Abstract:
A waveguide comprising a SF_WG portion between a first transmission line and a second transmission line, wherein the SF_WG portion has a width greater than or equal to 75 um.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a waveguide particularly though not solely to an SF_WG for MMW signals. 
       BACKGROUND 
       [0002]    The following abbreviations will be used in this specification: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 SF_WG 
                 Sommerfeld waveguide 
               
               
                   
                 MMW 
                 MilliMetre Wave 
               
               
                   
                 CPW 
                 Coplanar Waveguide 
               
               
                   
                 MSL 
                 Microstrip Line 
               
               
                   
                 PCB 
                 Printed Circuit Board 
               
               
                   
                 IC 
                 Integrated Circuit 
               
               
                   
                 EM 
                 ElectroMagnetic 
               
               
                   
                 TEM 
                 Transverse Electromagnetic Mode 
               
               
                   
                 TM01 
                 Transverse Magnetic Mode 01 
               
               
                   
                 GSG 
                 Ground Signal Ground 
               
               
                   
                 G-line 
                 Goubau-line 
               
               
                   
                   
               
             
          
         
       
     
         [0003]    Communications signals may be carried over air or some other solid medium such as a wire. In case of high frequency signals, special structures such as waveguides are sometimes used to minimise radiation leakage and interference among adjacent channels. However, for certain high frequency signals such as MMW signals, using TEM based transmission lines or integrated waveguides may result in a high propagation loss. 
         [0004]    Another transmission medium that can be used for MMW signals is a single metal wire SF_WG (or G-line) since this may have a lower propagation loss. However because of the special mode that a SF_WG operates in, the method of excitation is important. Depending on the application, the excitation can be from an antenna or a transmission line converter. An antenna may have a low converting efficiency because of the open EM-field. A more common prior art approach is using Sommerfeld wave excitations from a CPW. 
         [0005]      FIG. 1(   a ) shows an A-type converter  100 , where the wire width is 1 um (in  FIG. 1(   a ), the wire is too thin to be seen) and  FIG. 1(   b ) shows a B-type converter  104 , where the wire width is 5 um. The very thin wires may be required to achieve an acceptable impedance matching for a wide bandwidth. Wire width of 1 um may be practical for IC fabrication but it may be too thin for PCB fabrication. 
       SUMMARY OF THE INVENTION 
       [0006]    In general terms in a first aspect the invention proposes a SF_WG for inter-board or inter-chip connections, where the width of the SF_WG is greater than or equal to 75 um. 
         [0007]    In a second aspect the invention proposes a SF_WG with a length substantially similar to an integer multiple of half the wavelength at the central signal frequency. 
         [0008]    One or more embodiments may have the advantage of: 
         [0009]    simple, practical structure dimensions for fabrication; 
         [0010]    very wide bandwidth; 
         [0011]    low loss as compared with integrated waveguide and many other transmission lines; 
         [0012]    transmission from vertical and horizontal bending may be minimised; and/or suitable for multiple parallel channels. 
         [0013]    According a first particular expression of the invention, there is provided a waveguide according to claim  1 . 
         [0014]    According to a second particular expression of the invention, there is provided a waveguide according to claim  15 . One or more embodiments may be implemented according to claims  2  to  14  or claims  16  to  36 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    One or more example embodiments of the invention will now be described, with reference to the following figures, in which: 
           [0016]      FIG. 1(   a ) is a schematic of a first prior art CPW to SF_WG transition, 
           [0017]      FIG. 1(   b ) is a schematic of a second prior art CPW to SF_WG transition, 
           [0018]      FIG. 2  is a schematic of a MSL to SF_WG transition according to a first example embodiment, 
           [0019]      FIG. 3  is a schematic of a SF_WG on a PCB according to a second example embodiment, 
           [0020]      FIG. 4  is a schematic of a SF_WG for IC die interconnection according to a third example embodiment, 
           [0021]      FIG. 5  is a schematic of a MSL to SF_WG transition according to a fourth example embodiment, 
           [0022]      FIG. 6  is a schematic of a CPW to SF_WG transition according to a fifth example embodiment, 
           [0023]      FIG. 7  is a schematic of a CPW to SF_WG transition according to a sixth example embodiment, 
           [0024]      FIG. 8  is a schematic of a SF_WG vertical bending protection structure according to a seventh example embodiment, 
           [0025]      FIG. 9  is a schematic of a SF_WG horizontal bending protection structure according to an eighth example embodiment, 
           [0026]      FIG. 10  is a schematic of a 2-channel SF_WG according to a ninth example embodiment, and 
           [0027]      FIG. 11  is a graph of the test results obtained using a SF_WG according to the second example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    A number of example embodiments will now be described for die-to-die interconnection using a SF-WG. One or more example embodiments may avoid the very thin wire required in the prior art, which may allow both IC and PCB fabrication. 
         [0029]      FIG. 2  shows a MSL to SF_WG transition  200  according to the first example embodiment. A MSL  202  is attached to the top major surface of a dielectric substrate  204  connected to a first IC (not shown). A ground plane  206  is attached on the bottom major surface of the substrate  204 . The MSL  202  transitions into the SF_WG  208  by virtue of a notch  210  in the end  212  of the ground plane  206 . The shape of the notch  210  can be linear or nonlinear (e.g. exponential), for example a triangular notch. 
         [0030]    The MSL  202  width may be constant through to the SF_WG  208 . The MSL  202  width may be determined by the dielectric substrate thickness, dielectric constant and desired characteristic impedance. For example, if the dielectric material thickness is 130 um, material dielectric constant is 10 and desired characteristic impedance is 50 ohm, then the trace width (i.e. MSL  202  and SF_WG  208  width) may be 100 um. By the use of the notch  210  the MSL mode can be converted to Sommerfeld (TM01) mode with the loss minimised. Also the width of the SF_WG  208  may stay constant and may not need to be very thin. For example the width of the SF_WG may be greater than or equal to 75 um which may allow for easy PCB fabrication. 
         [0031]    The MSL to SF_WG transition  200  according to the first example embodiment from  FIG. 2  may be implemented on a PCB  300  as shown in  FIG. 3  or on a IC die  400  as shown in  FIG. 4 . 
         [0032]    The second example embodiment shown in  FIG. 3  has a SF_WG  302  attached on a PCB  300  between a first MSL  304  and a second MSL  306 . A first transition  308  is provided between the first MSL  304  and the SF_WG  302 , and a second transition  310  is provided between the second MSL  306  and the SF_WG  302 . A ground plane  312 , 314  is attached on the bottom of the PCB directly underneath the respective MSL  304 , 306 . 
         [0033]    The third example embodiment shown in  FIG. 4  has a bond wire SF_WG  402  attached between a first IC die  400  and a second IC die  404 . A first transition  406  is provided between a first MSL  410  on the first IC die  400  and the SF_WG  402 , and a second transition  408  is provided between a second MSL  412  on the second IC die  404  and the SF_WG  402 . Each of the transitions  406 ,  408  extends from its respective MSL  410 ,  412  to the bond wire SF_WG  402 . Each MSL  410 ,  412  forms a trace on one side of its respective dielectric substrate and a ground plane is formed on the other side of each dielectric substrate. The ground plane in each transition  406 ,  408  may split or open under the trace formed by the MSL  410 ,  412  either linearly or non-linearly. 
         [0034]    The disclosed transition according to the first example embodiment in  FIG. 2  is more suitable for the PCB substrate or wire over air application, although it can be used on a IC die. This is because this transition does not require a very thin trace for impedance matching as that in  FIG. 1 . However, for IC die, the transition structure is usually required to be small for reducing cost. Moreover, since the loss tangent of the IC substrate, for example, silicon is usually high (in one example, 0.9) whereas the PCB material has a relatively lower loss tangent (in one example, 0.05), the transition loss for the application of the disclosed transition according to the first example embodiment in  FIG. 2  on a IC die becomes larger than that on a PCB. 
         [0035]    The fourth example embodiment shown in  FIG. 5  has a bond wire SF_WG  500  attached between a first MSL  502  on a first IC die  504  and a second MSL  506  on a second IC die  508 . Unlike the third example embodiment shown in  FIG. 4  in which there is no requirement on the length of the bond wire SF_WG  402 , the length of the bond wire SF_WG  500  in the fourth example embodiment in  FIG. 5  is required to be an integer multiple of a half wavelength at the central signal frequency. Having the length of the bond wire SF_WG  500  as an integer multiple of a half wavelength at the central signal frequency ensures the conversion of the wave to Sommerfeld wave and provides good impedance matching. Furthermore, the width of each MSL  502 ,  506  is preferably the same as the width of the bond wire SF_WG  500 . However, there is no requirement on the shape of the bond wire SF_WG  500 . Similar to the third example embodiment in  FIG. 4 , there is also a ground plane associated with each MSL  502 ,  506 . 
         [0036]    The fifth example embodiment shown in  FIG. 6  has a single wire SF_WG  600  with a length that is an integer multiple of a half wavelength at the central signal frequency. The single wire SF_WG  600  is connected between two CPW (GSG)  602 ,  604 . There are two pairs of quarter wavelength wires  606 , 608 . Each pair of wires  606 , 608  is bonded at one end to a ground pad on one of the CPW (GSG)  602 ,  604  and acts as a balun. The other end of each pair of wires  606 ,  608  is attached to an interposer  616  on which the IC dies  618 , 620  are attached. Each pair of balun wires  606 , 608  are spread at an angle of about 45 degrees. 
         [0037]    The sixth example embodiment shown in  FIG. 7  is the same as the fifth example embodiment (i.e. it also comprises a single wire SF_WG  726  connected between two CPW (GSG)  722 , 724 ) except that a limited ground plane  700 , 702  is provided directly under each IC die  718 , 720  on the interposer  716 . Instead of being attached to the interposer  716 , the other end of each pair of balun wires  712 ,  714  is attached to the respective ground plane  700 , 702 . With the ground planes  700 , 702 , the sixth example embodiment in  FIG. 7  may achieve a more stable performance. 
         [0038]    One or more embodiments may be encapsulated in a dielectric material such as mould resin. In that case changes to the dimensions of the embodiments will be required according to the dielectric constant of the dielectric material. 
         [0039]    Bending of a SF_WG may result in radiation and propagation loss. Although the SF_WG  402  and  500  in the third and fourth example embodiments respectively are bent, the distance between the IC dies may be short and hence bending loss may not be as important as coupling impedance matching and mode transition. However, this may not be the case for the second example embodiment in  FIG. 3  and it may be preferable to reduce the radiation and propagation loss due to the bending of the SF_WG  302  in this embodiment. Bending of the SF_WG  302  in the second example embodiment in  FIG. 3  can be separated into 1) vertical bending (orthogonal to the substrate plane) and 2) horizontal bending (on the substrate plane). 
         [0040]    For type 1) bending, the radiation propagation loss may be reduced by the seventh example embodiment in  FIG. 8 . The SF_WG  800  is sandwiched by two dielectric layers  802 ,  804  with different dielectric constants. Dielectric layers  802 ,  804  may be made of any dielectric materials with low losses. The dielectric layers  802 ,  804  may have dielectric constants which differ only slightly from each other. 
         [0041]    For type 2) bending, the eighth example embodiment shown in  FIG. 9  may be used to reduce the radiation propagation loss. A metal patch  900  is provided under the SF_WG  902  and dielectric substrate  904 . The metal patch  900  may comprise two ends and a notch at each end. In one example, the metal patch  900  may comprise three sections  906 ,  908  and  910  with the sections  906 ,  908  respectively joined to the sections  908 ,  910  at an angle as shown in  FIG. 9 . The sections  906 ,  908 ,  910  may be arranged in a z shape and the angle between the sections  906 ,  908 ,  910  may take on any value. The notch at either end of the metal patch  900  may be shaped linearly or nonlinearly (e.g. exponentially), such as triangular shaped. This converts the SF_WG  902  to a MSL and because a MSL is not sensitive to bending, the eighth example embodiment as shown in  FIG. 9  may reduce losses caused by type 2) bending and in turn, may improve the performance of the SF_WG. 
         [0042]    The ninth example embodiment is shown in  FIG. 10  with a 2-channel SF_WG with each channel similar in structure to the second example embodiment. The channels may be separate structures attached together or may be integrated side by side. The bending of the 2-channel SF_WG in the ninth example embodiment in  FIG. 10  is merely an example and the multi-channel SF_WG may also be straight or bent in a different manner. 
         [0043]    The ninth example embodiment may also be protected from vertical and horizontal bending by using the seventh and eighth example embodiments, respectively. Also the third, fourth, fifth or sixth example embodiments may also be employed with multiple channels. 
         [0044]      FIG. 11  shows the test results for a 600 mm length SF_WG using the second example embodiment from  FIG. 3 . In  FIG. 11 , the S-parameters of the SF_WG are plotted. In general, S-parameters describe the response of an N-port network (in this case N=2) to voltage signals at each port. The first number in the subscript of each S-parameter represents the responding port, whereas the second number in the subscript represents the incident port. As shown in  FIG. 11 , the S 11  and S 22  show a wide bandwidth and the S 12 &amp;S 21  shows the loss is low. 
         [0045]    While example embodiments of the invention have been described in detail, many variations are possible within the scope of the invention as will be clear to a skilled reader.