WO1998011504A1

WO1998011504A1 - Passive transponder

Info

Publication number: WO1998011504A1
Application number: PCT/DE1996/001761
Authority: WO
Inventors: Pourang Mahdavi
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1996-09-12
Filing date: 1996-09-12
Publication date: 1998-03-19

Abstract

The invention deals with a passive transponder, which has a resonant circuit (LT, CT) having a transponder coil (LT), a control logic (20), a switching device (S) controlled by the control logic, a supply voltage capacitor (CS), a resistor (RL) lying parallel to the supply voltage capacitor (CS) and a rectifier circuit (D) to load the supply voltage capacitor (CS) as a function of the voltage induced by the transponder coil (LT) at the resonant circuit. The switching device (S) connects the resistor (RL) and the supply voltage capacitor (CS) through the rectifier circuit (D) to the resonant circuit (LT, CT) when closed, and it disconnects the resistor (RL) and the supply voltage capacitor (CS) from the resonant circuit (LT, CT) when open.

Description

Passive transponder

description

The present invention relates to a transponder and in particular to a passive transponder and a modulation method for modulating the data sent from the transponder to a reading station.

Microtransponder technology is increasingly used in the field of applied microelectronics and microsystem technology. A transponder is generally used to refer to a unit that is spatially separated from a basic system and that can receive, send, store and possibly also process data. Microtransponders are spatially separated system components that are made possible by microtechnology, i.e. Microelectronics or microsystem technology are realized. The distance between the transponder and the base unit can be divided into areas between a few millimeters, a few meters and even a few hundred meters. The different distance ranges are referred to as close range, middle range and wide range. A microtransponder can contain a memory with an identification code and / or sensors for various physical and / or chemical quantities as sources of information.

A schematic representation of an arrangement consisting of a reading station and a transponder is shown in FIG. Of particular interest are transponders which do not have their own energy source and which obtain their energy by means of a resonance circuit Lr, C _τ from an incident query field 10, which is obtained by an antenna of the reading station, which is caused by a coil L _s . In this case one speaks of passive transponders. Passive transponders can be built very small and have a theoretically unlimited lifespan. The areas of application for passive transponders are extremely broad and range from medical technology, for example implants with HF data coupling, to security technology, for example access controls with contactless chip cards, to CIM applications (CIM = computerized integrated manufacturing = computer-integrated manufacturing) ), for example a workpiece identification.

A more precise illustration of the typical arrangement of the known transponder in the query field of a reading station is shown in FIG. 2. The reading station generates a high-frequency interrogation field 10 via the antenna Lg. The transponder has a resonance circuit L _{T /} c _{τ which is} matched to the frequency of the interrogation field and which takes energy from the high-frequency interrogation field 10 and thereby generates a voltage. The voltage is rectified by a rectifier device, in the illustration in FIG. 2, the diode D, and also smoothed by a capacitor C _s , which is connected in parallel with the resonant circuit. The voltage U obtained in this way from the query field then represents the supply voltage for the actual transponder circuit 20, which can have, for example, a read-only memory and a control circuit. The resistance of the transponder circuit can be modeled by the useful resistance R _L shown in broken lines.

According to the overlay theorem, a voltage in the form of a returning wave is now generated by the transponder coil Iüp at the reading station, the amount of the voltage generated at the reading station being proportional to the current flowing through the transponder coil L _τ .

The data transmission from the transponder to the reading station takes place by switching on or off an additional modulation resistor R _M lying parallel to the resonant circuit L _τ , C _τ by means of an electronic switch S. This switch is controlled by the actual transponder circuit 20. By switching the switch S the more or less energy is taken from the interrogation field emitted by the antenna L _s in the rhythm of the switch S, as a result of which an alternating signal voltage ^u signal i is generated at the antenna of the reading station. The transponder data, which are stored in the read-only memory, for example, the switch S being controlled by the control circuit of the actual transponder circuit 20, are determined by processing this signal U _S i _gna ; Li. This method of transmitting data from the transponder to the reading station is known as absorption modulation and is described in detail, for example, by P. Neukomm, H. Baggentos, "Passive Telemetry with Absorption Modulation", ETH Zurich, Design & Electronics, September 18, 1990.

The method of absorption modulation has the disadvantage of the additional load on the energy source of the transponder, namely the resonant circuit L _τ , C _τ , due to the modulation resistance R _M. This causes a reduction in the supply voltage U _c and thus a reduction in the reading distance. There are minimal modulation losses if the modulation resistance R _{M of} the transponder circuit is large compared to the useful resistance, which is modeled by the resistance R _L. However, the greater the useful resistance RL, the weaker the signals Usignall 'arriving in the reading station, which increases the sensitivity of the system to interference, which in turn results in a reduction in the reading distance.

Regardless of the considerations made in the last paragraph, the absorption modulation method remains disadvantageous in that the absorption modulation destroys valuable energy in the modulation resistance R _M , with the result that the range of the transponder system is always smaller than that of a pure one Energy transfer would be possible.

Based on the prior art mentioned, the present invention is based on the object of a passive To provide a transponder that allows data to be transferred from the transponder to a reading station with a reduced sensitivity to interference and an increased reading distance.

This object is achieved by a passive transponder according to claim 1.

Another object of the present invention is to provide a modulation method for data transmission from a transponder to a reading station with a reduced sensitivity to interference and an increased reading distance.

This object is achieved by a modulation method according to claim 6.

The present invention provides a passive transponder with a resonance circuit having a transponder coil, a control logic, a switch controlled by the control logic, a supply voltage capacitor, a load lying in parallel with the supply voltage generator and a rectifier circuit for charging the supply voltage capacitor on account of one of the transponder coil Resonance circuit induced voltage, wherein the switch in its closed state connects the load and the supply voltage capacitor via the rectifier circuit with the resonance circuit and separates the load in its open state from the resonance circuit.

The passive transponder according to the invention solves the problem of additional energy _destruction in the modulation _resistor R _jf according to the prior art and the low efficiency of the modulation in the case of conventional absorption modulation.

The modulation method according to the invention for data transmission from a transponder to a reading station is characterized in that the modulation of the data transmitted from the transponder to the reading station is carried out by switching between a payload of the transponder and a load with a higher impedance with respect to the payload. This switching is carried out in the preferred embodiment of the present invention by opening a switch which is connected between the resonance circuit of the transponder and the payload. Because of the effect generated by this switching, the method according to the invention can be referred to as idle modulation.

The modulation losses of the transponder are minimized by the method of idling modulation for the transponder. Since this reduces the energy consumption of the transponder and the necessary strength of the query field, the reading distance of the transponder system increases. Furthermore, the transponder signals arriving in the reading station are stronger when using the idle modulation than is the case when using the absorption modulation method. This results in greater immunity to interference and a lower error rate in the transmission. With the same range, the antennas, i.e. the coils of the transponder constructed according to the invention can be made smaller. This in turn means that the transponders can be built smaller and cheaper.

Further developments of the present invention are set out in the dependent claims.

Preferred exemplary embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. Show it:

Figure 1 is a rough schematic representation of an arrangement of reading station and transponder.

2 shows a schematic illustration of a reading station and a transponder which is operated according to the method of Sorption modulation works;

3 shows a schematic illustration of a reading station and a transponder according to the invention;

4 shows a diagram which shows the characteristic curves of the reactive power P _L c _» taken from the interrogation field by the resonance circuit, the active power Pip received by the transponder and the voltage U _R generated by the transponder at the reading station as a function of the useful resistance R _{L of} the transponder; and

5 shows diagrams of the "transponder supply voltage" U _c and of the signals ^u signal ^{and u} signal 2 present at the reading station.

A preferred exemplary embodiment of the present invention is explained in more detail below with reference to FIGS. 3 to 5. Elements of the reading station / transponder arrangement shown in FIG. 3, which correspond to those of the arrangement shown in FIG. 2, are identified by the same reference numerals as in FIG. 2.

The reading station of the arrangement shown in FIG. 3 corresponds to the reading station of FIG. 2 and generates a high-frequency interrogation field 10 by means of the coil L _s .

According to the preferred embodiment of the present invention, the transponder has a parallel resonance circuit consisting of a coil Lr and a capacitor C _τ . An output of the resonance circuit is connected to the input terminal of a rectifier device, which in the preferred embodiment is formed by a diode D. The second connection of the diode D is connected to the first connection of a switch S. The second connection of the switch S is connected via a capacitor C _s to the second output of the parallel resonance circuit Lp, C _τ . The capacitor C _s serves as the supply voltage capacitor of the trans ponders. As shown in dashed lines in FIG. 3, the payload of the transponder can be represented as a resistor R _L , which is connected in parallel with the capacitor C _s .

The transponder also has an actual transponder circuit 20, which essentially corresponds to the transponder circuit described above with reference to FIG. 2. Such circuits are well known in the art.

In the transponder according to the invention shown in FIG. 3, the modulation resistance R _M is missing compared to the known transponder shown in FIG. 2. Furthermore, the switch S is no longer connected in parallel to the resonant circuit L _τ , Cm, but is connected in series with the same.

The difference between the known absorption modulation method and the idling modulation method according to the invention is described below with reference to the characteristic field, which is shown in FIG. 4. The characteristic curves shown in FIG. 4 are each shown in a standardized form depending on the useful resistance of the transponder circuit R _L. ^P LC corresponds to the reactive power taken from the interrogation field in the resonance circuit of the transponder, P _τ corresponds to the active power consumed by the transponder, and U _R corresponds to the amplitude of the voltage generated by the transponder at the reading station in the reading direction. The operating point of the transponder is usually selected close to point (A), since there is a power adjustment for the transponder circuit.

Starting from the operating point (A), at which the switch S is open in accordance with the known transponder, which is shown in FIG. 2, closing the switch in the known transponder has a switchover from the operating point (A) to that in FIG. 4 shown point (B) result, whereby a signal U _s ^ _{gnall is} generated at the reading station. The signal U _S j " _na ιι represents the data transmitted by the absorption modulation from the transponder to the reading station. ten.

3, the switch S is closed in the initial state, so that the transponder can be supplied with energy via the high-frequency interrogation field and the resonant circuit m, C _τ . At the beginning of the modulation phase, the switch S is switched on and off in the rhythm of the data to be transmitted by the circuit 20, which contains, for example, a read-only memory and a control circuit. In the characteristic curve of the voltage U _R , which is shown in FIG. 4, this means a switchover from the working point (A) to the working point (C). In contrast to the known method of absorption modulation, the transponder resonance circuit is not damped. With an open switch S, the resonance circuit can oscillate much more up to the open circuit voltage and thus draw the maximum energy from the query field. If the switch S is closed from this state, this energy is transferred in the form of the voltage from the capacitor C _τ into the supply voltage capacitor C _s . In the course of a modulation cycle, the transponder according to the invention therefore only produces capacitive recharge losses and no ohmic losses. Therefore, the efficiency of energy transmission in this circuit is higher than in the known circuit of FIG. 2.

As a result of the more favorable energy balance, point (C) can be selected at a greater distance from point (A) in the method according to the invention. For this reason, the voltage ^u signal2 ' ^d ^ - ^e arriving at the reading station also corresponds to the data transmitted ^{from the} transponder to the reading station, stronger than in the known method of absorption modulation, in which the modulation is effected by damping the transponder resonance circuit.

A comparison of these voltages is shown in FIG. 5. 5, the voltage ^u Siqnall '^ - by the conventional absorption modulation is generated. The voltage at the transponder circuit U _c in the known transponder is shown in the lower left diagram. The upper right diagram shows the voltage Usi _q nal ₂ generated by the idle modulation method according to the invention at the reading station. The voltage U _c present on the transponder according to the invention is shown in the lower right diagram of FIG. Furthermore, the modulation frequency that controls the switchover of the switch S is shown in the lower diagrams. This modulation frequency was chosen in such a way that, with the same component values, the minimum permissible voltage at the respective transponder circuit, for example 9 volts, is not fallen below.

The determined diagrams shown in FIG. 5 show that the transponder signal at the reading station in the inventive method of idle modulation is approximately four times larger than the signal that is generated in the known method of absorption modulation.

Claims

claims

1. Passive transponder with

a transponder coil (L _τ) comprising resonant circuit (L _τ, c _τ), a control logic (20), a controlled by the control logic switch (S), a power supply capacitor (C _s), one parallel to the supply-voltage capacitor (C _s ) lying load (R _L ) and a rectifier circuit (D) for charging the supply voltage capacitor (C _s ) due to a voltage induced by the transponder coil (L _τ ) in the resonance circuit, characterized in that

that the switch (S) in its closed state connects the load (R _L ) and the supply voltage capacitor (C _s ) via the rectifier circuit (D) to the resonant circuit (L _τ , C _τ ) and the load (R _L ) and the supply voltage capacitor (C _s ) in its open state separates from the resonant circuit (L _τ , C _τ ).

2. Passive transponder according to claim 1, characterized in that

that the rectifier circuit (D) is a diode.

3. Passive transponder according to claim 1 or 2, characterized in that

that the resonance circuit (L _τ , c _τ ) is a parallel resonance _circuit consisting of the parallel connection of a transponder coil (L _τ ) and a capacitor (C _τ ).

4. Passive transponder according to one of claims 1 to 3, characterized. that the load (RL) is formed by the useful resistance of the passive transponder.

5. Passive transponder according to one of claims 1 to 4, characterized in

that the switch (S) is connected in series with the resonant circuit (L _τ , C _τ ).

6. Modulation method for data transmission from a transponder to a reading station, characterized in that

that the modulation of the data transmitted from the transponder to the reading station takes place by switching between a payload of the transponder and a load which is more resistive with respect to the payload.

7. Modulation method according to claim 6, characterized in that

that the switching between the normal load of the transponder and the load, which is higher in terms of normal load, is carried out by opening a switch (S) which is connected between a resonant circuit (L _τ , c _τ ) of the transponder and the payload (RL) of the transponder .