EP1667281A1 - Mobile communication terminal - Google Patents

Abstract

The terminal has a printed circuit board (104) with grounding points, and mounted with a radiocommunication system transmitting and receiving in a frequency band. A resonant frequency shifting unit (107) is connected to the ground of the board by electromagnetic coupling. The unit (107) shifts the resonant frequency of a battery (106) outside the transmission and/or reception frequency band of the system.

Description

The present invention relates to a mobile communication terminal comprising at least one element having a resonant frequency in one of the transmission and / or reception frequency bands of the terminal, and an offset element for shifting said frequency outside. one of said frequency bands used by the terminal.

A mobile communication terminal, such as a telephone or any other electronic device equipped with at least one radiocommunication system, is composed of various elements, such as, for example, a printed circuit board, also called PCB (Printed Circuit Board ), on which are mounted the electronic components necessary for the operation of said system, an antenna, a screen, a keyboard, a speaker, a microphone, a battery, etc., all enclosed in a housing. However, any element, whatever it is since it is electrically conductive, has an electromagnetic behavior which is particularly related to its shape and its dimensions. If, moreover, they are electrically connected by at least one point to the ground of the printed circuit board, these elements are capable of emitting in a resonance frequency which, in most cases, does not disturb the behavior of the terminal in so far as this resonant frequency does not not found in one of the frequency bands used by the radiocommunication system of this terminal to transmit and / or receive radio waves.

However, it turns out that the shape and dimensions of some of these elements, such as in particular the battery, are of the same order of magnitude as the radio wave lengths transmitted or received by the radiocommunication system. Consequently, the resonant frequency of this element lies in a frequency band used by the terminal's radio communication system, which induces a disturbance that results in a decrease in the efficiency of the antenna used by this radiocommunication system, by a increase in noise level (desensitization) and harmonic level degradation.

In addition, a mobile communication terminal provides more and more services while maintaining small dimensions. For example, it is more and more common to encounter a terminal using several radio communication systems such as, for example, the GSM 900 system (Global System for Communication) which uses a frequency band centered around 900 MHz and the DCS system (Digital Communication System) which uses a frequency band centered around 1800MHz. Thus, the risk that a resonance frequency will disturb the operation of these multifrequency terminals increases with the number of frequency bands used.

Due to the small dimensions of a mobile communication terminal, high-frequency coupling phenomena can also be induced, which also results in a reduction in the efficiency of the antenna used by the radiocommunication system, desensitization and a deterioration of the harmonic level.

These resonance and coupling phenomena are relatively complex to predict, before any in-situ experiment. Thus, generally, when the problem is detected, the architecture of the terminal is frozen and therefore it becomes difficult to change the element responsible for these phenomena.

Fig. 1 represents a mobile communication terminal 100 (for example a telephone) according to the invention. This terminal 100 comprises the conventional elements inherent to this type of apparatus, namely a human / machine interface composed for example of a keyboard 101, a screen 102 and a speaker 103, a card of printed circuit) 104 on which are mounted the various electronic components necessary for the operation of the terminal and comprising a mass, and at least one antenna 105 transmitting and / or receiving in at least one frequency band F, and a battery 106 which is connected to the printed circuit board 104 for its power supply by two conductors 106 ₁ and 106 ₂ , one of which, 106 ₂ , is connected to the ground of the printed circuit board 104 and the other, said point of supply potential 106 ₁ , is connected to a point of the printed circuit board 104 which is powered by a supply potential.

où 1 et L sont respectivement la longueur et la largeur de l'élément 106 et H la distance entre la face inférieure de la batterie 106 et la carte de circuit imprimé 104, ceci lorsque celui-ci est rectangulaire ou parallélépipédique.However, the battery 106, because it is electrically connected by a ground point of the printed circuit board 104, behaves like a half-wave antenna. It has been shown that the value of this wavelength is given by the following relation: $$\frac{\lambda }{\mathrm{two}}\cong l+\mathrm{The}+H,$$

where 1 and L are respectively the length and the width of the element 106 and H the distance between the lower face of the battery 106 and the printed circuit board 104, when this latter is rectangular or parallelepipedic.

Thus, given the size of the battery 106, its resonant frequency is located in one of the frequency bands F of the antenna 105, as soon as the terminal is in operation. This resonant frequency disturbs the emission and reception of electromagnetic waves from this antenna 105 by decreasing its efficiency, increasing the noise level (desensitization) and degrading the level of harmonics.

It is shown in FIG. A diagram of the Stationary Wave Rate (TOS) of a terminal as just described as a function of frequency in the case where the present invention is not implemented (thin line curve) ). As can be seen, this diagram has three main resonant frequencies 11, 12 and 13. The resonance frequency 11, which has a value close to 900 MHz, corresponds to the resonance frequency of the antenna 105 operating in GSM mode. The resonant frequency 12, which has a value close to 1800 MHz, corresponds to the resonance frequency of the antenna 105 operating in DCS mode. However, it can be seen that the value of the TOS is strongly disturbed by the presence of the resonance frequency 13 emanating from the battery 106.

The present invention proposes to solve this problem. Thus, a mobile communication terminal to which the present invention applies is of the type which comprises a printed circuit board which is provided with a mass and on which are mounted in particular a radiocommunication system provided for transmitting and receiving in at least one frequency band, and at least one electrically conductive element including at least one point is electrically connected to a first point of said mass of said printed circuit board, said resonant element having a shape and dimensions such that it has a resonant frequency in one or more of the frequency bands of transmission and / or reception of said radio communication system of said terminal.

According to a feature of the present invention, said communication terminal further comprises at least one shift element connected to the ground of said printed circuit board by electromagnetic coupling and for shifting said resonance frequency of said resonant element out of said transmit and / or receive frequency bands of said radio communication system.

According to a second embodiment of the present invention, said shifting element consists of a metal plate located at a distance from said resonant element.

Insofar as said resonant element is rectangular or parallelepipedal, said offset element is advantageously placed along one of the two edges or one of the two faces extending in the width direction of the resonant element. For example, said electrical connection between the mass of said printed circuit board and said resonant element is located at one of its corners.

Advantageously, the surface of said plate and the distance between said plate and said resonant element are adjusted so that the value of said resonant frequency is shifted by said shift element to a resonance frequency not used by said radio communication system of said terminal.

• La Fig. 1 est un schéma représentant un terminal de communication mobile auquel la présente invention peut s'appliquer,
• La Fig. 2 est un schéma représentant l'élément de décalage de fréquence de résonance selon un premier mode de réalisation de la présente invention,
• La Fig. 3 est un schéma représentant l'élément de décalage de fréquence de résonance selon un seconde mode de réalisation de la présente invention,
• La Fig. 4 est un schéma représentant le modèle de ligne équivalent à un retour de masse capacitif, et
• La Fig. 5 est un diagramme illustrant le décalage d'une résonance parasite dans le cas d'un téléphone mobile fonctionnant sous le mode DCS 1800MHZ.

The characteristics of the invention mentioned above, as well as others, will appear more clearly on reading the following description of an exemplary embodiment, said description being given in relation to the attached drawings, among which:

• Fig. 1 is a diagram showing a mobile communication terminal to which the present invention may apply,
• Fig. 2 is a diagram showing the resonance frequency shift element according to a first embodiment of the present invention,
• Fig. 3 is a diagram showing the resonance frequency shift element according to a second embodiment of the present invention,
• Fig. 4 is a diagram representing the line model equivalent to a capacitive mass return, and
• Fig. 5 is a diagram illustrating the shift of a parasitic resonance in the case of a mobile phone operating in DCS 1800MHZ mode.

The present invention applies to a terminal such as that which has already been described in relation to FIG. 1. It therefore comprises a keyboard 101, a screen 102 and a speaker 103, a printed circuit board 104 on which are mounted the various electronic components necessary for the operation of the terminal and comprising a mass and, and at least one antenna 105 emitting and / or receiving in at least one frequency band F, and a battery 106 which is connected to the printed circuit board 104 for its power supply by two conductors 106 ₁ and 106 ₂ , one of which 106 ₂ , is connected to the ground of the printed circuit board 104 and the other, said point of supply potential 106 ₁ , is connected to a point on the printed circuit board 104 which is powered by a supply potential, for example at a potential of + 4 volts. It may be noted that the dimensions of the elements drawn on FIGS. are not to scale, some of them having been enlarged to allow a better representation of the elements in relation to the invention.

To solve the problem of parasitic resonance frequency from a terminal element such as battery 106, the present invention proposes to equip said terminal with an offset element 107 which is provided so that the resonance frequency the resonant element 106 is shifted in a band of frequencies not used by the antenna 105.

Fig. 2 shows a first embodiment of an offset element 107 applicable in the case where the body of the element 106 is electrically conductive and is, by the conductor 106 ₂ at the same DC potential as the ground of the circuit board The offset member 107 is made of a conductor which provides a direct connection between the body of the battery 106, at a point remote from the electrical conductors 106 ₁ and 106 ₂ , and the ground of the printed circuit board 104 . Thus, the resonance frequency of the battery 106 has been shifted because it now behaves like a quarter wave antenna $\frac{{\lambda }_{\mathrm{GSM}}}{4}\cong l+\mathrm{The}+H.$

For example, connecting the battery 106 to the ground by the shifting member 107 decreases the resonant frequency by half. So, the battery once associated with the shifting element 107, no longer resonates in the 1800 MHz DSC band but in the 900 MHz frequency band. For terminals not operating in this second band, this first embodiment is quite possible.

Note that the battery 106 may be embedded in a shell of insulating material. In this embodiment, it is necessary to practice a window in this shell so as to ensure the electrical connection of the element 107 with the body of the battery 106.

The shifting element 107 of this first embodiment as shown in FIG. 2, can not be realized when the electrically conductive body of the battery 106 is not at the same DC potential as the mass of the printed circuit board 104, because of the short-circuit at the DC voltage level of the power supply. he would then beget.

Moreover, the shifting element 107 of this first embodiment does not make it possible to define the offset value of the resonance frequency of the element 106. Thus, in the case of terminals using several frequency bands, it can happen that the new value of the resonant frequency disturbs another frequency band. In the preceding example, the resonance frequency disturbs the 1800 MHz band, that is to say the DSC band. Once offset, this frequency has a value of 900 MHz, thus disturbing a terminal using GSM 900MHz mode.

Fig. 3 represents the shifting element 107 according to a second embodiment of the invention. This element 107 consists of a metal plate connected to the ground of the printed circuit board 104 by a so-called return conductor 201 and connected only by electromagnetic coupling to the body of the battery 106. In this embodiment, the battery 106 It must be electrically isolated from the shifting element 107 and, in practice, it is, because it is embedded in a shell of insulating material.

It has been shown that this shifting element 107 has a capacitive behavior which will shift the resonance frequency of the element 106 by a value which depends essentially on the physical characteristics of this shifting element 107, such as its area and its distance. d "to the body of the battery 106.

The battery 106, of parallelepipedal shape, behaves like a rectangular patch antenna. The currents that flow on his body are concentrated mainly along its four edges. But, only two of these edges are radiant. These are the ones who are separated by the greatest distance, that is, the edges that extend in the direction of the width of the element 106. Indeed, the other two edges can be likened to two slots traversed by magnetic currents of opposite direction whose contributions cancel each other . To fully play its effect, the offset plate 107 is therefore advantageously placed along one of these two edges extending in the width direction of the battery 106.

The capacitive value C of the shifting element 107 depends on the surface of the metal plate 107 and its distance d to the battery 106.

dans laquelle C désigne la capacité, Z ₀ désigne l'impédance caractéristique (par exemple égale à 50 ohms), c désigne la vitesse de la lumière dans le vide, ε ₀ la permittivité du vide, d la distance entre la plaque 107 et la batterie 106 et S la surface de la plaque 107.An attempt has been made to model the battery 106 and the introduction of the shifting element 107 to determine the resulting frequency shift. To do this, the shift element 107 is analyzed using a transmission line model 400 such as that shown in FIG. 4. The capacitive mass return 400 is thus modeled by a characteristic impedance open transmission line 50 ohms and length L _oc . The influence of the capacitive value C of the ground return 400 on the resonance frequency of the battery which has the dimensions L and 1 and which is at a distance H from the printed circuit board 104 is then governed by the equation following by analogy with an article by CR Rowell et al. "Capacitively loaded PIFA for compact mobile telephone", IEEE Transactions on antennas and propagation, 45 (5), May 1997 concerning capacitively charged PIFA type antennas: $$\mathrm{The}+l+H+{\mathrm{The}}_{o\mathrm{vs}}=\frac{\lambda }{\mathrm{two}}\mathrm{with}{\mathrm{The}}_{o\mathrm{vs}}=\mathrm{VS}.Z_0.\mathrm{vs}\mathrm{and}\mathrm{VS}=\frac{{\epsilon }_0.S}{d}$$

wherein C denotes the ability, Z ₀ is the characteristic impedance (for example equal to 50 ohms), c denotes the speed of light in vacuum, ε ₀ the permittivity of vacuum, d is the distance between the plate 107 and the battery 106 and S the surface of the plate 107.

Thus, the value of the theoretical frequency of the resonance, once shifted, is related to the dimensions of the battery 106, at its distance H to the printed circuit board 104, to the distance between the metal plate 107 and the battery 106 and finally, on the surface of the metal plate 107, according to the following equation: $$f=\frac{\mathrm{vs}}{\lambda }=\frac{\mathrm{vs}}{2.\left(\mathrm{The}+l+H+\frac{{\epsilon }_0.Z_0.\mathrm{vs}}{d}S\right)}$$

According to the preceding numerical values, the resonant frequency of the battery 106 alone without a metal plate 107 is 1.7 GHz. Now, considering the distance d between the battery 106 and the metal plate 107 equal to 0.25 mm, and the surface of the metal plate equal to 110 mm 2, the capacitance introduced is 3.8 μF, so that the frequency of the resonance is now theoretically offset by 0.4 GHZ to reach the frequency of 1.3 GHz, not used by the terminal.

Fig. 5 shows, in bold lines, the diagram of the Stationary Wave Rate (TOS) as a function of the frequency in the case of the addition of an offset element 107 according to the second embodiment which has just been described. It can be seen in this second diagram that the frequency band around 1800 MHz and the frequency band around 900 MHz are not disturbed by the resonance frequency emanating from the battery 106. This frequency has been shifted around the frequency band. 1300 MHz.

The advantage of using an offset element 107 such as that just described in connection with FIG. 3 is that its dimension can be easily adjusted empirically in situ after the design and realization of a prototype of a terminal, moving to a non-disturbing value the resonance frequency of the battery 106.

The description that has just been given is in connection with a battery 106. It will be understood that other elements of the terminal could be treated in the same way if they have a shape and dimensions that could cause it to resonate. in a frequency band occupied by the terminal and therefore also that they are electrically at a ground point of the printed circuit board. The present invention therefore also applies to this type of elements.

Claims (7)

Mobile communication terminal comprising a printed circuit board (104) which is provided with at least one earth point and on which is mounted in particular a radiocommunication system designed to transmit and receive in at least one frequency band and at least one an electrically conductive element of which at least one point (106 ₁ ) is electrically connected to a first ground point of said card (104), said resonant element (106) having a shape and dimensions such that it has a resonance frequency in the one or more of the transmission and / or reception frequency bands of said radio communication system of said terminal, characterized in that it furthermore comprises at least one shifting element (107) connected to the earth of said card (104) by electromagnetic coupling and for shifting said resonant frequency of said resonant element (106) out of said one or more transmit frequency bands and / or reception of said radio communication system. Communication terminal according to claim 1, characterized in that said shifting element (107) consists of a metal plate remote from said resonant element (106). Communication terminal according to claim 2, the resonant element (107) being rectangular or parallelepipedal, characterized in that said shifting element (107) is placed along one of the two edges or one of the two faces of said element resonant (106) extending in the direction of its width. Communication terminal according to claim 3, characterized in that said electrical connection between the mass of said printed circuit board and said resonant element is located at one of the angles of said resonant element (106). Communication terminal according to one of claims 2 to 4, characterized in that the surface of said plate and the distance between said plate and said resonant element are adjusted so that the value of said resonance frequency is offset by said shift element (107). ) on a resonance frequency not used by said radio communication system of said terminal. Communication terminal according to claim 5, characterized in that said offset resonance frequency value f is determined by the following relation: $$f=\frac{\mathrm{vs}}{\lambda }=\frac{\mathrm{vs}}{2.\left(\mathrm{The}+l+H+\frac{{\epsilon }_0.Z_0.\mathrm{vs}}{d}S\right)}$$

wherein Z ₀ is the characteristic impedance, c denotes the speed of light in vacuum, ε ₀ the permittivity of vacuum, d is the distance between said metal plate and said resonant element, the surface S of the plate 1 and L respectively the length and width of said resonant element and H the height between said resonant element and said mass of the card. Communication terminal according to one of the preceding claims, characterized in that said resonant element (106) is a battery supplying said terminal.

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