RF Transponder and method of tuning its frequency

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DESCRIPTION

Air Core Coil RF Transponder and Air Core Manufacturing Method.

The present invention relates to RF ("radio frequency") transponders and, in particular, to air core coil RF transponders and to a method of fitting an RF transponder.

In radio frequency identification systems, the resonant frequency of the transponder is one of the most important factors in the reading function of transponders. The precise resonant frequency of the transponder helps to ensure the quality of the electrical functionality of the product.

Often some transponders have different read functions, and this difference is usually determined by differences in resonant frequencies. This, in turn, is determined by the technology used. For example, in low frequency transponders, such as for example 125 kHz transponders, and medium frequency transponders, such as for example 13.56 MHz, air core coil technology is commonly used.

Generally, in the manufacture of air-core coil transponders, air-core coils are made from conventional, individual, insulated lead wires, which are typically self-adhering. If self-adhesive lead wires are used, this can result in the coils being solid and difficult to deform if the number of turns is large. On the contrary, if the number of turns is small, for example between 1 and 50 turns, the coil could be brittle and easily deformable. If the coil is deformed, the impedance of the coil is also altered.

Furthermore, each coil comprises an inductor, a resistor, and a parasitic capacitance. If the distance between the turns of conductive wire and the winding process differs from one transponder to another, the internal parasitic capacitance will also be different, resulting in differences between the impedances of said coils. This, in turn, results in a difference between the resonant frequencies.

It is known to adjust the resonant frequency of transponders by modifying the capacitance and / or modifying the inductor of the resonant circuit of the transponders.

A resonant circuit with a very high Q (quality factor) is particularly effective in capturing high energies from the reading device and retransmitting that energy to the reading device, particularly at longer reading ranges. On the other hand, a high Q circuit does not allow large tolerances for the resonant frequency since such frequency differences create large differences in the coupling of the energy from the reader to the transponder and transmit less energy from the transponder to the reader. This not only reduces the maximum read distance, but also results in variations of the maximum read distance between transponders.

For this reason, a more accurate resonant frequency is necessary for a high-Q resonant circuit to maximize the read distance and to minimize differences between transponders.

In addition to resonant frequency control problems due to variations caused by differences between transponder coils, problems can arise from variations between associated electronic components. Consequently, for example, the ICs (integrated circuits) used have capacitors that introduce variations in the resonant frequencies and tolerance between the ICs of the capacitors, which consequently produce variations in the resonant frequency of the transponders.

Variations in the electrical parameters of the coils and / or variations produced by the tolerances of the electrical components require an effective technique of adjusting the resonant frequencies before, during or after manufacture.

Document US2002 / 0180602 discloses an RFID transponder according to the preamble of claim 1. The resonant frequency of the transponder can be adjusted by moving a ferrite core within the antenna winding.

It is an object of the present invention to provide RF air core coils having precise impedance.

It is also an object of the present invention to provide a method of adjusting the resonant frequency of RF transponders with air-core coils during and / or after the manufacture of said transponders.

Said objective is achieved by an RF transponder according to claim 1 and a method of adjusting said transponder according to claim 9. Preferred embodiments are disclosed in the dependent claims.

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Other features and advantages of the present invention will be apparent from the following description of the invention with reference to the accompanying drawings.

Figure 1 is an enlarged perspective view in which certain parts have been removed, for clarity, of a transponder showing certain features of the present invention.

Figure 2 is a cross-sectional plan view of a carrier that is part of the transponder of Figure 1.

Figure 3 is an equivalent circuit of a coil that is part of the transponder of Figure 1.

Figure 4 is a block diagram showing a process of testing the transponder to determine its resonant frequency.

Figures 5a-5d are cross-sectional views showing different orientations of the transponder carrier corresponding to different resonance frequencies.

Referring now to the drawings and in particular to Figures 1 and 2, an RF air core coil transponder (10) is shown having a housing (11) (only the lower half thereof is shown ) and a rigid spool shaped rotary carrier (12) made from a suitable electrical insulating material, such as plastic, for example PPS, mounted inside said casing (11). Like the spool (12), the housing (11) can also be made of plastic, for example PPS. Advantageously, the stiffness requirement of the spool (12) is achieved by making the spool (12) solid. A spool (14) is wound around the spool (12). Inside the spool (12) at least one body (16) is mounted to influence the field, both electric and magnetic, existing inside the coil (14). Although in the present embodiment the body (16) has a cylindrical shape, said body (16) can take any shape and can be composed of any material suitable for influencing the field, such as magnetic metals or ferrites. Aluminum has proven to be an effective material. Additionally, even though only one body (16) is shown, in the practice of the present invention there may be more than one body (16) influencing the field mounted on said reel (12). Another body (17) that influences the field is fixed in the lower half (11a) of the casing (11). Like body 16, body 17 can take any shape and can be composed of any suitable material, such as aluminum. An IC (integrated circuit) (18) containing electrical components of the transponder (10) is coupled to the spool (12).

Referring to Figure 3, an equivalent circuit (20) of coil (14) is shown. The equivalent circuit (20) includes an inductor (22) that represents the inductor of the coil (14), a resistor (24) that represents the internal resistance of the coil (14), and capacitors (26) and (28) that represent the internal parasitic capacitances of the coil (14). The inductor (22), the resistors (24) and the capacitors (26) and (28) constitute the impedance of the coil (14).

The resonant frequency of the coil (14) is a function of, among other variables, the respective values ​​of the inductor (22) and the capacitors (26) and (28). At the same time, said values, as is well known, are functions of, among other variables, the distances between the turns of the coil (14) and the shape of said coil (14). Therefore, if each transponder (10) had the same distance between the turns of the coil (14) and the same coil shape, the value of the inductor (22) and the capacitor (26) and (28) of the coil (14) of each transponder would be essentially the same, in the same way as the resonant frequency. Unfortunately, the design of the prior art transponders and their manufacture has not made it possible to achieve uniform values. The present invention, however, allows such uniform values ​​to be achieved.

Therefore, the rigidity of the spool (12) allows the coil (14) to maintain its shape and guarantee precise distances between turns, especially when a winding machine forms the coil. This, in turn, allows to achieve constant values ​​of inductor and parasitic capacitance, which in turn, results in a constant resonance frequency.

As mentioned hereinabove, bodies (16) and (17) influence the magnetic and / or electric field and, consequently, the total impedance of the coil (14) and the resonant frequency.

The ability of the bodies (16) and (17) to influence the magnetic and / or electric field not only depends, as mentioned above, on the type of material from which the bodies (16) and (17) are made and its shapes, particularly its surface areas, but also the angular position of the body (16).

In accordance with the present invention, the shape and composition of the bodies (16) and (17) is selected to achieve the desired resonance frequency. Then, the resonant frequency of the transponder (10) is checked, as can be seen in figure 4, with an appropriate test equipment (30). If the results of the check indicate that the transponder (10) being evaluated does not have the desired resonance frequency, the angular position of the body (16) is modified by rotating the holder.

With reference to Figures 5a-5d, different angular positions of the body (16) are shown therein. Modifying the position of said body (16) changes the impedance of the resonant circuit (20) and, consequently, changes the resonant frequency of the transponder (10). More specifically, changing the position of the body (16) with respect to the body (17) modifies the combined surface areas of the bodies

ES 2 297 656 T3 (16) and (17). Modifying said combined surface areas, in turn, changes the impedance of the coil (14). More specifically, the greater the combined surface area, the greater the impedance. Therefore, the combined surface areas of bodies 16 and 17 in Figure 5b have a greater surface area than in Figure 5a. As a result, the resonant circuit (20) of the transponder (10) of Figure 5b has a higher impedance than the resonant circuit (20) of the transponder (10) of Figure 5a and consequently a lower resonant frequency. Similarly, the resonant circuit (20) of the transponder (10) of Figure 5c has a higher impedance than the transponder (10) of Figure 5b and consequently a lower resonant frequency. The position of the body (16) shown in Figure 5d results in greater combined surface areas and therefore the resonant circuit (20) of Figure 5d has a higher impedance than the resonant circuits of Figures 5a-5c, resulting in the lower resonant frequency. In this way, adjusting the position of the body (16) in a clockwise direction decreases the resonant frequency of the transponder (10) with respect to the previous position.

Although the present invention has been described with reference to particular embodiments, many other variations, modifications, and uses will be apparent to those skilled in the art. Therefore, the present invention is not limited by the description herein, but by the appended claims.