Abstract:
A transmission device that establishes efficient match with an impedance mismatch section of a differential transmission system. The transmission device includes: a differential driver; a differential receiver; a differential line that connects between the differential driver and the differential receiver, the differential line including in-phase signal wiring and inverted-phase signal wiring; a delay increasing structure interposed in the differential line at an upstream of the impedance mismatch section; and a delay increasing structure interposed at a downstream of the impedance mismatch section. The delay increasing structure is interposed only in one of the in-phase signal wiring and the inverted-phase signal wiring, and the delay increasing structure is interposed only in another of the in-phase signal wiring and the inverted-phase signal wiring.

Description:
TECHNICAL FIELD 
     The present invention relates to a transmission device using differential lines, and more particularly, to a novel circuit configuration for impedance match. 
     BACKGROUND ART 
     With the recent advances in performance of information processing systems, gigabit-per-second (Gbps) class serial signaling using a differential signal has been widespread. Such a higher-speed signal, however, leads to a noticeable waveform deterioration due to impedance mismatch (discontinuous structure). 
     Most of the high-speed serial transmission standards therefore require that reflection characteristics fall below a prescribed value in a wide frequency range. 
     In the case of a transmission device using differential lines, on the other hand, it is known that a parasitic inductance may be generated in a package or a module because wire bonding is used to connect a mounted chip to board wiring. 
     It is also known that a parasitic capacitance may be generated between the differential lines because an in-phase signal and an inverted-phase signal are often made in close proximity to each other in order to increase the signal density in a bump of a chip, a solder ball of a package, and a differential through hole in a PCB board. 
     As the signal frequency band becomes higher, the impedance mismatch due to those parasitic components becomes more noticeable to deteriorate the reflection characteristics. It has therefore become more difficult to satisfy a reflection prescribed value of the standards, which is one major problem. 
     In view of the above, the technology for achieving impedance match by a capacitance circuit and the technology of using an input/output terminal of an IC as an impedance transformer have hitherto been proposed as the countermeasure for the problem (see, for example, Patent Literature 1 and Patent Literature 2). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 05-37209 A 
     [PTL 2] JP 2010-206084 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     The conventional transmission device has the following problem. According to the technologies described in Patent Literature 1 and Patent Literature 2, the technology for achieving impedance match and the design method therefor are originally derived from the technology for single-ended transmission, not assuming differential transmission. When applied to a differential transmission system, the conventional transmission device has been designed similarly to a general single-ended transmission system while imposing the symmetry condition on differential signals so that mode conversion may be prevented. An efficient technology cannot therefore be established for the differential transmission system. 
     The present invention has been made for solving the above-mentioned problem, and it is an object thereof to obtain a transmission device that establishes efficient match with an impedance mismatch section of a differential transmission system. 
     Solution to Problem 
     A transmission device according to one embodiment of the present invention includes: a differential driver; a differential receiver; a differential line that connects between the differential driver and the differential receiver, the differential line including in-phase signal wiring and inverted-phase signal wiring; a first delay increasing structure interposed in the differential line at an upstream of an impedance mismatch section; and a second delay increasing structure interposed at a downstream of the impedance mismatch section. The first delay increasing structure is interposed only in one of the in-phase signal wiring and the inverted-phase signal wiring, and the second delay increasing structure is interposed only in another of the in-phase signal wiring and the inverted-phase signal wiring. 
     Advantageous Effects of Invention 
     According to one embodiment of the present invention, the transmission characteristics can be improved and the reflection characteristics can be reduced in the impedance mismatch section on the differential lines. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a transmission device according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating an impedance mismatch section of  FIG. 1  in the form of an equivalent circuit. 
         FIG. 3  is a block diagram when a delay increasing structure of  FIG. 1  is constructed by a transmission line. 
         FIG. 4  is a block diagram when the delay increasing structure of  FIG. 1  is constructed by a capacitor element. 
         FIG. 5  is a block diagram when the delay increasing structure of  FIG. 1  is constructed by an inductor element. 
         FIG. 6  is an explanatory graph showing a differential reflection reducing effect according to the first embodiment of the present invention in the form of simulation results in the configuration example of  FIG. 3 . 
         FIG. 7  is an explanatory graph showing common reflection characteristics according to the first embodiment of the present invention in the form of simulation results in the configuration example of  FIG. 3 . 
         FIG. 8  is an explanatory graph showing a differential transmission increasing effect according to the first embodiment of the present invention in the form of simulation results in the configuration example of  FIG. 3 . 
         FIG. 9  is a block diagram illustrating a transmission device according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a block diagram illustrating a transmission device according to a first embodiment of the present invention, schematically illustrating a circuit when a communication device is constructed. 
     In  FIG. 1 , the basic configuration of the transmission device (communication device) includes a differential driver  1  and a differential receiver  2 . 
     The differential driver  1  and the differential receiver  2  are connected to each other by differential lines formed of in-phase signal wiring L 1  and inverted-phase signal wiring L 2  (two pieces of wiring), and differential signals are transmitted from the differential driver  1  toward the differential receiver  2 . 
     An impedance mismatch section  3  is interposed in the middle of the differential lines between the differential driver  1  and the differential receiver  2 . 
     Examples of the impedance mismatch section  3  include a wire bond in an IC chip, a bump of a chip, a solder ball of a package, a through hole in a PCB board, and a connector. 
     At the upstream of the impedance mismatch section  3  on the differential lines, a first delay increasing structure  4   a  (hereinafter sometimes referred to simply as “delay increasing structure  4   a ”) for increasing a delay of only one of the in-phase signal wiring L 1  and the inverted-phase signal wiring L 2  (in this case, the inverted-phase signal wiring L 2 ) is interposed. 
     At the downstream of the impedance mismatch section  3 , a second delay increasing structure  4   b  (hereinafter sometimes referred to simply as “delay increasing structure  4   b ”) is interposed in a line having the opposite polarity to the line for the upstream delay increasing structure (in this case, the in-phase signal wiring L 1 ). 
     Next, a description is given of the circuit operation according to the first embodiment of the present invention illustrated in  FIG. 1 . 
     First, the differential signals output from the differential driver  1  are transmitted through the differential lines formed of the in-phase signal wiring L 1  and the inverted-phase signal wiring L 2  (two pieces of wiring) and are directed toward the impedance mismatch section  3 . 
     In this case, because the delay increasing structure  4   a  is interposed at the upstream of the impedance mismatch section  3 , a part of the differential signals are converted into common-mode signals and thereafter the signals enter the impedance mismatch section  3 . 
     At this time, reflection occurs in the impedance mismatch section  3  because of the impedance mismatch, but a part of the reflected wave becomes a reflection component for the differential mode and another part becomes a reflection component for the common mode due to the delay increasing structure  4   a.    
     In this manner, the reflected wave generated by the impedance mismatch section  3  is dispersed into the two modes, and hence the reflection component for the differential mode is reduced as compared to the case where the signals are all reflected as the differential mode. 
     Most of the standards of high-speed serial signaling define only the differential-mode reflection component with respect to a differential-mode signal input but do not define the common-mode reflection component with respect to the differential-mode signal input. Consequently, the effect that the standards are more easily satisfied can be obtained owing to the effect of reducing the differential-mode reflection component with respect to the differential-mode signal input. 
     Now consider the case where the impedance mismatch section  3  is undesired coupling between the differential lines. 
     A description is now given of the case where the undesired coupling of the impedance mismatch section  3  is a differential capacitance  3   a  as exemplified by a circuit configuration example of  FIG. 2 . 
       FIG. 2  is a block diagram illustrating the impedance mismatch section  3  of  FIG. 1  in the form of an equivalent circuit. The illustrated equivalent circuit corresponds to the case where the differential capacitance  3   a  (parasitic capacitance) is generated between the differential lines when an in-phase signal and an inverted-phase signal are made in proximity to each other in order to increase the signal density in a bump of a chip, a solder ball of a package, a differential through hole in a PCB board, or the like. 
     In  FIG. 2 , the delay increasing structure  4   a  is interposed at the upstream of the impedance mismatch section  3  (inverted-phase signal wiring L 2 ), and hence at least a part of the signals are converted into the common mode and enter the impedance mismatch section  3 . 
     When the delay increasing structure  4   a  is equal to the half-wave length of the signals, the signals are all converted into the common mode and enter the impedance mismatch section  3 . 
     In this case, the common-mode signal is a mode in which the two pieces of wiring L 1  and L 2  constructing the differential lines change with the same potential, and hence the differential capacitance  3   a  is regarded as being absent equivalently. In other words, no reflection caused by the parasitic capacitance occurs and thus the transmission increases. 
     The signals that have passed through the impedance mismatch section  3  are converted into the differential mode again by the delay increasing structure  4   b , which is interposed at the downstream of the impedance mismatch section  3  on the line having the opposite polarity to the line for the upstream delay increasing structure (in-phase signal wiring L 1 ), and are input to the differential receiver  2 . 
     Note that, although the general delay increasing structures  4   a  and  4   b  have been described with reference to  FIGS. 1 and 2 , for example, the delay increasing structures may be constructed by transmission lines  5   a  and  5   b  as illustrated in  FIG. 3 , may be constructed by capacitor elements  6   a  and  6   b  connected to a GND  10  as illustrated in  FIG. 4 , and may be constructed by series inductor elements  7   a  and  7   b  as illustrated in  FIG. 5 . 
     It should be understood that  FIGS. 3 to 5  may be combined, and, for example, the present invention is applicable also to a ladder circuit in which the plurality of capacitor elements  6   a  and  6   b  ( FIG. 4 ) connected to the GND  10  and the plurality of series inductor elements  7   a  and  7   b  ( FIG. 5 ) are arranged alternately. 
       FIGS. 6 ,  7 , and  8  are explanatory graphs showing the effects obtained by the first embodiment of the present invention, each showing the simulation result in the configuration example of  FIG. 3  (the case where the delay increasing structures  4   a  and  4   b  are the transmission lines  5   a  and  5   b ). 
       FIG. 6  shows frequency characteristics when the signals are reflected in the differential mode, in which the horizontal axis represents the frequency (GHz) and the vertical axis represents the differential reflectance (dB). 
       FIG. 7  shows frequency characteristics when the signals are reflected in the common mode, in which the horizontal axis represents the frequency (GHz) and the vertical axis represents the common reflectance (dB). 
       FIG. 8  shows frequency characteristics when the signals are transmitted in the differential mode, in which the horizontal axis represents the frequency (GHz) and the vertical axis represents the differential transmission rate (dB). 
     In  FIG. 6 , it can be confirmed that the reflectance in the differential mode is suppressed by adding the delay increasing structure  4   a  (transmission line  5   a ) at the upstream as compared to the case where the delay increasing structure is not provided. 
     In  FIG. 7 , on the other hand, it can be confirmed that the reflectance in the common mode is increased by adding the delay increasing structure  4   a  (transmission line  5   a ) at the upstream as compared to the case where the delay increasing structure is not provided (−∞ dB). In other words, by converting the differential-mode signals temporarily into the common-mode signals, an influence reducing effect for the undesired coupling between the differential wirings can be obtained. Note that, even if the reflection in the common mode increases, the value thereof does not particularly deviate from the standards in most cases. 
     In  FIG. 8 , it can also be confirmed that the transmission characteristics in the differential mode become larger by adding the delay increasing structure  4   b  (transmission line  5   b ) at the downstream so that the signals are returned to the differential mode from the common mode as compared to the case where the delay increasing structure is not provided. 
     Note that, although the delay increasing structure  4   a  is interposed only in the inverted-phase signal wiring L 2  at the upstream of the impedance mismatch section  3  and the delay increasing structure  4   b  is interposed only in the in-phase signal wiring L 1  at the downstream of the impedance mismatch section  3  in  FIGS. 1 to 5 , the delay increasing structure  4   a  may be interposed only in the in-phase signal wiring L 1  at the upstream of the impedance mismatch section  3  and the delay increasing structure  4   b  may be interposed only in the inverted-phase signal wiring L 2  at the downstream of the impedance mismatch section  3  conversely. 
     As described above, the transmission device according to the first embodiment of the present invention ( FIGS. 1 to 8 ) includes the differential driver  1 , the differential receiver  2 , the differential line that connects between the differential driver  1  and the differential receiver  2  and includes the in-phase signal wiring L 1  and the inverted-phase signal wiring L 2 , the first delay increasing structure  4   a  interposed in the differential line at the upstream of the impedance mismatch section  3 , and the second delay increasing structure  4   b  interposed at the downstream of the impedance mismatch section  3 . 
     The first delay increasing structure  4   a  is interposed only in one of the in-phase signal wiring L 1  and the inverted-phase signal wiring L 2  (for example, the inverted-phase signal wiring L 2 ), and the second delay increasing structure  4   b  is interposed only in the other of the in-phase signal wiring L 1  and the inverted-phase signal wiring L 2  (for example, the in-phase signal wiring L 1 ). 
     The first and second delay increasing structures  4   a  and  4   b  respectively include the transmission lines  5   a  and  5   b  ( FIG. 3 ), the capacitor elements  6   a  and  6   b  ( FIG. 4 ), or the inductor elements  7   a  and  7   b  ( FIG. 5 ), and can be constructed by a ladder circuit formed of the capacitor elements  6   a  and  6   b  and the inductor elements  7   a  and  7   b.    
     In this manner, at the upstream of the impedance mismatch section  3 , the reflection reducing effect ( FIG. 6 ) in the differential mode owing to the dispersion of the reflected wave into the differential mode and the common mode and the influence reducing effect ( FIG. 7 ) for undesired coupling between differential wirings (differential capacitance  3   a ) owing to temporary conversion of the differential-mode signal into the common mode can be obtained as the effects obtained by interposing the delay increasing structure  4   a.    
     Further, at the downstream of the impedance mismatch section  3 , the transmission increasing effect ( FIG. 8 ) for the differential signal can be obtained as the effect obtained by interposing the delay increasing structure  4   b.    
     Further, the delay increasing structures  4   a  and  4   b  are constructed by, for example, the transmission lines  5   a  and  5   b  ( FIG. 3 ) or the like, and can therefore be realized at low cost. 
     Now, a specific description is given of the first effect (improvement on transmission characteristics and reflection characteristics). 
     When it is assumed that the impedance mismatch section  3  is, for example, the differential capacitance  3   a  ( FIG. 2 ), a large reflection generally occurs with respect to a differential input at high frequency, but a part of the incident differential signals are converted into the common mode by the delay increasing structure  4   a  interposed in one wiring (inverted-phase signal wiring L 2 ) at the upstream of the impedance mismatch section  3  (discontinuous structure) on the differential line. 
     For the common-mode signals, two differential signals have the same potential, and hence the differential capacitance  3   a  is regarded as being absent equivalently. Thus, no reflection occurs due to the parasitic capacitance and the transmission increases. The signals are further converted again from the common mode into the differential mode by the delay increasing structure  4   b  interposed at the downstream of the impedance mismatch section  3 . 
     As a result, the effects that the transmission characteristics are improved and the reflection characteristics are reduced in the impedance mismatch section  3  on the differential lines can be obtained. 
     Now, a specific description is given of the second effect (satisfaction of the standards of high-speed serial signaling). 
     According to the first embodiment of the present invention, the delay increasing structure  4   a  (asymmetric structure) is interposed at the upstream of the impedance mismatch section  3  so that the differential mode and the common mode are coupled to each other, and hence, when the differential-mode enters, reflection involving the mode conversion occurs so that a part thereof is reflected as the differential mode and another part is reflected as the common mode. 
     In this manner, the reflected wave of the impedance mismatch section  3  is dispersed into the two modes, and hence the differential-mode reflection component with respect to the differential-mode signal input is reduced. 
     Most of the standards of high-speed serial signaling define only the differential-mode reflection component with respect to a differential-mode signal input but do not define the common-mode reflection component with respect to the differential-mode signal input. Consequently, the effect that the standards are more easily satisfied can be obtained owing to the effect of reducing the differential-mode reflection component with respect to the differential-mode signal input. 
     Owing to the first and second effects described above, the transmission characteristics are improved and the reflection characteristics are reduced with respect to the differential signals. 
     Second Embodiment 
     Note that, although the first embodiment ( FIGS. 1 to 5 ) improves the reflection characteristics by interposing the delay increasing structure  4   a  on the upstream side of the impedance mismatch section  3  and improves the transmission characteristics of the differential signals by interposing the delay increasing structure  4   b  also on the downstream side, only the delay increasing structure  4   a  on the upstream side of the impedance mismatch section  3  may be interposed to improve only the reflection characteristics as illustrated in  FIG. 9 . 
       FIG. 9  is a block diagram illustrating a transmission device according to a second embodiment of the present invention, schematically illustrating a circuit when a communication device is constructed similarly to the above ( FIG. 1 ). 
     In  FIG. 9 , the same components as those described above (see  FIG. 1 ) are denoted by the same reference symbols as those used above to omit the detailed descriptions thereof. 
     This case is different from the above in that only the delay increasing structure  4   a  is interposed on the upstream side of the impedance mismatch section  3  and that the delay increasing structure  4   b  ( FIG. 1 ) on the downstream side is removed. 
     Also in this case, the delay increasing structure  4   a  is constructed by any one of the transmission line  5   a  ( FIG. 3 ), the capacitor element  6   a  ( FIG. 4 ), and the inductor element  7   a  ( FIG. 5 ). 
     The circuit configuration of  FIG. 9  can obtain the reflection reducing effect ( FIG. 6 ) in the differential mode owing to the dispersion of the reflected wave into the differential mode and the common mode from among the effects according to the first embodiment. 
     Because the delay increasing structure  4   b  at the downstream is not present, there is another advantage that this circuit configuration is applicable even when the impedance mismatch section  3  is located immediately close to the differential receiver  2 . 
     Note that, although the delay increasing structure  4   a  is interposed only in the inverted-phase signal wiring L 2  at the upstream of the impedance mismatch section  3  in  FIG. 9 , the delay increasing structure  4   a  may be interposed only in the in-phase signal wiring L 1 . 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  differential driver,  2  differential receiver,  3  impedance mismatch section,  3   a  differential capacitance,  4   a ,  4   b  delay increasing structure,  5   a ,  5   b  transmission line,  6   a ,  6   b  capacitor element,  7   a ,  7   b  inductor element,  10  GND, L 1  in-phase signal wiring, L 2  inverted-phase signal wiring