Abstract:
The present invention includes method and apparatus for a device including two capacitors separated by a gap within one package thereby the two capacitors are coupled to each other in such a way that the impedance between them are matched with respect to the other components along a transmission path.

Description:
FIELD OF THE INVENTION 
     The invention is related to the field of electrical high-speed interconnects and, in particular, to passive microwave devices. 
     BACKGROUND OF THE INVENTION 
     In conventional high-speed point-to-point differential links of 2.5 GBits/s, AC couplers connect devices with different termination voltages. Tiny capacitors function as AC couplers and are added to the connections when required. Conventional technology includes devices that have a termination voltage of 1.8V. However, newer devices have differential voltages such as 1.5 V or 1.2 V. In the future, termination voltage is expected to be even lower, e.g., 1.0V or 0.8V. At 2.5 GBits/s, it is still technically feasible to use tiny capacitors directly on the Printed Circuit Board (PCB). At increased speeds of 3.125 GBit/s, 10 GBit/s or more, signal conditions get progressively worse because discrete coupling capacitors between differential connections cause increased reflections at higher speeds. At 10 Gbit/s or higher, discrete coupling capacitors will be practically unusable. 
     SUMMARY 
     Various deficiencies of the prior art are addressed by the present invention of a package containing AC coupling devices for high-speed differential interconnects. The present invention includes method and apparatus for a device including two capacitors within one package thereby the capacitors are coupled to each other in such a way that the line impedance of the differential links is matched. 
     In accordance with one embodiment of the invention, a passive microwave device in connection with a differential communication link includes a package having two capacitors. The two capacitors are coupled in such a way that the capacitors are part of a differential transmission line with a differential impedance equal to the impedance of the other components of the differential communication link. 
     In accordance with another embodiment, the present invention includes a printed circuit board (PCB) having a high-speed differential AC coupling device. This embodiment also includes a transmitting device that sends a differential signal, a backplane having traces for transmitting the signal and a receiving device that receives the signal. The AC coupling device has a differential impedance substantially the same as the traces. 
     In accordance with a further embodiment, the present invention includes connecting a high-speed AC coupler between a transmitting device and a receiving device for transmission of differential signals along a differential transmission link. An impedance of the AC coupler is matched such that the impedance is substantially the same as the impedance of the rest of the differential transmission link. A high-speed differential signal is transmitted from the transmitting device to the receiving device. 
     The invention further provides other methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below. The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a high-level block diagram of a printed circuit board (PCB) having an AC coupled internal interface suitable for use with the present invention; 
         FIGS. 2(   a ),  2 ( b ) and  2 ( c ) depict the side, bottom and front views, respectively, of a high-level block diagram of a package having the coupling capacitors of the present invention; and 
         FIGS. 3(   a ),  3 ( b ) and  3 ( c ) depict the side, top and front views, respectively, of a high-level block diagram of a package of the present invention having high coupling capacitance. 
     
    
    
     However, the appended drawings illustrate only exemplary embodiments of this invention and are, therefore, not to be considered limiting of its scope, for the invention admits to other equally effective embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally described within the context of passive microwave devices. It will be appreciated by those skilled in the art that the invention may be utilized within the context of any electronic interconnect that utilizes AC coupling. 
       FIG. 1  depicts a high-level block diagram of a printed circuit board (PCB) having an AC coupled internal interface suitable for use with the present invention. The PCB  100  has a circuit pack # 1   110 , two backplane connectors  120  and  140 , a backplane  130 , a circuit pack # 2   150  and a package  160  that has two coupling capacitors. 
     The circuit pack # 1   110  includes a transmitting device  112  for transmitting a differential signal. The differential signal is an electrical signal having separate positive (P) and negative (N) portions. The differential signal is transmitted over a set of differential transmission lines  114 . The P portion follows one trace and the N portion follows a parallel trace. The differential signal follows the traces and passes through a set of solder pads  116  and vias  118 . The vias  118  are plated throughholes that connect top and bottom traces to traces in the inner layer. The vias  118  connect the transmission lines of the transmitting device to the transmission lines of the circuit pack # 1   110 . The differential signal continues on the circuit pack # 1   110  through the differential pair transmission lines  117  to vias  122  and to the backplane connector  120 . 
     The backplane connector  120  connects the differential signal from the vias  122  to the vias  126  through transmission lines  124 . The differential signal through the backplane connector continues on the transmission lines of the backplane  130 . 
     The backplane  130  includes differential transmission lines  134 , which receives the differential signal from the transmitting device  112  through vias  126  and connects to the backplane connector  140  through vias  142 . 
     The backplane connector  140  connects the differential signal from the vias  142  to the vias  146  through transmission lines  144 . The differential signal traveling through the backplane connector continues on to the set of differential transmission lines  154  of the circuit pack # 2   150 . 
     The circuit pack # 2  includes the set of differential transmission lines  147 , the receiving device  152  and a package of two capacitors  160 . The differential transmission lines  147  receive the differential signals through vias  152 , which enters the package  160  through solder pads  164 . The package has a capacitor Cp and a capacitor Cn. The positive portion of the differential signal passes through the capacitor Cp and the negative portion of the differential signal passes through the capacitor Cn. The capacitors function as an AC coupling interface. The differential signal then returns to the differential transmission lines  157  through solder pads  166  and vias  154 . Upon reaching the receiving device  152 , the differential signal passes through the vias  158  and the solder pads  156  to the receiving device  152 . 
     In one embodiment, 100-ohm impedance is designed along the differential transmission links between the P path and N path from the transmitting device to the receiving devices. The backplane connectors, solder pads, and vias increase the undesirable effects added to the signals, but the impact on the system is minimized by utilizing techniques well known in the art. 
     In one embodiment, the package includes two ceramic capacitors having multiple layers as described below. The capacitors are separated by a gap of a distance approximately the same as the distance of the differential transmission link. Thus, the impedance between the two ceramic capacitors is approximately the same as the impedance between the two paths of the differential transmission links. The package including the two capacitors is designed with the appropriate gap between the Cp and Cn such that the impedance is kept the same all along the paths of the signals. Thus, the capacitance can be set without the need to be concerned with the size of the capacitors. Therefore, distortions in the impedance profile even for high bit rates are acceptable. 
       FIGS. 2(   a ), ( b ) and ( c ) depict the side, bottom and front views, respectively, of a high-level block diagram of a package having the coupling capacitors of the present invention. In one embodiment, the package of  FIG. 2  is a surface mount device such as a mini-BGA. In another embodiment, the package is 8-pin QFN. In a further embodiment, the package is the standard 0603 with split pads. Other packaging techniques are possible. 
     The differential signal enters the package through solder pads A, B and exits the package at solder pads C, D. The transmission path between A and C functions as the capacitor for the positive portion of the signal and the transmission path between B and D functions as the capacitor for the negative portion of the signal. The material between the solder pads are multi-layered high-frequency ceramic having thin metallic surfaces. The details of the design of the package will be described below. 
       FIG. 2(   a ) depicts the side view of a high-level block diagram of the package having the coupling capacitors of the present invention. A first set of solder pads  210  receives a differential signal from the transmitting device. A plurality of metallic surfaces  212  are connected to the first set of pads  210  but not directly connected to a second set of pads  220 . A plurality of metallic surfaces  216  are connected to the second set of pads  220  but not in direct contact with the first set of pads. A non-conductive high-frequency ceramic material  214  is layered between the metal surfaces. In one embodiment, the capacitance between the first set of pads and the second set of pads is 1 nF. In another embodiment, the capacitance between the two sets of pads is 50 nF. Any capacitance value is possible depending on the number of layers of ceramics and the length between the sets of pads. In addition, because lengths, the number of layers, and the dielectric constant of the ceramics can be varied, different geometries can result in the same capacitance value. 
       FIG. 2(   b ) depicts the bottom view of a high-level block diagram of the package having the coupling capacitors of the present invention. The package has four solder pads located at the four corners of the package. Solder pad A  230  is located at the upper left hand corner of the package. Solder pad B  240  is located on the lower left hand corner of the package. Solder pad C  260  is located in the upper right hand corner of the package. Solder pad D  250  is located in the lower right hand corner of the package. There is a large capacitance between pads A and C and pads B and D compared to the capacitance between pads A and B and pads C and D, which is set, in accordance with the mechanical dimensions and the dielectric constant of the ceramic material, to achieve a differential impedance of approximately 100 Ohms. That distance between pads A and B and pads C and D is usually between 100 to 300 micrometers. The interior  235  of the package contains materials that are multi-layered high-frequency ceramic having thin metallic surfaces as described above. 
       FIG. 2(   c ) depicts the front view of a high-level block diagram of the package having the coupling capacitors of the present invention. A positive set of solder pads  270  receives and sends the positive portion of the differential signal from the transmitting device. A plurality of metallic surfaces  212  are connected to the front portion of the package but not connected to back portion of the package. A plurality of metallic surfaces  216  are connected to the back portion of the package but not in direct contact with the front portion. A non-conductive high-frequency ceramic material  214  is layered between the metal surfaces. In one embodiment, the impedance between the positive set of pads and the negative set of pads is approximately 100 ohms. In another embodiment, the impedance between the two sets of pads is approximately 95 ohms. The impedance should be in the range of between 90 to 105 ohms. Any impedance value is possible depending on the spacing between pads A and C to pads B and D. In one embodiment, the geometry of this device is designed using a 3-D field solver program. 
     In another embodiment, ground layers (not shown) are included above and below the metallic surfaces. The ground layers lower the impedance and provide shielding. In a further embodiment, ground pads (not shown) are added along the sides of the package. 
     The geometry of the package is determined using: 
     
       
         
           
             
               
                 
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             n=number of layers 
             d=thickness of layers 
             ∈ r =relative dielectric constant 
             ∈ 0 =absolute dielectric constant 
             w=width of capacitor structure 
             l=length of capacitor structure 
           
         
       
    
     The absolute dielectric constant ∈ 0  is 8.85 pF/m. In one embodiment, the ceramic material has a relative dielectric constant ∈ r  of 30 pF/m and a thickness of 25 E−6 m. For transmission of 10G, 8B/10B encoded binary signals, a capacitance needs to be at least 1000 pF. Different geometries are possible to achieve the desired capacitance. In one embodiment, each of the two capacitors is 0.5 mm high and 1.5 mm wide and has a length of 3.14 mm. The two capacitors are placed in parallel with a gap of about 0.7 mm in between them to achieve a differential impedance of about 100 ohms between the closest parallel sides. In this embodiment, the package is about 0.6 mm high, about 4 mm wide and about 3.5 mm long. 
     In another embodiment, in order to have approximately 100 nF capacity associated with the capacitors, a ceramic material with much higher ∈ r  has to be used. The typical material for this range of capacity is approximately 2000 pF/m. This value is too high to get the desired 100-Ohms differential impedance with a reasonable gap size between the two capacitors within one package. In one embodiment, a hybrid stackup is used where different materials are used for the capacitors than the material for the gap between the capacitors. 
       FIGS. 3(   a ),  3 ( b ) and  3 ( c ) depict the side, bottom and front views, respectively, of a high-level block diagram of a package of the present invention having high coupling capacitance. The structure is similar to the package of  FIG. 2  except a hybrid stackup is used. As can be seen by comparing  FIG. 2  and  FIG. 3 , there are some differences between the two packages. One difference is the arrangement of the layers. Another difference is the ceramic materials used in the layers. Other differences to increase the coupling capacitance may be utilized. 
       FIG. 3  ( a ) depicts the side view of a high-level block diagram of the package having the high coupling capacitance of the present invention. A first set of solder pads  310  receives a differential signal from the transmitting device. A plurality of vertical metallic surfaces (not shown) are connected to the first set of pads  310  but not directly connected to a second set of pads  320 . A plurality of metallic surfaces  316  are connected to the second set of pads  320  but not in direct contact with the first set of pads  310 . A non-conductive vertical slab of high-frequency ceramic material  314  is layered between each of the metal surfaces. The ceramic material  314  is either a higher frequency ceramic material or a lower frequency ceramic material. 
       FIG. 3(   b ) depicts the top view of a high-level block diagram of the package having the high coupling capacitance of the present invention. The package has four solder pads located at the four corners of the package. Solder pad A  330  is located at the upper left hand corner of the package. Solder pad B  340  is located on the lower left hand corner of the package. Solder pad C  360  is located in the upper right hand corner of the package. Solder pad D  350  is located in the lower right hand corner of the package. There is a large capacitance between pads A and C and pads B and D compared to the capacitance between pads A and B and pads C and D, which is set, in accordance with the mechanical dimensions and the dielectric constant of the ceramic materials. The interior  235  of the package contains materials that are multi-layered high-frequency ceramic having thin metallic surfaces. The package includes layers of metallization, m12, m14, m16, m18, and m20, connected to pad A. The package also includes layers of metallization, m1, m3, m5, m7, and m9, connected to pad B. The package also includes layers of metallization, m1, m13, m15, m17, and m19, connected to pad C. The package also includes layers of metallization, m2, m4, m6, m8, and m10, connected to pad D. The package includes higher frequency ceramic material, d2-d10 and d12-d20. The package also includes lower frequency ceramic material, d1, d11 and d21. 
       FIG. 3(   c ) depicts the front view of a high-level block diagram of the package having the high coupling capacitance of the present invention. A positive set of solder pads  270  receives and sends the positive portion of the differential signal from the transmitting device. The package includes layers of metallization, m12, m14, m16, m18, and m20, connected to pad A. The package also includes layers of metallization, m1, m3, m5, m7, and m9, connected to pad B. The package also includes layers of metallization, m11, m13, m15, m17, and m19, connected to pad C. The package also includes layers of metallization, m2, m4, m6, m8, and m10, connected to pad D. The package includes higher frequency ceramic material, d2-d10 and d12-d20. The package also includes lower frequency ceramic material, d1, d11 and d21. Any impedance value is possible depending on the spacing between pads A and C to pads B and D. In one embodiment, the geometry of this device is designed using a 3-D field solver program. 
     While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.