Patent Publication Number: US-6988898-B2

Title: Signal repeating device

Description:
TECHNICAL FIELD 
   The present invention relates to a stub structure for preventing signal reflection that will occur when transmitting a signal from a first board to a second board. 
   BACKGROUND ART 
     FIG. 1  is a diagram showing a configuration of a conventional board connection on a transmission line disclosed in Japanese patent application laid-open No. 4-28182/1992, for example. In  FIG. 1 , the reference numeral  1  designates a daughter card;  2  designates a transmission path of the daughter card  1 ;  3  designates a signal through hole (through hole used for a signal) of the daughter card  1 ;  4  designates a ground layer;  5  designates a ground through hole (through hole used for a ground) of the daughter card  1 ;  6  designates a backplane;  7  designates a transmission path of the backplane  6 ;  8  designates a signal through hole of the backplane  6 ;  9  designates a ground layer;  10  designates a ground through hole of the backplane  6 ;  11  designates a connector having its connector pin  11   a  inserted into the signal through hole  3  of the daughter card  1  and its connector pin  11   b  inserted into the signal through hole  8  of the backplane  6 ; and  12  designates a connector having its connector pin  12   a  inserted into the ground through hole  5  of the daughter card  1 , and its connector pin  12   b  inserted into the ground through hole  10  of the backplane  6 . 
   Next, the operation will be described. 
   The connector pin  11   a  of the connector  11  is inserted into the signal through hole  3  of the daughter card  1 , and the connector pin  11   b  of the connector  11  is inserted into the through hole  8  of the backplane  6 . 
   Thus, the transmission path  2  of the daughter card  1  is electrically connected to the transmission path  7  of the backplane  6 . 
   Accordingly, a signal output from a driver or the like installed in the daughter card  1  is transmitted from the transmission path  2  of the daughter card  1  to the transmission path  7  of the backplane  6  via the connector  11 . 
   However, if the characteristic impedance of the transmission path  2  of the daughter card  1  differs from that of the transmission path  7  of the backplane  6 , the impedance mismatching will bring about signal reflection, which prevents high-speed transmission of the signal. 
   In view of this, to minimize the signal reflection due to the impedance mismatching, the conventional transmission lines make a contrivance as to the placement of the ground and reducing the length of the fitting portion of the connector  11 . 
   With the foregoing arrangement, the conventional communication equipment can control the signal reflection as long as the transmission speed of the signal is within a certain limit. However, as the transmission speed of the signal further increases, a problem arises of being unable to control signal reflection sufficiently by only contriving the placement of the ground and the length of the fitting portion of the connector  11 . 
   The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a stub line for controlling the signal reflection even if the transmission speed of the signal is increased. 
   DISCLOSURE OF THE INVENTION 
   The signal transmitter in accordance with the present invention includes electrical short stubs connected to signal through holes in first and second boards. 
   This offers an advantage of being able to control the signal reflection even if the transmission speed of the signal is increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing a configuration of conventional communication equipment; 
       FIG. 2  is a diagram showing a configuration of an embodiment 1 of the signal transmitter in accordance with the present invention; 
       FIG. 3  is an enlarged perspective view of a backplane of the equipment of  FIG. 2 ; 
       FIG. 4  is a diagram illustrating an admittance diagram (Smith chart); 
       FIG. 5  is a view showing a configuration of an embodiment 2 of the communication equipment in accordance with the present invention; 
     FIG.  6 ( a ) is a plane view showing a layout of through holes, and FIG.  6 ( b ) is a cross-sectional view of some of the through holes; and 
     FIG.  7 ( a ) is a plane view showing a layout of through holes, and FIG.  7 ( b ) is a cross-sectional view of some of the through holes. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   The best mode for carrying out the invention will now be described with reference to the accompanying drawings to explain the present invention in more detail. 
   Embodiment 1 
     FIG. 2  is a diagram showing a configuration of an embodiment  1  of the communication equipment in accordance with the present invention; and  FIG. 3  is an enlarged perspective view of a backplane of the equipment of FIG.  2 . In  FIG. 2 , the reference numeral  1  designates a daughter card (first board);  2  designates a transmission path of the daughter card  1 ;  3  designates a signal through hole (through hole used for a signal) of the daughter card  1 ;  4  designates a ground layer;  5  designates a ground through hole (through hole used for a ground) of the daughter card  1 ;  6  designates a backplane (second board);  7  designates a transmission path of the backplane  6 ;  8  designates a signal through hole of the backplane  6 ;  9  designates a ground layer; and  10  designates a ground through hole of the backplane  6 . 
   The reference numeral  11  designates a connector (first connector) having its connector pin  11   a  inserted into the signal through hole  3  of the daughter card  1  and its connector pin  11   b  inserted into the signal through hole  8  of the backplane  6 ; and  12  designates a connector (second connector) having its connector pin  12   a  inserted into the ground through hole  5  of the daughter card  1 , and its connector pin  12   b  inserted into the ground through hole  10  of the backplane  6 . The connectors  11  and  12  constitute transmission lines. 
   The reference numeral  13  designates a short stub for electrically connecting the signal through hole  3  with the ground through hole  5 , and  14  designates a short stub for electrically connecting the signal through hole  8  with the ground through hole  10 . 
   Next, the operation will be described. 
   The connector pin  11   a  of the connector  11  is inserted into the signal through hole  3  of the daughter card  1 , and the connector pin  11   b  of the connector  11  is inserted into the through hole  8  of the backplane  6 . 
   Thus, the transmission path  2  of the daughter card  1  is electrically connected to the transmission path  7  of the backplane  6 . 
   Accordingly, a signal output from a driver or the like installed in the daughter card  1  is transmitted from the transmission path  2  of the daughter card  1  to the transmission path  7  of the backplane  6  via the connector  11 . 
   However, if the characteristic impedance of the transmission path  2  of the daughter card  1  differs from that of the transmission path  7  of the backplane  6 , the impedance mismatching will bring about signal reflection, which prevents high-speed transmission of the signal. 
   In view of this, to control the signal reflection, the present embodiment 1 has the electrical short stub  13  connected to the signal through hole  3  of the daughter card  1 , and the electrical short stub  14  connected to the signal through hole  8  of the backplane  6 . 
   In other words, the signal through hole  3  is electrically connected to the ground through hole  5  by the short stub  13  in the daughter card  1 , and the signal through hole  8  is electrically connected to the ground through hole  10  by the short stub  14  in the backplane  6 . 
   Here, the connecting position  11  of the short stub  13  to the transmission path is determined such that the normalized conductance g, which is obtained by dividing the imaginary component of the input admittance Y i  by the characteristic admittance Y 0  (=1/Z 0 ) of the transmission path  2 , becomes “1”. Here, the input admittance Y i  is defined as the admittance seen by looking into the load side, the connector  11 , from the signal source side, the daughter card  1 . 
   More specifically, as illustrated in the admittance diagram (Smith chart) of  FIG. 4 , considering that the input impedance of the connector  11  equals the load impedance Z L , its admittance point is denoted by A 1  in FIG.  4 . 
   Here, considering that the ground through hole  5  is inductive, a condition is set such that the load impedance Z L  (characteristic impedance) of the connector  11  becomes greater than the characteristic impedance of the transmission path  2  of the daughter card  1 . Then, the position of the standing wave moves so that the distance from the tip of the connector pin  11   a  to the connecting position of the short stub  13  to the transmission path can be sharply reduced to about 1/10 of the wavelength. Thus, the admittance point is moved from A 1  to A 1 ′ by setting the load impedance of the connector  11  such that the foregoing condition is satisfied. In this case, the short stub  13  can be fixed to the ground through hole  5  directly. 
   Next, a decision is made such that the normalized conductance g becomes “1”, which is obtained by dividing the imaginary component of the input admittance Y i  seen by looking into the connector  11  from the daughter card  1  side by the characteristic admittance Y 0  (=1/Z 0 ) of the transmission path  2 . Thus, the admittance point is moved from A 1 ′ to A 2  on a curve on which g=1. 
   Subsequently, a condition is set such that the length of the short stub  13  matches the ratio between the characteristic impedance (characteristic admittance) of the short stub  13  and the input reactance (susceptance) of the short stub  13 . Then, the inductance of the short stub  13  and the capacitance of the line (line from the tip of the connector pin  11   a  to the connecting position of the short stub  13 ) have their susceptance components canceled each other. Accordingly, the admittance point is moved from A 2  to A 3 , the origin of the Smith chart, by setting the length of the short stub  13  such that it meets the foregoing condition. Thus, the impedance matching is achieved. 
   Incidentally, when the backplane  6  is a signal source, the short stub  14  is provided, which electrically connects the signal through hole  8  to the ground through hole  10 . In this case, the connecting position l 2  of the short stub  14  is determined in the same manner as that of the short stub  13 . In other words, it is determined such that the normalized conductance g becomes “1”, which is, obtained by dividing the imaginary component of the input admittance Y i  seen by looking into the load side, the connector  11 , from the signal source, the backplane  6  side, by the characteristic admittance Y 0  (=1/Z 0 ) of the transmission path  7 . 
   As described above, the present embodiment 1 is configured such that it comprises the short stub  13  or  14  for electrically connecting the signal through hole  3  or  8  to the ground through hole  5  or  10 , respectively. Thus, the present embodiment 1 offers an advantage of being able to control the signal reflection even if the transmission speed of the signal is increased. 
   Specifically, it can improve the S/N, jitter and error rate of the device because the signal energy on the transmission path is transmitted to the final stage or another side receiver without loss. 
   Furthermore, the present embodiment 1 is configured such that it sets the load impedance Z L  of the connector  11  greater than the characteristic impedance of the transmission path  2  of the daughter card  1 . Thus, the present embodiment 1 offers an advantage of being able to reduce the distance from the tip of the connector pin  11   a  to the connecting position of the short stub  13  to about 1/10 of the wavelength. 
   Embodiment 2 
     FIG. 5  is a view showing a configuration of an embodiment 2 of the package in accordance with the present invention. In  FIG. 5 , the same reference numerals designate the same or like portions to those of  FIG. 2 , and their description will be omitted here. 
   In  FIG. 5 , the reference numeral  21  designates a printed circuit board on which an LSI  23  is mounted,  22  designates a ball,  23  designates the LSI corresponding to a board (second board) on a signal receiving side, and  24  designates bonding wires electrically connecting the ball  22  with the pins of the LSI  23 . The printed circuit board  21 , balls  22  and bonding wires  24  constitute a package. 
   Although the signal transmitting section consists of the connectors  11  and  12  in the foregoing embodiment 1, it may consists of the package electrically connecting the transmission path  2  of the board on the signal transmitting side with the pins of the LSI  23  mounted on the package as shown in FIG.  5 . 
   In this case, the connecting position l m  and length l s  of the short stub  13  are determined such that the inductive susceptance (reactance), the imaginary part of the admittance (impedance), of the short stub  13  is canceled by the capacitive susceptance (reactance), the imaginary part of the admittance (impedance), seen by looking into the LSI  23  side from the connecting position of the short stub  13 . 
   More specifically, the connecting position l m  of the short stub  13  is determined such that the normalized conductance g, which is obtained by dividing the imaginary component of the input admittance Y i  seen by looking into the LSI  23  from the signal transmitting side by the characteristic admittance Y 0  (=1/Z 0 ) of the transmission path  2 , becomes “1” by using a Smith chart, or the following expression (1).
 
 Y   i   =Y   0 ( Y   L  cos β l   m   +jY   0  sin β l   m )/( Y   0  cos β l   m   +jY   L  sin β l   m )  (1)
 
1/ Y   0   =Z   0 =(η/π)cos  h   −1 ( d/ φ)  (2)
 
where
         β: phase constant (β=ω/λ);   Y L : admittance of the signal transmission line;   η: wave impedance in free space;   φ: diameter of the through holes  3  and  5 ; and   d: distance between the signal through hole  3  and ground through hole  5 .       

   On the other hand, the length l s  of the short stub  13  is obtained such that when the susceptance seen by looking into the LSI  23  from the connecting position of the short stub  13  l m  is B[S], the susceptance seen by looking into the short connected side of the short stub  13  from the connecting position l m  of the short stub  13  becomes −B[S] by using a Smith chart or the following expression (3). The following expression (3) is obtained by placing the admittance Y L  of the signal transmitting section in the foregoing expression (1) at infinity (∞: short-circuited).
 
Y i   =−jY   0  cos β l   s   (3)
 
   As described above, the present embodiment 2 is configured such that the signal transmitting section consists of the package electrically connecting the transmission path  2  of the board on the signal transmitting side with the pins of the LSI  23  mounted on the package. Thus, the present embodiment 2 offers an advantage of being able to control the signal reflection even if the package is used as the signal transmitting section. 
   In addition, the present embodiment 2 is configured such that the connecting position l m  of the short stub  13  is determined considering the admittance Y L  of the signal transmitting section, the characteristic admittance Y 0  of the transmission path  2  of the board, the input admittance Y i  seen by looking into the signal transmitting section from the transmission path of the board, and the phase constant β. Accordingly, the present embodiment 2 offers an advantage of being able to control the signal reflection, even if the transmission speed of the signal is increased. 
   Furthermore, the present embodiment 2 is configured such that the length l s  of the short stub  13  is determined considering the characteristic admittance Y 0  of the transmission path  2  of the board, the input admittance Y i  seen by looking into the signal transmitting section from the transmission path of the board, and the phase constant β. Accordingly, the present embodiment 2 offers an advantage of being able to control the signal reflection, even if the transmission speed of the signal is increased. 
   Embodiment 3 
   Although the foregoing embodiment 1 is described by way of example including a single signal through hole  3  and ground through hole  5  placed in the daughter card  1 , a plurality of signal through holes  3  and ground through holes  5  can be placed in the daughter card  1 . In this case, the signal through holes  3  and ground through holes  5  can be disposed alternately at regular intervals as shown FIGS.  6 ( a ) and  6 ( b ). 
   Here, the connecting position l m  of the short stub  13  is determined by the foregoing expression (1), and the length l s  of the short stub  13  is determined by using the foregoing expression (3) or the Smith chart of FIG.  4 . 
   Thus, the present embodiment 3 offers an advantage of being able to determine the connecting position l m  and length l s  of the short stub  13  flexibly over a wide range. 
   Embodiment 4 
   The foregoing embodiment 3 is described by way of example including signal through holes  3  and ground through holes  5  disposed alternately at regular intervals. However, when transmitting a signal from the backplane  6  to the daughter card  1 , signal through holes  8  and ground through holes  10  can be disposed alternately at regular intervals as shown in FIGS.  7 ( a ) and  7 ( b ). 
   Here, the connecting position l m  of the short stub  14  is determined by the foregoing expression (1), and the length l s  of the short stub  14  is determined by using the foregoing expression (3) or the Smith chart of FIG.  4 . 
   Thus, the present embodiment 4 offers an advantage of being able to determine the connecting position l m  and length l s  of the short stub  14  flexibly over a wide range. 
   Industrial Applicability 
   As described above, the communication equipment in accordance with the present invention is applicable to reducing the signal reflection as much as possible which occurs when transmitting a signal from a first board to a second board that are connected with each other.