Pulse or digital communications – Pulse position – frequency – or spacing modulation
Reexamination Certificate
2000-06-23
2004-02-03
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Pulse position, frequency, or spacing modulation
C375S316000, C340S010300, C340S870030
Reexamination Certificate
active
06687293
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to radio frequency identification (RFID) tag devices, and more particularly, to a radio frequency identification tag device which calibrates its decoder timing from a received pulse position modulation (PPM) signal within one calibration symbol time.
BACKGROUND OF THE RELATED TECHNOLOGY
Radio frequency identification (RFID) tag devices may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards, keyless entry and the like. RFID tag devices will work under more hostile environmental conditions than bar code labels since the RFID tag device may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tag devices are used in conjunction with a radio frequency tag reader (interrogator) which generates a continuous wave (CW) radio frequency (RF) or electromagnetic carrier that activates the RFID tag device at close range. The RFID tag device is passive and may have no internal power sources, rather it uses some of the power in the CW RF or electromagnetic carrier of the RFID tag reader to power internal circuits that read a stored internal digital code and cause the RFID tag device to signal its stored internal digital code to the RFID tag reader.
The RFID tag device modifies the amplitude of the CW carrier of the RFID tag reader by tuning and detuning a resonant circuit tuned to the CW carrier. The RFID tag device comprises, for example, a parallel resonant circuit or antenna tuned to the frequency of the CW radio frequency or electromagnetic carrier, an RF to direct current (DC) converter, a circuit for tuning and detuning the parallel resonant circuit/antenna, logic which stores the internal digital code, logic which reads the internal digital code and causes the circuit for tuning and detuning the parallel resonant circuit/antenna to operate in co-operation with the internally stored digital code.
An excellent application for RFID tag devices is item level tagging such as retail and inventory management where a large number of RFID tags may be read and written in the same reader field. Read-write memory is incorporated in the RFID tag device and may be allocated for device operation (program) and user data such as for example, but not limited to, inventory number, product expiration date, weight, product description, etc. The RFID tag device may have, for example, two operational modes: 1) “tag talks first” and 2) “reader talks first” modes. The “tag talks first” mode is when the RFID tag device transmits its data as soon as it is energized by the RFID tag reader. The “reader talks first” mode is when the RFID tag device does not transmit unless being commanded to do so by the RFID tag reader.
The RFID tag reader sends command signals to the RFID tag device by modulating its RF or electromagnetic carrier signal. These command signals may be represented by appropriately timed gap pulses using, for example, Pulse Position Modulation (PPM) of the RF or electromagnetic carrier signal. PPM is a digital transmission scheme whereby data is represented by the temporal location of a pulse or pulses within a time window known as a symbol frame.
It is desirable for power and space considerations of the RFID tag device to utilize an onboard oscillator for supplying the clock timing required for decoding the PPM transmission symbols. The frequency of the internal oscillator of the RFID tag device, however, may vary as much as plus or minus 25 percent because of changes in the semiconductor fabrication process, operating voltage and/or temperature. This much variation in the RFID tag device's internal oscillator clock frequency would make accurate decoding of the PPM transmission impossible if left uncorrected.
Known methods of matching the RFID tag device internal clock oscillator frequency to the external PPM frequency involves adjusting the internal clock oscillator frequency and requires several cycles of calibration symbols to accurately lock the internal clock oscillator to the PPM frequency transmitted by the RFID tag reader. A phase locked loop has been used to adjust the internal clock oscillator frequency in this manner. U.S. Pat. Nos. 4,648,133 and 5,354,319 disclose phase locked loops for controlling the frequency of the PPM decoder clock oscillator so as to lock to the external PPM signal. U.S. Pat. No. RE. 31,254 discloses calculating an error component between a local oscillator and an external frequency source with a software program algorithm running on a microprocessor.
Therefore, what is needed is a simpler, faster and more effective way of calculating the relative frequency relationship between a PPM receiver/decoder oscillator and an externally transmitted PPM signal, and then calibrating the PPM decoder circuit to the required timing precision for correct PPM decoding within one calibration symbol time.
SUMMARY OF THE INVENTION
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing in an RFID tag device a circuit that calculates the relative frequency relationship between an internal oscillator of the RFID tag device and an external PPM source such as a RFID tag reader, and then calibrates the RFID tag device PPM decoder circuit to the required precision for reliable PPM symbol decoding. In the embodiment of the present invention, the PPM decoder is calibrated to the difference between the external PPM frequency source (i.e., RFID tag reader) and the internal clock-oscillator of the RFID tag device, which is preformed in a single measurement during one calibration symbol time.
The RFID tag reader sends the command and acknowledgement signals to the RFID tag device by modulating the continuous wave (CW) carrier signal. The RFID tag reader uses, for example, two classes of encoding signals for modulation. They are (a) 1-of-16 PPM for data transmission, and (b) fast read commands that consist of gap pulse sequences. The gap pulse sequences are controlled by pulse width and time spacing between pulses to encode the command and operating parameters. The RFID tag reader also sends time reference pulses to calibrate the time base of the decoder in the RFID tag device.
The RFID tag reader uses 1 of 16 PPM for control commands such as tag acknowledgement, read a tag block, write a tag block, etc. The 1 of 16 PPM uses the pulse positions in one of sixteen possible time slots as the communication mechanism for sending 4 bit symbols (2
4
=16). All communications begin with a code violating calibration sequence composed of, for example but not limited to, three pulses in pulse positions zero, six and fourteen.
The symbol frame start and end are not explicitly transmitted and are recovered by knowledge of the last symbol received, the count to the next received pulse and counts per pulse width. An equation describing this relationship is:
(sym)
n
=CBP/CPP−16+(sym)
n−1
where CBP=number of internal oscillator counts between pulses
CPP=number of internal oscillator counts per pulse width
(sym)
n−1
=previous received symbol
(sym)
n
=new symbol received
Initiating synchronization is achieved by recognizing the code violating calibration symbol and determining the “counts per pulse width” (CPP) of the internal oscillator of the RFID tag device. Maintaining synchronization requires the ability to use the new pulse to correct for any accumulated error between the RFID tag device and the transmitted PPM time bases, and to maintain the time base of the RFID tag device time base to sufficient accuracy between the PPM pulses. For a maximum pulse separation of 31 pulse positions, the maximum allowed error is preferably ½ pulse position. This allows a maximum error of one part in 62, or +/−1.6%.
Timing for detecting (demodulating) these commands from the PPM radio frequency (RF) or electromagnetic transmission is generated by a clock-oscillator internal to the
Alexander Samuel
Loyer Stephen R.
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