Abstract:
Provided is a system adopting a differential signaling system including a low frequency signaling line arranged to be adjacent to a pair of differential signaling lines in parallel to each other, for transmitting a signal having a frequency which is smaller than a frequency of a signal to be transmitted through the pair of differential signaling lines, in which a transmission end of the low frequency signaling line is connected to a ground pattern through a first capacitive element, and a reception end of the low frequency signaling line is connected to the ground pattern through a second capacitive element. Thus, it is possible to provide, easily and at a low cost, a differential signaling system in which a common mode noise is eliminated without increasing the number of pins.

Description:
This application is a divisional of U.S. patent application Ser. No. 11/486,089, filed Jul. 14, 2006, the contents of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a differential signaling structure adopting a differential signaling system, in which a radiation noise from an electronic instrument is reduced. 
     2. Related Background Art 
     In recent years, in a signal transmission between electronic instruments, it is necessary to improve a data transmission rate so as to be compatible with a high-speed operation of the electric instruments. To improve the data transmission rate, there are required a higher frequency of a signal to be transmitted and a higher switching speed of the devices used for the signal transmission. To comply with the higher frequency and the higher speed of the signal to be transmitted, it is necessary to take measures against radiation noises. For this reason, a differential signaling system has been used in place of a conventional signaling system of a single-ended signaling system. In particular, a low voltage differential signaling (LVDS) system has a great effect in reducing the radiation noises because a signal wave form thereof is a low-amplitude voltage, in addition to the effect of canceling magnetic fields generated by anti-phase currents flowing through a differential signal pair with other. 
       FIG. 8  is a schematic diagram showing a general circuit structure adopting the LVDS system. In  FIG. 8 , reference numeral  1000  denotes a printed circuit board, reference numeral  100  denotes a transmission side circuit element, reference numeral  101  denotes a reception side circuit element, and reference numeral  300  denotes a ground pattern. Reference numeral  1001  denotes a printed wiring board, and the transmission side circuit element  100  and the reception side circuit element  101  are mounted on the printed wiring board  1001 . Between the transmission side circuit element  100  and the reception side circuit element  101 , a differential signaling line  8  is arranged by providing signaling lines  1  and  2  that have the same electrical characteristics, thereby performing differential signaling with a low-amplitude voltage. 
     A terminating resistor  3  having a value substantially equal to a differential impedance of the differential signaling line is provided between in the vicinity of input terminals of the reception side circuit element  101  and connected to the signaling lines  1  and  2 . By providing the terminating resistor  3 , the entire anti-phase currents flowing through the signaling lines  1  and  2  are thermally consumed, thereby making it possible to suppress the distortion of a wave form and the generation of radiation noises due to reflection. The signaling lines  1  and  2  are arranged to be adjacent in substantially parallel to each other and have the same length. As a result, the anti-phase currents flowing through the signaling lines  1  and  2  generate magnetic fields having substantially the same quantity in opposite directions to be cancelled out, thereby making it possible to suppress generation of the radiation noises. 
     In  FIG. 8 , in addition to the differential signaling line  8  for transmitting a high frequency signal, there are provided three low frequency signaling lines  4 ,  5  and  6 , and a ground line  7 . The low frequency signaling lines  4 ,  5 , and  6  are connected to the transmission side circuit elements  200 ,  202  and  204 , respectively, and to the reception side circuit elements  201 ,  203  and  205 , respectively, and transmit a signal having an extremely small frequency as compared with the differential signaling line  8 . The signal transmitted through the low frequency signaling lines  4 ,  5  and  6  have a small frequency, so that the radiation noise is not a problem even by adopting the transmission system of the single-ended signaling system. Both ends of the ground pattern line  7  each are connected to a ground pattern  300 , and the ground line  7  constitutes return paths for the differential signaling line  8  and the low frequency signaling lines  4 ,  5  and  6 . 
     The differential signaling system represented by the LVDS system is effective in reducing the radiation noises due to the high frequency signal. However, to comply with the higher frequency and the higher speed of the signal, a standard for the radiation noises becomes more stringent year after year, so that the differential signaling system is not sufficient enough to deal with the radiation noises. 
     Even when two signaling lines of the differential signaling line are designed to have completely the same electrical characteristics, an in-phase current component is generated in the differential signaling line due to a time lag within the transmission side circuit element, a difference between build up and build down characteristics thereof, and the like. The differential signaling system is effective for an anti-phase signal, but not capable of suppressing the radiation noises generated due to an in-phase signal. The radiation noise generated in the differential signaling line due to the in-phase current component is called a common mode noise. 
     In a case of a circuit structure shown in  FIG. 8 , the in-phase current component flows through the differential signaling line  8 , thereby generating the common mode noise. With regard to the in-phase current component flowing from the transmission side circuit element  100  to the reception side circuit element  101 , there is no path through which the in-phase current component flows past the reception side circuit element  101 . As a result, the in-phase current component returns to the transmission side circuit element  100  through a stray capacitance and the like of the printed wiring board while straying, whereby the radiation noises is generated. 
     Japanese Patent Application Laid-Open No. H11-205118 proposes an application of a center tap terminal circuit to a differential signaling system shown in  FIG. 9 . In  FIG. 9 , reference numerals  10  and  11  denote resistors which are designed to have about a half value of a differential impedance of a differential signaling line. The resistors  10  and  11  are connected in series to signaling lines  1  and  2  between the signaling lines  1  and  2  in the vicinity of input terminals of the reception side circuit element  101 . Reference numeral  12  is a capacitor provided between a connection point of the resistors  10  and  11  connected in series and a ground pattern  300  to connect the connection point to the ground pattern. An in-phase current component generated in the differential signaling lines  1  and  2  flows to the ground pattern  300  through the resistors  10  and  11  having the same value, and the capacitor  12 . Then, the in-phase current component returns to a reception side circuit element  100  through a ground line  7 , which is connected to the ground pattern, as a return path. Thus, it is possible to suppress the radiation noise. 
     Japanese Patent Application Laid-Open No. 2001-007458 proposes an application of a center tap terminal circuit to a differential signaling system shown in  FIG. 10 . In  FIG. 10 , the radiation noise is reduced by devising an arrangement of in-phase current components and ground lines which become return paths for the current components. In  FIG. 10 , reference numerals  13  and  14  denote ground lines that are newly provided to be adjacent to and in substantially parallel to signaling lines  1  and  2 , and that are each connected to the ground pattern  300 . With this structure, the in-phase current components generated in the signaling lines  1  and  2  flow through a center tap terminal constituted of the resistors  10  and  11  and the capacitor  12 , and then returns to the reception side circuit element  100  through the two ground lines  13  and  14 . In this case, the magnetic fields generated by the in-phase current component flowing through the signaling lines  1  and  2  and the magnetic fields generated by the return current flowing through the ground lines  13  and  14  cancel out each other in the vicinity of a pole, thereby making it possible to reduce the radiation noise. 
     However, in the case of the differential signaling system shown in  FIG. 9 , when the ground line  7  serving as the return path for the in-phase current is apart from the signaling lines  1  and  2 , a current loop becomes large. As a result, the effect of reducing the radiation noise is not obtained sufficiently. 
     In addition, in the case of the differential signaling system shown in  FIG. 10 , it is necessary to add two ground lines each time of adding a pair of differential signaling lines. This increases the number of connector pins and cable cores. Further, this causes an increase in packing density and contour size of a printed wiring board on which the connector is mounted, an increase in cross-sectional area of the cable, and the like, thereby increasing a manufacturing cost of a circuit, and preventing an electric instrument from being small-sized. A higher speed system requires more lines which need to be switched to the differential signaling system, thereby making the problems more serious. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide, easily and at a low cost, a structure for reducing radiation noise adopting a differential signaling system, in which the problem of the radiation noise inherent to the differential signaling system can be solved without increasing the number of pins, in a signal transmission between electric instruments, in a case where the differential signaling system is carried out in order to reduce the radiation noise in association with an increase in frequency and device switching speed for improvement of a data transmission rate. 
     To solve the above-mentioned problems, the present invention provides a differential signaling structure including: a pair of differential signaling lines provided between a transmission side circuit element and a reception side circuit element; and a low frequency signaling line arranged to be adjacent to and in parallel to the differential signaling lines, for transmitting a signal having a frequency smaller than a frequency of a signal to be transmitted through the differential signaling lines, in which a transmission end of the low frequency signaling line is connected to a ground pattern through a first capacitive element, and a reception end of the low frequency signaling line is connected to the ground pattern through a second capacitive element. 
     Further, the present invention provides a differential signaling structure including: a pair of differential signaling lines provided between the transmission side circuit element and the reception side circuit element; and a low frequency signaling line arranged to be adjacent to and in parallel to the differential signaling lines, for transmitting a signal having a frequency smaller than a frequency of a signal to be transmitted through the differential signaling lines, in which, in the vicinity of an input terminal of the reception side circuit element of the pair of differential signaling lines, two resistors are arranged which are connected in series to the pair of differential signaling lines, and which have a resistance value that is about a half of a resistance value matching a differential impedance; the second capacitive element is connected between a connection point of the two resistors connected in series with each other and the transmission end of the low frequency signaling line; and the reception end of the low frequency signaling line is connected to the ground pattern through a third capacitive element. 
     Further, the present invention provides a printed circuit board including: a pair of differential signaling lines provided between a transmission side circuit element and a reception side circuit element; and a low frequency signaling line arranged to be adjacent to and in parallel to the differential signaling lines, for transmitting a signal having a frequency smaller than a frequency of the signal to be transmitted through the differential signaling lines, wherein a transmission end of the low frequency signaling line is connected to the ground pattern through the first capacitive element, and a reception end of the low frequency signaling line is connected to the ground pattern through the second capacitive element. 
     The above and other objects of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a circuit structure adopting a differential signaling system according to a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram showing the circuit structure adopting the differential signaling system according to the first embodiment of the present invention; 
         FIG. 3  is a schematic diagram showing a circuit structure adopting a differential signaling system according to a second embodiment of the present invention; 
         FIG. 4  is a schematic diagram showing a circuit structure adopting a differential signaling system according to a third embodiment of the present invention; 
         FIG. 5  is a schematic diagram showing another embodiment of the present invention; 
         FIG. 6  is a schematic diagram showing still another embodiment of the present invention; 
         FIG. 7  is a graph showing experimental results according to the present invention; 
         FIG. 8  is a schematic diagram showing a conventional circuit structure; 
         FIG. 9  is a schematic diagram showing the conventional circuit structure; and 
         FIG. 10  is a schematic diagram showing the conventional circuit structure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a schematic diagram showing a circuit structure according to a first embodiment of the present invention. It should be noted that the same reference numerals are given to the members which are the same as those of  FIGS. 8 ,  9  and  10 . In this embodiment, only the parts different from prior arts will be described. 
     In  FIG. 1 , reference numeral  15  denotes a capacitor which is provided so as to connect the vicinity of an output terminal of a transmission side circuit element  200  of a low frequency signaling line  4  and a ground pattern  300  to each other, and reference numeral  16  denotes a capacitor which is provided so as to connect the vicinity of an input terminal of a reception side circuit element  201  of the low frequency signaling line  4  and the ground pattern  300  to each other. Referring to  FIG. 10  for comparison, a ground line  14  arranged to be adjacent to one side of a signaling line  2  constituting a differential signaling line is removed, and the low frequency signaling line  4  is arranged in its position, that is, to be adjacent to and in substantially parallel to the signaling line  2 . With this structure, an in-phase current component flowing through differential signaling lines  1  and  2  reaches the ground pattern  300  through resistors  10  and  11  and a capacitor  12 . Further, the in-phase current component returns to a reception side circuit element  100  through a ground line  13 , and at the same time, returns to the reception side circuit element  100  through the capacitor  16 , the low frequency signaling line  4 , and the capacitor  15 . In this case, the magnetic field generated by the in-phase current component flowing through the differential signaling lines  1  and  2  and the magnetic field generated by a return current flowing through the ground line  13  and the low frequency signaling line  4  cancel out each other in the vicinity of a pole, thereby making it possible to suppress generation of radiation noise. 
     At this time, through the low frequency signaling line  4 , a low frequency signal is transmitted from the transmission side circuit element  200  to the reception side circuit element  201 . As a result, it is necessary that the return current is caused to flow in a state where the return current does not substantially affect the low frequency signal. In the low frequency signal, when a voltage of the signal to be transmitted from the transmission side circuit element  200  is suppressed to an attenuation factor of 10% or less at the reception side circuit element  201 , a failure in signal transmission is not caused. 
       FIG. 2  is a schematic diagram showing a circuit structure of the low frequency signaling line  4 . In  FIG. 2 , reference symbol Vs denotes a signal source of a transmission side circuit element  200  of a low frequency signal, and reference symbol Zo denotes an output impedance of a transmission side circuit element  200  of a low frequency signal. A point A denotes a signal reception terminal of the reception side circuit element  201 . A voltage generated when a voltage of the signal source Vs is received at the signal reception terminal A is determined by an input impedance Zi of the reception side circuit element  201  and by an impedance caused when capacitors C 1  and C 2  are connected in parallel with each other. The impedance is reduced when the capacitors C 1  and C 2  are connected in parallel with each other, so that a voltage amplitude is reduced as compared with a case of an input impedance Zi in absence of the capacitors C 1  and C 2 . 
     Assuming that a minimum pulse width of the signal to be transmitted through the low frequency signaling line  4  is set to τmin, a frequency of the signal to be transmitted is represented as the following reciprocal which is a reciprocal of twice the minimum pulse width τmin:
 
1/(2×τmin)  Formula (1).
 
Accordingly, within a bandwidth of equal to or less than the frequency indicated by Formula (1), it is sufficient that the attenuation factor of the voltage is set to equal to or less than 10%.
 
     In general, in a CMOS-IC, the output impedance Zo is extremely small as compared with the input impedance Zi of the reception side circuit element  201  of a low frequency signal, and is represented as the following formula:
 
Zo&lt;&lt;Zi  Formula (2).
 
     Thus, in order to suppress the attenuation factor of the voltage amplitude to 10% or less, when it is assumed that the impedance of a parallel circuit including the capacitors C 1  and C 2  is Zc, it is sufficient that Zc is set to a value ten or more times Zi as represented by the following formula.
 
 Zc&gt; 10 ×Zi   Formula (3).
 
     This is because a total impedance caused when Zi is connected in parallel with Zc is represented as the following formula:
 
( Zc×Zi )/( Zc+Zi )=(10/11)× Zi   Formula (4)
 
and the attenuation factor of the impedance becomes (10 μl), in other words, 10% or less. The impedance Zc of the parallel circuit including the capacitors C 1  and C 2  at the frequency of f is given by the following formula:
 
 Zc= 1/(2 π×f ×( C 1 +C 2)  Formula (5).
 
Then, f is obtained by substituting the frequency given by Formula (2) as represented by the following formula:
 
 Zc =τmin/(π×( C 1 +C 2))  Formula (6).
 
When Formula (6) is substituted for Formula (3), the following formula can be obtained.
 
 C 1 +C 2&lt;((τ min )/(10 ×π×Zi ))  Formula (7).
 
     In other words, the total value of the C 1  and C 2  makes it a condition that Formula (7) is satisfied. Formula (7) determines a necessary condition for receiving the signal having the minimum pulse width τmin at the reception side circuit element  201  without a failure. 
     By setting such the condition, the attenuation factor of the wave form amplitude of the low frequency signal is suppressed to be reduced by 10% at a maximum, thereby making it possible to achieve reduction in radiation noise without causing any failures in operation. 
     Second Embodiment 
       FIG. 3  is a schematic diagram showing a circuit structure according to a second embodiment of the present invention. It should be noted that the same reference numerals are given to the members which are the same as with those of  FIG. 1  representing the first embodiment. In this embodiment, only the parts different from the first embodiment will be described. 
     In  FIG. 3 , reference numeral  17  denotes a capacitor which is provided so as to connect the vicinity of an output terminal of a transmission side circuit element  202  of a low frequency signaling line  5  and a ground pattern  300 , and reference numeral  18  denotes a capacitor which is provided so as to connect the vicinity of an input terminal of a reception side circuit element  203  of the low frequency signaling line  5  and the ground pattern  300 . The low frequency signaling line  5  is arranged to be adjacent to and in substantially parallel to a differential signaling line  1 . In this case, the low frequency signaling lines  4  and  5  for carrying out the low frequency signaling have the same electrical characteristics, and are arranged such that each distance from the lines  4  and  5  to differential signaling lines  1  and  2  is set to be equal. In addition, a capacitor  15  and the capacitor  17  have the same capacitance value, and a capacitor  16  and the capacitor  18  also have the same capacitance value. With the above-mentioned structure, a return current of an in-phase current passing through the differential signaling lines  1  and  2  flows through each of the low frequency signaling lines  4  and  5  at the same level. As a result, the magnetic field generated by the in-phase current and the magnetic field generated by the return current cancel out each other in contrast to each other and in a balanced manner, thereby making it possible to further reduce the radiation noise. 
     It should be noted that each capacitance value of the capacitors  15 ,  16 ,  17  and  18  can be obtained in the same manner as in the first embodiment. 
     Third Embodiment 
       FIG. 4  is a schematic diagram showing a circuit structure according to a third embodiment of the present invention. It should be noted that the same reference numerals are given to the members which are the same as those of  FIG. 3  representing the second embodiment. In this embodiment, only the parts different from the second embodiment will be described. 
     In  FIG. 4 , reference numeral  19  denotes a capacitor which is provided so as to connect the vicinity of an output terminal of a reception side circuit element  201  of a low frequency signaling line  4  and a midpoint between center tap terminating resistors  10  and  11 , and reference numeral  20  denotes a capacitor which is provided so as to connect the vicinity of an input terminal of a reception side circuit element  203  of a low frequency signaling line  5  and the midpoint between the center tap terminating resistors  10  and  11 . The low frequency signaling lines  4  and  5  are arranged to be adjacent to and in substantially parallel to differential signaling lines  1  and  2 . A return current of an in-phase current flowing through the differential signaling lines  1  and  2  is divided in each direction of the capacitors  19  and  20  at the midpoint between the center tap terminating resistors  10  and  11 . Further, the return current flows through the low frequency signaling lines  4  and  5  to a ground pattern  300  through capacitors  15  and  17 , respectively, and then returns to a transmission side circuit element  100  of a differential signal. 
     With this structure, as compared with the second embodiment, even when the number of capacitors is reduced by one, the same effect of reducing the radiation noise can be obtained, thereby making it possible to reduce packaging area and the manufacturing cost. 
     It should be noted that each capacitance value of the capacitors  15 ,  17 ,  19  and  20  can be obtained in the same manner as in the first embodiment. 
     As the transmission side circuit elements  100 ,  200 ,  202  and  204  according to the first, second and third embodiments, various ICs may be used. In a similar manner, as the reception side circuit elements  101 ,  201 ,  203  and  205  according to the first, second and third embodiments, various ICs may be used. 
     Further, as shown in  FIG. 5 , the transmission side circuit elements  100 ,  200 ,  202  and  204  and a ground pattern  301  may be set as different terminals of the same IC package  2000 , and the reception side circuit elements  101 ,  201 ,  203  and  205  and a ground pattern  302  may be also set as different terminals of the same IC package  2001 . 
     Further, as shown in  FIG. 6 , the transmission side circuit elements  100 ,  200 ,  202  and  204  may be set as different terminals of the same connector  3000 , and the reception side circuit elements  101 ,  201 ,  203  and  205  may be also set as different terminals of the same connector  3001 . In this case, signaling lines  1  to  6  are arranged within a cable  3002 . 
     Experimental Example 
     In the differential signaling system shown in  FIG. 3  according to the second embodiment, a strength of a generated electric field was obtained by simulation. 
     A structure in which experimental results shown in  FIG. 7  were obtained will be described. In each case of a differential signaling system shown in FIGS.  3 ,  9  and  10 , transmission side circuit elements  100 ,  200 ,  202  and  204  were arranged on a left side of a printed circuit board  1000 , and reception side circuit elements  101 ,  201 ,  203  and  205  were arranged on a right side of the printed circuit board  1000 . Signaling lines  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7  and  13  each have a wire diameter of 0.1 mm and a length of 50 mm. The signaling lines were arranged to be parallel with each other at an interval of 2 mm. Terminating resistors  10  and  11  were set to 50Ω, and a capacitor  12  was set to 0.1 μF, thereby constituting a center tap terminal circuit. Further, the electric field strength obtained in a case where capacitors  15 ,  16 ,  17  and  18  were set to 50 pF The strength of the electric field generated in this case is represented as the symbol “o” as shown in  FIG. 7 . It should be noted that the simulation result indicates the electric field strength obtained when an object to be measured was arranged at a level of 80 cm from a ground pattern surface by the 3m method. Further, the electric field strength obtained in a case where capacitors  15  and  17  were set to 10 pF, and capacitors  16  and  18  were set to 90 pF is indicated as the symbol “x” as shown in  FIG. 7 . Further, for comparison, the electric field strength in the differential signaling system shown in  FIG. 9  is represented as the symbol “*”, and the electric field strength obtained in a case where a ground line was arranged at both adjacent sides of the differential signaling line shown in  FIG. 10  is represented as the symbol “Δ”. 
     As apparent from  FIG. 7 , the electric field strengths of “o” and “x” indicating the differential signaling system according to the present invention are lowered by 10 dB or more as compared with the electric field strength of “1” indicating the conventional differential signaling system shown in  FIG. 9 , and radiation noise is suppressed to a large extent. In addition, it is apparent that the electric field strengths of “o” and “x” have substantially the same value as compared with the electric field strength of “Δ” indicating the conventional differential signaling system shown in  FIG. 10 . 
     Therefore, it is found that it is possible to obtain the same effect of reducing the radiation noise by using an extremely simple method according to the present invention, as compared with the differential signaling system shown in  FIG. 10 . 
     This application claims priorities from Japanese Patent Application Nos. 2005-209881 filed on Jul. 20, 2005, and 2006-186912 filed on Jul. 6, 2006, which are hereby incorporated by reference herein.