Abstract:
A method which relates to fabricating a dielectric waveguide (WG) on a PCB for RF communication between ICs on the PCB. The WG can replace a baseband copper bus and resulting in the PCB being smaller and/or cheaper. The WG may be printed, stamped, cut or prefabricated onto the PCB.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 from Singapore Patent Application Number 201106265-0, filed on Aug. 26, 2011. The entire contents of the above application is incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to chip-to-chip RF communications on a PCB and an on-PCB dielectric waveguide. 
       BACKGROUND 
       [0003]    Copper tracks are typically used for chip-to-chip communications on a printed circuit board (PCB). However, the copper tracks have limited bandwidth for data transmission. Moreover, the energy expended is increased when the data transmission rate increases. Copper tracks may also be employed in a parallel configuration between the chips. This may increase the data transmission rate and avoid channel loss difference at low frequency and high frequency, but the power consumption may be even higher. 
         [0004]    Parallel copper tracks also result in a large footprint, requiring the use of a large circuit board. Thus, it may be difficult to have a compact and sleek casing using parallel copper tracks. 
         [0005]    Alternatively, parallel-to-serial conversion can also be carried out using a pair of copper tracks. However, this alternative still suffers from high power consumption for high data transmission rate applications. 
       SUMMARY 
       [0006]    In general terms the invention relates to fabricating a dielectric waveguide (WG) on a PCB for RF communication between integrated circuits (ICs) on the PCB. This may have the advantage that the WG can replace a baseband copper bus and thus the PCB can be smaller and/or cheaper. The WG may be printed, stamped, cut or prefabricated onto the PCB. 
         [0007]    In a specific expression of the invention there is provided a method for providing chip-to-chip RF communications on a PCB, the method including providing a dielectric waveguide made from a dielectric material, and connecting a coupler at each end of the dielectric waveguide for coupling the dielectric waveguide to at least two chips. 
     
    
     
       DESCRIPTION OF FIGURES 
         [0008]    In order to ensure that the embodiments of the invention may be fully understood and readily put into practical effect, there is provided, by way of non-limitative example-only embodiments, the following illustrative figures which are referenced by the foregoing description. 
           [0009]      FIG. 1  is a schematic diagram of a system for chip-to-chip RF communications of an embodiment; 
           [0010]      FIGS. 2(   a ) to ( e ) are examples of cross-sectional shapes of a dielectric waveguide of an embodiment of the present invention; 
           [0011]      FIG. 3  is a plan view image of the coupler shown in  FIG. 1 ; 
           [0012]      FIG. 4  is a schematic side view of the coupler of  FIG. 3 ; 
           [0013]      FIG. 5  is a process flow chart for a first method of forming a dielectric waveguide; 
           [0014]      FIG. 6  is a process flow chart for a second method of forming a dielectric waveguide; 
           [0015]      FIG. 7  is a process flow chart for a third method of forming a dielectric waveguide; 
           [0016]      FIG. 8  is a schematic view of a PCB including a dielectric waveguide; 
           [0017]      FIG. 9  is a graph of simulated propagation losses for the PCB of  FIG. 8 ; 
           [0018]      FIG. 10  is photograph of a PCB with a hand painted dielectric waveguide; 
           [0019]      FIG. 11  is a plot of actual propagation losses for the PCB of  FIG. 10 ; 
           [0020]      FIG. 12  is an image of a PCB using copper tracks; 
           [0021]      FIG. 13  is an image of a PCB using the system of an embodiment of the present invention; 
           [0022]      FIGS. 14(   a ) to ( d ) is a diagram of examples of forming the dielectric waveguide; 
           [0023]      FIG. 15  is a graph showing propagation losses of an on-PCB dielectric waveguide and a microstrip line (MSL); 
           [0024]      FIG. 16  is a schematic view of a PCB without any dielectric waveguide; 
           [0025]      FIG. 17  is a graph of simulated propagation losses for the PCB of  FIG. 16 ; 
           [0026]      FIG. 18  is a plan view image of the coupler shown in  FIG. 1  coupled with a dielectric waveguide; and 
           [0027]      FIG. 19  is a side view image of the coupler shown in  FIG. 1  coupled with a dielectric waveguide. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    There is provided a system which facilitates chip-to-chip RF communications, whereby the system is implementable on PCBs with existing copper tracks. The system enables chip-to-chip RF communications on PCBs in place of copper track connections between the chips. There is also provided methods of incorporating a dielectric waveguide of the system on PCBs. 
         [0000]    The system  20  is shown in  FIG. 1  with a first signal source  28  being connected to a second signal source  30  via a dielectric waveguide  22  with couplers  24 ,  26  at respective ends  32 ,  34  of the dielectric waveguide  22 . The sources  20 ,  30  may be integrated circuits or “chips”. 
         [0029]    The on-PCB dielectric waveguide has a higher data bandwidth compared to transmission via copper tracks. The dielectric waveguide is typically a high pass channel with low channel attenuation.  FIG. 15  is a graph showing propagation losses of an on-PCB dielectric waveguide and a microstrip line (MSL). It should be noted that the propagation losses of the dielectric waveguide is low for a wide range of frequencies compared to the increasing losses by the MSL as the frequencies increase. Although the MSL has high loss at high frequency, the loss is minimized at high frequency when the length of the MSL is small. Thus, it is possible to combine a short MSL and a dielectric waveguide and still have low propagation losses at a broad range of frequencies. 
         [0030]    Referring to  FIG. 1 , there is provided the system  20  for chip-to-chip RF communications. It is appreciated that the system  20  may be incorporated on a PCB, whereby the PCB surface may be either a dielectric or a metallic layer. As such, the system  20  can be provided over either metal tracks on the PCB or a dielectric substrate. The system  20  may replace a conventional copper bus for chip-to-chip communications. 
         [0031]    The system  20  includes a dielectric waveguide  22  made from a dielectric material. The dielectric material may be selected from, for example, Polytetrafluoroethylene (PTFE) or a composite material of PTFE and ceramic. Referring to  FIG. 2 , there are shown some examples of cross-sectional shapes of the dielectric waveguide  22 . The dielectric waveguide  22  may have cross-sectional shapes like, for example, quadrilateral ( FIG. 2(   a )), circular ( FIG. 2(   b )), semi-circular ( FIG. 2(   c )), elliptical ( FIG. 2(   d )), and polygonal ( FIG. 2(   e )). It should be appreciated that the cross-sectional shapes may be determined by a process used to form the dielectric waveguide  22 . In addition, the cross-sectional shape should allow the dielectric waveguide  22  to adhere to the PCB surface. 
         [0032]    The system  20  also includes a coupler  24 ,  26  at each end  32 ,  34  of the dielectric waveguide  22 . Each coupler  24 ,  26  couples the dielectric waveguide  22  to a signal source  28 ,  30 . The signal source  28 ,  30  may be a semiconductor chip. An intrinsic impedance of the dielectric material is matched to the output impedance of the coupler  24 ,  26 . The impedances of the coupler  24 ,  26  and the dielectric material may be, for example, 50 ohms. The impedances of the coupler  24 ,  26  and the dielectric material should be matched. The coupler  24 ,  26  and the dielectric material of the dielectric waveguide  22  have substantially similar high pass frequency responses. The dielectric waveguide  22  has high pass characteristics with a cut-off frequency being dependent on a cross-sectional area of the dielectric waveguide  22 . Referring to  FIGS. 3 and 4 , each coupler  24 ,  26  includes two metal layers  60 ,  62  and a PCB substrate  64  located between the two metal layers  24 ,  26 . It should be appreciated that the dimensions of the coupler  24 ,  26 , denoted in  FIG. 3 , are merely illustrative and should not be taken to be restrictive. The coupler  24 ,  26  may be either a discrete module on the PCB or a part of an IC chip. Thus, the coupler  24 ,  26  can be added after fabrication of a PCB. 
         [0033]    A first metal layer  60  at a first face  61  of the PCB substrate  64  of the coupler  24 ,  26  may be in a form of a polygonal shape (an asymmetrical pentagon is shown) when viewed in a plan view as shown in  FIG. 3(   b ). The first metal layer  60  includes a MSL which is coupled to a contact of the signal source  28 ,  30  and transitions to a planar horn antenna  68 . The planar horn antenna  68  is also high pass. A spanning angle of the two metal paths of the planar horn antenna  68  should be controlled to obtain an identical cut-off frequency as the dielectric waveguide  22 , which is desirable when matching the planar horn antenna  68  to the dielectric waveguide  22 . A distal edge  72  of the first metal layer  60  away from the MSL  66  may denote a planar horn-like transmission region of the coupler  24 ,  26 . 
         [0034]    A second metal layer  62  (as shown in  FIG. 3(   c )) at a second face  63  of the PCB substrate  64  acts as a ground plate for the coupler  24 ,  26  and does not overlap with the first metal layer  60 . The metal used for the first metal layer  60  and the second metal layer  62  may include, for example, copper. The dielectric waveguide  22  is coupled to the coupler  24 ,  26  in a manner as shown in  FIGS. 18 and 19 , whereby the dielectric waveguide  22  includes an overlapping portion  19  for placement on the coupler  24 ,  26 . 
         [0035]    Referring to  FIG. 8 , there is shown a schematic view of the PCB  64  with the dielectric waveguide  22 , with the couplers  24 ,  26 . It should be appreciated that port  1  and port  2  in  FIG. 8  are from signal source  1  ( 28 ) and signal source  2  ( 30 ), respectively.  FIG. 9  shows a simulated plot of propagation losses for the PCB  64 . The line “P 21 ” shows a higher level of RF signal reception at port  2  from port  1  compared to the line “P 31 ” which shows a lower level of RF signal reception at port  3  from port  1  (without the dielectric waveguide  22 ). As earlier simulation results, shown in  FIG. 16  based on a setup shown in  FIG. 15 , have shown that propagation losses at port  2  and port  3  are similar in the absence of the dielectric waveguide  22  on the PCB  64 , it is evident that the dielectric waveguide  22  minimizes propagation losses. 
         [0036]    Referring to  FIG. 10 , there is shown a photograph of a plan view of a PCB  65  with a hand painted dielectric waveguide  23 , with the couplers  25 ,  27 .  FIG. 11  shows a plot of actual propagation losses for the PCB  65 . The line “Port 5 ” shows a higher level of RF signal reception at port  5  from port  4  compared to the line “Port 6 ” which shows a lower level of RF signal reception at port  6  from port  4  (without the dielectric waveguide  23 ). The mode of propagation in the dielectric waveguide  23  depends on a size of the dielectric waveguide  23  and a type of the couplers  25 ,  27 . For example, a planar horn coupler results in TE mode propagation in the WG. In addition to minimizing propagation losses, it should be appreciated that using the system  20  may minimize electromagnetic interference and reduce power consumption compared to the use of copper tracks for chip-to-chip communications. 
         [0037]    Referring to  FIGS. 5 to 7 , there are shown a plurality of methods for forming a dielectric waveguide  22  on a PCB.  FIG. 5  shows a “printing” method  70  for forming the dielectric waveguide  22 . The “printing” method  70  includes laying a dielectric waveguide  22  of melted dielectric material on the PCB ( 72 ), and solidifying the channel  22  of dielectric material ( 74 ). The dielectric material may be selected from, for example, PTFE, a composite material of PTFE and ceramic and so forth. It should be appreciated that the “printing” method  70  is low cost and versatile as a path of the dielectric waveguide  22  may be easily varied to connect various signal sources together. Furthermore, the dielectric  7  waveguide  22  also is able to be formed on existing copper tracks on any PCB. The “printing” method  70  is denoted graphically in  FIG. 14(   a ). 
         [0038]      FIG. 6  shows a process of an “injection stamping” method  80  for forming the dielectric waveguide  22 . The “injection stamping” method  80  includes injecting melted dielectric material into an injection mold, the injection mold being for forming the dielectric waveguide  22  ( 82 ), and subsequently stamping the dielectric material to the PCB ( 84 ) with sufficient pressure to ensure a desired cross-sectional shape and an appropriate density. Furthermore, the channel  22  also is able to be formed on existing copper tracks on any PCB. The “injection stamping” method  80  is denoted graphically in  FIG. 14(   b ). 
         [0039]      FIG. 7  shows a process of a “cutting” method  90  for forming the dielectric waveguide  22 . The “cutting” method  90  includes adhering a layer of dielectric material to the PCB ( 92 ), cutting the dielectric waveguide  22  from the layer of dielectric material ( 94 ), and removing excess portions of the layer of dielectric material ( 96 ). Furthermore, the dielectric waveguide  22  also is able to be formed on existing copper tracks on any PCB. The “cutting” method  90  is denoted graphically in  FIG. 14(   c ). 
         [0040]    It may also be possible to form the dielectric waveguide  22  on the PCB by either adhering or mounting the dielectric waveguide  22  on the PCB, whereby the dielectric waveguide  22  is pre-fabricated. The pre-fabricated dielectric waveguide  22  may be formed using, for example, injection molding, vacuum forming, and compression molding. This method of either adhering or mounting the dielectric waveguide  22  is denoted graphically in  FIG. 14(   d ). 
         [0041]    It should be noted that when the system  20  is used, less copper is correspondingly used. A single dielectric waveguide is able to replace a plurality of copper tracks. Thus, even when the use of copper for the couplers is taken into consideration, the use of dielectric waveguides is more economical than the use of the plurality of copper tracks. 
         [0042]    As illustrated in  FIGS. 11 and 12 , which have identical measurement scales,  FIG. 11  shows a PCB board using a plurality of copper tracks for chip-to-chip communications while  FIG. 12  shows a PCB board with the same functions as that shown in  FIG. 11  using the system  20 . The more compact dimensions of the PCB in  FIG. 12  as compared to the PCB in  FIG. 11  is evident. As such, it is evident that the use of the system  20  results in a smaller footprint on the PCB. It should be appreciated that IC chip and waveguide dimensions also affect a size of the PCB. It should also be noted that the methods for forming the dielectric waveguide  22  enables flexibility in a configuration of a PCB, as the dielectric waveguide  22  can be either removed or reconfigured, and the dielectric waveguide  22  may be formed over existing copper tracks. The aforementioned methods also cost less compared to incorporating a plurality of copper tracks on a PCB. 
         [0043]    Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations in details of design, construction and/or operation may be made without departing from the present invention.