Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-12-20
2002-03-12
Lee, Benjamin C. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572100, C340S010300, C340S010400, C235S451000, C235S492000, C235S384000, C342S044000, C342S051000
Reexamination Certificate
active
06356198
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority from prior French Patent Application No. 98-16383, filed Dec. 21, 1998, the entire disclosure of which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic circuits, and more specifically to electromagnetic transponders that are interrogated contactless and wireless by a read/write terminal.
2. Description of Related Art
Conventional electromagnetic transponders are transceivers that typically lack an independent power supply, and instead extract the power required by the electronic circuits included therein from a high frequency field radiated by the antenna of the read/write unit. Such electromagnetic transponders are based on the use of oscillating circuits, on the transponder side and on the read/write unit side. These circuits are coupled by a close magnetic field when the transponder enters the field of the read/write unit. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, and on the intensity of this excitation.
FIG. 1
schematically shows a conventional data exchange system between a read/write unit
1
and a transponder
10
. Generally, unit
1
is formed of an oscillating circuit formed of an inductance L
1
in series with a capacitor C
1
between an output terminal
2
of an antenna coupler
3
and a terminal
4
at a reference potential (generally, ground). Coupler
3
receives a signal f
1
, provided by an oscillator
5
, that forms a high frequency carrier. Signal f
1
is used in the absence of a data transmission from terminal
1
to transponder
10
, as an “energy source” for activating transponder
10
if the transponder enters the field. If necessary, a modulator
6
provides a data signal based on data, received from an input e that is the output of an electronic system (not shown).
The junction point of capacitor C
1
and inductance L
1
forms, in the example shown in
FIG. 1
, a terminal rx for sampling a data signal, received from a transponder
10
and intended for a demodulator
7
. An output s of the demodulator communicates the data received from transponder
10
to the electronic system of the read/write unit. Demodulator
7
receives, from oscillator
5
, a signal f
2
(most often, with the same frequency as signal f
1
) to enable the demodulation. The demodulation may be performed from a signal sampled across antenna coupler
3
and not across inductance L
1
. By the excitation from signal f
1
inductance L
1
of unit
1
generates a high frequency field of small intensity. On the side of transponder
10
, an inductance L
2
, in parallel with a capacitor C
2
, forms a parallel oscillating circuit (called a reception resonant circuit) for capturing the field generated by the oscillating circuit of unit
1
. The resonant circuit (L
2
-C
2
) of transponder
10
is tuned on the frequency of the oscillating circuit (L
1
-C
1
) of unit
1
.
Terminals
11
and
12
of the resonant circuit (corresponding to the terminals of capacitor C
2
) are connected to two AC input terminals of a rectifying bridge
13
that is formed, for example, of four diodes D
1
, D
2
, D
3
, and D
4
. In the circuit of
FIG. 1
, the anode of diode D
1
and the cathode of diode D
3
are connected to terminal
11
. The anode of diode D
2
and the cathode of diode D
4
are connected to terminal
12
. The cathodes of diodes D
1
and D
2
form a positive rectified output terminal
14
. The anodes of diodes D
3
and D
4
form a reference terminal
15
of the rectified voltage. A capacitor Ca is connected in parallel with rectified output terminals
14
and
15
of bridge
13
to filter the rectified voltage provided by the bridge.
When transponder
10
is in the field of unit
1
, a high frequency voltage is generated across the resonant circuit. This voltage, rectified by bridge
13
and smoothed by capacitor Ca, provides a supply voltage Va to electronic circuits of the transponder via a voltage regulator
16
. The electronic circuits of the transponder have been symbolized in
FIG. 1
by a block
17
. This block P generally is a chip (most often integrating regulator
16
) containing at least one memory and one processor. To transmit data from transponder
10
to unit
1
, block
17
controls a stage of modulation (back modulation) of the resonant circuit (L
2
-C
2
).
This modulation stage is generally formed of an electronic switch (a transistor
18
) and a resistor R, associated in series between terminals
14
and
15
. Transistor
18
is controlled at a much smaller frequency (generally by a factor of at least 10) than frequency f
1
of the excitation signal of the oscillating circuit of unit
1
. The oscillating circuit of the transponder is thus submitted to an additional damping as compared to the load formed of regulator
16
and circuit
17
when switch
18
is closed. The voltage decreases across winding L
2
so that the transponder takes a greater amount of energy from the high frequency field.
In the system range, two effects of the presence of the transponder in the field can be made out. At a relatively large distance (that is, substantially at the range limit), the oscillating circuit of the terminal operates with no interference. At a smaller distance, there is an increase in the charge of the oscillating circuit of unit
1
. Accordingly, the amplitude across inductance L
1
decreases. Thus, there is a transfer of the amplitude modulation performed by transponder
10
to read/write unit
1
. which can then detect the presence of transponder
10
in its field. Another method consists of detecting the phase variation due to this change of charge on the oscillating circuit of unit
1
.
Different types of modulation may be used for the data exchanges between unit
1
and transponder
10
. Most often, an amplitude modulation representing either the entirety of signal f
1
(all or nothing modulation), or a small portion (on the order of 10%) of this amplitude is used, according to the supply need of transponder
10
. It should be noted that, whatever the type of modulation used (for example, phase or frequency modulation) and whatever the type of data coding (NRZ, NRZI, or Manchester), the transmission of the modulation is performed digitally, by skip between two binary levels.
The matching frequency of the oscillating circuits conditions the transmission rate since the frequency of modulation, by switch
18
on the transponder side must be clearly smaller than the carrier frequency used to supply the transponders. Accordingly, the higher the supply carrier frequency, the greater the data flow rate can be. For example, conforming to a standard ISO 14443 of small distance transponder systems (distance smaller than twenty centimeters), the frequency of carrier f
1
is 13.56 MHZ and the frequency of the control pulses of switch
18
, on the transponder side, is 847 kHz (16 times less).
Conventional transponder systems such as those described hereabove suffer from several drawbacks. A first drawback is that “transmission gaps”, that is, distances between the transponder and the terminal at which the terminal does not detect the transponder even though the transponder is in its field, can occur. Such transmission gaps occur when the transponder is very close to the read/write unit, that is, when the distance between both inductive coupling elements L
1
and L
2
is small as compared to the system operating range. For example, for an application to small distance systems under a 13.56 MHZ frequency, the range is on the order of 10 centimeters and detection losses appear when the transponder is at less than three centimeters from the read/write unit. A conventional solution for overcoming this problem is to force a minimum interval between the transponder and unit
1
. However,
Bardouillet Michel
Wuidart Luc
Bongini Stephen
Fleit Kain Gibbons Gutman & Bongini P.L.
Jorgenson Lisa K.
Lee Benjamin C.
STMicroelectronics S.A.
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