data transfer rates. In one embodiment, supply voltage  34  provides five (5) volts, while resistors  53  and  41  have resistances of 12 and 24 ohms, respectively. Capacitors  52 ,  42  have capacitances of 150 and 50 picoFarads, respectively, and inductors  40 ,  48  have inductance of 25 and 3 microHenries, respectively. In addition, bias transistor  39  and amplitude transistor  50  may be metal oxide semiconductor field-effect transistors (MOSFET&#39;s). 
     FIG. 4  is a graph illustrating an output signal  72  produced by amplifier  28 , and which has an envelope that achieves 100% amplitude modulation. In particular, output signal  72  represents the voltage across resistor  53  (FIG.  3 ). The envelope of signal  72  switches between a maximum voltage (V MAX ) and a minimum voltage (V MIN ) for a first time period T 1 . During a second time period T 2 , output signal  72  has a voltage of approximately 0 volts. 
   The simulation illustrated in  FIG. 4  assumes that amplifier transistor  50  and bias transistor  39 , as well as supply voltage  34 , are initially off. At 0.1 μs, controller  24  begins a first cycle of amplitude modulating output signal  72  by switching amplifier transistor  50  at a high frequency, such as 13.56 MHz, e.g., with a 50% duty cycle. In addition, controller  24  maintains bias transistor  39  in an off state. At approximately 4.2 μs, and after switching amplifier transistor  50  for a time period T1, controller  24  simultaneously activates bias transistor  39  and deactivates amplifier transistor  50 . During this period, current flows through inductor  40  to ground via resistor  41  and transistor  39 , causing output signal  72  to have an amplitude of substantially zero volts. 
   At approximately 6.2 μs, and after delaying for a time period T2, controller  24  begins a second cycle  76  of amplitude modulation. In particular, controller  24  deactivates bias transistor  39  and begins switching amplifier transistor  50 . In this manner, the current flowing through inductor  40  is maintained at all times, preventing current decay which is typically experienced by conventional Class-E amplifiers. 
   As a result, the envelope of output signal  72  more quickly reaches 100% amplitude during the second cycle  76 , and for subsequent cycles, than for the first cycle  74 . Consequently, amplifier  28  is able to achieve higher data rates than conventional Class-E amplifiers. In addition, a shorter rise time during amplitude modulation is advantageous when communicating with multiple tags simultaneously and, in particular, avoiding collisions between communications from the various tags. Yet another advantage of biasing the current through the inductor  40  is the attenuation of ringing within output signal  72  that otherwise is inherent in Class-E amplifiers due to the RLC components. 
     FIG. 5  is a schematic diagram illustrating another example amplifier  80  for use within reader  12 . In particular, amplifier  80  produces an output signal having less than 100% amplitude modulation, such as 10% amplitude modulation. Similar to amplifier  28  described above, amplifier  80  includes a number of components  62 ,  64 ,  66 ,  68 ,  70  that are arranged as a typical Class-E amplifier, outlined by dotted lines  75 . 
   In addition to these components, amplifier  80  includes a modulation transistor  60  connected in parallel to resistor  58 , which connects inductor  62  to supply voltage  54 . In particular, modulation transistor  60  may be a MOSFET transistor having a source and a drain connected across resistor  58 . By activating and deactivating modulation transistor  60 , controller  24  varies the supply voltage received by inductor  62 , and causes amplitude modulation of the output signal produced by amplifier  80 . 
     FIG. 6  is a graph illustrating a portion of an envelope of output signal  82  produced by amplifier  80 , in which the envelope achieves 10% amplitude modulation. In particular, the envelope of signal  82  modulates between peak voltage amplitudes of a first maximum voltage (V MAX1 ) and a second maximum voltage (V MAX2 ). V MAX2  may be, for example, approximately 90% of V MAX1 . 
   The simulation illustrated in  FIG. 6  assumes that at 0.0 μs (not shown), controller  24  begins a first modulation cycle by switching amplifier transistor  64  at a high frequency, such as 13.56 MHz, with a 50% duty cycle. In addition, controller  24  deactivates modulation transistor  60 , causing current to flow through resistor  58 , and supply voltage  54  to drop across resistor  58 . Consequently, at 3.8 μs the envelope of signal  82  has a peak voltage of V MAX2 . 
   At approximately 4.8 μs, and after switching amplifier transistor  64  for a time period, controller  24  activates modulation transistor  60 , causing current to bypass resistor  58 , and increasing the envelope of signal  82  from V MAX2  to V MAX1 . In this manner, controller  24  amplitude modulates output signal  82 . In one embodiment, 10% amplitude modulation is achieved by selecting supply resistor  58  to have a resistance of 2 ohms. The various components of amplifier  80 , however, can be selected to achieve other desired modulation schemes. 
   Various embodiments of the invention have been described, including embodiments for producing amplitude modulated RF signals to communicate with an RFID tag. The apparatus makes use of many of the components of a conventional class E amplifier. Notably, advantages may be achieved by incorporating the embodiments described above into a single RFID reader. The reader may, for example, readily support multiple modulation schemes, such as 100% and 10% amplitude modulation, by incorporating the embodiments described above. Accordingly, the reader could readily selectively use the embodiments to support a variety of tag types. In addition, the reader may also use the bias transistor while achieving amplitude modulation with the modulation transistor. This may be advantageous in reducing the rise time during a modulation scheme of less than 100% amplitude modulation. These and other embodiments are within the scope of the following claims.