Abstract: An RF identification system for generating data indicative of an item that is tagged, and one bit of variable data representative of an external stimulus. An ID code reader is provided and an RF identification tag is coupled to the item that is to be monitored and the ID code reader queries the tag. The tag comprises two RF transponder chips that store a data relating to the item, and an excitation coil coupled thereto. A switch is coupled to the chips and excitation coil, and a capacitor is coupled across the switch. A sensor is coupled to the switch for generating one bit of variable data representative of an external stimulus, such as the pressure in a tire. The ID code reader comprises an exciter and RF excitation coil, and a receiver and an RF receiver coil. The receiver includes phase detection circuitry for detecting a state of the switch by detecting the phase shift between signals received from each of the transponder chips. When the switch is shorted, the effect of the capacitor is nullified.

Abstract: Security devices which are difficult to reproduce include a grid screen metallization pattern. The grid screen metallization pattern may be laid down over a hologram or diffraction grating formed as a surface relief pattern on a substrate, to form a visually identifiable, semi-transparent security device. Additionally, the metallization pattern may include resonant structures in which information about the security device is encoded. In some embodiments of these security devices, the metallization pattern is disposed in accurate registration with the underlying hologram or diffraction grating. These security devices are made by methods which include printing an oil pattern on the substrate. Areas on which oil is deposited do not receive metal during a metallization step. Since these methods do not use caustics, metallization patterns including features which would otherwise trap and hold caustics are possible.

Abstract: In a method for automatically identifying an unknown number of transponders, these transponders are put into a dead state by a collision signal transmitted by the reader. Then, each transponder generates its own random dead time and the transponder with the shortest dead time is the first to transmit its data to the reader, while the other transponders are put into an idle state. When the reader has completely received the data of the first transponder, the idle states of the other transponders end and they generate another dead time for themselves. The entire process is then repeated, i.e., the transponder with the shortest dead time transmits its data to the reader, etc. until all transponders have transmitted their data to the reader individually and successively.

Abstract: A radar transponder for a stable and reliable response transmission upon reception of a weak radar signal having a reflection-conscious circuitry free from damage by a transmit effect, including a reception damage by the leakage of a powerful transmit wave signal and a transmission damage by the reflection of the powerful transmit wave signal. The reflection-conscious circuitry may include a circulator coupled to the transmit/receive antenna for selecting the powerful transmit wave signal or the weak radar signal, a receiving circuit provided after the circulator for receiving the weak radar signal, and an SPDT switch provided between the circulator and the receiving circuit for passing an input of the weak radar signal to the receiving circuit in a receiving period with its ground function dormant and for leading an input of the powerful transmit wave signal to a ground using the ground function in a transmitting period.

Abstract: A wireless equipment diagnosis system which can perform inspection and maintenance precisely and accurately, and which comprises sensors which measure information on each equipment being diagnosed, a transponder which transmits measured data from the sensors after modulating the measured data; a portable interrogator which acquires the measured data by receiving transmitted signals from the transponder; and a host computer which diagnoses each equipment by implementing predetermined signal processing using information stored in advance and data transferred from the acquired data.

Abstract: A device used in a multimedia communication system is formed with a transceiver unit for transmitting and receiving information signal to and from the inside of a room and a base unit adapted to be mounted to a wall of the room to detachably support the transceiver unit. The transceiver unit has a plug surface on which a plug projects, a circular wall projecting on the plug surface to surround the plug, and a guide wall projecting on the plug surface inside the circular wall and around the plug. The base unit comprises a disk member which is provided with a receptacle exposed in a receptacle surface defined on the disk member. The disk member has a diameter slightly smaller than an inner diameter of the circular wall so that the disk member can fit within the circular wall when the transceiver unit is attached to the base.

Abstract: An on-board unit includes at least one receiving and transmitting antenna for a bidirectional transmission of data which works in accordance with the transponder method. To compensate for interference fading, a high-frequency amplifier is connected in an incoming circuit to the demodulator/modulator, through which the transmission range of the transmission distance is improved as well. The same amplifier and the demodulator/modulator can be used in downlink and uplink operation. In uplink operation, the demodulator/modulator can alternatively subject the carrier signal to amplitude shift keying or phase shift keying.

Abstract: Transponder ?A,B! formed of a bottom-closed elongate tubular enclosure ?12! having a mouth ?12'! at an upper end and circuit-receiving space within the enclosure for fully receiving an electronic transponder circuit assembly ?15!. A cap ?14,114! closes the mouth, the enclosure and cap each being of tough synthetic resin material which will not be damaged by being dropped and is intrinsically capable of absorbing shock to a substantial degree when so dropped. The circuit assembly is completely immersed in silicone oil or gel cushioning medium ?16! so that it completely surrounds the circuit assembly in all directions, the cushioning medium providing absorption of impact energy. The silicone cushioning medium when within the closure has its fluid level extending above the seal and above an upper end ?15'! of the circuit assembly. The cap is hermetically joined to the enclosure by ultrasonic welding. The transponder repeatedly survives a 2-meter drop test without failure of the circuit assembly.

Abstract: For the digital data communication between a vehicle (transponder) and a beacon, an automatic gain control wherein the path attenuation between the transponder and the brake increases disproportionately with the distance. A two-stage automatic gain control is included for the gain control. After the first amplifier stage, the output signal is measured and supplied, together with a setpoint value contained in a setpoint memory, to a control amplifier. The output signal of the control amplifier is used both for automatically controlling the first amplifier stage, as well as the second amplifier stage. The automatic control makes it possible to cover only one single lane when automatically debiting highway tolls for a motor vehicle, for example. Influences caused by other motor vehicles on a nearby lane are eliminated.

Abstract: A method for automatically categorizing the general variety of motor-vehicle types moving through a given checkpoint, according to their predominant characteristics; basically involving features such as length, height, axle count, existence of a trailer, and specific envelop contour (i.e.--motorcycle, car, truck, bus, limousine, etc.). Advantageously employing Doppler-based CW-radar for ultimate real-time accuracy, resolving even closely spaced vehicles having differing acceleration/deceleration rates; the system will establish a vehicle's classification during stop-and-go or at speed. Real-time velocity information is read from successively radar targeted vehicles, while combined with real-time information acquired via at least one special electronic profiling-window apparatus employing beam emitters/receptors or lasers in making incremental transverse-slice readings of traversing vehicles.

Abstract: A programmable digital radio is configured to perform transponder functions. A receiver in the programmable digital radio receives analog signals from an interrogator and digitizes the analog signals to produce digital interrogation signals. The digital interrogation signals are processed digitally and valid interrogations are detected. Processing includes detection of pulses, determination of each pulse's threshold, determination of intervals between pulses and whether the intervals meet mode criteria, and determination of priority of output of digital messages encoded in the pulses. At least one message is supplied to a transmit module in the programmable radio. The transmit module performs diversity processing on the received message to determine to which received signal to reply, produces reply data and modulates the reply data to produce intermediate frequency signals. The intermediate frequency signals are then converted to analog signals which are transmitted to the interrogator.

Abstract: A moving object identification system with a novel coding scheme achieves 75% transmission efficiency. With this system, a transmittable range is expanded. Logical value assignment for residual resonant oscillations in a residual resonant signal is performed by comparing a residual frequency with a threshold frequency. A wave energy reduction of an ASK modulated signal is used to decrease the energy of the signal in order to distinguish similar residual frequencies in the case of extremely close communication. Reliability of Write/Read instructions in this system is improved by employing a test writing before an actual writing and a voltage monitoring device.

Abstract: To enable communications between a vehicle-installed device and an antenna unit even when the vehicle-installed device passes through an area where neighboring communication areas overlap each other, a toll collection point of a multiple-lane express road is provided with a gantry having antenna units provided corresponding to the lanes. The antenna units are controlled so that neighboring units alternately transmit signals. The antenna units transmit signals according to a predetermined cycle and transmit an unmodulated carrier wave. Upon receiving a signal, a vehicle-installed device generates a response signal by using a control circuit and transmits a signal by reflection, fixes a communication party for further communications and assumes a status of waiting for an interrogation signal.

Abstract: A method and device for transferring a measurement signal from a revolving roll used in a paper making machine, in which detectors are arranged in the transverse direction of the roll, preferably uniformly spaced. Measurement signals generated by the detectors are passed to a measurement signal reading unit for reading the measurement of the detectors which is installed preferably at the end of the roll. From the measurement signal reading unit, the signals are transmitted wirelessly further as telemeter signals to a reading unit which is placed outside the roll and connected to a PC or equivalent computer. In the measurement signal reading unit for reading the measurement of the detectors, a transponder is arranged into which an unique code is coded for each detector. The signals are processed in the entire measurement process as analog signals.

Abstract: A method of identifying an object and determining in which of at least two areas the object is located is disclosed. The method includes the steps of: focusing a first directional antenna (18) on a first area (28a), focusing a second directional antenna on a second area (28b), transmitting a first field strength pulse (44) from said first directional antenna (18), transmitting a second field strength pulse (46) from said second directional antenna (18), and comparing in a transponder (14) the first field strength pulse (44) to the second field strength pulse (46) to determine in which of the two areas (28a,28b) the transponder (14) is located. Other devices, systems and methods are also disclosed.

Abstract: The invention relates to an identification system comprising an interrogator and a plurality of transponders. The interrogator includes transmitter means for transmitting an interrogation signal to the transponder, receiver means for receiving a response signal from the transponder, and processor means for identifying the transponder from data in the response signal. Clock extraction means are further provided for extracting a transponder clock signal from the response signal which is used to synchronise the modification of the interrogation signal with the response signal from the transponder. Each transponder comprises receiver means for receiving the interrogation signal, a transponder clock generator, a code generator, transmitter means, and a modulator connected to the code generator, so that on receipt of the interrogation signal the transponder transmits a response signal containing data which identifies the transponder.

Abstract: An identification and/or sensor system includes an identification and/or sensor configuration having resonators for determining at least one parameter to be interrogated. The resonators have a quality being good enough to effect a storage of energy in the resonators. A transmitting and receiving apparatus transmits interrogation signals by radio to the identification and/or sensor configuration and receives and evaluates reply signals transmitted back from the identification and/or sensor configuration. The transmitting and receiving apparatus is a broadband device.

Abstract: An antipilferage tag is disclosed which includes a resonant circuit adapted to receive an RF signal and to transmit a response signal when interrogated by said RF signal. The tag includes circuit components constituted by or fabricated from a metallized layer supported by a dielectric material.

Abstract: Airplanes have transponders which receive and transmit signals to communicate information regarding the flight of the airplane. A transponder testing apparatus is integrated into the transponder and verifies that the transponders are operating in accordance to Federal Aviation Regulations.

Abstract: A method and system is provided for communicating permanent or semi-permanent information to a transponder (14) from an interrogator (12), preferably operated by a toll agency or other authorized entity. This permanent or semi-permanent information is communicated by means of special instructions valid only during a special mode or maintenance mode which is entered by transmitting a special access code to the transponder (14) from the authorized interrogator (12). The transponder (14) will preferably acknowledge to the authorized interrogator (12) that it is, in fact, operating in the maintenance mode so the interrogator (12) can transmit the special instructions in confidence.

Abstract: To provide a communications system to accurately detect when a mobile transceiver has entered an area where communication with a fixed transceiver is possible, and to start-up the internal circuitry of the mobile transceiver, a communications system transmits a start-up pilot signal of a prescribed period from a transponder mounted in a fixed station to a transponder mounted in a vehicle. In the transponder, the received signal upon being received by an antenna is detected in a detector and amplified in an amplifier. When this signal is greater than a reference voltage, a pulse is output from a level discriminator. This pulse signal is converted into a pulse of constant pulse width in a discriminator pulse generator and integrated in an integrator. When the integrated value exceeds a reference voltage, a start-up discriminator sends a drive signal to actuate the internal circuitry of the mobile transceiver.

Abstract: A transponder is disclosed comprising a resonant RLC circuit with one or more components that can be connected or disconnected non-destructively using switches that respond to an externally applied magnetic field. The switching in or out of the components causes a change in the resonant frequency of the original RLC circuit that is detected by the base station sweeping a carrier signal through a range of frequencies to detect a returned signal that changes when the carrier signal frequency matches the resonant frequency of the resonant circuit/tag. The resonant frequency changes when one or more of the circuit components is connected or disconnected. Each switch transfers at a unique magnetic field intensity so that electrical contact is achieved at a predetermined value of an externally applied magnetic field. The detected changes are used to create a code that is used to pass information from the resonant circuit/tag to the base station.

Abstract: A electronic license plate system having an identification device embedded in a license plate portion and electronics stored in a license plate holder ensures that the proper identification device is connected to the electronics. The identification device is constructed in a manner such that tampering with the device will render is inoperable. Also, if an improper license plate is mounted to the license plate holder of a vehicle, a security system will disable the electronics of the system or will transmit a alarm signal.

Abstract: A transponder antenna is carried by a substrate and is sandwiched between the substrate and a cover sheet. A rectangular opening extends through the cover sheet and receives a signal processing chip adhesively carried in a recess below a cap which covers the opening. The chip is spaced with respect to the antenna so as to facilitate inductive coupling between the chip and antenna.

Abstract: A transponder detection system for detecting the presence at a detection area of a vehicle on which the transponder is mounted. Two antenna arrays are located on opposite sides of the detection area, and have their boresights directed to the detection area. For each array, a sum and difference signal of signals received from the transponder are measured. The sum channel signal is used to establish whether a transmission has occurred, and the difference channel signal is used to isolate any transponder that is at the detection area.

Abstract: A duplex radio communication system includes an Interrogator which generates a first modulated signal by modulating a first information signal onto a radio carrier signal. The Interrogator transmits the first modulated signal to at least one remote Tag of the system. The remote Tag receives and processes the first modulated signal received at its Antenna. A Backscatter Modulator uses a second information signal to modulate the reflection of the first modulated signal from the Antenna, the reflected signal being a second modulated signal. The Interrogator receives and demodulates the second modulated signal to obtain the second information signal.

Abstract: A radio communication system includes an Interrogator for generating and transmitting a first modulated signal by modulating a first information signal onto a radio carrier signal, the first information signal indicating in which of multiple response modes a receiving Tag should respond. One or more Tags of the system receive the first modulated signal and decode it to obtain the first information signal. A Backscatter Modulator modulates the reflection of the first modulated signal using a second information signal whose content, data rate, or modulation is determined by said first information signal, thereby forming a second modulated signal. The Interrogator receives and demodulates the second modulated signal to obtain the indicated second information signal.

Abstract: A system for wireless communication between at least one first station and a plurality of second stations using a relay station is disclosed. The relay station contains, among other features, an antenna array and a multi-channel transponder. The antenna array has a plurality of antenna elements which are divided into two sets. The first set is used to provide transmission or reception using beams having a first beamwidth and the second set is used to provide transmission or reception using beams having a second beamwidth. The multi-channel transponder, which is connected to the antenna array and a feeder link antenna, receives feeder link signals from at least one of the first stations and converts the signals into drive signals for the antenna array elements. The multi-channel transponder has a first channel bandwidth for channels transponding signals for transmissions having the first beamwidth and a second channel bandwidth for channels transponding signals for transmissions having said second beamwidth.

Abstract: A transponder used in a remote identification system requiring an interrogator and a transponder. The transponder which is thin and compact includes a transmit/receive antenna which is thin for receiving an interrogating signal and a rewrite signal emitted from the interrogator, and for emitting a response signal towards the interrogator. There is a modulator/demodulator for modulating a signal according to a code inherent in the transponder to emit the same from the antenna as the response signal without newly applying energy to the interrogating signal, and for demodulating the rewrite signal. A controller for applying a modulating signal generated according to the inherent code to the modulator/demodulator and for storing a signal demodulated by the modulator/demodulator. A liquid crystal display is used for displaying data including bar codes which can be read by a bar code reader. The bar code can be rewritten.

Abstract: A protocol is used to coordinate the use of a common communication medium by one or more interrogating commander stations and an unknown plurality of responding responder stations. Each commander station and each responder station is equipped to broadcast messages and to check for error in received messages. When more than one station attempts to broadcast simultaneously, an erroneous message is received and communication is interrupted. To establish uninterrupted communication, a commander station broadcasts a command causing each responder station of a potentially large first number of responder stations to each select a random number from a known range and retain it as its arbitration number. After receipt of such a command, each addressed responder station transmits a response message containing its arbitration number. Zero, one, or several responses may occur simultaneously.

Abstract: A method and apparatus with adaptive transponder plucking is provided in which oscillations from a resonant circuit (12 and 14) are maintained by coupling power from a capacitor (18) through a switch (26) to the resonant circuit (12 and 14). The switch (26) is controlled by adaptive pluck circuitry. The adaptive pluck circuitry includes peak level detector (22) and comparator (24). When the peak level of an oscillating signal from the resonant circuit (12 and 14) falls below a reference voltage, the switch (26) is activated to couple power to the resonant circuit (12 and 14).

Abstract: A method of determining the exact location of a transponder with respect to a reader antenna, with no additional components to the existing system required, is disclosed. An existing pin of the reader IC, the RSTP pin, provides an analog fieldstrength signal from which the distance to the reader antenna is deduced via known in the art signal processing methods. A grid or even an exact position location along a route is also achievable with initial set-up of at least two transponders a predetermined distance apart and a reader antenna.

Abstract: A system and method which conserves energy in the operation of a transponder or tag (14) by providing that the transponder (14) be enabled or awakened in multiple stages. A threshold detector (62) is provided which measures the power level of received RF energy. If the RF energy received by the detector (62) exceeds a pre-determined level, the transponder (14) then employs a modulation detector (64) to ascertain whether it has been awakened by a valid interrogation signal from an interrogator (12) or whether the RF energy received was merely a spurious burst of RF energy from some other source. If a pre-determined modulation is detected by the modulation detector (64), the transponder (14) is then fully activated to its normal operational state.

Abstract: An electronic license plate architecture is described wherein a license plate portion has a conventional license plate having visual information thereon and an identification device integrated therewith. The identification device stores restricted information. A license plate holder supports the license plate portion and houses communications and signal processing electronics. An antenna may be integrated with the license plate portion or may be housed in the license plate holder.

Abstract: A transponder device (10) includes a discharge circuit (18) to control the discharge of voltage from a charge capacitor (14). The discharge circuit (18) prevents transmission of a data telegram by the transponder device (10) until a voltage of the charge capacitor (14) reaches a threshold voltage level. The threshold voltage level is sufficient to ensure that a complete transmission of the data telegram is performed. Upon reaching the threshold voltage level, a transponder analog and digital circuit (19) initiates transmission of the data telegram. A control logic circuit (32) activates a counter (36) upon the start of the data telegram transmission. The counter (36) generates a control signal that is used to bypass the discharge circuit in order that the charge capacitor (14) can be connected directly to the transponder analog and digital circuit (19).

Abstract: A diode modulator which is not part of the rectifier power supply circuit of an RF Transponder is controlled by the transponder electronic logic and memory circuitry to conduct RF current from a second terminal to a first terminal of the RF Transponder antenna in a first state of the transponder electronic circuitry, and does not conduct appreciable current in a second state of the transponder electronic circuitry, thereby changing the loading of the RF Transponder antenna between the first state and the second state of the transponder electronic circuitry, and thereby changing the intensity of the RF radiation backscattered from the RF Tag. The present invention has further advantages that the same diode used as a modulator can also be used in the transponder receiver circuit.

Abstract: Apparatus and method of identifying a plurality of transponders (10-16) entering into an inquiry field (18) of an interrogation unit (20) are provided. The transponders (10-16) are each assigned a unique identification code. Upon receipt of an interrogation pulse from the interrogation unit (20), each transponder (10-16) responds with its respective identification code. Depending on the spatial distance of each transponder (10-16) with respect to the interrogation unit (20), the interrogation unit (20) detects and receives the strongest response. The received identification code is stored. The interrogation unit (20) repeatedly sends interrogation pulses containing any stored identification code until all of the transponders (10-16) are identified and read.

Abstract: A vehicle ID radar system has a simple construction and obtains high identifying performance by making a general identification based on a plurality of responses. There are provided an antenna for obtaining a plurality of question signals from a base station radar unit, a receiver for processing the signals received from the antenna, a question code demodulator for demodulating question codes from the received signals, a question code decoder for decoding the question codes and reading responses respectively corresponding to the plurality of questions from a plurality of data bases, a response code generator for generating response signals, and a transmitter for modulating the response signals and supplying them to the antenna.

Abstract: In one aspect, the present invention provides an air coil 20. The air coil 20 may be made from a strip of electrically conductive material 22 which has an insulating material overlying it. For example, the conductive material may comprise copper. The strip 22 is wound into a coil to perform a plurality of windings. The insulating material electrically insulates each of the winding from adjacent ones of the windings.

Abstract: An improved transponder for use in identifying an controlling vehicles wherein transponders mounted on vehicles respond to signals from a fixed location interrogator providing information relative to the vehicle or other data as required. The transponder disclosed in particular utilizes an improved method of providing frequency shift keyed (FSK) signals in response to a trigger or command signal received from a ground mounted loop or antenna. As disclosed, the transponder of the invention generates FSK signals through programming multiple microprocessor loops of predetermined duration, each loop providing microprocessor highs and lows for transmission to the interrogating station. Utilization of the inherently accurate microprocessor cycle time to generate distinct FSK signals avoids problems with ordinarily used analog frequency generating devices.

Abstract: A radio frequency automatic identification system detects targets which include a plurality of radio frequency resonators. Information is attributed to the target in accordance with the radio frequency response characteristics of the target, such as the resonant frequencies of resonators present, and/or the spatial locations within the target of the resonators. Preferred resonators include thin dipoles, which may be metallizations on a plastic film substrate. Information may be attributed to a target by fabricating the target with resonators disposed at locations to encode information in accordance with a predetermined encoding system; alternatively, resonators may be randomly disposed, and a translation code applied to convert the radio frequency information to particular information in a particular format. Readers for use in the system read the radio frequency response characteristics of a target in the near field of a radiating aperture which is illuminated by a radio frequency source.

Abstract: Apparatus and method for conveying information from an object by delayed reflection of an interrogating ultra-high range resolution radar signal which is digitized into a series of programmable spaced pulses by an electronic license plate circuit. The electronic license plate includes a plurality of unique programmable delay lines connected through a coupler network to an antenna.

Abstract: In a communication system for data transmission from a moving vehicle to a stationary beacon (1) which has a transceiver in which a signal is broadcast by the beacon (1) and received by an antenna (5, 5') of an on-board unit (4) of the vehicle and after a modulation with a data signal is transmitted back again to the beacon (1), where the data are extracted from the returned signal, the reliability of transmission for the data is increased by providing that the on-board unit (4) has at least two modulators (6, 6'), with which independently of one another the received signal is modulatable with the data signal and can be transmitted back in different ways. The transmission in different ways of the signals modulated independently of one another can be done by means of spatial separation of the antennas (5, 5') and/or by means of polarizing the broadcast signals differently from one another.

Abstract: The invention relates to an identification system comprising an interrogator and a plurality of transponders. The interrogator includes transmitter means for transmitting an interrogation signal to the transponder, receiver means for receiving a response signal from the transponder, and processor means for identifying the transponder from data in the response signal. Clock extraction means are further provided for extracting a transponder clock signal from the response signal which is used to synchronise the modification of the interrogation signal with the response signal from the transponder. Each transponder comprises receiver means for receiving the interrogation signal, a transponder clock generator, a code generator, transmitter means, and a modulator connected to the code generator, so that on receipt of the interrogation signal the transponder transmits a response signal containing data which identifies the transponder.

Abstract: This invention relates to a shielded transponder 12 which is capable of being mounted on any surface, whether ferromagnetic or not. The transponder includes an antenna which in use is susceptible to detuning as a result of being mounted on a ferromagnetic surface. The shield 16 prevents this detuning and enables the transponder 12 to be mounted on any surface. The shield 16 may produce slight detuning, however, this can be compensated for by pretuning of the transponder.

Abstract: A device for wireless transfer of information includes a data carrier in the form of a thin card, and a transponder that receives a first microwave signal, modulates it and codes it with data, and reradiates a second microwave signal without adding new energy thereto. The transponder includes at least one antenna device formed using microstrip conductors. The antenna device includes a patch antenna with an antenna layer that acts against a ground plane where the antenna layer and the ground plane are of substantially equal size. The device is characterized in that, it is equipped with a separate card holder with a reflective surface that is larger than said ground plane. The reflective surface increases the range by reradiating the second microwave signal and reducing sensitivity to back lobe reflections.

Abstract: A transponder system (10) includes an interrogation unit (12) that transmits a first frequency F1 representing a binary zero, and a second frequency F2, representing a binary one. These signals are transmitted from an interrogator resonant circuit (22) within a radio frequency module (16). The transponder system (10) also includes a transponder unit (14) that receives a transmit frequency signal (51) from the interrogation unit (12) at a transponder resonant circuit (42). The transponder resonant circuit (42) is tuned to the first frequency F1 and the second frequency F2. A transponder control module (44) detects a change in the transmit frequency signal (51) from the interrogation unit (12) by a decrease in the voltage amplitude with the transponder resonant circuit (42).

Abstract: The present invention uses a novel adaptation of a tree splitting algorithm applied to Radio Frequency (RF) tagging technology to identify many tags in the RF field of a base station.The invention uses the tree splitting algorithm to identify a single tag in a field of a plurality of radio frequency tags. Once the single tag is identified, the identified tag is placed in a Data.sub.-- Exchange state where the base station can access data from the tag memory by using information that identifies the tag.

Abstract: A transponder arrangement is described having an interrogator unit (10) which transmits at least one RF programming sequence and at least one RF interrogation pulse. The responder unit (12) includes a responder unit receiver (130) for receiving data transmitted by the RF programming sequence from the interrogator unit (10). The responder unit (12) then upon receipt of the RF interrogation pulse transmits data, which may have been modified by the programming sequence from the interrogator unit (10), back to the interrogator unit (10) in the form of a modulated RF carrier. The responder unit (12) further comprises a responder unit energy accumulator (136) which stores energy contained in the RF interrogation pulse and a responder unit end of burst detector (142) which upon detection of a decreasing power level of the RF interrogation pulse sends a RF threshold signal.

Abstract: A quasi-passive identification system having an interrogator and a passive ransponder. The interrogator has a signal generator that generates radio-frequency energy under the control of a manual actuator. A preset encoder encodes the radio-frequency energy with an interrogation code to produce an interrogation signal that is radiated by an antenna. The transponder has an receiver for receiving the interrogation signal and separating it into received radio-frequency energy and a coded signal. A decoder decodes the coded signal and looks for a match between the coded signal and the interrogation code. A response transmitter, which is controlled by the decoder, encodes the received radio-frequency energy with the response code and radiates the response signal. When a receiver in the interrogator receives the response signal, the response signal is decoded and analyzed for a proper response. A response indicator is energized when a proper response is received.