Abstract:
A portable device comprising a housing ( 105, 110 ); a circuit board ( 125 ) located within the housing; and a tubular shield member ( 130 ) mounted at a first end ( 132 ) on the circuit board ( 125 ) so as to surround electronic components ( 14 ) on the circuit board to be shielded, wherein the device is arranged such that a metal portion of an exterior wall ( 120 ) of the housing electromagnetically seals off the shield member at its other second end ( 134 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention is concerned with electromagnetic shielding in electronic devices, especially radio devices. The present invention has particular applicability to radiotelephone transceivers manufactured in the format of a PC card or PCMCIA card. 
     Electromagnetic signals are generated as a by-product during normal operation of an electrical circuit. Electromagnetic compatibility (EMC) is defined as the ability of a device to function properly in its intended electromagnetic environment and not be a source of unacceptable pollution to that environment. In some situations, shielding is required to provide electromagnetic compatibility between a sensitive or noisy part of the circuit and the rest of the circuit. One typical example is the isolation of a frequency synthesiser from the rest of the RF section in a cellular radiotelephone. 
     FIG. 1 shows a conventional approach to providing electromagnetic shielding in this situation. 
     Referring to FIG. 1, a radiotelephone housing  10  has located within it a printed circuit board  12 . The printed circuit board  12  carries surface mount components. Some of these components  14 , for example the frequency synthesiser, require shielding from other components  16 , for example the rest of the RF section  16 . To this end, a shield box  18  is used to enclose electromagnetically the components  14  and thus provide for EMC between the components  14  and the component  16 . 
     However, because of the mechanical tolerances required in the height of the shield box  18 , resulting from the manufacturing of the shield box  18  itself or the surface mount process, a gap must be designed for between the roof  18   a  of the shield box and the housing  10 . This space plus the thickness of the roof  18   a  of the shield box  18  equates to the cost in terms of height which this conventional approach to shielding incurs. The ‘height cost’ is illustrated in FIG. 1 by double headed arrow h 1 . 
     The present invention is concerned with reducing the height cost which this conventional approach inevitably incurs. 
     BRIEF SUMMARY OF THE INVENTION 
     With this in mind, the present invention provides, in one aspect, a portable device comprising a housing; a circuit board located within the housing; and a shield member mounted at a first end on the circuit board so as to surround electronic components on the circuit board to be shielded, wherein the device is arranged such that a conductive portion of an exterior wall of the housing electromagnetically seals off the shield member at the other second end. 
     By virtue of these features, the need for a roof  18   a  to the shield box  18  in FIG. 1 is eliminated. Consequently, the thickness of the roof  18   a  no longer counts towards the height cost of the shielding and the aforementioned design tolerances associated with it are thus reduced. 
     The height cost of the shielding associated with the present invention is less than that associated with the conventional approach in FIG.  1 . This means that either components having a higher maximum height can be used or the overall volume of the device reduced if components with the same maximum height are used. 
     In addition, the tubular shield member of the present invention is even simpler to manufacture than the shield box  18 . 
     Preferably, a gasket is located at the second end of the shield member to facilitate electromagnetic sealing. Alternatively, a conductive adhesive can be used to adhere the gasket to the shield member and thus provide ohmic contact therebetween. 
     Both of these embodiments of the invention enjoy the advantage that no special structural modification to the housing wall is required to implement them. 
     In other embodiments, a gasket can be used to mount the first end of the shield to the printed circuit board. 
     It will be appreciated that the height savings which are available when employing the present invention are particularly useful in connection with a PC card based product, such as the PCMCIA card based cellular transceiver which is described hereinafter, which is extremely constrained height/thickness -wise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are hereinafter described with reference to the accompanying drawings, in which: 
     FIG. 1 shows a conventional approach to provide electromagnetic shielding; 
     FIG. 2 shows a perspective view of a cellular radiotelephone transceiver in the format of a PC card; 
     FIG. 3 shows a cross-sectional view of the PC card illustrating the present invention; 
     FIG.  4 ( a ) shows in isolation and in more detail the gasket in FIG. 3; 
     FIG.  4 ( b ) shows the area in a circle marked X in FIG.  4 ( a ) in cross-section along the central longitudinal axis of the limb; and 
     FIG.  4 ( c ) shows in area in a circle market Y in FIG. 3 in enlarged form. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a cellular radiotelephone transceiver embodied in the form of a PC card or PCMCIA card  100 . It is designed to be inserted in the standard PCMCIA connector of a laptop or high-performance palmtop computer and provide an integrated solution to wireless communication including speech, data and fax services, and Short Message Services (SMS) over digital cellular networks. 
     Mechanically, the card  100  has a housing consisting of a main portion  105  which is received within the PCMCIA connector of a host computer and an extended portion  110  by which the card  100  is handled by a user when inserting and withdrawing the card  100 . The extended portion  110  is thicker than the main portion  105  and is used to house the larger surface mount components. Clearly, within the main portion  105  of the card  100  the overall thickness or height of the card is tightly constrained by the requirement that the card  100  be insertable within a standard PCMCIA connector. Height considerations within the main portion  105  are thus of paramount importance. 
     FIG. 3 shows a partial cross-sectional view of the main portion  105  of the card  100 . The walls  120  of the housing of the main portion  105  are made from metal. Within the housing, a printed circuit board  125  carrying surface mount components is located. Obviously, it is these components which provide the functionality of the card  100 . However, the details of these components do not form part of the present invention, except to say (as in FIG. 1) that some components  14 , providing, for example, the frequency synthesiser, require shielding from other components  16 , say, for example, the rest of the RF section. To this end, a tubular shield member  130  is surface mounted at one end, hereinafter its first end  132 , to the printed circuit board  125  so as to surround the components  14 . The tubular shield member  130  is of rectangular cross-section and is defined by side walls designated  131   a ,  131   b ,  131   c ,  131   d  (only side walls  131   b ,  131   d  are visible in FIG.  3 ). In other embodiments, the tubular shield member  130  need not be rectangular in cross-section. The solder connection  133  at the first end of the shield member  130  provides an ohmic connection to the printed circuit board  125  and so prevents leakage of electromagnetic radiation between the printed circuit board  125  and the shield member  130  at its first end  132 . 
     At the second end  134  of the tubular shield member  130 , a gasket  140  is fitted. The gasket is shown in more detail in FIGS.  4 ( a ),  4 ( b ),  4 ( c ). Referring to FIG.  4 ( a ), the gasket  140  comprises a planar network or framework of interconnected planar limbs  142   a ,  142   b ,  143   b ,  143   d  corresponding to a wall  131   a ,  131   b ,  131   c ,  131   d  of the tubular shield member  130 . Each limb  142  is provided with a plurality of tangs  144  upstanding centrally from the plane of the limbs  142  along the whole of their length. As illustrated in FIG.  4 ( a ) some tangs have been omitted and replaced with a line to aid the overall clarity of the drawing. Each tang  144  is depressible, as illustrated by arrow D, towards the plane of its associated limb  142  such that the tangs  144  can adopt a range of angles relative thereto, for example from its natural bias position shown in FIG.  4 ( b ) to a position in which it is co-planar with its associated limb. The limbs  142   b  and  142   d  are also provided with a pair of projections  146  equipped with barbs  148  which upstand in the opposite direction to the tangs  144 . 
     Returning to FIG. 3, the function of the tangs  144  is to provide adequate electromagnetic sealing between the gasket  140  and the shield member  130 . The range of angles which each tang  144  can adopt compensates for dimensional tolerances between the shield member  130  and the wall  120  to prevent unacceptable electromagnetic leakage therebetween. In connection with FIG. 3, it is pointed out that the tangs  144  are depicted as converging towards each other in order to make clearly distinguishable the tangs  144  in the FIG. 3 view. In fact, as can be seen from FIG.  4 ( a ), each tang  144  extends axially along the length of its associated limb. Also, the tangs  144  are shown at an angle which seems to approach that of its natural-bias position in FIG.  4 ( b ) i.e. in slightly exploded form. This is again to more clearly illustrate the operation of the gasket  140 . In practice, each tang  144  would to slightly varying degrees of approximation be co-planar with its associated limb subject to the tolerance considerations discussed above. 
     The function of the pairs of projections  146  on the limbs  142   b  and  142   d  is to bite into the side walls  131   b  and  131   d  respectively, and thereby secure the gasket  140  to the shield member  130 . The biting action of the barbs  148  on the projections  146  associated with the limb  142   b  is shown in FIG.  4 ( c ). 
     The gasket  140  may be separate from or integral with the shield member  130 .