US20070290808A1 - Backscatter interrogators, communication systems and backscatter communication methods - Google Patents

Abstract

The present invention provides backscatter interrogators, communication systems and backscatter communication methods. According to one aspect of the present invention, a backscatter interrogator includes a data path configured to communicate a data signal; a signal generator configured to generate a carrier signal; and a modulator coupled with the data path and the signal generator, the modulator being configured to spread the data signal to define a spread data signal and amplitude modulate the carrier signal using the spread data signal, the modulator being further configured to phase modulate the carrier signal.

Description

TECHNICAL FIELD
• The present invention relates to backscatter interrogators, communication systems and backscatter communication methods.
BACKGROUND OF THE INVENTION
• Backscatter communication systems are known in the art. In a backscatter system, one transponder, such as an interrogator, sends out a command to a remote communications device. After the interrogator transmits the command, and is expecting a response, the interrogator switches to a CW mode (continuous wave mode). In the continuous wave mode, the interrogator does not transmit any information. Instead, the interrogator just transmits radiation at a certain frequency. In other words, the signal transmitted by the interrogator is not modulated. After a remote communications device receives a command from the interrogator, the remote communications device processes the command. The remote communications device of the backscatter system modulates the continuous wave by switching between absorbing RF radiation and reflecting RF radiation. For example, the remote communications device alternately reflects or does not reflect the signal from the interrogator to send its reply. Two halves of a dipole antenna can be either shorted together or isolated from each other to modulate the continuous wave.
• One example of a backscatter system is described in commonly assigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996, and incorporated herein by reference. Another example of a backscatter system is described in U.S. Pat. No. 5,649,296 to MacLellan et al. which is also incorporated herein by reference.
• One application for backscatter communications is in wireless electronic identification systems, such as those including radio frequency identification devices. Of course, other applications for backscatter communications exist as well. Most presently available radio frequency identification devices utilize a magnetic coupling system. An identification device is usually provided with a unique identification code in order to distinguish between a number of different devices. Typically, the devices are entirely passive (have no power supply), which results in a small and portable package. However, such identification systems are only capable of operation over a relatively short range, limited by the size of a magnetic field used to supply power to the devices and to communicate with the devices.
• Another wireless electronic identification system utilizes a large, board level, active transponder device affixed to an object to be monitored which receives a signal from an interrogator. The device receives the signal, then generates and transmits a responsive signal. The interrogation signal and the responsive signal are typically radio-frequency (RF) signals produced by an RF transmitter circuit. Because active devices have their own power sources. The active devices do not need to be in close proximity to an interrogator or reader to receive power via magnetic coupling. Therefore, active transponder devices tend to be more suitable for applications requiring tracking of objects that may not be in close proximity to the interrogator, such as a railway car.
• Spread spectrum modulation techniques are known in the art. Utilization of spread spectrum modulation provides distinct advantages ins some communication applications. For example, some spread spectrum modulation techniques enable desired signals to be distinguished from other signals (e.g., radar, microwave ovens, etc.) operating at approximately the same frequencies.
• Federal Communication Commission (FCC) regulations require that spread spectrum systems meet various requirements. For example, spread spectrum systems operating in the 2.4-2.485 GHz band should comply with FCC rule 15.247 which states, in relevant part, that the power spectral density cannot be more than +8 dBm in any given 3 kHz band. Further, the maximum power output is 1 Watt into a 6 dBi gain antenna. The minimum 6 dB bandwidth for a direct sequence spread spectrum is 500 kHz. In addition, the energy within restricted bands of 0-2.390 GHz and 2.4835-2.5 GHz should be lower than 500 uV/m at three meters. Communication systems operating within this specified band should be designed with regard to these regulations.
• Amplitude modulation (AM) communication techniques enable communications with the use of relatively straightforward detectors. Typically, such AM detectors can be efficiently implemented with the utilization of relatively few components. However, drawbacks exist with the utilization of amplitude modulation techniques. For example, approximately half the total power of AM communications resides within the carrier. This limits the power which can be used for communicating data if AM modulation and spread spectrum techniques are utilized within the above specified frequency band.
• Therefore, there exists a need to provide communication systems which comply with radio frequency regulations while also providing robust communications.
SUMMARY OF THE INVENTION
• The present invention includes backscatter interrogators, communication systems and backscatter communication methods.
• One aspect of the invention provides a backscatter interrogator. The backscatter interrogator includes a data path configured to communicate a data signal and a signal generator configured to generate a carrier signal. The carrier signal comprises a microwave signal in preferred embodiments. The interrogator also provides a modulator coupled with the data path and the signal generator. The modulator is configured to spread the data signal to define a spread data signal and amplitude modulate the carrier signal using the spread data signal. The modulator is further configured to phase modulate the carrier signal to reduce the power within the carrier signal.
• A second aspect of the present invention provides a backscatter interrogator including a data path configured to communicate a data signal and a signal generator configured to output a microwave carrier signal. The backscatter interrogator further includes a modulator coupled with the data path and the signal generator. One configuration of the modulator is operable to spread the data signal and selectively invert the spread data signal. The modulator is further configured to band limit the inverted spread data signal and modulate the carrier signal using the band limited data signal.
• Another aspect of the present invention provides a backscatter communication system including an interrogator and an electronic communication device. The interrogator is configured to spread a data signal. The interrogator is further configured to amplitude modulate a carrier signal using the data signal and phase modulate the carrier signal using the data signal. The interrogator is arranged to output the amplitude modulated and phase modulated carrier signal. The electronic communication device is configured to output a reply signal responsive to reception of the amplitude modulated and phase modulated carrier signal.
• Another aspect of the invention provides a backscatter communication method including the steps of providing a data signal and providing a carrier signal. This method also includes spreading the data signal, amplitude modulating the carrier signal using the spread data signal, and phase modulating the carrier signal. The amplitude modulated and phase modulated carrier signal is thereafter communicated.
• Another aspect of the invention provides a method of communication in a backscatter system including an interrogator and a communication device. The method includes providing a data signal, providing a carrier signal and spreading the data signal. The method also includes modulating the carrier signal using the spread data signal, communicating the modulated carrier signal, and suppressing the carrier signal during the communicating.
• Yet another aspect of the invention provides a backscatter communication method including the steps of providing a data signal and a carrier signal. The invention also includes spreading the data signal, selectively inverting the spread data signal, amplitude modulating the carrier signal using the data signal, and communicating the modulated carrier signal.
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 illustrating a communication system embodying the invention.
• FIG. 2 is a front view of an employee badge according to one embodiment of the invention.
• FIG. 3 is a front view of a radio frequency identification tag according to another embodiment of the invention.
• FIG. 4 is a circuit schematic of a transponder included in the system ofFIG. 1.
• FIG. 5 is a block diagram of an interrogator in accordance with one embodiment of the invention.
• FIG. 6 is a circuit schematic of RF circuitry included in the interrogator ofFIG. 5.
• FIG. 7 is a circuit schematic of one embodiment of circuitry included in the RF configuration ofFIG. 6.
• FIG. 8 is an illustrative representation of a tri-level communication signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).
• FIG. 1 illustrates awireless communications system10 embodying the , invention.Communications system10 includes a first transponder including aninterrogator26 and ahost computer48 in communication withinterrogator26.Communications system10 further includes anelectronic communications device12, such as the device disclosed in U.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996. In one embodiment,wireless communications device12 comprises a wireless identification device such as the Microstamp (™) integrated circuit available from Micron Communications, Inc., 3176 S. Denver Way, Boise, Id. 83705.Interrogator26 communicates with thecommunications device12 via an electromagnetic link, such as via an RF link (e.g., at microwave frequencies, in one embodiment). While other embodiments are possible, in the illustrated embodiment thecommunications device12 includes atransponder16 having areceiver30 and a transmitter32 (FIG. 4).Communications device12 further includes apower source18 connected totransponder16 to supply power totransponder16.Communications device12 further includes at least one antenna connected totransponder16 for wireless transmission and reception. In the illustrated embodiment,communications device12 includes at least oneantenna46 connected totransponder16 for radio frequency transmission bytransponder16, and at least one receiveantenna44 connected totransponder16 for radio frequency reception bytransponder16. In the illustrated embodiment, the transmitantenna46 is a dipole antenna, and the receiveantenna44 is a loop antenna. In the illustrated embodiment, thetransponder16 is in the form of an integrated circuit. However, in alternative embodiments, all of the circuitry oftransponder16 is not necessarily all included in a single integrated circuit.
• Power source18 is a thin film battery in the illustrated embodiment; however, in alternative embodiments, other forms of power sources can be employed. If thepower source18 is a battery, the battery can take any suitable form. Preferably, the battery type will be selected depending on weight, size, and life requirements for a particular application. In one embodiment, thebattery18 is, a thin profile button-type cell forming a small, thin energy cell more commonly utilized in watches and small electronic devices requiring a thin profile. A conventional button-type cell has a pair of electrodes, an anode formed by one face and a cathode formed by an opposite face. In an alternative embodiment, the battery comprises a series connected pair of button type cells.
• Thecommunications device12 can be included in any appropriate housing or packaging.
• FIG. 2 shows but one example of a housing in the form of acard11 comprising plastic or other suitable material. Theplastic card11houses communications device12 to define anemployee identification badge13 including thecommunications device12. In one embodiment, the front face ofbadge13 has visual identification features such as an employee photograph or a fingerprint in addition to identifying text.
• FIG. 3 illustrates but one alternative housing supporting thedevice12. More particularly,FIG. 3 illustrates aminiature housing20 encasing thedevice12 to define a tag which can be supported by an object (e.g., hung from an object, affixed to an object, etc.).
• Although two particular types of housings have been disclosed, thecommunications device12 can be included in any appropriate housing.Communications device12 is of a small size that lends itself to applications employing small housings, such as cards, miniature tags, etc. Larger housings can also be employed. Thecommunications device12, housed in any appropriate housing, can be supported from or attached to an object in any desired manner.
• Interrogator unit26 includes a plurality of antennas X1, R1, as well as transmitting and receiving circuitry, similar to that implemented in thedevice16. Thehost computer48 acts as a master in a master-slave relationship withinterrogator26. Thehost computer48 includes an applications program for controlling theinterrogator26 and interpreting responses, and a library of radio frequency identification device applications or functions. Most of the functions communicate withinterrogator26. These functions effect radio frequency communication betweeninterrogator26 andcommunications device12.
• Communications system10 includes a transmit antenna X1, and a receive antenna R1 connected tointerrogator26. In operation, theinterrogator26 transmits an interrogation signal or command27 (“forward link”) via the antenna X1. Thecommunications device12 receives the incoming interrogation signal via itsantenna44. Upon receiving thesignal27, thecommunications device12 responds by generating and transmitting a responsive signal or reply signal29 (“return link”). Theinterrogator26 is described in greater detail below.
• In one embodiment, theresponsive signal29 is encoded with information that uniquely identifies, or labels theparticular device12 that is transmitting, so as to identify any object or person with whichcommunications device12 is associated.
• In the embodiment illustrated inFIG. 1,multiple communications devices12 can be employed; however, there is no communication betweenmultiple devices12. Instead, themultiple communications devices12 communicate withinterrogator26.FIG. 1 illustrates thecommunications device12 as being in thehousing20 ofFIG. 3. The system would operate in a similar manner if thedevice12 is provided in a housing such as thehousing10 ofFIG. 2, or any other appropriate housing.Multiple communications devices12 can be used in the same field of an interrogator26 (i.e., within communications range of an interrogator26). Similarly,multiple interrogators26 can be in proximity to one or more of thedevices12.
• The above describedsystem10 is advantageous over prior art devices that utilize magnetic field effect systems because, with thesystem10, a greater range can be achieved, and more information can be obtained (instead of just an identification number).
• As a result, such asystem10 can be used, for example, to monitor large warehouse inventories having many unique products needing individual discrimination to determine the presence of particular items within a large lot of tagged products.
• FIG. 4 is a high level circuit schematic of thetransponder16 utilized in the devices ofFIGS. 1-3. In the embodiment shown inFIG. 4, thetransponder16 is a monolithic integrated circuit. More particularly, in the illustrated embodiment, theintegrated circuit16 comprises a single die, having a size of 209×116 mils², including thereceiver30, thetransmitter32, a micro controller ormicroprocessor34, a wake up timer andlogic circuit36, a clock recovery anddata recovery circuit38, and a bias voltage andcurrent generator42.
• In one embodiment, thecommunications devices12 switch between a “sleep” mode of operation, and higher power modes to conserve energy and extend battery life during periods of time where nointerrogation signal27 is received bydevices12, using the wake up timer andlogic circuitry36.
• In one embodiment, a spreadspectrum processing circuit40 is included intransponder16. In this embodiment, signals transmitted and received byinterrogator26, and transmitted and received bycommunications device12 are modulated spread spectrum signals. Spread spectrum modulation is described below.
• Many modulation techniques minimize required transmission bandwidth. However, the spread spectrum modulation technique employed in the illustrated embodiment requires a transmission bandwidth that is up to several orders of magnitude greater than the minimum required signal bandwidth. Although spread spectrum modulation techniques are bandwidth inefficient in single user applications, they are advantageous where there are multiple users, as is the case with the instant radio frequency identification system24. The spread spectrum modulation technique of the illustrated embodiment is advantageous because the interrogator signal can be distinguished from other signals (e.g., radar, microwave ovens, etc.) operating at the same frequency. The spread spectrum signals transmitted bycommunications device12 and byinterrogator26 are pseudo random and have noise-like properties when compared with the digital command or reply. The spreading waveform is controlled by a pseudo-noise or pseudo random number (PN1) sequence or code. The PN code is a binary sequence that appears random but can be reproduced in a predetermined manner by thedevice12. More particularly, incoming spread spectrum signals are demodulated bycommunications device12 or byinterrogator26 through cross correlation with a version of the pseudo random carrier that is generated bycommunications device12 itself orinterrogator26 itself, respectively. Cross correlation with the correct PN sequence unspreads the spread spectrum signal and restores the modulated message in the same narrow band as the original data.
• A pseudo-noise or pseudo random sequence (PN sequence) is a binary sequence with an autocorrelation that resembles, over a period, the autocorrelation of a random binary sequence. The autocorrelation of a pseudo-noise sequence also roughly resembles the autocorrelation of band-limited white noise. A pseudo-noise sequence has many characteristics that are similar to those of random binary sequences. For example, a pseudo-noise sequence has a nearly equal number of zeros and ones, very low correlation between shifted versions of the sequence, and very low cross correlation between any two sequences. A pseudo-noise sequence is usually generated using sequential logic circuits. For example, a pseudo-noise sequence can be generated using a feedback shift register.
• A feedback shift register comprises consecutive stages of two state memory devices, and feedback logic. Binary sequences are shifted through the shift registers in response to clock pulses, and the output of the various stages are logically combined and fed back as the input to the first stage. The initial contents of the memory stages and the feedback logic circuit determine the successive contents of the memory.
• The illustrated embodiment employs direct sequence spread spectrum modulation. A direct sequence spread spectrum (DSSS) system spreads the baseband data by directly multiplying the baseband data pulses with a pseudo-noise sequence that is produced by a pseudo-noise generator. A single pulse or symbol of the PN waveform is called a “chip.” Synchronized data symbols, which may be information bits or binary channel code symbols, are added in modulo-2 fashion to the chips before being modulated. The receiver performs demodulation. For example, in one embodiment the data is amplitude modulated. Assuming that code synchronization has been achieved at the receiver, the received signal passes through a wideband filter and is multiplied by a local replica of the PN code sequence. This multiplication yields the unspread signal.
• A pseudo-noise sequence is usually an odd number of chips long. In the illustrated embodiment, one bit of data is represented by a thirty-one chip sequence. A zero bit of data is represented by inverting the pseudo-noise sequence.
• Spread spectrum techniques are also disclosed in “Spread Spectrum Systems,” by R. C. Dixon, published by John Wiley and Sons, Inc., incorporated herein by reference.
• In operation, the interrogator sends out a command that is spread around a certain center frequency (e.g, 2.44 GHz). After the interrogator transmits the command, and is expecting a response, the interrogator switches to a CW mode (continuous wave mode). In the continuous wave mode, the interrogator does not transmit any information. Instead, the interrogator just transmits 2.44 GHz radiation. In other words, the signal transmitted by the interrogator is not modulated. After thecommunications device12 receives the command from the interrogator, thecommunications device12 processes the command. Ifcommunications device12 is in a backscatter mode it alternately reflects or does not reflect the signal from the interrogator to send its reply. For example, in the illustrated embodiment, two halves of a dipole antenna are either shorted together or isolated from each other to send a reply.
• In one embodiment, the clock for the entireintegrated circuit16 is extracted from the incoming message itself by clock recovery anddata recovery circuitry38. This clock is recovered from the incoming message, and used for timing for themicro controller34 and all the other clock circuitry on the chip, and also for deriving the transmitter carrier or the subcarrier, depending on whether the transmitter is operating in active mode or backscatter mode.
• In addition to recovering a clock, the clock recovery anddata recovery circuit38 also performs data recovery on valid incoming signals. The valid spread spectrum incoming signal is passed through the spreadspectrum processing circuit40, and the spreadspectrum processing circuit40 extracts the actual ones and zeros of data from the incoming signal. More particularly, the spreadspectrum processing circuit40 takes the chips from the spread spectrum signal, and reduces each thirty-one chip section down to a bit of one or zero, which is passed to themicro controller34.
• Micro controller34 includes a serial processor, or I/O facility that receives the bits from the spreadspectrum processing circuit40. Themicro controller34 performs further error correction. More particularly, a modified hamming code is employed, where each eight bits of data is accompanied by five check bits used by themicro controller34 for error correction.Micro controller34 further includes a memory, and after performing the data correction,micro controller34 stores bytes of the data bits in memory. These bytes contain a command sent by theinterrogator26. Themicro controller34 responds to the command.
• For example, theinterrogator26 may send a command requesting that anycommunications device12 in the field respond with the device's identification number. Status information is also returned tointerrogator26 fromcommunications device12 whencommunications device12 responds.
• The transmitted replies have a format similar to the format of incoming messages. More particularly, a reply starts with a preamble (e.g., all zeros in active mode, or alternating double zeros and double ones in backscatter mode), followed by a Barker or start code, followed by actual data.
• The incoming message and outgoing reply preferably also include a check sum or redundancy code so thatintegrated circuit16 orinterrogator12 can confirm receipt of the entire message or reply.
• Interrogator26 provides a communication link between a host computer andtransponder16.Interrogator26 connects to thehost computer48 via an IEEE-1284 enhanced parallel port (EPP). The interrogator communicates withtransponder16 via the RF antennas X1 and R1.
• A Maximal Length Pseudo Noise (PN) Sequence is used in the Direct Sequence Spread Spectrum (DSSS) communications scheme in the forward link. In one embodiment, the sequence is generated by a linear feedback shift register. This produces a repeating multiple “chip” sequence.
• A zero bit is transmitted as one inverted full cycle of the PN sequence. A one bit is transmitted as one full non-inverted cycle of the PN sequence.
• After sending a command, the interrogator sends a continuous unmodulated RF signal with an approximate frequency of 2.44175 GHz. Return link data is Differential Phase Shift Key (DPSK) modulated onto a square wave subcarrier with a frequency of approximately 600 kHz (e.g., 596.1 kHz in one embodiment). A data0 corresponds to one phase and data1 corresponds to another, shifted 180 degrees from the first phase. The subcarrier is used to modulate antenna impedance oftransponder16. For a simple dipole, a switch between the two halves of the dipole antenna is opened and closed. When the switch is closed, the antenna becomes the electrical equivalent of a single half-wavelength antenna that reflects a portion of the power being transmitted by the interrogator. When the switch is open, the antenna becomes the electrical equivalent of two quarter-wavelength antennas that reflect very little of the power transmitted by the interrogator. In one embodiment, the dipole antenna is a printed microstrip half wavelength dipole antenna.
• Referring toFIG. 5, one embodiment ofinterrogator26 is illustrated. The depictedinterrogator26 includes enhanced parallel port (EPP)circuitry50, DPSK (differential phase shift keyed)circuitry52, and RF (radio frequency)circuitry54, as well as a power supply (not shown) and a housing or chassis (not shown). In the illustrated embodiment, the enhancedparallel port circuitry50, theDPSK circuitry52, and the RF ifcircuitry54 respectively define circuit card assemblies (CCAs).
• The interrogator uses an IEEE-1284 compatible port in EPP mode to communicate withhost computer48. TheEPP circuitry50 provides digital logic required to coordinate sending and receiving a message withtransponder16. TheEPP circuitry50 buffers data to transmit fromhost computer48, converts the data to serial data, and encodes it. TheEPP circuitry50 then waits for data from thetransponder16, converts it to parallel, and transfers it tohost computer48. In one embodiment, messages include up to 64 bytes of data.
• The EPP mode interface provides an asynchronous, interlocked, byte wide, bi-directional channel controlled by a host device. The EPP mode allows the host computer to transfer, at high speed, a data byte to/from the interrogator within a single host computer CPU I/O cycle (typically 0.5 microseconds per byte).
• RF circuitry54 interfaces with the transmit and receive antennas X1, R1. Exemplary transmit operations ofRF circuitry54 are described with reference toFIGS. 6-8.RF circuitry54 modulates the data for transmission totransponder16, provides a continuous wave (CW) carrier for backscatter communications with transponder16 (if backscatter communications are employed), and receives and downconverts the signal received from transponder16 (which comprises a backscatter signal in one embodiment).
• Referring toFIG. 6,RF circuitry54 includes adata path73,signal generator75,amplifier76 andmodulator77. An exemplary signal generator comprises a frequency synthesizer configured to tune the RF continuous wave carrier. In the described embodiment,signal generator75 is configured to generate a microwave carrier signal (e.g., approximately 2.44 GHz).
• RF circuitry54 defines a transmitter, and is configured to receive data fromEPP circuitry50.Modulator77 is coupled withdata path73 andsignal generator75.Data path73 comprises an interconnection such as a wire configured to communicate a data signal tomodulator77.Modulator77 receives the data fromEPP circuitry50 and amplitude modulates the data onto a carrier in the described embodiment. More particularly,modulator77 turns the RF on and off (ON OFF KEY).
• According to one embodiment,modulator77 is configured to spread the data signal to provide spread spectrum communications including a spread data signal.Modulator77 is further configured to amplitude modulate the carrier signal using the spread data signal. In a preferred embodiment,modulator77 is configured to phase modulate the carrier signal. Phase modulation is utilized to provide suppression of power within the carrier signal in accordance with a preferred embodiment of the invention. Following such amplitude and phase modulation,interrogator26 is preferably configured to communicate the carrier signal.
• Modulator77 can be configured to spread the data signal using the PN1 pseudo-noise sequence. The first pseudo-noise sequence code (PN1) is encoded with data received bycommunications path73. In a preferred embodiment,modulator77 is thereafter configured to invert the spread data signal. Phase modulation of the carrier signal is responsive to selected inverting of the spread data signal. Encoding the pseudo-noise sequence with the data signal forms a plurality of chips. The chips individually correspond to one of two values, such as logical high (1) or logical low (0).Modulator77 is configured in a preferred embodiment to selectively invert the chips of the spread signal to implement phase modulation.
• In one embodiment,modulator77 is configured to randomly invert the spread data signal.Modulator77 is operable in the preferred embodiment to invert the spread data signal using a second pseudo-noise sequence (PN2). The second pseudo-noise sequence code (PN2) is utilized to modulate the phase of the RF carrier in the described embodiment. The second pseudo-noise sequence (PN2) utilized to provide random inversion of the spread data signal comprises a 1,023 bit sequence in one embodiment. Such can be implemented using logic circuitry and shift registers configured in a feedback arrangement in one embodiment.
• Randomly changing the phase of the RF carrier signal implements phase modulation and provides the desired effect of spreading the carrier energy over a wider band width. Thus, the communication power can be increased while decreasing the amount of power present in the carrier.
• The random code or pseudo-noise sequence (PN2) utilized to implement selected inversion of the spread data signal is preferably band limited to avoid excessive spectral spreading. The carrier signal can be modulated using the band limited data signal.
• Modulator77 can be arranged to implement the inversion operations responsive to the spread data signal being a predetermined value. For example,modulator77 can be configured to invert one or more chips of the spread data signal responsive to the spread data signal being a logical low value (e.g., zero Volts). The inverting is preferably coordinated or synchronized with the spread data signal.
• In particular,modulator77 is preferably configured such that the second pseudo-noise sequence (PN2) changes state, and implements inversion of the spread data signal, at predetermined places corresponding to the spread data signal. It is preferred to restrict state changes of the PN2 pseudo-noise sequence to periods of time wherein the carrier signal is off during amplitude modulation. For example, during the presence of multiple adjacent zeros within the spread data signal, sufficient time typically exists for the second pseudo-noise sequence to complete a state change while the carrier signal is turned off. Such is preferred to reduce inducing of an amplitude variation within the output communicated signal which may cause error within the associated remote communication device inasmuch as the second pseudo-noise sequence pulses have a limited rise and fall time.
• It is desired to reduce communicated energy present in restricted frequency bands in many applications.Modulator77 is configured in one embodiment to band limit or filter signals withinRF circuitry54. A preferred configuration is to filter baseband signals withinmodulator77 before conversion to RF frequencies.Modulator77 includes a filter configured to band limit the spread data signal in the described embodiment.
• Referring toFIG. 7, anexemplary modulator77 is illustrated. The depictedmodulator77 includeslogic circuitry65, summingamplifier64,low pass filter66,attenuator68, andmixer69.Modulator77 is coupled withEPP circuitry50,code generator60,code generator61,signal generator75, andpower amplifier76.
• The depictedlogic circuitry65 comprises an exclusive NORdevice62 and an exclusive ORdevice63. Exclusive NORdevice62 is coupled withEPP circuitry50 viadata path73 andcode generator60.Device62 is configured to encode the pseudo-noise sequence (PN1) fromcode generator60 with data received viapath73, and output a spread data signal. The spread data signal is applied to exclusive ORdevice63 as well ascode generator61.
• Code generator61 is configured to change state according to second pseudo-noise sequence (PN2) and responsive to the spread data signal being a predetermined value. The PN2 output ofcode generator61 is applied to exclusive ORdevice63 and summingamplifier64. In the described embodiment,code generator61 is configured to change the state of the outputted signal according to pseudo-noise sequence (PN2) and responsive to the spread data being a logic low value as received from exclusive NORdevice62. The pseudo-noise sequence (PN2) outputted fromcode generator61 and applied to exclusive ORdevice63 randomly inverts the spread data signal.
• The output of exclusive ORdevice63 is also applied to summingamplifier64. Summingamplifier64 is configured to combine the pseudo-random noise sequences PN1, PN2. Summingamplifier64 is further as configured to output a tri-level or tri-state signal to implement the amplitude modulation and phase modulation of the carriersignal using mixer69. The carrier signal is modulated using the tri-level signal corresponding to the inverted data signal in the described embodiment. Combination of the spread data signal and the PN2 pseudo-noise sequence provided bycode generator61 is preferred to reduce the amount of hardware utilized withinmodulator77. Summingamplifier64 preferably includes an offsetadjustment67 utilized to balance the amplitude outputted frommixer69 to provide maximum carrier suppression in the preferred embodiment.
• In the depicted embodiment, the tri-level or state output of summingamplifier64 is applied tolow pass filter66,attenuator68 andmixer69.Low pass filter66 is configured in the described embodiment to reduce spreading of energy into restricted bands adjacent the desired band for communication. In one embodiment,low pass filter66 comprises a seven pole filter having a cutoff frequency of 20 MHz. In particular, filter66 preferably limits the rise and fall time of the spread data signal represented within the tri-level signal as described below.
• Attenuator68 is configured to adjust the amplitude of the outputted signal oflow pass filter66.Discrete components78 are providedintermediate attenuator68 andmixer69. The illustrateddiscrete components78 include a series capacitor and a resistor coupled to ground.Discrete components78 are operable to block DC components within the ouput ofattenuator68.
• The tri-level signal can comprise three states in the described embodiment. A logical high value can be represented be either a positive voltage or negative voltage within the tri-level signal.Communication device12 is configured to interpret both positive and negative voltages as a logical high value in the described embodiment. A logical low value is preferably approximately zero Volts within the described tri-level signal.
• Mixer69 is configured to invert the phase of the carrier signal responsive to the reception of a negative value within the tri-level signal emitted from summingamplifier64. A positive voltage within the tri-level signal results in no change of the phase of the carrier signal.Mixer69 turns the RF carrier signal off responsive to the tri-level signal being zero Volts.
• The output ofmixer69 is applied toamplifier76 in the described embodiment. The depictedamplifier76 comprises a power amplifier coupled tomodulator77 and configured to amplify the signal.Amplifier76 is operable responsive to control fromEPP circuitry50 in the illustrated embodiment.
• Referring toFIG. 8, operation ofmodulator77 is illustrated. Plural signals90-96 are shown inFIG. 8.Signal90 represents the data output ofEPP circuitry50.Signal91 corresponds to the first pseudo-noise sequence (PN1) signal outputted fromcode generator60.Signal92 represents the output of exclusive NORdevice62.Signal93 corresponds to the second pseudo-noise sequence (PN2) signal outputted fromcode generator61.Signal94 represents the output of exclusive ORdevice63.Signal95 represents the output of summingamplifier64 andsignal96 represents the output oflow pass filter66.
• Low pass filter66 is preferably configured to reduce the presence of harmonic information within the tri-level signal.Filter66 can be configured to pass fundamental frequency information and any other harmonic information necessary for proper operation ofremote communication device12.
• For example,low pass filter66 can be configured to pass fundamental frequency and third harmonic information in one application (e.g., the remote communication device is sensitive to slow rising and falling edges). It is preferred to adjustfilter66 to provide optimal bandwidth efficiency while effectively communicating data to the associated remoteelectronic communication device12.
• During continuous wave (CW) transmissions for the backscatter mode,modulator77 is configured to apply the carrier signal to transmit antenna X1.Transponder16 is operable to backscatter the signal with a DPSK modulated sub carrier. Following receipt of the command communication signal frominterrogator26,communication device12 can be configured to output a reply signal. In backscatter configurations,device12 is operable to modulate the CW emission frominterrogator26. The backscattered reply signal is received via receive antenna R1 ofinterrogator26.
• Although not shown inFIGS. 6-8,RF circuitry54 can include a quadrature downconverter configured to coherently downconvert the received reply signal.RF circuitry54 can also include automatic gain controls (AGCS) coupled to the quadrature downconverter and configured to set the amplitude of signals I and Q. The I and Q signals, which contain the DPSK modulated sub-carrier, are passed on toDPSK circuitry52 for demodulation.
• 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.

Claims (8)

1-47. (canceled)

48. A method of operating an interrogator comprising:

encoding a first pseudo-noise sequence with a data signal to create a first spread data signal;

randomly inverting the first spread data signal using a second pseudo-noise sequence to create a second spread data signal, wherein one or more chips in the first spread data signal are inverted when the first spread data signal has a logical low value; and

creating a tri-state signal from the second pseudo-noise sequence and the second spread data signal.

49. The method ofclaim 48, further comprising:

reducing harmonic information within the tri-state signal using a low-pass filter; and

applying an output of the low-pass filter to an attenuator.

50. The method ofclaim 48, wherein randomly inverting the first spread data signal implements phase modulation.

51. The method ofclaim 48, further comprising:

modulating a carrier signal using the tri-state signal.

52. The method ofclaim 48, further comprising:

band limiting the first spread data signal.

53. The method ofclaim 48, further comprising:

inverting a phase of a carrier signal mixed with a negative value of the tri-state signal.

54. The method ofclaim 48 further comprising:

mixing a carrier signal with the tri-state signal.

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