Abstract:
A method for use in a RFID reader, the method comprising providing an output signal from an amplifier of a transmitter, attenuating the output signal via a step attenuator, detecting and comparing the attenuated output signal to a reference value, and adjusting power to the amplifier until the attenuated output signal is substantially equal to the reference value. The attenuated output signal drives a local oscillator signal of a receiver thereby canceling noise generated by the transmitter in the receiver.

Description:
PRIORITY DATA 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/657,120 entitled “RFID DEVICE AND METHOD,” filed Feb. 28, 2005. 
    
    
     BACKGROUND 
     RFID or radio frequency identification technology has been used in a variety of commercial applications such as inventory tracking and highway toll tags. In general, a transceiver tag or transponder transmits stored data by backscattering varying amounts of an electromagnetic field generated by an RFID reader. The RFID tag may be a passive device that derives its electrical energy from the received electromagnetic field or may be an active device that incorporates its own power source. The backscattered energy is then read by the RFID reader and the data is extracted therefrom. 
     The RFID reader includes a transmitter that provides the electrical energy or information to the RFID tag. To accomplish this, the transmitter employs a power amplifier to drive an antenna with an unmodulated or modulated output signal. Traditionally, in order to generate highly controlled (i.e., shaping the modulation wave in order to minimize unwanted spectral content) amplitude modulation (AM) for the output signal, a highly linear power amplifier running in Class-A mode has been used. However, RFID readers that utilize Class-A power amplifiers are inefficient, require more of heat-sinking, and have poor noise figure. Additionally, these readers are not operable under certain applications such as Power Over Ethernet (POE) which have maximum power consumption requirements. 
     Various methods have been used to control the power output of the RFID reader. Many of them involve calibrating each individual power output setting step during the reader production process. This requires complex algorithms or lookup tables and time consuming calibration procedures. What is needed is a method for controlling the power output of the RFID reader that allows for accurate steps in the power output setting without requiring large firmware overhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a simplified schematic diagram of an embodiment of an RFID reader. 
         FIG. 2  is a simplified schematic diagram of an embodiment of a carrier signal generator in  FIG. 1 . 
         FIG. 3  is a simplified schematic diagram of an embodiment of a controller in  FIG. 1 . 
         FIG. 4  is a simplified schematic diagram of an embodiment of a power amplifier system of a transmitter in  FIG. 1 . 
         FIG. 5  is a detailed circuit diagram of an embodiment of a power amplifier in  FIG. 4 . 
         FIG. 6  is a simplified flowchart of an embodiment of a method for controlling output power of an RFID reader. 
         FIG. 7  is a simplified flowchart of an embodiment of a method for calibrating an RFID reader. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified schematic diagram of an embodiment of an RFID reader  100 . Although the reader  100  is described in the context of RFID, it may be adapted for use in non-RFID applications. The reader  100  comprises a receiver  300  that uses multiple mixers or multipliers. An example of a receiver that may be used in this embodiment is described in co-pending U.S. patent application Ser. No. 10/992,958, filed Nov. 19, 2004, entitled “HOMODYNE RFID RECEIVER AND METHOD,” which is incorporated herein by reference. The reader  100  also comprises a transmitter  200  that is coupled to the receiver  300  and a controller  500 . A demodulator  400  such as an amplitude shift keying (ASK) demodulator is coupled to the receiver  300  and the controller  500 . The receiver  300  may be followed by an optional subcarrier demodulator depending on the RFID protocol used. 
     The transmitter  200  comprises a power amplifier (PA) system  210  coupled to a forward power tap  230 . The forward power tap  230  includes a directional coupler to feed a portion of the output signal coming from the PA system  210  to a step attenuator  240 . The output of the step attenuator  240  is split, a portion  242  is fed to a power detector  250  and the other portion  244  is provided to the receiver  300  to drive a local oscillator (LO) signal therein. The output  228  of the power detector  250  is coupled to the controller  500 . The forward power tap  230  is also coupled to a circulator  260  which is coupled to an antenna  270 . An example of an antenna that may be used in this embodiment is described in co-pending U.S. patent application Ser. No. 11/362,951 entitled “CIRCULARLY POLARIZED SQUARE PATCH ANTENNA,” which is incorporated herein by reference. The circulator  260  is operable to isolate the receive path from the transmit path when one antenna is used. Alternatively, the reader  100  may employ two antennas, one for the transmitter  200  and one for the receiver  300  including an optional antenna switch. The circulator  260  is also coupled to a reverse power tap  280 . The reverse power tap  280  may use a directional coupler to obtain a portion  282  of a reflected transmitted power and feeds this into a power detector in the controller  500  to detect mismatch. The other portion  284  is the received signal coming from the reverse power tap  280  which is received from the antenna  270  and is fed into the receiver  300  for processing. 
     Referring also to  FIG. 2 , the transmitter  200  also comprises a carrier signal generator  220  that is coupled to the input  213  of the PA system  210 . The carrier signal generator  220  may include a reference clock  223  coupled to a synthesizer  222  which is then coupled to a pre-amplifier  221 . The reference clock  223  may be a 20 MHz clock with a total tolerance that is compliant with local regulations and RFID protocol standards. After sufficient buffering, the reference clock signal also drives a clock signal  224  in the controller  500 . The synthesizer  222  is able to generate frequencies in the range of 860 MHz to 930 MHz with a step size of 250 kHz for FCC regulated areas or 200 kHz for ETSI regulated areas or smaller that is equal to 20 MzHz divided by a whole number. The synthesizer  222  uses the 20 MHz reference clock signal as its reference input  225  and outputs a VCO signal  226 . The pre-amplifier amplifies the VCO signal  226  for input  213  to the PA system  210 . The transmitter  200  may also comprise a temperature sensor  290  that is coupled to the controller  500  and provides its output to the controller. 
     Referring also to  FIG. 3 , the controller  500  comprises a digital signal processor (DSP)  510  coupled to an analog-to-digital converter (ADC)  520 , a digital-to-analog converter (DAC)  530 , and memory  540 . The DSP&#39;s timing is driven by the reference clock of the carrier signal generator  220 . The output  229  of the demodulator  40  is also coupled to the DSP  510 . The ADC  520  receives analog signals from various components of the RFID reader  100  and converts these signals to digital signals so that the DSP  510  can evaluate and process the data. The DAC  530  converts digital signals from the DSP  510  to analog signals to control the various components of the reader  100 . It is understood that the controller  500  may include other circuitry that supports communications between the DSP  510  and other components of the reader and that other types of microcontrollers can be use that provide similar functionality. 
     In operation, the carrier signal generator  220  generates a radio frequency (RF) carrier signal  227  that is provided to the PA system  210  to modulate with an information signal generated by the controller. The transmission output signal  214  from the PA system  210  of the transmitter  200  includes the carrier signal  227  modulated by the information signal. The method of modulation will be described in detail below. The transmission output signal  214  is radiated by the antenna  270  to an RF transponder or RFID tag (not shown). The signal radiated back from the RFID tag in response to the transmitted signal is captured by the antenna  270  and delivered to the receiver  300  by the reverse power tap  280 . The receiver  300  is operable to mix the received signal with components of the LO signal provided by the step attenuator  240 . The resultant baseband signals may be further demodulated by the demodulator  400  and the data extracted by the controller  500  for further processing. Details of the PA system  210  and operations thereof are described below with reference to  FIGS. 4-7 . 
       FIG. 4  is a simplified schematic diagram of the PA system  210  of the transmitter  200  in  FIG. 1 . The PA system  210  comprises a class-C power amplifier  219 , a transistor  216  having an emitter-follower configuration  218 , and an input  231  to a bias circuit  215 . Alternatively, the transistor  216  may have a source-follower configuration. As stated above, the transmission output signal  214  from the PA system  210  of the transmitter  200  includes the carrier signal  227  modulated by the information signal. The carrier signal  227  is generated by the carrier signal generator  220  and is coupled to the input  213  of the PA  219 . The information signal is generated by the controller  500  and is called MODULATION DAC  212 . This signal  212  is coupled to the base of the transistor  216 . A supply signal  211 , PA_PWR that is controlled by the DSP  510 , is coupled to the collector of the transistor  216 . 
     Amplitude modulation (AM) takes place by changing a DC power supply voltage  217  of the PA  219 . This is accomplished by driving the voltage for the emitter-follower circuit  218  with the MODULATION DAC signal  212 . A signal that is not modulated is generated by driving the emitter-follower circuit  218  to the high side of the power supply voltage  211 . Thus, the voltage level of PA_PWR  211  determines the output power of the PA  219 . The output signal  214  of PA  219  is a continuous wave (CW) RF signal since PA_PWR  211  is such that the emitter voltage reaches its ceiling. In order for the PA  219  to modulate the signal  227 , the DSP  510  ( FIG. 2 ) has access to the DAC  530  and generates the MODULATION DAC  212  signal. The output at the emitter which is the supply voltage  217  of PA  219  follows the input at the base which is the MODULATION DAC waveform  212 . The emitter voltage decreases depending on the required modulation depth. As a result, the DSP  510  is capable of shaping the modulation spectrum to achieve a bandwidth compliant with regulations and produces very linear modulation. Alternatively, if there is any deviation of linearity then the DSP  510  can pre-distort the information (MODULATION DAC) waveform  212  so that the net result is a very linear operation. The DSP  510  can also implement PA  219  on/off functionality and power ramping requirements by using the MODULATION DAC signal  212 . 
     Additionally, phase modulation can also be implemented by this configuration. This is accomplished by hard-switching the phase of the input signal  213  to the PA  219  and at the same time shaping the envelope of the carrier signal by the method discussed above. Hard-switching the phase is done by inverting the input signal so that there is a hard toggle between 0 and 180 degrees. The resulting modulation is a true phase-reversal amplitude shift keying (PRASK) modulation as described in the C1G2 Electronic Product Code (EPC) standard for RFID. 
       FIG. 5  is a detailed circuit diagram of the power amplifier in  FIG. 4 . The PA  219  operates in Class-C mode and thus, is classified as a Class-C power amplifier. The PA  219  comprises a transistor  800  having a gate, source, and drain. As an example, the transistor  800  may be a common source N-channel enhancement mode MOSFET. It is understood that the other types of transistors may be used to provide the same functionality for the power amplifier. The gate of the transistor  800  is coupled to a transmission line/inductor  812 . The transmission line/inductor  812  is coupled to a capacitor  801  which is coupled to the input signal  213  of the PA  219 . The transmission line/inductor  812  is also coupled to a variable capacitor  802 , capacitors  805 ,  806 , and an inductor  807 . The other side of the capacitor  802  is coupled to ground. The capacitors  805  and  806  are coupled in parallel with each other with the other side coupled to ground. The other side of the inductor  807  is coupled to the bias circuit  215 . The source of the transistor  800  is coupled to ground. The drain of the transistor  800  is coupled to a transmission line/inductor  813  which is coupled to a variable capacitor  803 , capacitors  804 ,  808 ,  809 ,  810 , and an inductor  811 . The capacitors  808 ,  809 , and  810  are coupled in parallel with each other with the other side coupled to ground. The other side of the inductor  811  is coupled to the supply voltage  217 . The other side of capacitor  804  is coupled to the output signal  214  of the PA  219 . 
     The PA  219  operates in Class-C mode to achieve high efficiency resulting in low power consumption. To facilitate proper power amplifier operation, the PA  219  utilizes high-Q LC tank circuits both at the input  213  and at the output  214 . Q represents a quality factor of the tank circuit and is defined as the ratio of energy stored during one complete RF cycle to energy consumed. Thus, using high-Q LC tank circuits means that there is little loss in the input and output which provides for proper tuning and impedance matching of the PA  219 . For Class-C operation, the gate DC bias voltage is set just below the transistor  800  pinch-off point such that the PA  219  does not draw a drain current without a drive signal. With a sufficiently high level input signal  213 , the transistor  800  may operate in a saturation region. Biasing is implemented by the bias circuit  215  which comprises a digital or analog potentiometer to allow for adjustment of this gate voltage. 
     The consequence of using the Class-C amplifier as described above is that linearity is extremely poor because the conduction angle is much less than 180 degrees. Thus, Class-C amplifiers are not suitable for amplifying amplitude-modulated signals. In order to remedy this, modulation is performed by changing (modulating) the power supply voltage  217  as discussed above in  FIG. 4 . Therefore, the configuration of the PA system  210  allows for low DC power consumption because of Class-C operation for the PA  219  and highly controlled amplitude wave shaping by means of the drain envelope modulator  218 . Because the PA  219  has low DC power consumption and the PA is the largest contributor to the overall power consumption of the RFID reader  100 , this allows the reader to be operated under Power over Ethernet (PoE) specifications while using a full maximum output power as specified by local regulations. According to IEEE 802.11af, PoE allows for a maximum of approximately 13 Watts of available DC power. Thus, the existing Ethernet wiring can supply the DC voltage for the RFID reader without a need for a complete power supply infrastructure. 
       FIG. 6  is a simplified flowchart of an embodiment of a method for controlling output power of the RFID reader  100 . Referring also to  FIGS. 1-3 , in block  610 , a power control loop starts by providing a portion of the output signal  214  from the PA  219  of the transmitter  200 . This is provided by the directional coupler of the forward power tap  230 . In block  620 , the output signal is then attenuated by the step attenuator  240 . The step attenuator  240  may be any commercially available, off-the-shelf calibrated, programmable attenuator which provides accurate steps of attenuation. An example is a 32-step programmable attenuator with steps of 1 dB. The output of the programmable step attenuator is split. In block  632 , part of the attenuated output signal  244  drives the LO signal of the receiver  300  of the reader  100 . In block  631 , part of the attenuated output signal  242  is fed into the power detector  250  where the output signal is detected (or rectified). The rectified power detector output is fed into the ADC  520  of the controller  500  which provides a measured number representing a power level of the attenuated output signal. 
     The controller  500  includes the DSP  510  that compares the measured number with a reference value stored in memory  540  of the controller  500 . The reference value is determined during calibration which is described in detail below. In decision block  640 , the DSP  510  determines whether the measured number is equal to the reference value. In block  650 , if drift is detected between the measured number and the reference value, a power supply voltage to the power amplifier can be adjusted accordingly. The controller  500  includes the DAC  530  by which the DSP  510  generates a signal called PA_FDBK  531 . The voltage level of PA_FDBK  531  is dependent on the amount of drift that was detected. The DSP  530  includes firmware that determines D/A values based on A/D values. The PA_FDBK signal  531  in turn generates the power supply voltage called PA_PWR  211  which is fed back into the PA system  210  of the transmitter  200  closing the power control loop. This PA_PWR  211  voltage signal is generated by a regulator in response to a request voltage signal, PA_FDBK  531 . The PA_PWR  211  will be adjusted until the attenuated output signal is equal to the reference value stored in the controller. In block  660 , when the attenuated output signal (measured value) is equal to the reference value the power level  217  to the PA  219  is maintained the same. As a result, the power control loop tries to get a constant power level going into the power detector  250 . 
     As noted above, the reference value is calibrated one time during production. The only time the RFID reader  100  is actively controlling the power is when the output signal  214  of the amplifier  219  is a continuous wave (CW) RF signal or in other words not modulating. Thus, the PA_PWR  211  voltage signal supplied to the PA system  210  directly translates to a certain output power that is transmitted or radiated by the RFID reader  100 .  FIG. 7  is a simplified flowchart of an embodiment of a method for calibrating an RFID reader  100 . In block  710 , a measurement is taken from the antenna  270  of the RFID reader  100  to determine the output power. In decision block  720 , it is determined whether the measurement equals a pre-selected power setting, for example, 1 Watt coming off the antenna  270 . In block  730 , if the measurement does not equal the pre-selected power setting then the power signal, PA_PWR  211  is adjusted accordingly. In block  740 , if the measurement equals the pre-selected power setting then the output power of the power detector  250  is measured. In block  750 , the reference value is calibrated by setting the reference value to this measured value of the power detector  250 . In block  760 , the reference value is stored in memory  540  of the controller  500 . Thus, the stored reference value represents what is needed at the output of the power detector  250  to get 1 Watt coming off the antenna  270 . The calibration process is done with a certain setting for the step attenuator  240 . This setting is also stored in memory  540  for use during execution of the power control loop any time the calibrated power level has to be reproduced. The DAC value that is necessary for proper supply voltage setting may also be stored in memory for use during start-up. 
     The calibration process discussed above allows for accurate power setting and accurate steps in power setting without having to recalibrate the reference value for each power setting step. Continuing with the example above, during start-up the power control loop can be improved by using best estimate start-up values that were determined during calibration and stored in memory  540 . Additionally, an operating temperature measured by the sensor  290  is made available to the DSP  510  during start-up which allows the DSP to compensate the best estimate start-up values according to know temperature-power dependencies. The RFID reader  100  is calibrated to transmit 1 Watt off the antenna  270  and the step attenuator  240  is set by the DSP  510 . If the desired output power is ½ Watt which is 3 dB lower than 1 Watt, the programmable step attenuator  240  (steps of 1 dB) is set 3 dB different from what the attenuator was set for 1 Watt. The power detector  250  in the power control loop still tries to get to the same reference value that was stored in memory but in this situation there is less attenuation (3 dB less) than before. The power control loop will try to get the power level going into the attenuator 3 dB lower which means that the output signal coming off the antenna  270  is 3 dB lower than the calibrated 1 Watt value. Thus, the power setting of the RFID reader  100  accurately follows each step of the programmable step attenuator  240  without having to recalibrate the reference value for each power setting step. 
     As discussed above, referring again to  FIG. 1 , part of the attenuated output signal  244  is used to drive the LO signal for the receiver  300 . The fact that the power control loop tries to get the power level going into the power detector  250  at one constant level means that the power level going to the receiver LO is also constant with the same value. This guarantees a proper level going into mixers of the receiver  300  which is an advantage of implementing the power control loop. The power level going to the receiver LO is constant only during reception where the output signal is CW. During transmission, if the output signal  214  was modulated then the receiver LO signal would also be modulated and this would not work for the receiver  300 . However, with RFID, the reader  100  never receives signals when the reader is transmitting signals. Thus, the power control loop provides for proper receiver operation using the attenuated output signal  244  to drive the receiver LO signal. 
     Additionally, in RFID, one factor that must be accounted for is how the transmitter  200  is affecting noise input into the receiver  300 . Driving the receiver LO by the method discussed above, cancels out noise that may be generated by the power amplifier  219  and increases receiver  300  sensitivity. For an RFID backscatter homodyne based system, frequency changes (and phase changes) in the transmitted RF signal will cancel out with the received tag signal as long as the transmitted signal is derived from the same frequency source (carrier signal generator  220 ) as the LO signal for the receiver  300  mixers. However, added phase noise that are generated in active elements, such as amplifiers, that come after a master oscillator will not cancel out if the receiver LO signal was derived directly from the master oscillator. The present arrangement of the power control loop solves the added phase noise problem because the receiver LO signal is driven by the attenuated output signal  244  that comes after the power amplifier system  210  of the transmitter  200 . Thus, the power control loop provides for phase noise cancellation that may be generated by the transmitter  200  resulting in an increase in RF signal margin in the receive path. AM noise may not be cancelled out. However, the advantage of using a Class-C amplifier over a Class-A amplifier is that the Class-C amplifier generates lower level AM noise and therefore, results in less deterioration of receiver sensitivity. 
     The method described herein provides a low-cost and efficient way to control the power output of an RFID reader that allows accurate steps in power settings and at the same time cancels added RF amplifier phase noise in the receiver which increases receiver sensitivity. The method described herein does not require large firmware overhead such as complex algorithms or lookup tables for each power setting or time consuming calibration procedures. 
     The system described herein provides a high efficiency power amplifier resulting in low power consumption and a highly controlled modulator resulting in very linear modulation. The system described herein is suitable for applications such as Power over Ethernet without the need for a power supply infrastructure. The existing Ethernet wiring is able to supply the DC voltage for the system. 
     Although embodiments of the present disclosure have been described in detail, those skilled in the art should understand that various changes, substitutions and alterations may be made without departing from the spirit and scope of the present disclosure. Accordingly, all such changes, substitutions and alterations are intended to be included within the scope of the present disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.