Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Phase comparison
Reexamination Certificate
1999-06-03
2001-12-11
Nguyen, Vinh P. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Phase comparison
C324S650000, C340S870030
Reexamination Certificate
active
06329808
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronics and, more particularly, to a system for detection of a load modulation signal by inductive coupling.
BACKGROUND OF THE INVENTION
The present invention relates to the reception of binary data sent by an integrated circuit with contactless operation. An integrated circuit with contactless operation may include a contactless chip card, an electronic label, and an electronic token, for example.
FIG. 1
is a standard circuit diagram of a detection system
10
and a load modulation system
20
cooperating with each other by inductive coupling. The system
10
includes an adjustable capacitor C
1
and a coil L
1
connected in series to form a resonant circuit. The capacitor C
1
is connected by a resistor R
1
to the output of a voltage/current amplifier
1
. The voltage/current amplifier
1
receives at an input an AC voltage V
0
having a frequency F
0
generated by an oscillator
2
. The voltage V
0
, converted into current by the amplifier
1
, forms the excitation signal for the resonant circuit L
1
C
1
having a natural frequency Fp
1
set near the excitation frequency F
0
.
The load modulation system
20
includes an antenna coil L
2
forming a resonant circuit with a capacitor C
2
having a natural frequency Fp
2
tuned to the frequency F
0
. The coil L
2
is parallel-connected with a load modulation circuit, which in this case, is a resistor R
2
series connected with a switch Tm. The switch Tm is controlled by a binary signal S
1
having a carrier frequency F
1
that is to be detected by the detection system
10
. The load modulations applied to the coil L
2
as a function of the signal S
1
effect the coil L
1
by inductive coupling. An AC voltage Vm having a frequency F
0
and modulated in amplitude and in phase is observed at the terminals of the antenna circuit L
1
C
1
.
According to the method illustrated in
FIG. 1
, detection of the load modulation signal S
1
is done by a phase comparator
3
receiving the voltages V
0
and Vm. The comparator
3
delivers a voltage Vj proportional to the phase shift of these two signals. The voltage Vj is filtered by a bandpass filter
4
set to the carrier frequency F
1
of the signal S
1
. The output of the bandpass filter
4
is made binary by a comparator
5
with a threshold Vref. At the output of the comparator
5
is the signal S
1
having a frequency F
1
.
Detecting the signal S
1
by phase comparison has the advantage of providing a good signal-to-noise ratio, but requires the natural frequency Fp
1
of the antenna circuit L
1
C
1
to be substantially mismatched with respect to the frequency F
0
. The phase j of the voltage Vm is not sensitive to the load modulations when the natural frequency Fp
1
of the antenna circuit L
1
C
1
is exactly equal to the excitation frequency F
0
.
FIG. 2
shows the phase curves j of the voltage Vm as a function of the natural frequency Fp
1
for various resistance values of 0, v1, v2 for the load resistor R
2
. When Fp
1
is equal to F
0
, the phase curves intersect at the same point and the sensitivity of the system
10
to the load modulations is zero. In setting the natural frequency Fp
1
of the circuit L
1
C
1
to a point F
0
′ close to F
0
, high sensitivity is obtained. The phase j is a function of the load or load resistor R
2
.
The natural frequency Fp
1
of the circuit L
1
C
1
must be precisely adjusted by setting the capacitor C
1
, and must be regularly checked to correct any drifts of the capacitor caused by environmental conditions (temperature, humidity, etc.) and aging. This constraint makes the method undesirable for implementation in a system with limited maintenance. For example, a system of limited maintenance with respect to contactless chip cards is a contactless chip card reader located in a public place, or a station for automatic testing of contactless integrated circuits, etc. For this reason, it is generally preferred to use a second method which includes demodulation of the amplitude of the voltage Vm at the terminals of the resonant circuit L
1
C
1
to extract the signal S
1
therefrom. This second method, however, has the drawback of providing a poor signal-to-noise ratio.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system and a method for the detection of a load modulation by phase comparison that does not have the drawbacks of the above described method.
According to the present invention, the method for detection by inductive coupling of a load modulation signal received in an antenna circuit is provided by exciting a detection coil by a signal with a predetermined frequency. The method further includes the step of comparing the phase of the excitation signal with the phase of a signal received by the antenna circuit. The antenna circuit does not form a resonant circuit near the frequency of the excitation signal, and the phase of the excitation signal is compared with the phase of the current flowing through the detection coil.
According to one embodiment applied to the detection of a load modulation signal, the system includes a detection coil and an integrated circuit coil respectively formed by primary and secondary windings of a transformer. The load modulation signal is sent by an integrated circuit comprising a modulation circuit modulating the load of the coil. The secondary winding is connected to contacts of the integrated circuit.
The present invention also relates to a system for detection by inductive coupling of a load modulation signal. The system comprises an antenna circuit including a coil, a circuit for applying to the antenna circuit an excitation signal having a predetermined frequency, a phase comparator receiving at a first input the excitation signal and receiving at a second input a signal picked up in the antenna circuit, and a circuit to extract the load modulation signal from a phase signal delivered by the phase comparator. The antenna circuit does not form a resonant circuit near the frequency of the excitation signal, and the phase comparator receives at the second input a signal delivered by a detection circuit for detecting current flowing through the coil.
According to one embodiment, the detection circuit for detecting the current comprises a transformer having a primary winding series-connected with the antenna coil. The secondary winding of the transformer is parallel-connected with a resistor to convert the current flowing through the primary winding into voltage.
According to another embodiment, the circuit for extracting the load modulation signal includes a bandpass filter centered on a carrier frequency of the load modulation signal.
The present invention also relates to a station for the reception of data by inductive coupling and load modulation. The station includes a detection system according to the present invention for the detection of a load modulation signal, and a decoder circuit to extract data elements from the load modulation signal.
The present invention also relates to a station for the testing of an integrated circuit with contactless operation using an antenna coil. The station includes a detection system according to the present invention for the detection of a load modulation signal emitted by the integrated circuit.
According to one embodiment, the coil of the detection system and the coil of the integrated circuit are primary and secondary windings of a transformer.
According to another embodiment, the station includes a decoder circuit to extract data elements from the detected load modulation signal.
REFERENCES:
patent: 4602253 (1986-07-01), Kreft
patent: 4654658 (1987-03-01), Walton
patent: 4928087 (1990-05-01), Kreft et al.
patent: 5343079 (1994-08-01), Mohan et al.
patent: 5717722 (1998-02-01), Mori
patent: 0 492 569 A2 (1991-12-01), None
patent: 0 704 928 A2 (1995-09-01), None
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Jorgenson Lisa K.
Nguyen Vinh P.
Patel Paresh
STMicroelectronics S.A.
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