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Claims  |
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What is claimed is:
1. An electronic identification tag for use with an identification reader,
said reader interrogating said tag by generating a reversing magnetic
field, said tag comprising:
a means for storing information to be communicated to an electronic
identification reader;
a means for varying the absorption of power from said reversing magnetic
field in accordance with said stored information, the generation of said
magnetic field being started at some arbitrary time, said magnetic field
being zero prior to said arbitrary time and consisting of a steady-state
component and a transient component subsequent to said arbitrary time;
a means for delaying said variation in absorption of power for a
predetermined period of time, said delay time being the time required for
said transient associated with starting the generation of said reversing
magnetic field to decrease to a level where said varying absorption of
power from said reversing magnetic field is distinguishable from said
transient.
2. The identification tag of claim 1 further comprising:
a means for extracting power from said reversing magnetic field, said power
being used by said tag in performing its function.
3. The identification tag of claim 2 wherein said stored information is
binary data, each bit of said binary data causing the absorption of power
from said reversing magnetic field to vary according to a first function
when said bit is a "0" and according to a second function when said bit is
a "1".
4. The identification tag of claim 2 wherein said stored information is
binary data, each bit of said binary data causing the absorption of power
from said reversing magnetic field to vary according to a first periodic
function when said bit is a "0" and according to a second periodic
function when said bit is a "1", the values of said first periodic
function for any half-period separations in time being equal in magnitude
and opposite in sign, the values of said second periodic function for any
half-period separations in time being equal in magnitude and opposite in
sign, the durations of said first and second periodic functions being
equal to a bit period.
5. The identification tag of claim 4 wherein said bit period is a multiple
of the periods of said first and second functions and said first and
second function periods are multiples of the period of said reversing
magnetic field, the beginning of said bit period coinciding with some one
of said first function periods, some one of said second function periods,
and the reversing magnetic field period first occurring after said delay
time.
6. The identification tag of claim 2 further comprising at least one
environmental sensor, said sensor providing information related to the
environment in which said tag is located, variation of absorbed power by
said power absorbing means being in accordance with information supplied
by said information storage means and with information supplied by said
sensor.
7. The identification tag of claim 2 or claim 6 wherein said power
extraction means supplies power in the form of a DC voltage and said delay
means comprises a threshold detector and a timing device, said threshold
detector producing a signal when said voltage exceeds a threshold level,
said timing device producing a signal at a predetermined time after said
threshold detector signal occurs.
8. The identification tag of claims 2 or 6 wherein said power extraction
means comprises a coil and a capacitor in parallel and an AC-to-DC
converter, said coil and a capacitor being resonant at the frequency of
said reversing magnetic field, the AC input voltage to said converter
being the voltage appearing across said coil and capacitor, the output
from said converter being said DC voltage.
9. The identification tag of claim 8 wherein said power absorption means
comprises a resistive load connected across said coil and capacitor and a
signal generator, said signal generator producing an extended signal as a
function of time when said delaying means permits said variation in
absorption of power from the reversing magnetic field to occur, said
extended signal being functionally related to information supplied by said
information storage means, said extended signal controlling the resistance
of said resistive load.
10. An electronic identification tag having information to be communicated
to an electronic identification reader, said reader interrogating said tag
by generating a reversing magnetic field, said tag comprising a coil, a
capacitor, a variable load, an AC-to-DC voltage converter, a delay means,
a signal generator, a first connecting means, a second connecting means, a
third connecting means, a fourth connecting means, and a fifth connecting
means; said coil and capacitor having first and second connecting
terminals, said variable load, voltage converter, and delay means having
first, second, and third connecting terminals, and said signal generator
having first, second, third, and fourth connecting terminals; first
terminals of all said devices being connected together by said first
connecting means, said second terminals of said coil, capacitor, variable
load, and voltage converter being connected together by said second
connecting means, said third terminal of said voltage converter being
connected to said second terminals of said delay means and said signal
generator by said third connecting means, said third terminal of delay
means being connected to said third terminal of said signal generator by
said fourth connecting means, and said fourth terminal of said signal
generator being connected to said third terminal of said variable load by
said fifth connecting means; said coil and capacitor being resonant at a
predetermined frequency, said coil producing an AC voltage across its
terminals in response to said reversing magnetic field encircled by said
coil, said voltage converter producing a DC voltage at its third terminal
in response to said AC voltage impressed across its first and second
terminals by said first and second connecting means, said DC voltage being
impressed across the first and second terminals of said delay means and
the first and second terminals of said signal generator by said first and
third connecting means, said DC voltage causing said delay means to
provide a "start" signal after a predetermined delay to the first and
third terminals of said signal generator by said first and fourth
connecting means, said DC voltage in conjunction with said "start" signal
causing said signal generator to supply a signal to the first and third
terminals of said variable load by said first and fifth connecting means,
said signal generator signal being uniquely related to said tag
information stored within said signal generator, the magnitude of said
variable load being varied in accordance with the magnitude of said signal
supplied by said signal generator, the absorption of power by said
variable load from said reversing magnetic field being in proportion to
said magnitude of said variable load.
11. The identification tag of claim 10 further comprising at least one
environmental sensor, the information produced by said sensor being part
of said tag information and being supplied to said signal generator.
12. The identification tag of claims 10 or 11 wherein said devices
comprising said tag with the exception of said coil and capacitor are
packaged in the form of integrated circuitry on a substrate, said
substrate also serving as a platform for mounting said capacitor and said
coil.
13. The identification tag of claims 10 or 11 wherein said devices
comprising said tag with the exception of said coil and capacitor are
packaged in the form of integrated circuitry on a substrate, said
capacitor being in the form of a chip, said integrated circuitry substrate
being mounted on said capacitor chip, said coil being attached to said
capacitor chip.
14. The identification tag of claims 10 or 11 wherein all of said devices
comprising said tag with the exception of said coil and capacitor are
packaged in the form of integrated circuitry on a substrate, said
capacitor also being incorporated in said substrate by means of integrated
circuit processing techniques, said substrate also serving as a platform
for mounting said coil.
15. The identification tag of claims 10 or 11 wherein all of said devices
comprising said tag with the exception of said coil and capacitor are
packaged in the form of integrated circuitry on a substrate, said coil and
said capacitor also being incorporated in said substrate by means of
integrated circuit processing techniques.
16. An electronic identification reader for use with an electronic
identification tag that absorbs power from a reversing magnetic field in
accordance with information to be communicated to said reader, said reader
operating in at least one of a plurality of modes, said modes being
characterized by mode control data, said reader comprising:
a means for storing mode control data;
a means for generating a reversing magnetic field;
a means for starting said generating means;
a means for stopping said generating means;
a means for obtaining a measure of the time-dependent absorption of power
from said magnetic field by an electronic identification tag, said power
absorption measure representing information being communicated by said
tag; and
a means for extracting said information from said power absorption measure,
said information extracting means operating in at least one of a plurality
of information extracting modes, each of said information extracting modes
defining a specific functional relationship between said information and
said time-dependent absorption of power by said tag, said information
extracting modes being characterized by said mode control data, the
specification of the information extracting mode in said mode control data
permitting the extraction of information from said time-dependent
absorption of power by said tag.
17. The identification reader of claim 16 wherein said means for generating
a reversing magnetic field includes generating said magnetic field at any
one of a plurality of frequencies, said magnetic field frequency being
specified in said mode control data.
18. The identification reader of claim 16 further comprising a means for
mode control, said mode control means causing said information extracting
means to operate in at least one of said plurality of said modes in
accordance with said mode control data.
19. The identification reader of claim 18 wherein said means for mode
control is at least one switch.
20. The identification reader of claim 18 wherein said means for mode
control is a plug-in memory device.
21. The identification reader of claim 18 wherein said means for mode
control is a reprogrammable memory device.
22. The identification reader of claim 17 further comprising a means for
mode control, said mode control means causing said reversing magnetic
field to be generated at a particular frequency and causing said
information extracting means to operate in at least one of said plurality
of said information extracting modes in accordance with said mode control
data.
23. The identification reader of claims 18 or 22 wherein said mode control
means causes said information extracting means to operate simultaneously
in a plurality of said information extracting modes.
24. The identification reader of claims 18 or 22 wherein said mode control
means causes said information extracting means to operate sequentially in
a plurality of said information extracting modes.
25. The identification reader of claim 18 further comprising means for
processing information wherein said extracted information is transformed
into processed information in accordance with a functional relationship
between said processed information and said extracted information, said
functional relationship being one of a plurality of functional
relationships, said plurality of functional relationships including one
for which said processed information is identical to said extracted
information, each of said operating modes also involving one of said
plurality of functional relationships.
26. The identification reader of claim 25 wherein said extracted and
processed information is binary data, each pair of bits of said extracted
information being transformed into a single bit of said processed
information, "01" and "10" pairs being transformed into "0" and "1"
respectively.
27. The identification reader of claim 25 wherein said extracted and
processed information is binary data, each bit of said extracted
information after the first bit being transformed into a bit of said
processed information, said processed bit being a "0" if said extracted
bit and the prior extracted bit are the same, said processed bit being a
"1" if said extracted bit and the prior extracted bit are different.
28. The identification reader of claim 25 further comprising a means for
reconstituting said processed information in accordance with a functional
relationship between said reconstituted information and said processed
information, said functional relationship being one of a plurality of
functional relationships, said plurality of functional relationships
including an identity relationship for which said reconstituted
information is identical to said processed information, said plurality of
functional relationships also including a plurality of error-correcting
functional relationships, said reconstituted information for each of said
plurality of error-correcting functional relationships being an error-free
version of said processed data, each of said information extracting modes
also based on one of said plurality of functional relationships.
29. The identification reader of claim 25 further comprising a means for
detecting errors in said processed information in accordance with a
functional relationship among the pieces of information, said
error-detecting functional relationship causing an error flag to be raised
when at least one error in the processed information is detected, said
functional relationship being one of a plurality of error-detecting
functional relationships, each of said information extracting modes also
based on one of said plurality of error-detecting functional
relationships.
30. The identification reader of claims 28 or 29 further comprising a means
for decrypting an encrypted portion of the tag information in accordance
with a functional relationship between said encrypted information and said
decrypted information, said decrypting functional relationship being one
of a plurality of decrypting functional relationships, each of said
information extracting modes also based on one of said plurality of
decrypting functional relationships.
31. The identification reader of claim 30 further comprising a means for
correcting errors in said decrypted information to obtain error-corrected
information in accordance with a functional relationship between said
error-corrected information and said decrypted information, said
functional relationship being one of a plurality of functional
relationships, each of said information extracting modes also based on one
of said plurality of functional relationships.
32. The identification reader of claim 25 further comprising a means for
displaying said information.
33. The identification reader of claim 32 wherein said display means has an
enabling control and said tag information includes the tag class, said
display means being enabled if said tag class is included in said mode
control data.
34. The identification reader of claim 16 wherein said generating means
comprises a first capacitor, a coil, and a second capacitor connected in
series in the same sequence, said capacitors being equal in capacitance,
said series combination being resonant at a predetermined frequency.
35. The identification reader of claim 17 wherein said generating means
comprises a plurality of capacitor pairs, a connecting means, and a coil,
any one of said plurality of capacitor pairs being selectable for
connection by said connecting means to said coil, the capacitors
comprising each of said pairs being equal in capacitance, the capacitors
selected for connection being connected one at each end in series with
said coil, each of said capacitor pairs in series with said coil having a
different resonant frequency.
36. The identification reader of claims 34 or 35 wherein said generating
means further comprises first and second drivers, said drivers supplying
signals to ends of said series combination of the same form and frequency
but of opposite phase thereby causing said series combination to be driven
in push-pull, said frequency being the resonant frequency of the series
combination.
37. The identification reader of claims 34 or 35 wherein said means of
obtaining a measure of absorption of power comprises an envelope detecting
device, the input signal to said envelope detecting device being the
alternating voltage appearing across said coil, the output signal of said
envelope detecting device being the amplitude of said alternating coil
voltage.
38. The identification reader of claim 37 wherein said envelope detecting
device comprises a full-wave rectifier and a filter, said full-wave
rectifier being connected across said coil, the output of said full-wave
rectifier feeding into said filter, the bandwidth of said filter being
chosen to maximize the signal-to-noise ratio, the variable portion of the
output of said filter being the envelope of said alternating coil voltage
and a measure of the power absorption from the field of said coil.
39. The identification reader of claim 37 wherein said means for obtaining
a power absorption measure further comprises a DC cancelling circuit, the
signal from said envelope detecting device feeding into said DC cancelling
circuit, the output signal from said DC cancelling circuit being the
envelope detecting device output signal diminished by the envelope
detecting device output signal at some predetermined time after the
beginning of the generation of said reversing magnetic field.
40. The identification reader of claim 39 wherein the DC cancelling circuit
has an input terminal, an output terminal, and a common input/output
terminal, said DC cancelling circuit comprising a capacitor having first
and second terminals, a single-pole/single-throw switch having first and
second terminals, a connecting means, and a switch control means, the
second terminal of said capacitor being connected by said connecting means
to the first terminal of said switch, the first terminal of said capacitor
being the input terminal of said DC cancelling circuit, the second
terminal of said capacitor being the output terminal of said DC cancelling
circuit, the second terminal of said switch being the common input/output
terminal of said DC cancelling circuit, said switch control means closing
said switch at the beginning of the generation of said reversing magnetic
field and opening said switch at said predetermined time after the
beginning of the generation of said reversing magnetic field.
41. The identification reader of claim 16 wherein said information
extracting means comprises an analog-to-digital converter and a digital
processor, the function of said analog-to-digital converter being to
obtain amplitude samples of said power absorption measure at discrete
intervals of time and to provide digital representations of said amplitude
samples to said digital processor, the function of said digital processor
being to extract from said digital representations said information being
communicated by a tag.
42. The identification reader of claim 16 wherein the information being
communicated by said tag is binary data, each bit of said binary data
causing the absorption of power by said tag from said reversing magnetic
field to vary according to a first function or a second function of finite
duration, different tags utilizing either the same or different first and
second function pairs.
43. The identification reader of claim 16 wherein the information being
communicated by a tag is binary data, each bit of said binary data causing
the absorption of power from said reversing magnetic field to vary
according to a first periodic function or a second periodic function, the
duration of said first and second periodic functions being a bit period,
the values of said first periodic function at half-period intervals being
equal in magnitude and opposite in sign, the values of said second
periodic function at half-period intervals being equal in magnitude and
opposite in sign, said function pair being either the same or different
for different tags, all of said bit periods having the same duration.
44. The identification reader of claim 43 wherein said information is being
communicated by a first tag, a second tag, a third tag, a fourth tag, and
a fifth tag, the first and second function pairs for said first tag being
those associated with on-off keying, the first and second function pairs
for said second tag being those associated with coherent phase-shift
keying, the first and second function pairs for said third tag being those
associated with differentially-coherent phase-shift keying, the first and
second function pairs for said fourth tag being those associated with
coherent frequency-shift keying, the first and second function pairs for
said fifth tag being those associated with non-coherent frequency-shift
keying.
45. The identification reader of claim 16 wherein the process performed by
said information extracting means proceeds sequentially in a plurality of
stages, the extraction of information requiring the successful completion
of all of said process stages, the successful completion of each of said
process stages occurring within a predetermined time period after said
magnetic field generation starts, said information extracting means
producing a "fail" signal whenever any one of a preselected group of said
process stages is not successfully completed within the predetermined time
period corresponding to that process stage, said magnetic field generation
starting means is activated by a user of said reader, and said magnetic
field generation stopping means is activated when either a "fail" signal
occurs or extraction of information is completed.
46. The identification reader of claim 42 wherein said information
extracting means comprises a means for recognizing power absorption by a
tag, said power absorption recognition process being the first of a
plurality of stages comprising the information extraction process.
47. The identification reader of claim 46 wherein said information
extracting means further comprises a bit synchronizing means for
identifying the beginnings of bit periods, the bit synchronizing process
being the second of said plurality of stages comprising the information
extraction process.
48. The identification reader of claim 47 wherein said information
extracting means further comprises a means for determining which of said
first and second functions most closely resembles said power absorption
measure over a bit period, the function determining process being the
third of said plurality of stages comprising the information extraction
process.
49. The identification reader of claim 43 wherein said information
extracting means comprises a means for recognizing power absorption by a
tag, said power absorption recognition process being the first of a
plurality of stages comprising the information extraction process.
50. The identification reader of claim 49 wherein said power absorption
recognition means comprises a plurality of function-pair means, each of
said function-pair means comprising four replica generators, four
correlators, and a calculating device, the functions of said replica
generators being to generate inphase and quadrature replicas, said inphase
replicas being replicas of said first and second periodic functions, said
quadrature replicas being replicas of said first and second periodic
functions delayed by one-quarter periods, the function of said correlators
being to multiply said power absorption measure by each of said replicas
and to integrate the results over a correlation interval, said correlation
interval extending from the present to one bit period in the past, thereby
obtaining inphase and quadrature replica correlations with said power
absorption measure for said first and second periodic functions, said
correlations being repeated at intervals no greater than one-half period,
the function of said calculating device being to determine from said
inphase and quadrature replica correlations when power absorption by a tag
is occurring.
51. The identification reader of claim 43 wherein said information
extracting means comprises a means for recognizing power absorption by a
tag and a bit synchronizing means for identifying the beginnings of bit
periods, said power absorption recognition process being the first of a
plurality of stages comprising the information extraction process, said
bit synchronizing process being the second of said stages comprising the
information extraction process.
52. The identification reader of claim 51 wherein said bit synchronizing
means comprises a plurality of function-pair means, each of said
function-pair means comprising four replica generators, four correlators,
and a calculating device, the functions of said replica generators being
to generate inphase and quadrature replicas, said inphase replicas being
replicas of said first and second periodic functions, said quadrature
replicas being replicas of said first and second periodic functions
delayed by one-quarter periods, the function of said correlators being to
multiply said power absorption measure by each of said replicas and to
integrate the results over a correlation interval, said correlation
interval extending from the present to one bit period in the past, thereby
obtaining inphase and quadrature replica correlations with said power
absorption measure for said first and second periodic functions, said
correlations being repeated at intervals no greater than one-half period,
the function of said calculating device being to recognize the occurrence
of a zero-crossing time from said inphase and quadrature replica
correlations, said zero-crossing time being the point in time when one
function of said function pair controls the power absorption of the tag
during the first half of said correlation interval and the other function
of said function pair controls the power absorption during the second half
of said correlation interval, the beginning of a bit period being one-half
bit period after said zero-crossing time.
53. The identification reader of claim 43 wherein said information
extracting means comprises a means for recognizing power absorption by a
tag, a bit synchronizing means for identifying the beginnings of the bit
periods for a tag, and a bit identifying means for determining whether a
"0" or a "1" was communicated by the tag, said power absorption
recognition process being the first of a plurality of stages comprising
the information extraction process, said bit synchronization process being
the second of said stages, and said bit identification process being the
third of said stages.
54. The identification reader of claim 53 wherein said bit identifying
means comprises a plurality of function-pair means, each of said
function-pair means comprising four replica generators, four correlators,
and a calculating device, the functions of said replica generators being
to generate inphase and quadrature replicas, said inphase replicas being
replicas of said first and second periodic functions, said quadrature
replicas being replicas of said first and second periodic functions
delayed by one-quarter periods, the function of said correlators being to
multiply said power absorption measure by each of said replicas and to
integrate the results over a correlation interval, said correlation
interval extending from the present to one bit period in the past, said
correlation interval coinciding with said tag bit period, thereby
obtaining inphase and quadrature replica correlations with said power
absorption measure for said first and second periodic functions, said
correlations being repeated at intervals equal to a bit period, the
function of said calculating device being to identify the power absorption
function controlling the absorption of power by the tag from said inphase
and quadrature replica correlations thereby identifying the bit that was
communicated by the tag.
55. The identification reader of claim 43 wherein said bit period is a
multiple of the periods of said first and second functions and said first
and second function periods are multiples of the period of said reversing
magnetic field, the beginning of said bit period coinciding with some one
of said first function periods, some one of said second function periods,
and some one of said reversing magnetic field periods.
56. An electronic identification reader for use with an electronic
identification tag that absorbs power from a reversing magnetic field in
accordance with information to be communicated to said reader, said reader
operating in at least one of a plurality of modes, said modes being
characterized by mode control data, said reader comprising:
a means for storing mode control data;
a means for generating a reversing magnetic field;
a means for starting said generating means;
a means for stopping said generating means;
a means for obtaining a measure of magnetic field variations brought about
by an electronic identification tag, said magnetic field variation measure
representing information being communicated by said tag; and
a means for extracting said information from said magnetic field variation
measure, said information extracting means being capable of operating in a
plurality of information extracting modes, each of said information
extracting modes defining a specific functional relationship between said
information and said time-dependent magnetic field variation brought about
by said tag, said information extracting modes being characterized by said
mode control data, the specification of the information extracting mode in
said mode control data permitting the extraction of information from said
time-dependent absorption of power by said tag.
57. The identification reader of claim 56 wherein said means for generating
a reversing magnetic field includes generating said magnetic field at any
one of a plurality of frequencies, said magnetic field frequency being
specified in said mode control data.
58. The identification reader of claim 56 further comprising a means for
mode control, said mode control means causing said information extracting
means to operate in at least one of said plurality of said modes in
accordance with said mode control data.
59. The identification reader of claim 58 wherein said means for mode
control is at least one switch.
60. The identification reader of claim 58 wherein said means for mode
control is a plug-in memory device.
61. The identification reader of claim 58 wherein said means for mode
control is a reprogrammable memory device.
62. The identification reader of claim 57 further comprising a means for
mode control, said mode control means causing said reversing magnetic
field to be generated at a particular frequency and causing said
information extracting means to operate in at least one of said plurality
of said information extracting modes in accordance with said mode control
data.
63. The identification reader of claims 58 or 62 wherein said mode control
means causes said information extracting means to operate simultaneously
in a plurality of said information extracting modes.
64. The identification reader of claims 58 or 62 wherein said mode control
means causes said information extracting means to operate sequentially in
a plurality of said information extracting modes.
65. The identification reader of claim 56 wherein said information
extracting means comprises an analog-to-digital converter and a digital
processor, the function of said analog-to-digital converter being to
obtain amplitude samples of said magnetic field variation measure at
discrete intervals of time and to provide digital representations of said
amplitude samples to said digital processor, the function of said digital
processor being to extract from said digital representations said
information being communicated by a tag.
66. The identification reader of claim 65 wherein said information
extracting means further comprises a notch filter tuned to the frequency
of said reversing magnetic field and inserted between said means for
obtaining a magnetic field variation measure and said analog-to-digital
converter for the purpose of suppressing signals having the frequency of
said reversing magnetic field.
67. An electronic identification system including a reader and at least one
tag, each of said tags comprising:
a means for storing information to be communicated to an electronic
identification reader;
a means for varying the absorption of power from a reversing magnetic field
in accordance with said stored information, the generation of said
magnetic field being started at some arbitrary time, said magnetic field
being zero prior to said arbitrary time and consisting of a steady-state
component and a transient component subsequent to said arbitrary time;
a means for delaying said variation in absorption of power for a
predetermined period of time, said delay time being the time required for
said transient associated with starting the generation of said reversing
magnetic field to decrease to a level where said varying absorption of
power from said reversing magnetic field is distinguishable from said
transient;
said reader which operates in at least one of a plurality of modes, said
modes being characterized by mode control data, said reader comprising:
a means for storing mode control data;
a means for generating a reversing magnetic field;
a means for starting said generating means;
a means for stopping said generating means;
a means for obtaining a measure of the time-dependent absorption of power
from said magnetic field by an electronic identification tag, said power
absorption measure representing information being communicated by said
tag; and
a means for extracting said information from said power absorption measure,
said information extracting means being capable of operating in a
plurality of information extracting modes, each of said information
extracting modes defining a specific functional relationship between said
information and said time-dependent absorption of power by said tag, said
information extracting modes being characterized by said mode control
data, the specification of the information extracting mode in said mode
control data permitting the extraction of information from said
time-dependent absorption of power by said tag.
68. The identification system of claim 67 wherein said means of obtaining a
measure of absorption of power in said reader comprises an envelope
detecting device and a DC cancelling circuit, the input signal to said
envelope detecting device being the alternating voltage appearing across
said coil, the output signal of said envelope detecting device being the
amplitude of said alternating coil voltage, the signal from said envelope
detecting device feeding into said DC cancelling circuit, the output
signal from said DC cancelling circuit being the envelope detecting device
output signal diminished by the envelope detecting device output signal at
some predetermined reference time after the beginning of the generation of
said reversing magnetic field, said delay time in said tag being longer
than said predetermined reference time of DC cancelling circuit.
69. The identification system of claim 67, said identification tag further
comprising:
a means for extracting power from said reversing magnetic field, said power
being used by said tag in performing its function.
70. A method of communicating information in response to the presence of a
reversing magnetic field, the generation of said magnetic field being
started at some arbitrary time, said magnetic field being zero prior to
said arbitrary time and consisting of a steady-state component and a
transient component subsequent to said arbitrary time, said method
comprising the steps:
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