FIELD OF INVENTION

The present invention relates generally to a signal transmission device and more particularly to a signal transmission device providing a shielded conductor. The present invention also relates to a portable radio communication device having such an signal

transmission device.

BACKGROUND

Coaxial cables are made of a central conductor

surrounded by an isolator/dielectric and a shield or circular conductor. These cables are used for

interconnecting various entities such as antennas and radio circuits in order to receive and transmit radio signals. A coaxial cable may typically have an outer diameter of about 0.8 mm.

The reception and transmission of radio signals is the core activity of portable radio communication devices, such as mobile phones.

The dimensions of these devices have for many years become smaller and smaller. At the moment the height is in focus. It can nowadays be as low as 10 mm. For these types of devices this thus means that also the size of the coaxial cable is a major concern. It is therefore desirable to reduce the size of coaxial cables. One way to reduce the size is through providing the coaxial cable using a flexible carrier, such as flexfilm.

The company Techno Core does for instance develop coaxial cables that are small. These coaxial cables are produced using a Liquid Crystal Polymer (LCP) film. On this film there is provided a central main conductive trace in-between two secondary conductive traces. The whole film is then surrounded by a

separate shield.

Small sized coaxial cables are also known through for instance US 7,084,820, JP 2003123552, CN 101447256 and CN 201017724.

However, there is still room for improvement. The present invention is therefore directed towards providing a signal transmission device using a

flexible carrier with conductive traces for forming a shielded connection that can be made even smaller.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a signal transmission device that can be used as shielded conductor with small dimensions.

The invention is based on the realization that a foldable carrier carrying conductive material can be used for providing both conductor and shield of a shielded conductor.

According to the present invention there is provided a conductive trace carrier having a first and a second side and a first part having a midsection, a first lateral section and a second lateral section, where the midsection is provided between the first and second lateral sections, the midsection is provided with a first central primary conductive trace on the first side of the carrier and a first and a second secondary conductive trace on each side of the primary conductive trace on the first side of the carrier, the first part of the carrier is foldable for folding the first lateral section over the first side of the midsection and the second lateral section under the second, opposite side of the midsection in order to cover the midsection, and the first lateral section is provided with a first solid area of conductive material while the second lateral section is provided with a second solid area of conductive material in order to form a shielded conductor when the first part of the carrier is folded.

A portable electronic device comprising such a signal transmission device is also provided.

Further preferred embodiments are defined in the dependent claims. The signal transmission device according to the invention has several advantages. It makes it possible to provide a very thin shielded signal conductor, which is of advantage when the signal transmission device is provided in very thin portable radio communication devices. Other advantages of the signal transmission device are that it is economical to produce since a small amount of cheap components are used. BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a front view of a portable radio

communication device according to the present

invention;

Fig. 2 shows a perspective view of a signal

transmission device according to a first embodiment of the invention including a first and a second part of a foldable conductive trace carrier; Fig. 3 shows a side view of the first and second parts of the foldable conductive trace carrier as seen from a first end of the first part when in the process of being folded,

Fig. 4 shows a perspective view from the first end of the first part of the foldable conductive trace carrier from fig. 2 and 3 as seen from the first end of the first part and also being in the process of being folded;

Fig. 5 shows a side view from the first end of the first part of the foldable conductive trace carrier after being folded;

Fig. 6 shows a perspective view from the first end of the first part of the foldable conductive trace carrier, with which a connecting part has been joined; and

Fig. 7 shows a perspective view from the first end of the first part with connecting part, when a connector has been attached to the connecting part, and

Fig. 8 shows a perspective view from a first end of a first part of a foldable conductive trace carrier according to a variation of the invention, where a single sided flexfilm is used for providing a signal transmission device.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of a

preferred embodiment of a foldable signal transmission device and portable radio communication device

according to the invention will be given. In the description, for purposes of explanation and not limitation, specific details are set forth, in order to provide a thorough understanding of the present

invention. However, it will be apparent to one skilled in the art that the present invention may be utilized in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known units, entities and circuits are omitted so as not to obscure the description of the present invention with unnecessary details.

The invention is generally directed towards a signal transmission device that is based on a foldable conductive trace carrier and a portable electronic device including such a signal transmission device, where the foldable conductive trace carrier is

supposed to be folded for providing a shielded

conductor for transmitting signals such as radio signals . Fig. 1 shows a front view of a portable radio

communication device 10, such as a mobile phone. The portable radio communication device can however be another type of device, such as a lap top computer or a palm top computer, a TV receiver or an FM radio receiver. The device 10 is, as an example, provided with a display 12 placed close to an upper end of the device and a keypad 14 placed close to a lower end of the device. These are here provided on the casing of the device 10. It should however be realized that the device may just as well be provided without a display and/or without a keypad. The device 10 is also

provided with at least one antenna. However, all antennas are provided inside the device, i.e. in the interior of the device. In order to provide an element such as an antenna with signals or to receive signals from such an element, the element needs to be connected to other elements and entities using conductors. As an antenna receives radio signals a conductor used for it normally has to be shielded. The present invention is directed towards providing a signal transmission device that provides such a shielded conductor for an element such as an antenna element. According to the invention this conductor is provided through the use of a foldable conductive trace carrier, such as a flexfilm. Fig. 2 shows a perspective view of a signal

transmission device 16 comprising a foldable

conductive trace carrier including a first 22 and a second 24 part. Fig. 3 shows a side view of the first and second parts of the foldable conductive trace carrier as seen from a first end of the first part when it is in the process of being folded, fig. 4 shows a perspective view of the first end of the first part of the foldable conductive trace carrier as it is being folded and fig. 5 shows a side view from the first end of the first part of the foldable conductive trace carrier after being folded.

As can be seen in fig. 2, the foldable conductive trace carrier has two sides a first side 18, here also termed an upper side, and a second opposite side 20, here also termed bottom side. When the carrier is unfolded these sides are provided in two parallel planes. In fig. 2 and 3 the first part is presented partially folded, while the second part is straight or planar and non-folded and thereby it can be seen that the first and second sides 18 and 20 are parallel. The foldable conductive trace carrier is in the first embodiment of the invention provided through a

flexfilm, which may be a polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or a polyimide (PI) flexfilm. The foldable conductive trace carrier does in a first embodiment of the invention include at least two parts, a first part 22 and a second part 24, where the first part 22 is foldable, elongated and stretches from a first end 26 to a second end 28 along a first direction Dl. The second part 24 is joined to the first part 22 at this second end 28 and is here also elongated but with the direction of elongation perpendicular to the direction of elongation of the first part. It should however be realized that this second part may have any shape. As is more clearly seen in fig. 3 — 5, the first part 22 here includes a midsection 32 provided between a first and second lateral section 34 and 36. It is this midsection 32 that the second part 24 is joined to. These sections may all be essentially planar, but may get slightly curved at the areas where they are folded.

The first and second lateral sections 34 and 36 can here each be folded around a dedicated axis pointing in the first direction Dl, where each such axis is provided at opposing sides or edges of the midsection. Both these sections can thus be folded at right angles to the first direction Dl. The first part 22 of the carrier is thus foldable for placing the first lateral section 34 over the first side 18 of the midsection 32 as well as for placing the second lateral side 36 under the second, opposite side 20 of the midsection 32 in order to cover the midsection 32.

The second part 24 is here provided with an element, which is here an antenna element 30, which may be provided through placing a patterned area of

electrically conducting material, such as copper, on the second part 24. It is here possible that also the second part is foldable. It may for instance be foldable around an axis that is perpendicular to the axes around which the lateral sections of the first part are foldable and therefore also perpendicular to the first direction Dl. The second part may thus be foldable in the first direction Dl or in a direction that is opposite to the first direction Dl. It can be foldable in other directions. However, there is no requirement that this second part is foldable. In fact, the second part of the carrier need not be provided. The second end of the first part of the carrier may for instance be connected to a separate element not provided on the carrier, for instance via a connector placed at the second end of the first part.

The midsection 32 of the first part 22 is according to the invention provided with a number of conductive traces 38 , 40 , 42 on the first side. These conductive traces may also with advantage be provided through copper that has been placed on the carrier. Of these traces there is a first central primary conductive trace 38 . This trace stretches in the first direction Dl on the first side 18 of the carrier from the first end 26 of the first part to the second end 28 of the first part. On both sides of this primary conductive trace 38 there are provided secondary conductive traces 40 and 42 . These are placed on an equal

distance from the primary conductive trace 38 and run in parallel with it from the first 26 to the second end 28 of the first part and on the first side of the carrier. There is here a first secondary trace 40 provided adjacent the first lateral section 34 and a second secondary trace 42 adjacent the second lateral section 36 . The first lateral section 34 furthermore also includes conductive material. However, this material is provided on the second side 20 of the carrier. This material here includes a first solid area 44 of conductive material, with advantage copper. This first solid area 44 here furthermore stretches along the first direction Dl for the whole length of the first part of the carrier, i.e. all the way from the first 26 to the second end 28. The first solid area 44 furthermore also stretches at right angles to the first direction from a first position covering the first secondary conductive trace 40 to a second position at least partly covering the second secondary conductive trace when the first part of the carrier is folded.

Also the second lateral section 36 includes conductive material, for instance copper. However this material is provided on the first carrier side 18. On this side there is a second solid area 46 of conductive

material. The second solid area 46 is here joined with the second secondary conductive trace 42 of the midsection 32 along the first direction Dl for the whole length of the first part 22 of the carrier. They are thus formed as a unitary unit. The second solid area 46 then stretches from a first position where it is joined to the second secondary conductive trace 42 at right angles to the first direction to a second position where the second solid area 46 at least partly covers the first secondary conductive trace 40 when the first part of the carrier is folded.

These solid areas 44 and 46 are provided together with the secondary conductive traces 40 and 42 in order to provide shielding for the primary conductive trace 38, i.e. in order to provide a shielded conductor. In order to do this the first secondary conductive trace 40 is electrically connected to the conductive

material of the first lateral section, and then more particularly to the first solid area 44 of the first lateral section. This may be done through providing a group of vias including at least one via 54 going through the carrier. Here vias filled with conductive material like copper run through the midsection 32 from the first 18 to the second side 20.

In order to provide this connection the midsection 32 may furthermore be provided with a third solid area 48 of conductive material, like copper, on the second carrier side, where this third solid area then

stretches from the first end 26 to the second end 28. This third solid area 48 is furthermore with advantage aligned with the first secondary conductive trace 40 in order for the vias to directly connect the first secondary conductive trace 40 with the third solid area 48. The vias may then be provided regularly, for instance at equally spaced distances, along the first direction from the first to the second end. In this first embodiment the third solid area 48 is

furthermore joined with the first solid area 44. They may thus be provided as a unitary unit.

As the lateral sections 34 and 36 are then folded around the midsection 32 a shielded primary conductor 38 that may be provided as a coaxial cable is

provided. The folded sections can in one embodiment be separated by a dielectric, which may be air. However, in the first embodiment of the invention they are separated by a first 50 and a second 52 layer of adhesive material. This helps in sealing the device and keeping it closed for shielding the primary conductive trace. The adhesive material has

furthermore with advantage also dielectric properties, in which case insulation of the primary conductive trace 38 from the secondary conductive traces 40 and 42 and the solid areas 44, 46 and 48 is obtained. The first layer 50 of adhesive material is then inserted between the first lateral section 34 and the

midsection 32 while the second layer 52 of adhesive material is inserted between the second lateral section 46 and the midsection 32. The adhesive

material may with advantage be an acrylic adhesive or some other type of low loss adhesive.

Since the conductive material is provided on different sides of the carrier material, the distance between the first solid area and the primary conductive trace and the distance between the second solid area and the primary conductive trace may differ from each other. In order to make these distances equal for improving the voltage distribution in the signal transmission device, it is then possible make the second layer 52 of adhesive material thicker than the first layer 50 of adhesive material.

The coaxial cable provided through the folded first part of the flexible carrier is in the first

embodiment of the invention connected to the element on the second part of the carrier. This connection is provided at the second end of the first part. When this element is an antenna element, this element may be connected to only the central primary conductive trace. In addition to this connection the element may also be connected to the shielding. Whether an antenna element is connected to the shielding or not is dependent on the type of antenna.

In this way it is possible to provide a very thin shielded signal conductor which is of advantage when the signal transmission device is provided in very thin portable radio communication devices. The total thickness of the signal transmission device can be in an interval of 0.3 — 0.35 mm and with advantage 0.32 mm, which is a considerable size reduction as compared with a conventional coaxial cable. Other advantages of the signal transmission device are that it is

economical to produce since a small amount of cheap components are used. The use of a double sided

flexible carrier also enables a more flexible design. The signal transmission device can furthermore be designed to provide a defined interface impedance at the first end, for instance an impedance of 50 Ω.

In order to get connected to other elements than the antenna element 30, it is possible to provide the signal transmission device with a connector. Fig. 6 and 7 are directed towards such a case. Here fig. 6 shows a perspective view of the first end of the first part of the foldable conductive trace carrier to which a connecting part has been joined and fig. 7 shows a perspective view of the first part with connecting part, when a connector has been attached to this connecting part. As can be seen in theses drawings the carrier further includes a connecting part 56 which is joined to the first end 26 of the first part of the carrier. This connecting part 56 then stretches out in an opposite direction of the first direction. Here the primary conductive trace 38 and the secondary conductive traces 40 and 42 all stretch out from the first end onto the connecting part 56 . However, the lateral sections 34 and 36 and the solid areas of conducting material do not. They end at the first side 26 . In this way it is possible to place and mount a connector 58 on this connecting part 56 . The connector 58 may be a surface mount connector soldered to the primary conductive trace and optionally also to one or both of the secondary traces.

It should be realized that it is possible to have all the conductive material provided on one and the same side of the carrier, as is shown in fig. 8 . This figure shows the same elements as in fig. 3 . The difference here is that the first solid area 44 of conductive material on the first lateral section 34 is provided on the first side of the carrier. It is in this case also joined with the first secondary

conductive trace 40 . They thus form one unit

stretching from the first to the second end. The use of a single-sided flexfilm, i.e. a flexfilm carrying conductive material on only one side improves the economy of the production of the device even further. It can furthermore be mentioned that whether a double- sided or a single-sided flexfim is used, i.e. a flexfilm having conductive material on two sides or one side, can furthermore be decided by the second part of the carrier and more particularly on if this second part of the carrier uses one or two sides. Two sides may for instance be used for carrying more than one antenna pattern.

It should here be realized that as a further variation of the invention it is possible to include further traces on the first part of the connector, for

instance DC signal traces. Such traces may be of interest if the element connected to the second end of the first part of the carrier is a speaker. In this case at least one lateral section is wider than the midsection and the side of this second which is provided with a solid area of conductive material also includes a tertiary conductive trace stretching in the first direction from the first end to the second end of the first part separated from said solid area. Here either the first, second or both the first and second lateral sections may be provided with such a tertiary conductive trace. This tertiary conductive trace may then be provided on a part of the lateral section, which when the first part is folded,

stretches out beyond the edge where the midsection is joined to the other lateral section.

Various variations of a signal transmission device according to the invention have been described above. However, the person skilled in the art realizes that these can be varied within the scope of the appended claims without departing from the inventive idea. The invention is furthermore also directed towards a portable radio communication device including such a signal transmission device. Therefore the invention is only to be limited by the following claims.