D E S C R I P T I O N

"Electronic device for radio frequency identification and method for adjusting the resonance frequency of said electronic device."

Technical field of the invention

The invention relates to an electronic device for radio frequency identification of the type used in remote detection and identification, preferably of animals. The invention also relates to a method for adjusting the resonance frequency of said identification device and to an identification tag into which said identification device is incorporated.

Background of the invention

Radio frequency identification systems, known as RFID systems, are becoming increasingly widely used for the automatic identification of people, goods, commercial products and pets or farm animals. These identification systems generally comprise a reader device, which may be fixed or portable and which generates an interrogation signal that receives a response from an identification element, called a tag or transponder, which returns an identification number that has previously been saved onto it.

Depending on their power supply, RFID systems may be classified into two broad types: passive systems and active systems. In active systems, the transponders require their own power supply in order to work, so they are usually equipped with batteries that can last for several years. In passive systems, however, the transponders do not require a power supply as they take their energy from the radio frequency magnetic field generated by the reader device, i.e. the interrogation signal sent by the reader device has the dual purpose of transmitting information and supplying energy to said identification element.

RFID systems may also be classified according to the frequency range in which they operate, i.e. according to the communication frequencies of the transponder with the reader: LF (Low Frequency) normally below 135 KHz, HF (High Frequency) within the range of 13.56 MHz, UHF (Ultra High Frequency) within the range of 868-956 MHz or Microwave within the range of 2.45 GHz.

Finally, RFID systems may be classified on the basis of another characteristic, the communication mode, into systems based on full duplex transmission 'FDX' or half duplex transmission 'HDX'. FDX systems are based on

the principle that the response signal is sent as soon as the interrogation signal is received, and the reader receives the response signal repeatedly without interruption whilst the interrogation signal is maintained. Furthermore, whilst FDX transponders do not store energy in order to respond with their entire data content, the transponders used in HDX systems, which unlike the previous ones are based on the fact that the response signal is sent only after the interrogation signal has stopped, do store energy in order to respond with their entire data content at once.

Of these types of RFID systems, the conventionally used systems for identifying animals are passive systems that operate at frequencies below 135kHz in which the transponders are positioned in external ear tags, internal ruminal boluses or directly injected under the skin (subcutaneous).

One of the most important characteristics of HDX systems is the range at which they can operate, i.e. the distance at which the reader and the identification element incorporated into the transponder or tag are able to communicate correctly. There are many factors that influence the range e.g. the energy sent by the reader, which is limited by the regulations governing the sector, the geometrical dimensions (inner and outer diameter, thickness, number of turns...) and electric parameters (quality) of the coils of the reader and the transponder, the level of atmospheric interference, the type of modulation used for communication, the efficiency of the rectifier and the consumption of the transponder or the proximity in resonance frequency between the LC circuits called resonant tanks of the reader and the transponder.

When large communication distances are to be achieved (e.g. more than one metre in free space for low frequencies, LF or HF) very high quality resonant tanks are used in the reader and the resonance frequency is precisely calibrated. In these cases it is essential to adjust the resonant tank of the transponder to compensate for the tolerances that always exist in the manufacturing of components such as inductors and capacitors.

To adjust the resonance frequency of the tank of a HDX transponder, it is possible to include integrated capacitors in the integrated circuit of the transponder, the values of which maintain a predefined relationship, which are connected or disconnected as required, depending on the values of the external components and the tolerance of the capacitors included in the integrated circuit.

One possibility for connecting and disconnecting the capacitors is to use field-effect transistors in series with the trimming capacitors. These transistors may be put into a conductive or non-conductive state, e.g. by reading the values of a

non-volatile memory previously programmed with the correct values. The problem lies in the fact that a minimum voltage is required to read said non-volatile memory, which may never be achieved if there is too large a difference between the resonance frequency of the tanks of the reader and the transponder. A procedure that overcomes this drawback consists of using field-effect transistors in series with the trimming capacitors, which can be put permanently into a conductive or non-conductive state by means of a control gate terminal, as disclosed in EP0407848B1. However, this solution presents two drawbacks. The first drawback relates to the large area of silicon that is needed, as said field-effect transistors must present a sufficiently low resistance in series in conduction so as not to affect the quality of the resonant tank. The second drawback is that said transistors, as well as the circuit needed to activate them and ensure a sufficient retention, are not elements that are available from all foundries, so they must be custom made. Another possibility, although it is always used in systems other than HDX animal identification systems, is to use elements for connecting/disconnecting the capacitors whose electrical resistance varies drastically and permanently after a current higher than a certain value passes therethrough for a predetermined time.

These elements are available in most integrated circuit manufacturing technology libraries, as they do not require an extra manufacturing step or any postprocessing and they make use of physical effects that are independent of the manufacturing technologies. Their greatest disadvantage lies in the irreversibility of the process and the fact that the high level of energy needed to change the resistivity means that it is not possible to obtain it from the electromagnetic field generated by the reader, making it necessary to use an external power supply and physical contact to make the adjustment. Furthermore, with this solution it is not possible to compensate for variations in the value of the inductor or capacitor that occur during the usual process of subsequently over-moulding or encapsulating the integrated circuit of the transponders during manufacturing of the ear tags, in which no electrical connection is left between the circuit and the outside. For example, imagine an air core inductor that is inserted into a plastic piece that must subsequently be hermetically sealed by over-moulding in a typical identification application. The over-moulding is carried out by heating said plastic piece, which may cause mechanical stress to the inductor and result in a change in the value thereof. Once the over-moulding has been carried out there is no physical contact with the electric module, so it is not possible to supply the necessary energy to

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change the resistivity of the aforementioned elements.

An objective of the present invention is therefore to disclose an alternative electronic device, tag and method that make it possible to improve the adjustment to the resonance frequency between the resonant tanks of the reader and the animal identification tag, and in particular to improve the link between the tag and the reader, increasing the reading distance between the two devices by means of elements that are commonly available from foundries.

Explanation of the invention The electronic device for identifying animals, people or objects by radio frequency that is the object of the invention is of the type based on half duplex communication technology, 'HDX', which comprises a chip or integrated circuit equipped with at least one modulation capacitor, a rectifier, a memory unit and a logical control unit; a resonant tank, connected to said chip, which comprises at last one inductor; and at least one resonance capacitor, which is connected to the ends of the inductor; and a load capacitor for supplying power to said device.

Essentially, the device is characterised in that the integrated circuit comprises a thick trimming circuit for adjusting the resonance frequency of the device, which comprises a plurality of integrated capacitors that can be connected in parallel to the resonance capacitor using corresponding resistive elements, the resistive elements being adapted to irreversibly and sufficiently alter their resistivity after a certain current value passes therethrough for a predetermined time to bring about the definitive connection/disconnection of the corresponding integrated capacitors to the resonance capacitor; and a thin trimming circuit for adjusting the resonance frequency of the device, which comprises a plurality of integrated capacitors that can be reversibly connected or disconnected in parallel to the resonance capacitor by means of corresponding transistors.

Advantageously, the thick trimming circuit makes it possible to discard defective chips before the finished product is produced, whilst the thin trimming circuit makes it possible to finish adjusting said resonance frequency after encapsulation, at the same time as the identification number of the electronic device is saved in the memory unit.

According to another characteristic of the invention, the thick trimming circuit comprises transistors, controlled by a fuse control unit, whereby a current passes to the respective resistive elements.

According to another characteristic of the invention, the thin trimming circuit

comprises a logical adjustment unit adapted to alter the state of conduction or non- conduction of the corresponding transistors based on the reading of data stored in the memory unit of the device.

In a preferred embodiment, the resistive elements are adapted to present a high resistance in their normal state and a low resistance after a high enough current pulse has passed therethrough for a certain predetermined time. Preferably, the resistive elements are zap-type zener diodes.

In another preferred embodiment, the resistive elements are adapted to present a low resistance in their normal state and a high resistance after a predetermined current pulse has passed therethrough. Preferably, the resistive elements are polysilicon-type fuse elements.

According to another characteristic of the invention, the value of the integrated capacitors of the thick trimming circuit is greater than the value of the integrated capacitors of the thin trimming circuit. According to another characteristic of the invention, the device comprises a pair of terminals that connect to the inductor and terminals that connect to the fuse control unit and the resistive elements.

According to another characteristic, the device is encapsulated in a module and its terminals are accessible from outside. According to another aspect of the invention, it also discloses a tag for identifying animals, people or objects, which is essentially characterised in that it houses a module according to claim 10 by over-moulding with a plastic material.

Furthermore, according to another aspect of the invention, it also discloses a method for adjusting the resonance frequency of an electronic device according to any of claims 1 to 10 in a tag for identifying animals, people or objects.

Essentially, the method is characterised in that it comprises the steps of encapsulating the device in a capsule; a thick trimming step to adjust the resonance frequency of the electronic device by applying the necessary current to bring about a change in resistivity in at least one of the resistive elements, connecting the corresponding capacitor to the resonance capacitor of the resonant tank of the device, thus adjusting the resonance frequency; over-moulding the capsule on the tag; and a thin trimming step to adjust the frequency of the electronic device by switching at least one of the transistors to a conduction state, connecting the corresponding capacitor to the resonance capacitor of the resonant tank of the electronic device.

Brief description of the drawings

In the attached drawings an embodiment of the device of the invention is illustrated by means of a non-limiting example. Specifically: Fig. 1 is a diagram of a device according to the invention; and Fig. 2 is a perspective view of a tag containing the device shown in Fig. 1 housed in a module.

Detailed description of the drawings

The electronic device 1 for the radio frequency identification of animals shown in Fig. 1 is of the type based on half duplex communication technology, 'HDX'. Said device 1 comprises, in a known manner, a chip or integrated circuit 23 that performs different functions such as the generation and storage of a supply voltage, the generation of the internal clock needed for the device 1 to work, the demodulation of the data received and other functions that are normal in this type of devices; and a resonant tank 20, which is external to said integrated circuit 23 and connected thereto, which acts as an emitting and receiving aerial for the device 1.

As can be observed in Fig. 1 , the chip or integrated circuit 23 of the device 1 is equipped with a modulation capacitor 9, a rectifier 1 1 , a memory unit 13 and a logical control unit 15. In the example shown in Fig. 1 , it can be observed that the resonant tank 20 comprises an inductor 2 and a resonance capacitor 3 that is connected to the ends of the inductor 2 and is outside the chip or integrated circuit 23, unlike other embodiments that are not shown, which contemplate the integration of said resonance capacitor 3 in the chip. The inductor 2 consists of a thin cable wound around 500 times in a loop with an approximate diameter of 25 millimetres.

As well as the resonant tank 20, the device 1 comprises a load capacitor 4 for supplying power to the chip or integrated circuit 23.

Characteristically, the integrated circuit 23 comprises a first thick trimming circuit 21 for adjusting the resonance frequency of the device 1 to the working frequency of the reader, and a second thin trimming circuit 22 for making finer adjustments between the resonance frequency of the device 1 and the working frequency of the reader. Both trimming circuits, the thick trimming circuit 21 and the thin trimming circuit 22, include a number of integrated capacitors 5.1 to 5m and 5m+1 to 5m+n, respectively, the capacitors of which may be added to that of the resonance capacitor 3 depending on the conductive state of the electronic components connected thereto.

The integrated capacitors 5.1 , 5.2, 5m of the thick trimming circuit 21 may also be connected in parallel to the resonance capacitor 3, via corresponding resistive elements 6.1 , 6.2, 6m, or they may remain disconnected according to the operating needs, which depend to a large extent on the values of the external components and the tolerance of the capacitors included in the integrated circuit 23. Said resistive elements 6.1 , 6.2, 6m are adapted to irreversibly and sufficiently alter their resistivity after a certain current value has passed therethrough for a predetermined time, bringing about the definitive connection/disconnection of the corresponding integrated capacitors 5.1 , 5.2, 5m to the resonance capacitor 9. The resistivity of said resistive elements 6.1 , 6.2 and 6m is altered by corresponding transistors 7.1 , 7.2 and 7m that are controlled by a fuse control unit 14, causing current to circulate through said resistive elements 6.1 to 6m with a certain intensity for a predetermined time, thereby bringing about an irreversible change in their resistivity. Depending on the transistors 7.1 , 7.2 and 7m that have been activated, different capacitors are connected in parallel to the resonance capacitor 3.

In the embodiment shown in Fig. 1 , the resistive elements 6.1 , 6.2 and 6m of the device 1 are zap-type zener diodes, which are adapted to present a high resistance in their normal state and a low resistance after a high enough current pulse has passed therethrough for a predetermined time. This change in resistivity in the zener diodes occurs due to the fact that after passing current pulse through the metal of the terminals of the diode, they melt, giving rise to a low resistance element.

In another embodiment that is not shown, the resistive elements 6.1 , 6.2 and 6m of the device 1 are polysilicon-type fuses, which are adapted to present a low resistance in their normal state and a high resistance after a predetermined current pulse has passed therethrough. Thus, after a suitable current pulse has passed therethrough a phenomenon known as electromigration occurs in the fuse, causing a sudden increase in its resistance.

Moreover, the thin trimming circuit 22 comprises a plurality of integrated capacitors 5m+1 , 5m+2 and 5m+n, of a lower value than the capacitors 5.1 to 5m of the thick trimming circuit 21 , which can be reversibly connected or disconnected in parallel to the resonance capacitor 3 via corresponding field-effect transistors 8m+1 , 8m+2 and 8m+n. A logical adjustment unit 15 is responsible for altering the conductive state of said transistors 8m+1 , 8m+2 and 8m+n based on the reading of data stored in the memory unit 13 of the device 1 . These data form part of a specific block of the memory, called an adjustment block, and they require a minimum

energy and voltage for their activation.

It is clear that the integrated capacitors 5.1 to 5m and 5m+1 to 5m+n cannot have a different value, being chosen so that in a single capacitor or any possible combination of the existing capacitors results in a behaviour of the device 1 that is in line with the conventional working frequencies of readers. Specifically, in the case shown in Fig. 1 , the values of the capacitors 5.1 , 5.2, 5m and 5m+1 , 5m+2, 5m+n are 12pF, 24pF, 48pF and 2pF, 4pF, 8pF, respectively. Therefore, this embodiment achieves a proportional relationship between them. It should also be mentioned that the modulation capacitor 9 is also used as a reference capacitor for adjusting the resonance frequency.

The device 1 also has a pair of terminals 17.1 and 17.2 that make it possible to connect to the inductor 2 of resonant tank 20 from outside and terminals 16, shown in the diagram in Fig. 1 , which provide access to the fuse control unit 14 and the resistive elements 6.1 to 6m of the thick trimming circuit 21. The example in Fig. 2 shows the male part of an ear tag 100 for radio frequency identification of animals housing a module 101 that comprises a device 1 according to the invention by over-moulding in a plastic material.

The device 1 is encapsulated in the module, which houses all the elements except the pair of terminals 17.1 and 17.2 that connect to the inductor 2 and the terminals 16 that connect to the fuse control unit 14 and the resistive elements 6.1 , 6.2 and 6m of the thick trimming circuit 21 of the device 1. This type of terminals 16 contains e.g. the clock signal, the reference or mass signal and the power data signal. However, in another embodiment that is not shown there is a terminal 16 for each resistive element 6.1 to 6m. As regards the complete adjustment of the resonance frequency of the device

1 , it is carried out according to a method comprising several steps, including a thick trimming step, wherein the resistivity of one or more resistive elements 6.1 to 6m is changed to connect the corresponding capacitors 5.1 to 5m to the resonance capacitor 3 of the device 1 , and a subsequent thin trimming step, wherein one or more transistors 8m+1 to 8m+n are switched to a conduction state to connect the corresponding capacitors 5m+1 to 5m+n to the resonance capacitor 3 of the device 1.

The first step consists of encapsulating the device 1 in a capsule or module 101 that hermetically seals the integrated circuit 23, only leaving external access to the terminals 16 and 17.1 to 17.2 that provide access to the resistive elements 6.1 to 6m and to the inductor 2, respectively.

Then there is a thick trimming step for adjusting the frequency, wherein the necessary current is applied to the terminals 16 in order to bring about a change in the resistivity in the corresponding resistive elements 6.1 , 6.2 or 6m and thus the corresponding capacitors 5.1 , 5.2 or 5m are connected to the resonance capacitor 3 of the resonant tank 20 of the device 1 , thus making it possible to adjust the resonance frequency of the device 1 to the point where the power supply voltage is sufficient, even at large distances, to recover the data from an EEPROM type nonvolatile memory of a memory unit 13, where the necessary information is stored for a subsequent step for fine-tuning the resonance frequency. The connection or disconnection of these capacitors 5.1 to 5m is carried out by the transistors 7.1 to 7m, which are connected by their source terminal to a external pin that is common to all of them and by their drain terminal to respective resistive elements 6.1 to 6m, which are controlled by the fuse control unit 14, which makes it possible for the necessary current to pass therethrough in order to change the resistance of the elements 6.1 to 6m. As the energy supplied by the reader is not sufficient to provide the necessary current, this adjustment step is carried out using an external source that can supply said current by electrical contact through the terminals 16.

In order to carry out this thick trimming step correctly, first of all the resonance frequency of the device 1 must be measured with all the resistive elements 6.1 to 6m connected or disconnected, according to the type of resistive elements that is chosen, and so that in the EEPROM memory with a value such that all the transistors 8m+1 to 8m+n do not conduct. Then the same measure is repeated by adding the capacitor 9 via its corresponding transistor 10, which is controlled by a logical control unit 15 that is responsible for altering the conduction state of said transistor 10. It should be mentioned that no large leakages in the capacitor are expected within a single integrated circuit 23, especially if they are positioned close together, so the leakage of the capacitors 6.1 to 6m will be calculated on the basis of the leakage of the capacitor 9 in relation to its expected value. In order to reduce the leakage between capacitors still further, a layout technique known as a common centroid configuration is used. Depending on the frequencies measured and the leakage of the capacitor 9, it is decided which elements are to be used to pass the necessary current to change their resistivity and thus to add more capacitors to the circuit. As mentioned above, either of the two types of resistive elements 6.1 to 6m, zap-type zener diodes and polysilicon fuses, are valid, although it must be taken into account which type is used both in the design and for the capacitor selection

algorithm, as the zener zap is in a state of disconnection by default, whereas the polysilicon fuse is in a state of connection by default.

At this point, when measuring and trying to adjust the resonance frequency using the thick trimming circuit 21 , it is possible to determine whether the device 1 in question is valid or not. If it is defective, the product may be withdrawn before it is moulded into the tag 100, allowing devices 1 to be withdrawn from the manufacturing process in time and thus avoiding unnecessary production costs.

After the thick trimming step for adjusting the resonance frequency of the device 1 , the capsule is over-moulded into the end product, e.g. in the ear tag 100 shown in Fig. 2.

Lastly, a fine-tuning step is carried out, which can be performed at the same time as the identification number is recorded in the memory unit 13 of the device 1 . In this step, the logical adjustment unit 12 of the device 1 reads the EEPROM memory address of the memory unit 13 that is assigned for fine-tuning and controls the transistors 8m+1 to 8m+n, switching them to a state of conduction or non- conduction in accordance with the data read from the memory. When they adopt a conduction state, the corresponding capacitors 5m+1 to 5m+n are added in parallel to the resonant tank 20, adjusting the resonance frequency as much as possible. When the ear tag 100 enters the range of action of the reader's magnetic field, depending on the values of the elements of the resonant tank 20 and the result of the thick trimming, the device 1 might not yet be properly tuned, meaning that the power supply voltage will not be sufficient to achieve the full operating capacity of the tag 100, but it will be sufficient to read a value of the EEPROM memory of the memory unit 13, so that after tuning the charging is completed and the full operating capacity of the tag 100 is achieved. The calculation of the value that must be stored in an address in the general memory can be precisely evaluated by measuring the frequency that occurs when all the capacitors of the integrated circuit are connected and disconnected, as the relationship between them is known. Once the tag 100 has been put in transmission with the reader, it does not run down the capacitors so that the chip or integrated circuit 23 of the device 1 still has some remaining energy so that if the chip is interrogated again by a reader, the device 1 already has enough energy to immediately carry out the fine tuning. Said voltage may vary between 1 or 2 Volts.

Advantageously, this type of fine tuning can be carried out at any time and it is a reversible adjustment. Moreover, this type of adjustment makes it possible to adapt the device 1 of the tag to the fixed frequency of the reader.

Electronic devices 1 housed in capsule-type modules containing all the elements except the terminals for their respective connections, as shown in Fig. 1 , achieve a greater degree of protection of the silicon material of the chip or integrated circuit 23 against the impact of the over-injection of plastic when it is encapsulated in the capsule and when, for example, an animal tries to bite another animal's ear tag inside which said module is housed. In fact, the fine tuning means that once the end product is finished, and after having undergone mechanical stress or sudden temperature changes due to the over-moulding and/or plastic injection operations, the resonance frequency may be adjusted to the tag 100 remotely, compensating for any possible variations that might have occurred in the values of the inductor 2 or the resonance capacitor 3.

As well as external ear tags 100 like that shown in Fig. 2, other embodiments that are not shown contemplate the possibility of the device 1 being housed inside a ruminal bolus or as a glass transponder or directly injected under the skin (subcutaneous).