Abstract:
The invention relates to a device for guiding electromagnetic waves from a wave guide ( 10 ), in particular a multi-band wave guide, to a transmission line ( 20 ), in particular a micro strip line, arranged at one end of the wave guide ( 10 ), comprising coupling means ( 30 - 1, . . . , 30 - 7 ) for mechanical fixation and impedance matching between the wave guide ( 10 ) and the transmission line ( 20 ). It is the object of the invention to improve such a structure in the way that manufacturing is made easier and less expensive than according to prior art. According to the present invention that object is solved in the way that the coupling means comprises at least one dielectric layer ( 30 ) being mechanically connected with the main plane of the transmission line, the geometric dimension of that at least one dielectric layer extending along the propagation direction of the electromagnetic waves being correlated with the center frequency of electromagnetic waves in order to achieve optimised impedance matching.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a device for guiding electromagnetic waves from a wave guide, in particular a multi-band wave guide, to a transmission line, in particular a microstrip line, arranged at one end of the wave guide, comprising coupling means for mechanical fixation and impedance matching between the wave guide and the transmission line. 
   One problem for devices of that kind is to ensure a good transmission of electrical power in the wave guide to transmission line transition. Poor transition results in large insertion loss and this may degrade the performance of the whole module, e.g. a transceiver module. 
   2. Description of the Related Art 
   A device with a structure known in the prior art is shown in  FIG. 9 . There is shown a wave guide  10  and a transmission line  20 , in particular a micro strip structure which are attached to each other for enabling transition of electromagnetic waves from the wave guide  10  to the transmission line  20 . The transmission line  20  comprises a substrate  22  which is attached to a ground plane  24  for achieving good transition characteristics. The substrate  22  of the transmission line is typically made from low or high temperature co-fired ceramic LTCC or HTCC. 
   Impedance matching between the wave guide  10  and the transition line  20  is completed by providing a patch  26  in the transition area between the wave guide  10  and the transition line  20 . Moreover, for improving impedance matching there is provided a separate slab  12  from dielectric material fastened within the wave guide  10 . The slab  12  is, for example, attached within said wave guide  10  between machined shoulders  14 . 
   The prior art approach for achieving impedance matching is based on a complex structure which can only be realised in a difficult and expensive manufacturing process. Moreover, quite often so-called back-shorts are used i.e. a metal part is attached behind the micro strip  20  opposite the opening of the wave guide  10  in order to achieve impedance matching. Attaching the back-short further increases the complexity of the structure. 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention to improve the known device for guiding electromagnetic waves in a way that the manufacturing process is made easier and less expensive. 
   More specifically, the object is solved for the structure described above in the way that the coupling means comprises at least one dielectric layer being mechanically connected with the main plane of the transmission line, the geometric dimension of that at least one dielectric layer which extends along the propagation direction of the electromagnetic waves being correlated with the center frequency of the electromagnetic waves. 
   Because the mechanical fixation function and the electrical impedance matching function are integrated into one single component the manufacturing process of the layer structure is easy and inexpensive. 
   Impedance matching is achieved according to the present invention by varying the thickness of the at least one dielectric layer between the wave guide and the transmission line. The layer structure can, even if it comprises several layers, be considered as only one element used for achieving impedance matching. Thus, the adjustment process for achieving impedance matching is facilitated. 
   A preferred example is that the transmission line is an integral part of the coupling means. In that case the entire transition structure is co-fired in a multilayer ceramics manufacturing process. 
   A further preferred feature to enable optimised impedance matching is to provide metallised vias within a layer in order to build up a fence-like structure to further guide the waves after the have left the end of the wave guide. 
   Further preferably, there is at least one additional layer provided between the transmission line or the at least one layer and the wave guide, the additional layer comprising an air-filled cavity. The additional layer strengthenes the mechanical stability of the structure and the air-filled cavity ensures that the additional layer does not influence the transition characteristics of the structure. 
   It is advantageous that the cavity is aligned with an opening of the wave guide because in that case the influence of the additional layer to the transition characteristics of the structure is reduced to a minimum. 
   Furthermore, it is advantageous that the attachment of the wave guide to the layer adjacent to the wave guide is a solder ball connection because in that case self-aligning characteristics of the solder ball connections can be used. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail in the following accompanying figures, which are referring to preferred embodiments, wherein: 
       FIG. 1  discloses a first embodiment of a structure according to the present invention; 
       FIG. 2  is a diagram illustrating the transition characteristics of a wave guide to microstrip transition according to the present invention; 
       FIG. 3  is a diagram illustrating the relationship between the centre frequency and the dielectric thickness for optimal impedance matching in a structure according to the present invention; 
       FIG. 4  is a diagram illustrating the transition characteristics of a wave guide to micro strip transition or to a structure according to the present invention wherein the thickness of the layers in the structure is varied; 
       FIG. 5  shows a second embodiment of the structure according to the present invention; 
       FIG. 6  illustrates a manufacturing process for layers comprising vias; 
       FIG. 7  shows a third embodiment of a structure according to the present invention; 
       FIG. 8  is a top view of the structure shown in  FIG. 7 ; and 
       FIG. 9  shows a structure for guiding waves known from the prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a structure for guiding electromagnetic waves according to a first embodiment of the invention. The structure comprises a wave guide  10  and a transmission line  20 , the substrate layer  22  of which is arranged perpendicular to the longitudinal axis of the wave guide  10  for transition of electromagnetic waves from the wave guide  10  to the transmission line  20 . There are two layers  30 - 1  and  30 - 2  provided as coupling means, the layers  30 - 1 ,  30 - 2  being arranged between the substrate layer  22  of the transmission line  20  and the wave guide  10 , wherein the dielectric thickness of the layers  30 - 1 ,  30 - 2  is adjusted in a way described below. 
   Each of the layers  30 - 1 ,  30 - 2  comprises metallised through-holes  40 , called “vias”, forming a fence-like structure surrounding the area of each layer  30 - 1 ,  30 - 2 , respectively, through which the wave should be guided. Vias of different layers are interconnected with each other and with a metallised layer  24  at the bottom side of the substrate layer  22  of the transmission line  20 . 
   The influence of a variation of the thickness of the layers  30 - 1  and  30 - 2  on the transition characteristics of the structure according to  FIG. 1  will be illustrated in more detail by referring to  FIGS. 2 to 4 . 
     FIG. 2  illustrates the electrical characteristic of the structure according to  FIG. 1 .  FIG. 2  shows the frequency curves of the transmission coefficient (S 12 ), the reflection coefficient (S 11 ) measured from port  1  and the reflection coefficient (S 22 ) measured from port  2 , respectively. More specifically, it can be seen that at a centre frequency of 58 GHz and a thickness of the dielectric layer of 250 microns the characteristics are quite good. The curve S 11 , representing the return loss of the structure for different frequencies, shows that the return loss at the centre frequency of 58 GHz is smaller than 13.5 dB, while the insertion loss, represented by the curve S 12 , is 0.8 dB. 
   Moreover, the −1.5 dB bandwidth reaches from 55 . . . 64 GHz, meaning that the transition is not sensitive to tolerances or manufacturing process fluctuations. 
     FIG. 3  illustrates that the centre frequency of the pass-band of the structure according to  FIG. 1  has a linear dependency of the dielectric substrate thickness. That dependency, which is the result of a finite-element method simulation, means that just by selecting a suitable dielectric thickness one can easily adjust the centre frequency of the transition. 
     FIG. 4  illustrates the insertion losses for a wave guide to micro strip transition of a structure according to  FIG. 1  for different thicknesses of the dielectric layers. The insertion loss represented by the parameter S 12  is illustrated in  FIG. 4  for a dielectric thickness of 200 and 500 microns. The centre frequency of the −1.5 dB bandwidth lies in the case of a dielectric thickness of 200 microns at 63 GHz whereas for a layer thickness of 500 microns the centre frequency lies at 45 GHz. In both cases the bandwidth is approximately 7.5 GHz. 
   As illustrated above besides varying the thickness of the layers impedance matching can further be influenced and be improved by placing via-fences in the dielectric layer(s) and/or the substrate to define lateral dimensions of the continuation of the wave guide and thus, effect inter alia the insertion loss. 
     FIG. 5  shows a second embodiment of a structure according to the present invention in which three layers,  30 - 1 ,  30 - 2 ,  30 - 3 , between the substrate  22  of the transmission line  20  and the wave guide  10  comprises vias  40 . Quite often it is sufficient to optimise just only the dimensions of the layer  30 - 1  directly beneath the micro strip ground plane  24  and to keep elsewhere in the substrate the dimensions equal to the cross-sectional area of the metal wave guide  10 . In general it appears that the larger the dimensions of the wave guide continuation structure in the dielectric substrate of the layers  30 - 1 ,  30 - 2 ,  30 - 3  and the transmission line  20 , the smaller the insertion loss. 
   According to the present invention the preferred material for the dielectrical layers is low or high temperature co-fired ceramic LTCC or HTCC. 
   The process for manufacturing said layers comprising vias is illustrated in  FIG. 6 . In a first step S 1 , the substrate is generated by mixing solvents, ceramic powder and plastic binder (PMIX) and generating substrate tapes (CAST “GREEN” TAPE). After drying and stripping (method step S 2 ) and cutting out to size (method step S 3 ) vias are punched into said substrate (method step S 4 .) Normally the diameter of the vias is about 100 to 200 μm. After punching of the vias, the vias of each individual layer are filled by a conductor paste like silver, copper or tungsten, see method step printing into vias S 5 . After that, several layers are collected and are fired together as known from a normal manufacturing step of co-fired ceramic technology. These final method steps are illustrated in more detail in  FIG. 6  wherein after method step S 5  conducting pads with a given surface pattern are screened on the layer according to method step S 6 , several layers are laminated together in method step S 7  and after that, the layer assembly is fired according to method step S 8 . Finally braze pins are attached to the fired layer assembly of Electroless Plate (Ni, Au) according to method step S 9 . 
     FIG. 7  shows a third embodiment for a structure for guiding electromagnetic waves according to the present invention. It substantially corresponds to the structure shown in  FIG. 5  however, the implementation of the vias in the layers is shown in more detail and layers  30 - 4 ,  30 - 5 ,  30 - 6 , and  30 - 7  are additionally comprised within the structure. 
   Whereas in  FIG. 5  all layers  30 - 1 , . . .  30 - 3  have the same thickness, the thickness of layer  30 - 2  in  FIG. 7  has been varied in order to achieve good impedance matching. For example, for achieving good impedance matching at a particular frequency of 60 GHz it has been found that the appropriate thickness of layers  30 - 1  and  30 - 4  to  30 - 7  shall be 100 μm, whereas the thickness of layer  30 - 2  is proposed to be 150 μm. 
   The vias in the dielectric substrate layers do not only influence the impedance matching but also have an important roll in the mechanical design of the structure because they preferably connect the ground planes  24 ,  31 ,  32  of the transmission line  20  and of different layers  30 - 1 ,  30 - 2 . In that way the vias ensure mechanical stability of the structure. However, if there are only very few layers provided between the transmission line  20  and the coplanar wave guide  10  the resulting structure may still be mechanically fragile. To prevent this, additional layers  30 - 4 ,  30 - 5 ,  30 - 6 ,  30 - 7  may be added to the substrate. These additional layers preferably build up an air-filled cavity  50  aligned to the opening of the coplanar wave guide  10  in order not to change the desired electric characteristics of the structure by changing the dielectric thickness and consequently the resulting centre frequency. The structure can further be strengthened by using a metal base plate  37  having a slot  4  aligned with the opening of the coplanar wave guide  10 . 
   The ground plane  24  of the transmission line  20  as well as the ground planes  31 ,  32  and  37  of layers  30 - 1 ,  30 - 2  and  30 - 7  have slots slot  1 , slot  2 , slot  3 , slot  4  in order to ensure a proper transition of electromagnetic waves from the wave guide  10  to the transmission line  20 . These slots may be delimited by the via fences  41 ,  42  of the respective layers  30 - 1 ,  30 - 2 . However, the air-filled cavity  50  and the co-ordinated slot  4  in base plane  37  of layer  30 - 7  can be limited either by the dielectric substrate material itself or by the substrate material and vias  44 ,  45 ,  46 , and  47  placed on each side of the cavity  50 . While quite often the design rules prevent to place the vias close to the cavity  50  a better solution is to place the vias  50  half-wavelength away from the cavity edge; e.g. in  FIG. 7  the vias  44 ,  45 ,  46 , and  47  are placed at a distance of 860 μm away from the cavity edge. Half-wavelength distance of the vias from the wave guide opening or the cavity edge in that part of the structure which is close to the wave guide  10  is preferably selected because at that distance the reflection coefficient ρ is ρ=−1, which means that such an arrangement gives almost equal performance to the case that the cavity walls have been totally metallised (half-wavelength demand comes from the fact that standing waves have a half-wavelength periodicy meaning that in effect the cavity walls seem to be in zero potential). The proposed half-wavelength arrangement also prevents any electromagnetic leakage into/from the structure. 
   The vias obviously improve the transition of electromagnetic waves from a wave guide  10  to a transition line  20  but they are not mandatory in every layer. 
     FIG. 8  shows a top view of the structure according to  FIG. 7  wherein arrow  60  indicates the view direction of  FIG. 7 . Reference numeral  20  indicates the transmission line, in particular a micro strip structure having a width g of g=110 μm. The transmission line  20  has a dielectric thickness of 100μ (see  FIG. 7 ) and extends a distance c=130 μm over slot  1  in the micro strip ground plane  24  (see  FIG. 7 ). The area covered by slot  1  in the ground plane  24  measures in the example according to  FIG. 8  e×d wherein e=1840μ and d=920 μm. 
   Slots  2  and  3  of  FIG. 7  are represented by the thick dashed line in  FIG. 8  covering an area of h×a wherein h=1200μ and a=3760 μm. The thick dashed line also represents the via fences  41  and  42  since these via fences should be placed as close as possible to the edge of the respective ground planes  31  and  32  (see  FIG. 7 ). 
     FIG. 8  further shows a top view on vias  44  of layer  30 - 4  (see  FIG. 7 ). It is apparent that these via fences  44  and the via fences  45 ,  46 ,  47  of the beneath layers  30 - 5 ,  30 - 6  and  30 - 7  are positioned at a distance f, wherein f=860 μm from the edge of slot  3  which substantially corresponds to the edge of air cavity  15 ; the reasons for placing vias  44 – 47  at a distance to the edge of the air cavity  50  have been explained above. 
   Slot  4  represents the cross-sectional area a×b of the air cavity in layers  30 - 4 ,  30 - 5 ,  30 - 6 , and  30 - 7  according to  FIG. 7 . In the example of  FIG. 8 , a=3760μ and b=1880μ, wherein that area corresponds to the cross-sectional area of the opening of wave guide  10  and is aligned thereto. 
   The wave guide  10  can be attached to the adjacent layer  30 - 7  by using different mechanical approaches: e.g. by soldering or even using solder balls, e.g. BGA (Ball Grid Array) type of solder attachment. Using a solder ball connection has the advantage that self-aligning effects of the technology can be used. On the other hand when using solder ball connections there may be small air gaps between the connection between the wave guide  10  and the adjacent layer, however these very small air gaps do not substantially influence the electrical characteristics of the structure; thus, no direct contact between the wave guide  10  and the ceramic material of the layer is required. 
   Although the invention has been described for the usage of multilayer ceramics the substrate material of the transmission line  20  and of the layers  30 -i, where i=1, 2, 3, 4, 5, 6, or 7, may also be laminate material. The transmission line may be a micro strip, a stripline or a coplanar wave guide.