Communications: electrical – Selective – Interrogation response
Reexamination Certificate
2000-04-05
2004-08-31
Horabik, Michael (Department: 2635)
Communications: electrical
Selective
Interrogation response
C340S010200, C340S010500, C340S010510, C340S870030, C340S870030, C342S030000
Reexamination Certificate
active
06784785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read/write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read only transponders, that is, adapted to operating with a terminal only reading the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
2. Discussion of the Related Art
Electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal. 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's antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, on the intensity of this excitation, and on the transponder power consumption.
FIG. 1
very schematically shows, in a functional way, a conventional example of a data exchange system between a read/write terminal
1
(STA) and a transponder
10
(CAR).
Generally, terminal
1
is essentially formed of an oscillating circuit formed of an inductance L
1
in series with a capacitor C
1
and a resistor R
1
, between an output terminal
2
p
of an amplifier or antenna coupler
3
(DRIV) and a terminal
2
m
at a reference potential (generally, the ground). Amplifier
3
receives a high-frequency transmission signal Tx, provided by a modulator
4
(MOD). The modulator receives a reference frequency, for example, from a quartz oscillator
5
and, if necessary, a data signal to be transmitted. In the absence of a data transmission from terminal
1
to transponder
10
, signal Tx is used as a power source only, to activate the transponder if said transponder enters the field. The data to be transmitted come from an electronic system, generally digital, for example, a microprocessor
6
(&mgr;P).
The connection node of capacitor C
1
and inductance L
1
forms, in the example shown in
FIG. 1
, a terminal for sampling a data signal Rx, received from a transponder
10
and intended for a demodulator
7
(DEM). An output of the demodulator communicates (if necessary via a decoder (DEC)
8
) the data received from transponder
10
to microprocessor
6
of read/write terminal
1
. Demodulator
7
receives, generally from oscillator
5
, a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C
1
and resistor R
1
and not across inductance L
1
. Microprocessor
6
communicates (bus EXT) with different input/output (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits. The circuits of the read/write terminal draw the energy necessary for their operation from a supply circuit
9
(ALIM), connected, for example, to the electric supply system.
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) intended for capturing the field generated by series oscillating circuit L
1
C
1
of terminal
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 terminal
1
.
Terminals
11
,
12
, of resonant circuit L
2
C
2
, which correspond to the terminals of capacitor C
2
, are connected to two A.C. input terminals of a rectifying bridge
13
formed, for example, of four diodes D
1
, D
2
, D
3
, D
4
. In the representation 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 to rectified output terminals
14
,
15
of bridge
13
to store power and smooth the rectified voltage provided by the bridge. It should be noted that the diode bridge may be replaced with a single-halfwave rectifying assembly.
When transponder
10
is in the field of terminal
1
, a high frequency voltage is generated across resonant circuit L
2
C
2
. This voltage, rectified by bridge
13
and smoothed by capacitor Ca, provides a supply voltage to electronic circuits of the transponder via a voltage regulator
16
(REG). These circuits generally include, essentially, a microprocessor (&mgr;P)
17
(associated with a memory not shown), a demodulator
18
(DEM) of the signals that may be received from terminal
1
, and a modulator
19
(MOD) to transmit information to terminal
1
. The transponder is generally synchronized by means of a clock (CLK) extracted, by a block
20
, from the high-frequency signal recovered across capacitor C
2
before rectification. Most often, all the electronic circuits of transponder
10
are integrated in a same chip.
To transmit the data from transponder
10
to unit
1
, modulator
19
controls a stage of modulation (back modulation) of resonant circuit L
2
C
2
. This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals
14
and
15
. Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal
1
(for example, 13.56 MHz). When switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping as compared to the load formed of circuits
16
,
17
,
18
,
19
and
20
, so that the transponder draws a greater amount of power from the high frequency field. On the side of terminal
1
, amplifier
3
maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and phase variation of the current in antenna L
1
. This variation is detected by demodulator
7
of terminal
1
, which is either a phase demodulator or an amplitude demodulator. For example, in the case of a phase demodulation, the demodulator detects, in the half-periods of the sub-carrier where switch T of the transponder is closed, a slight phase shift (a few degrees, or even less than one degree) of the carrier of signal Rx with respect to the reference signal. The output of demodulator
7
(generally the output of a band-pass filter centered on the sub-carrier frequency) then provides an image signal of the control signal of switch T that can be decoded (by decoder
8
or directly by microprocessor
6
) to restore the binary data.
FIGS. 2A and 2B
illustrate a conventional example of data transmission from terminal
1
to a transponder
10
.
FIG. 2A
shows an example of shape of the excitation signal of antenna L
1
for a transmission of a code 1011. The modulation currently used is an amplitude modulation with a 106-kbit/s rate (one bit is transmitted in approximately 9.5 &mgr;s) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from oscillator
5
(period of approximately 74 ns). The amplitude modulation is performed either in all or nothing or with a modulation ratio (defined as being the difference of the peak amplitudes between the two states (0 and 1), divided by the sum of these amplitudes) smaller
Bardouillet Michel
Enguent Jean-Pierre
Wuidart Luc
Brown Vernal
Horabik Michael
STMicroelectronics S.A.
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