Protection of a radio frequency transmit-receive terminal against electromagnetic disturbances

An antenna circuit for a device of transmission/reception by inductive coupling, including a first inductive element in parallel with a capacitive element and, between each node of the parallel association and two terminals of a switch, a second inductive element.

BACKGROUND

Technical Field

The present disclosure generally relates to electronic circuits and, more specifically, to contactless transmit-receive terminals capable of operating in a first so-called reader mode, where the terminal communicates with a distant electromagnetic transponder, and in a so-called card mode, where the terminal operates as an electromagnetic transponder with respect to another terminal.

Description of the Related Art

A contactless card reader emits a magnetic field towards an oscillating circuit of a card generally having no autonomous power supply. In the reader-to-card direction, the data are generally coded and transmitted in amplitude modulation of a carrier of excitation of an oscillating circuit of the reader. In the card (transponder) to reader direction, the card circuits modulate the load formed by the oscillating circuit of the card on the magnetic field of the reader, with a circuit generally called a retromodulation circuit, for example, by short-circuiting the antenna circuit by means of a switch.

A radio frequency terminal transmitting data by inductive coupling, capable of operating in card mode and in reader mode, is capable of generating the data frame coding and decoding according to different protocols. Near field communication circuits (NFC) are capable of operating in card mode and in reader mode. Such circuits are commonly used in portable telecommunication devices such as cell phones. Various standards set the communication protocols.

In card mode, the power supply of the circuits is generally induced by the magnetic field generated by the reader, which forms the communication channel between the card and the reader. The alternating current (A.C.) signal exciting the oscillating circuit of the reader then forms a remote-supply carrier for the card.

In reader mode, the terminal is powered (for example, by a battery or by a connection to the power distribution system) to emit the magnetic field towards a card.

In dedicated devices, the antenna (or oscillating circuit inductance) of the reader is often tuned to the transmit frequency (for example, on the order of 13.56 MHz for ISO standard 14443) and matched to the impedance of the A.C. signal generator, while the antenna (inductance of the resonant circuit) of a card is often tuned to the operating frequency of the system (for example, 13.56 MHz).

If the reader antenna is tuned, but mismatched, the transmission is not optimal. Further, the reflected wave due to the mismatching disturbs the detection of the signal retromodulated by the card. Conversely, if the card antenna is matched but off-tune, the power recovery is not optimized.

As a result, antenna circuits are generally not compatible with an operation in card mode and in reader mode. In particular, it is difficult to use a single antenna to design a device which can operate both in reader mode and in card mode.

Another issue is linked to the need to associate filters against electromagnetic disturbances (electromagnetic interference, or EMI, filters) to the transmission circuit. Such filters are generally interposed between the electronic circuits of the transceiver device and the antenna.

U.S. Pat. No. 7,665,664 describes a reader by inductive coupling equipped with means for extracting a power supply voltage in a card mode operation by extracting the electric power directly in the antenna circuit of the reader without using a dedicated antenna coil. In this circuit, EMI-type low-pass filters are incorporated into the antenna circuit and are electrically interposed between terminals of the antenna circuit intended to be connected to a transmit circuit and series capacitors connected across the antenna. The architecture is of differential type and the antenna includes a grounded midpoint, two capacitors in parallel between each terminal of the antenna and the ground taking part in the tuning of the oscillating circuit. In card mode, the input terminals of the antenna circuit are short-circuited by a rectifying bridge. The capacitive elements taking part in the oscillating circuit as well as those in series with the low-pass filters then generate disturbances, which result in a detuning of the antenna in card mode, a loss of amplitude of the received signal, and in addition or alternatively, a deformation of the received wave.

BRIEF SUMMARY

An embodiment overcomes all or part of the disadvantages of known devices of transmission/reception by inductive coupling.

Another embodiment provides an architecture in which the provision of filters against electromagnetic disturbances does not adversely affect the operation in card mode.

Another embodiment provides a device in which the operation is improved both in card mode and in reader mode.

Thus, an embodiment provides an antenna circuit for a device of transmission/reception by inductive coupling, including a first inductive element in parallel with a capacitive element and, between each node of the parallel association and two terminals of a switch, a second inductive element.

According to an embodiment, the terminals of the switch are intended to be connected to a circuit for generating an A.C. signal intended to excite the oscillating circuit formed by the antenna circuit.

According to an embodiment, said nodes of the parallel association are intended to be connected to a circuit for interpreting detected signals.

Another embodiment provides a device of transmission/reception by inductive coupling. The device includes an antenna circuit, a circuit for generating an A.C. signal intended to excite the antenna circuit, a circuit for interpreting signals detected by the antenna circuit, and a circuit for controlling the switch.

According to an embodiment, the switch is in off position when the device will transmit the A.C. signal and operate in reader mode.

According to an embodiment, the switch is in on position when the device will transmit data and operate in card mode.

According to an embodiment, the second inductive elements are sized so that, at the frequency of the A.C. transmission signal, the antenna circuit is matched to the output impedance of the A.C. signal generation circuit.

According to an embodiment, the sum of the second inductive elements is selected so that the oscillating circuit is tuned to a frequency of an A.C. signal received from another device in the card mode operation.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and will be detailed. In particular, the data transmission protocols in reader mode or in card mode have not been detailed; the described embodiments being compatible with usual protocols, which are generally standardized, and thus easily available. Further, the applications of the reader-card devices have not been detailed; the described embodiments being here again compatible with the different current uses of dual-mode or combined devices. For example, they may be used as terminals of communication with electronic tags capable of switching to the card mode to transfer data from a terminal to a neighboring terminal, all the way to a server. They may also be used as portable devices capable of operating, according to applications, in card mode (in a transport application, for example) and in reader mode.

FIG. 1is a block diagram of an example of a combined, or reader-card, device1such as described in US patent application 2009/0247079 of the applicant (B8794, 08-RO-027/029). This diagram is functional in that the different elements used for the card mode and for the reader mode have not been detailed and have been shown in separate blocks. Thus, a card mode block11(CM) symbolizes the different circuits used in card mode, a reader mode block12(RM) symbolizes the different circuits used in reader mode, and a control block13(CTRL) symbolizes the different circuits used to select the mode and to synchronize the operation. In practice, some circuits may however be shared by the two functions.

An oscillating circuit or antenna circuit2includes, between terminals21and22of an antenna L1, a capacitive element C2. Terminals21and22are further connected to block11of the card mode to sample an alternating current (A.C.) signal detected in the field of a reader. The circuits of the reader mode include two output amplifiers31and32respectively connected by identical capacitive elements C1to terminal21and to terminal22. A switch K connects the respective outputs23and24of amplifiers31and32.

Switch K is controlled by control block13towards an off position when the device must operate in reader mode and towards an on position when the device must operate in card mode. In reader mode, switch K is off and capacitors C1take part in the impedance matching circuit at the carrier frequency. In card mode, capacitive elements C1are both in series and this series association is in parallel on capacitive element C2. Half the value of a capacitive element C1thus adds to the value of capacitive element C2. Amplifiers31and32are of three-state type to have, in card mode, a high output impedance state.

To add filters against electromagnetic disturbances to the architecture of the device ofFIG. 1, it could have been devised to insert therein the low-pass filters provided by above-mentioned document U.S. Pat. No. 7,665,664. However, this does not optimize the system operation. In particular, in card mode, the capacitive elements of the filters generate additional disturbances.

FIG. 2is a functional block diagram of an embodiment of a combined device.

As compared with the circuit ofFIG. 1, capacitive elements C1have been replaced with inductive elements L′. The rest of the circuit is not modified. For simplification, output amplifiers31and32(FIG. 1) of the reader circuit have not been illustrated but are present.

Thus, inductances L′ of same value are present between two output terminals23and24of block12(or two input terminals of antenna circuit2), and respective terminals21and22of the antenna, here referred to as L. A capacitive element Cp is in parallel on inductance L between terminals21and22. Different reference numerals have been used for inductive and capacitive elements L and Cp, with respect to elements L1and C2ofFIG. 1, to show that the values of these elements are not necessarily identical.

Switch K has the same function as previously, that is, it is off in a reader mode operation where inductive elements L′ take part, with capacitive element Cp, in the forming of a low-pass filter against electromagnetic disturbances, and it is on in a card mode operation to place the two inductances L′ in series and to connect this series association in parallel with antenna L, thus taking part in an increase of the value of the inductive element of the oscillating circuit.

FIGS. 3A and 3Billustrate the two configurations of the oscillating circuit of the device ofFIG. 2according to the switch position.

FIG. 3Aillustrates the equivalent electric diagram of circuit2ofFIG. 2in reader mode. The series resistor (parasitic resistor of antenna L) placed in parallel (referred to as Rp) on inductance L has been illustrated in dotted lines, as well as series resistors Rs (output impedances) of circuit12.

FIG. 3Billustrates the equivalent electric diagram of circuit2ofFIG. 2in card mode with, similarly, resistor Rp placed in parallel on the inductance. Inductive elements L′ show an equivalent inductance L′/2 which adds to inductance L of the antenna.

The passing from the parallel equivalent model (FIG. 3A) to the series equivalent model (FIG. 3B), and conversely, is usually performed with the following relations (to simplify the notations, reference is made to L, Rp, L′, and Cp to designate the values of the concerned elements):
Xs=Qs·Rs,
where Xs is the series admittance of the antenna circuit and where Qs is the quality factor of the series elements of the antenna circuit.

Admittance Xs is preferably matched to the output admittance of the transmit circuits. Factor Qs is equal to

Series resistance Rs is negligible as compared with the resistance, referred to as Rp, of inductive element L placed in parallel on the oscillating circuit.

It can then be written that Xs is approximately equal to √{square root over (Rs·Rp)}.

In the configuration ofFIG. 3B(parallel structure), parallel admittance Xp is due to the presence of capacitance Cp and can be written as Rp/Qp, where Qp is the quality factor of the parallel elements.

The oscillating circuit components are sized according to the tuning and matching frequencies. It is in particular desired for the quality factors of the parallel and series elements to be equal and to be able to written as:

Series and parallel admittances Xs and Xp approximately equal to √{square root over (Rs·Rp)} are thus obtained.

Inductive elements L′ are sized so that, at the frequency of the A.C. transmission signal, the antenna circuit is matched to the output impedance of the A.C. signal, and that their sum is such that the oscillating circuit is tuned to a frequency of an A.C. signal received from another device in the card-mode operation. Capacitive element Cp is sized by taking into account the value of elements L′ to form an EMI low-pass filter.

Taking into account the above-discussed conditions, the values of elements Cp and L′ can be determined as follows.

Consider a buffer (amplifier31or32) of internal impedance Rs and an antenna L of impedance R1+jLω. It is started by calculating the parallel equivalent model of this circuit.

The equivalent model of the antenna is made parallel by the following equations:

Q=⁢XsR⁢⁢1=⁢L⁢⁢ωR⁢⁢1,Rp=(Q2+1)⁢R⁢⁢1andLp=⁢Xpω=⁢Rpω⁢⁢Q,
where Rp designates resistance R1placed in parallel on inductance L, Lp designates the corresponding value of inductance L (placed in parallel) on resistor Rp, Q the quality factor of the antenna alone, and Xp the parallel admittance of the complete antenna circuit.

The matching circuit can then be replaced with a matching with two elements and a so-called “L” layout.

To model the circuit, it is desired to cancel the imaginary part of the charge impedance with a theoretical capacitance Cshunt in parallel on the antenna. This capacitance forms with inductance Lp a resonant circuit at the frequency at which the matching is desired to be performed. Thereby, at the resonance frequency, couple Lp, Cshunt resonates and has an infinite impedance. Thus, the imaginary part of the antenna is annihilated.

Cshunt=1(2·π·f)2·Lp,⁢wherebyXshunt=1Cshunt·ω
where f is the matching frequency and Xshunt is the admittance of capacitance Cshunt.

Previously-defined admittances Xs and Xp are calculated by means of the following equations.

Knowing Xs and Xp, the values of Cp and of L′ can be deduced:

It is now possible to share a same antenna for a device intended to operate in reader mode and in card mode while being compatible with a filtering of electromagnetic disturbances in reader mode. The provided circuit reduces the number of capacitive elements to be used and thus decreases the bulk.

It should be noted that the specific layout of the capacitive and inductive elements of the oscillating circuits allows an L matching resulting in a circuit tuned in card mode.

Various embodiments have been described, various alterations and adaptations are within the abilities of those skilled in the art. In particular, the selection of the values to be given to the inductive elements according to the antenna used is within the abilities of those skilled in the art based on the functional indications given hereabove and on the matching and tuning frequencies respectively intended for the reader mode and for the card mode. Further, although the embodiments have been more specifically described in relation with an example applied to standards 14443, they more generally apply to any radio frequency transmit/receive system, where a device is capable of operating in reader mode and in card mode. Further, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove. In particular, the function of switch K may be ensured by switches internal to blocks11and13.