Abstract:
An impedance conversion device has four conductors arranged so that the first and second conductors form a transmission line having a first characteristic impedance, the third and fourth conductors form a transmission line having the first characteristic impedance, the first and third conductors form a transmission line having a second characteristic impedance, and the second and fourth conductors form a third transmission line having the second characteristic impedance. The second and fourth conductors are interconnected at proximate ends through a resistance equal to the first characteristic impedance. The third and fourth conductors are interconnected at proximate ends through a resistance equal to the second characteristic impedance. The lateral dimensions of the impedance conversion device are small enough to permit insertion in a stacked pair line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an impedance conversion device, and in particular to an impedance conversion device that can be inserted into a stacked pair line. 
     2. Description of the Related Art 
     An example of a conventional impedance conversion device that can be inserted in a transmission line is given in Japanese Patent Application Publication No. 10-224123. The disclosed device is designed for insertion into a microstrip line, however, and is too wide in the direction orthogonal to the line for insertion into a stacked pair line. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an impedance conversion device that is narrow enough for insertion into a stacked pair line. 
     The invented impedance conversion device comprises first, second, third, and fourth conductors, each having a first end and a second end. The conductors are arranged so that the first and second conductors form a first transmission line having a first characteristic impedance, the first and third conductors form a second transmission line having a second characteristic impedance different from the first characteristic impedance, the second and fourth conductors form a third transmission line having the second characteristic impedance, and the third and fourth conductors form a fourth transmission line having the first characteristic impedance. 
     A first resistor having a resistance equal to the first characteristic impedance is connected between the second ends of the second and fourth conductors, which are mutually proximate. A second resistor having a resistance equal to the second characteristic impedance is connected between the first ends of the third and fourth conductors, which are mutually proximate. 
     The four conductors transmit a signal that is input at the first ends of the first and second conductors and output at the second ends of the first and third conductors. The fourth conductor preferably has a length not exceeding one-fourth of the fundamental wavelength of the transmitted signal. 
     The impedance of the transmitted signal is converted efficiently, and the dimensions of the impedance conversion device in the directions orthogonal to the longitudinal direction of the conductors are comparatively small, permitting the impedance converting device to be formed in a confined space and in particular to be inserted into a stacked pair line. Use of this impedance conversion device can contribute to a reduction in the size of microelectronic parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  is a perspective view of an impedance conversion device embodying the present invention; 
         FIG. 2  is a top plan view of the impedance conversion device in  FIG. 1 ; 
         FIG. 3  is a bottom plan view of the impedance conversion device in  FIG. 1 ; 
         FIG. 4  is a side elevation view of the impedance conversion device in  FIG. 1 ; 
         FIG. 5  is a sectional view through line V-V in  FIGS. 2-4 ; 
         FIG. 6  is a sectional view through line VI-VI in  FIGS. 2-4 ; 
         FIG. 7  is a sectional view through line VII-VII in  FIGS. 2-4 ; 
         FIG. 8  is a top plan view of a structure used in time-domain reflectometry; 
         FIG. 9  is a bottom plan view of the structure in  FIG. 8 ; 
         FIG. 10  depicts a time-domain reflectometer, and a coaxial cable and probes connected thereto; 
         FIG. 11  shows exemplary waveforms obtained by time-domain reflectometry using the structure in  FIGS. 8 and 9 ; 
         FIG. 12  schematically depicts the impedance conversion device in  FIG. 1  with a direct current source connected on its input side and a load resistor connected on its output side; 
         FIG. 13  schematically depicts the impedance conversion device in  FIG. 1  with a pulse generator connected on its input side, a load resistor connected on its output side, and an oscilloscope connected to measure the voltage on the output side; 
         FIG. 14  is a top plan view of an impedance conversion device used in time-domain reflectometry; 
         FIG. 15  is a bottom plan view of an impedance conversion device used in time-domain reflectometry; 
         FIG. 16  shows exemplary waveforms obtained with the measurement setup shown in  FIG. 13 ; 
         FIG. 17  shows exemplary waveforms obtained with the measurement setup shown in  FIG. 13  with the output side left electrically open; 
         FIG. 18  shows exemplary waveforms obtained with the measurement setup shown in  FIG. 13  with the central part of the conductor lengthened; 
         FIG. 19  is a top plan view of another structure used in time-domain reflectometry; 
         FIG. 20  is a bottom plan view of the structure in  FIG. 19 ; 
         FIG. 21  shows an exemplary waveform obtained by time-domain reflectometry using the structure in  FIGS. 19 and 20 ; 
         FIG. 22  is a perspective view illustrating crosstalk between mutually adjacent conductors; 
         FIG. 23  is a sectional view illustrating crosstalk between mutually adjacent conductors; 
         FIG. 24  is a sectional view illustrating another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An impedance conversion device embodying the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
     As shown in  FIGS. 1-7 , the impedance conversion device comprises first, second, third, and fourth strip-like conductors  11 ,  12 ,  13 ,  14 , first and second resistors  15 ,  16 , and a dielectric sheet  17 . The first to fourth conductors  11 ,  12 ,  13 ,  14  extend in mutually parallel straight lines. 
     The dielectric sheet  17  has a first surface or upper surface  17   a  (uppermost in FIGS.  1  and  4 - 7 ) and a second surface or lower surface  17   b . The first and third conductors  11 ,  13  are disposed side by side on the upper surface  17   a  of the dielectric sheet  17 , spaced apart from each other in a direction orthogonal to their lengths and parallel to the upper surface  17   a  and lower surface  17   b  of the dielectric sheet  17 . The second and fourth conductors  12 ,  14  are similarly disposed side by side on the lower surface  17   b  of the dielectric sheet  17 . 
     The first conductor  11  and the second conductor  12  are disposed on opposite sides of the dielectric sheet  17 , facing each other in a direction orthogonal to the upper surface  17   a  and lower surface  17   b  of the dielectric sheet  17 . The third conductor  13  and the fourth conductor  14  are similarly disposed on opposite sides of the dielectric sheet  17 , facing each other. 
     As shown in  FIGS. 2-4 , the impedance conversion device  1  has an input part or region  1   a , a central part or region  1   b , and an output part or region  1   c . The input region  1   a  is the region near the input end  1   d  of the impedance conversion device  1 ; the output region  1   c  is the region near the output end  1   e  of the impedance conversion device  1 . The central region  1   b  is the region between the input region  1   a  and the output region  1   c . The input region  1   a , the central region  1   b , and the output region  1   c  are mutually contiguous. 
     The first conductor  11  extends across the input region  1   a , the central region  1   b , and the output region  1   c  of the impedance conversion device  1 ; the first conductor  11  has an input part  11   a , a central part  11   b , and an output part  11   c  disposed in the input region  1   a , the central region  1   b , and the output region  1   c , respectively. 
     The second conductor  12  extends across the input region  1   a  and the central region  1   b  of the impedance conversion device  1 , and has an input part  12   a  and a central part  12   b  disposed in the input region  1   a  and the central region  1   b , respectively. 
     The third conductor  13  extends across the central region  1   b  and the output region  1   c  of the impedance conversion device  1 , and has a central part  13   b  and an output part  13   c  disposed in the central region  1   b  and the output region  1   c , respectively. 
     The fourth conductor  14  extends only across the central region  1   b , and has a central part  14   b  disposed in the central region  1   b.    
     The first conductor  11  and second conductor  12  form a transmission line having a first characteristic impedance z 1 . 
     The second conductor  12  and fourth conductor  14  form a transmission line having a second characteristic impedance z 2  different from the first characteristic impedance z 1 . 
     The first conductor  11  and the third conductor  13  form a transmission line having the second characteristic impedance z 2 . 
     The third conductor  13  and the fourth conductor  14  form a transmission line having the first characteristic impedance z 1 . 
     The first conductor  11  is disposed so that one end (the input end)  11   d  is at the input end  1   d  of the impedance conversion device  1 , and the other end (the output end)  11   e  is at the output end of the impedance conversion device  1 . 
     The second conductor  12  is disposed so that one end (the input end)  12   d  is at the input end  1   d  of the impedance conversion device  1 , and the other end (the output end)  12   e  is at the boundary  1   g  between the central region  1   b  and the output region  1   c  of the impedance conversion device  1 . 
     The third conductor  13  is disposed so that one end (the input end)  13   d  is at the boundary  1   f  between the input region  1   a  and the central region  1   b  of the impedance conversion device  1 , and the other end (the output end)  13   e  is at the output end  1   e  of the impedance conversion device  1 . 
     The fourth conductor  14  is disposed so that one end (the input end)  14   d  is at the boundary  1   f  between the input region  1   a  and the central region  1   b  of the impedance conversion device  1 , and the other end (the output end) is at the boundary  1   g  between the central region  1   b  and the output region  1   c  of the impedance conversion device  1 . 
     The output end  12   e  of the second conductor  12  and the output end  14   e  of the fourth conductor  14  are both disposed on the lower surface  17   b  of the dielectric sheet  17  and are mutually proximate. The input end  13   d  of the third conductor  13  and the input end  14   d  of the fourth conductor  14  are disposed on the lower surface  17   b  and the upper surface  17   a  of the dielectric sheet  17 , respectively, and are mutually proximate. 
     A first resistor  15  is mounted on the lower surface  17   b  of the dielectric sheet  17 . The first resistor  15  interconnects the output end  12   e  of the second conductor  12  and the output end  14   e  of the fourth conductor  14 , and has a resistance R 1  equal to the first characteristic impedance z 1 . 
     A second resistor  16  is formed so that it extends through the dielectric sheet  17 . The second resistor  16  interconnects the input end  13   d  of the third conductor  13  and the input end  14   d  of the fourth conductor  14 , and has a resistance R 2  equal to the second characteristic impedance z 2 . 
     The value (the absolute value) of the first characteristic impedance z 1  is, for example, fifty ohms (50Ω), and the value (the absolute value) of the second characteristic impedance z 2  is, for example, 82Ω. 
     The first to fourth conductors  11  to  14  have identical cross-sectional configurations, for example, a thickness (the vertical dimension in  FIGS. 5-7 ) of 40 micrometers, and a width (the horizontal dimension in  FIGS. 5-7 ) of 0.8 millimeters. (The dimensions in the drawings are not shown proportional to the actual dimensions.) 
     The dielectric sheet  17  has a thickness of 170 micrometers; the distance between the first conductor  11  and the second conductor  12  and the distance between the third conductor  13  and the fourth conductor  14  are equal to the thickness of the dielectric sheet  17 . 
     The distance between the first conductor  11  and the third conductor  13  and the distance between the second conductor  12  and the fourth conductor  14  are identically 100 micrometers (0.1 millimeters). 
     The first to fourth conductors parallel each other in the central region  1   b , which therefore may be referred to as the ‘quadri-parallel’ part below. In contrast, the input region  1   a  and the output region  1   c  may be referred to as ‘duo-parallel’ parts, as only the first and second conductors  11  and  12  are parallel in the input region  1   a , and only the first and third conductors  11  and  13  are parallel in the output region  1   c.    
     The length of the central region  1   b  of the impedance conversion device, that is, the length of conductor  14  (the length in the longitudinal direction in which conductors  11  to  14  extend) preferably does not exceed one-fourth of the fundamental wavelength of the signal that is transmitted, and is preferably at least ten times as long as the larger of the two distances that separate the first conductor  11  from the second conductor  12  and the first conductor  11  from the third conductor  13 . More specifically, the length is preferably longer than 1/64 of the fundamental wavelength of the transmitted signal. 
     When the impedance conversion device  1  is configured as above, its input impedance Zin is equal to the first characteristic impedance z 1  (50Ω) and its output impedance Zout is equal to the second characteristic impedance z 2  (82Ω). Impedance conversion therefore takes place. This was confirmed by using TDR (time domain reflectometry) to measure the impedance of the transmission lines. 
     TDR is carried out by transmitting a pulsed signal and observing the reflection of the pulse from the circuit under test; TDR detects changes in impedance along the transmission path of the signal. 
       FIGS. 8 and 9  are a top plan view and a bottom plan view of a structure used for time-domain reflectometry, corresponding respectively to  FIGS. 2 and 3 . The structure is similar to the impedance conversion device  1  shown in  FIGS. 1-7 ; a dielectric sheet  117  (corresponding to the dielectric sheet  17  in  FIG. 1 ) has a first conductor  111  and a third conductor  113  mounted on its upper surface  117   a , and a second conductor  112  and a fourth conductor  114  mounted on its lower surface  117   b . The first to fourth conductors  111  to  114  correspond to the first to fourth conductors  11  to  14  in  FIGS. 1-7 , with the same thickness and width as the first to fourth conductors. The first conductor  111  and the second conductor  112  face each other across the dielectric sheet  117 ; the third conductor  113  and the fourth conductor  114  face each other across the dielectric sheet  117 . Resistors  15  and  16  are not yet connected; the first to fourth conductors  111  to  114  are of equal length (LT=80 millimeters). 
     Strip-like leads  121  to  124  formed of the same material as the conductors are mounted at the ends  111   h  to  114   h  of the first to fourth conductors  111  to  114  (the left ends in  FIGS. 8 and 9 ); connecting pads  131  to  134  are mounted at the ends of the leads  121  to  124 . The length LL of the leads  121  to  124  is twelve millimeters. 
     Measurements were made of the impedance of each of the transmission lines formed by conductor  111  and conductor  112 , conductor  112  and conductor  114 , conductor  113  and conductor  114 , and conductor  111  and conductor  113 . As shown in  FIG. 10 , the TDR apparatus  51  had a coaxial cable  52  terminating in probes  53   a  and  53   b  for launching signal pulses and receiving reflected waves; the probes  53   a  and  53   b  were placed in contact with the conductors forming the transmission line so that signals could be input and their reflections received. 
     Specifically, to measure the impedance of the transmission line formed by conductor  111  and conductor  112 , connecting pads  131  and  132  of conductor  111  and conductor  112  were contacted by probes  53   a  and  53   b ; to measure the impedance of the transmission line formed by conductor  113  and conductor  114 , connecting pads  133  and  134  of conductor  113  and conductor  114  were contacted by probes  53   a  and  53   b . To measure the impedance of the transmission line formed by conductor  111  and conductor  113 , the other ends  111   i  and  113   i  of conductor  111  and conductor  113  were contacted by probes  53   a  and  53   b ; and to measure the impedance of the transmission line formed by conductor  112  and conductor  114 , the other ends  112   i  and  114   i  of conductor  112  and conductor  114  were contacted by probes  53   a  and  53   b.    
     Exemplary waveforms that appeared on the display of the TDR apparatus  51  are shown in  FIG. 11 . In  FIG. 11 , curves B 5   a , B 5   b , B 5   c , and B 5   d  indicate the waveforms obtained when conductor  111  and conductor  112 , conductor  113  and conductor  114 , conductor  111  and conductor  113 , and conductor  112  and conductor  114 , respectively, were contacted by probes  53   a  and  53   b ; the zero levels of different waveforms are mutually offset for visibility. 
     The leftmost regions RXa to RXd of these curves indicate the impedance of the coaxial cable  52  (50Ω); the regions adjacent to regions RXa to RXd on the right correspond to the sections in which probes  53   a  and  53   b  make contact with connecting pads  131  to  134  or the ends  111   i  to  114   i  of conductors  111  to  114 ; the central regions RPa to RPd indicate the impedance of conductors  111  to  114  (the impedance of the transmission line comprising conductors  111  and  112 , the transmission line comprising conductors  113  and  114 , the transmission line comprising conductors  111  and  113 , and the transmission line comprising conductors  112  and  114 ); and the rightmost regions ROa to ROd indicate the impedance at the electrically open ends. Regions RLa and RLb of curves B 5   a  and B 5   b , which are between the central regions RPa and RPb and the regions RCa to RCd corresponding to the contact sections of probes  53   a  and  53   b , indicate the impedance of the leads  121  to  124 ; regions RLc and RLd of curves B 5   c  and B 5   d , which are between the central regions RPc and RPd and the regions ROc and ROd corresponding to the electrically open ends, indicate the impedance of the leads  121  to  124 . 
     The values shown in Table 1 can be read from the measured waveforms as the impedance of each pair of conductors. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Conductor Pair 
                 Impedance 
               
               
                   
                   
               
             
             
               
                   
                 111, 112 
                 49.0 Ω 
               
               
                   
                 113, 114 
                 49.1 Ω 
               
               
                   
                 111, 113 
                 82.0 Ω 
               
               
                   
                 112, 114 
                 77.6 Ω 
               
               
                   
                   
               
             
          
         
       
     
     The impedance conversion efficiency and waveform distortion of the novel impedance conversion device  1  were studied under various conditions. 
     In the first case studied, a load resistor  18  with a value equal to the second characteristic impedance z 2  (82Ω) was connected between the output ends of the impedance conversion device  1 , that is, between the output ends  11   e  and  13   e  of conductors  11  and  13 , as shown in  FIG. 12 . In  FIG. 12 , conductors  11  to  14  are shown as coplanar to simplify the depiction of their electrical connection relationships and the depiction of resistors  15  and  16  is also simplified. 
     When a direct current voltage Vin is supplied from a direct current source  60  to the input end of the impedance conversion device  1  in  FIGS. 1-7 , that is, the input ends  11   d  and  12   d  of conductors  11  and  12 , as shown in  FIG. 12 , (electromagnetic coupling among conductors  11  to  14  may be ignored in this case), the voltage Vout that appears across the output ends  11   e  and  13   e  is given by the following equation:
 
 V out= V in×{ R 2/(2 ×R 2+ R 1+ R in)}
 
     where Rin is the internal resistance of the direct current source  60 . 
     The internal resistance Rin is generally made equal to the input impedance R 1 ; when Rin=R 1 , the above equation becomes:
 
 V out= V in×{ R 2/(2 ×R 2+2 ×R 1)}  (1)
 
If R 1 =50Ω and R 2 =82Ω, then:
 
                         Vout   =       ⁢     Vin   ×     {     82   /     (       2   ×   50     +     2   ×   82       )       }                   =       ⁢     Vin   ×     (     82   /   264     )                     (   2   )               
If the value of Vin is five hundred millivolts (500 mV), then:
 
 V out=500×82/264=155 mV  (3)
 
     Next, the voltage that appeared at the output end when a voltage pulse train was applied from a pulse generator  61  to the input end of the impedance conversion device  1  in  FIGS. 1-7 , as shown in  FIG. 13 , was observed using an oscilloscope  65 . In  FIG. 13 , conductors  11  to  14  are shown as being coplanar and resistors  15  and  16  are depicted in the same simplified way as in  FIG. 12 . 
     The experimental impedance conversion device  1  shown in  FIGS. 14 and 15  was used in this measurement. The experimental device  1  shown in  FIGS. 14 and 15  is substantially the same as the impedance conversion device  1  shown in  FIGS. 1-7 , but has leads  121  and  122  disposed at the input ends  11   d  and  12   d  of conductors  11  and  12  and connecting pads  131  and  132  disposed at the ends of leads  121  and  122 , similar to the structure shown in  FIGS. 8 and 9 . The dielectric sheet  17  extends farther than in  FIGS. 1-7 . 
     Measurements were made by connecting resistors  15  and  16  as shown in  FIG. 13 , in the same way as described with reference to  FIGS. 1-7 ; a load resistor  18  having a resistance (RL) equal to the second characteristic impedance z 2  (82Ω) was connected across the output ends (load ends)  11   e  and  13   e  of conductors  11  and  13 . The central parts  11   b ,  12   b ,  13   b ,  14   b  of conductors  11 ,  12 ,  13 ,  14  had a length of two millimeters (2 mm). 
     A pulse generator  61  having an internal resistance Rin equal to the first impedance z 1  (50Ω) and was used. The probes  63   a  and  63   b  of the pulse generator  61  were placed in contact with the connecting pads  131  and  132  on the input side. An oscilloscope  65  having high-impedance differential probes  66   a  and  66   b  was used. The measured waveforms are shown in  FIG. 16 . 
     In  FIG. 16 , curves B 6   a , B 6   b , B 6   c , B 6   d , and B 6   e  indicate waveforms obtained when the amplitude of the supplied pulses was 500 mV and the frequency of the pulse train was 100 MHz, 500 MHz, 1 GHz, 2 GHz, and 3 GHz, respectively. 
     The wave height values and rise times (the time required for the voltage level to increase from 20 percent to 80 percent of the wave height) determined from the measured waveforms are shown in Table 2. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Input frequency 
                 Wave height (mV) 
                 Rise time (ps) 
               
               
                   
               
             
             
               
                 500 MHz 
                 255.1 
                 67.3 
               
               
                  1 GHz 
                 222.2 
                 53.1 
               
               
                  2 GHz 
                 255.1 
                 66.5 
               
               
                  3 GHz 
                 259.2 
                 59.5 
               
               
                   
               
             
          
         
       
     
     The difference between the wave height values obtained experimentally and the value obtained from equation (3) (the value of the output voltage when direct current is applied) is due to electromagnetic coupling in the transmission line. 
     For example, when the frequency is 500 MHz, the measured wave height was 255.1 mV. The difference between this value and the value obtained from equation (3) (255.1 mV−155 mV=100.1 mV) represents a voltage component induced by electromagnetic coupling, and indicates that impedance conversion has been carried out effectively. 
     Next, similar measurements were made with the output ends of the impedance conversion device  1 , more specifically the output ends  11   e  and  13   e  of conductors  11  and  13 , left electrically open. The measurement conditions were the same as described above, except that to leave output ends  11   e  and  13   e  electrically open, the load resistor  18  was omitted. The measured waveforms are shown in  FIG. 17 . The wave height values determined from the measured waveforms are shown in Table 3. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Input frequency 
                 Wave height (mV) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 100 MHz 
                 880 
               
               
                   
                 500 MHz 
                 880.1 
               
               
                   
                  1 GHz 
                 537.9 
               
               
                   
                  2 GHz 
                 391.2 
               
               
                   
                  3 GHz 
                 619.3 
               
               
                   
                   
               
             
          
         
       
     
     As shown in  FIG. 17  and Table 3, the voltage level becomes higher when output ends  11   e  and  13   e  are left electrically open. Even when the output ends are left electrically open so that the circuit has no direct current connection, adequate energy is transmitted to the output ends of conductors  11  and  13 . When there is no direct current connection, although energy is transmitted only by electromagnetic coupling, total reflection takes place at the load ends  11   e  and  13   e , so twice as much voltage is obtained, and the apparent loss of energy due to impedance conversion is virtually nil. 
     When the output ends  11   e  and  13   e  of the impedance conversion device  1  are connected to a CMOS circuit gate, they are in nearly the same state as when left electrically open, so presumably the results will be nearly the same as shown in  FIG. 17  and Table 3. 
     Though the resistor  16  (R 2 =50Ω) connected between conductors  13  and  14  causes mismatch reflection, and reflection this has a frequency dependence, if there were no mismatch, the waveforms should be smooth. The reason for the mismatch will be explained later with reference to  FIG. 21 . 
     In the above examples ( FIGS. 16 and 17 ), the central part had a length of two millimeters;  FIG. 18  shows the measured waveforms for another experimental device in which the central part had a length of twenty millimeters. In  FIG. 18 , waveforms B 8   a , B 8   b , B 8   c , B 8   d , and B 8   e  were obtained with pulse train frequencies of 100 MHz, 500 MHz, 12 GHz, 2 GHz, and 3 GHz, respectively. The wave height values determined from the measured waveforms are shown in Table 4. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Input frequency 
                 Wave height (mV) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 500 MHz 
                 311.0 
               
               
                   
                  1 GHz 
                 244.8 
               
               
                   
                  2 GHz 
                 397.0 
               
               
                   
                  3 GHz 
                 251.4 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 18  and Table 4 show a decrease in voltage and an increase in waveform distortion. The reason is thought to be the long distance between boundaries  1   f  and  1   g , which causes a relatively long elapse of time from reflection at one boundary to reflection at the other boundary, leading to multiple reflections that distort the waveforms. 
     As described above, the characteristic impedance of the duo-parallel parts  1   a  and  1   c  and the characteristic impedance of the quadri-parallel part  1   b  are slightly different. Multiple reflections therefore occur. In order to avoid multiple reflection resonance, the quadri-parallel part should have a length not exceeding one-fourth of the fundamental wavelength of the signal that is transmitted. If the specific inductive capacity of the transmission line is four, then the electromagnetic wave speed is 1.5×10 8  m/s, and if the frequency of the pulse train supplied from the pulse generator  61  is 3 GHz, it follows that the wavelength is 50 millimeters, one-fourth of which is 12.5 millimeters. 
     The length of the quadri-parallel part  1   b  need only be sufficient for electromagnetic waves to reshape the electromagnetic space between the parallel conductors. Interference between the conductors is caused by the spreading of the electromagnetic waves in a direction orthogonal to their direction of propagation, and the spreading speed is the same as the speed with which the electromagnetic waves propagate along the transmission line. Reshaping of the electromagnetic space is possible if an electromagnetic wave can travel back and forth between the conductors about five times; the length corresponding to the delay time is a length ten times as long as the larger of the two distances separating the conductors (the larger of the distance (170 micrometers) between the first conductor  11  and the second conductor  12  and the distance (100 micrometers or 0.1 millimeter) between the first conductor  11  and the third conductor  13 ). Thus, if the larger of the two distances between the conductors is 170 micrometers, ten times that length is 1.7 millimeters; the quadri-parallel structure is effective if its length is equal to or greater than this value. 
     The characteristic impedance of the quadri-parallel part  1   b  and the characteristic impedance of the duo-parallel part  1   a  and  1   c  were confirmed to be different using time-domain reflectometry.  FIGS. 19 and 20  show the structure used in this time-domain reflectometry experiment. The structure shown in  FIGS. 8 and 9  was further modified by removing the parts near the ends  113   i  and  114   i  of the third conductor  113  and the fourth conductor  114 . The length LS of the removed parts was 25 millimeters; the section with the removed parts constituted the duo-parallel part. The remaining section (the section with no parts removed), which had a length LD of 55 millimeters, constituted the quadri-parallel part. Connecting pads  131  and  132  of the first conductor  111  and the second conductor  112  of this structure were contacted by probes  53   a  and  53   b  of the TDR apparatus  51 . The measured waveforms are shown in  FIG. 21 . The longitudinal axis in  FIG. 21  is enlarged compared to that in  FIG. 11 . 
     In  FIG. 21 , region RXa corresponds to a section of the coaxial cable  52 , region RCa corresponds to leads  121  and  122 , region RPa 1  corresponds to the quadri-parallel part (length LD), region RPa 2  corresponds to the duo-parallel part (length LS), and region ROa corresponds to the electrically open ends. 
     The impedance of the quadri-parallel part (length LD) shown in  FIG. 21  is 48Ω, and the impedance of the right-side region RPa 22  (excluding the region RPa 21  adjacent to the region RPa 1  corresponding to the quadri-parallel part) of the duo-parallel part (length LS) is 51.2Ω; reflection occurs due to this difference. The upper limit described above on the length of the quadri-parallel part  1   b  is set in order to prevent reflection from occurring repeatedly and leading to multiple reflections. 
     In the region RPa 2  corresponding to the duo-parallel part, the characteristic impedance changes gradually in the region RPa 21  adjacent to the region RPa 1  corresponding to the quadri-parallel part. This part corresponds to 125 picoseconds of time, which is the sum of the slump due to the rise time of the step waveform of the TDR apparatus  51  (35 picoseconds, the same as the slump at the contact section RCa and the electrically open end ROa) and the time taken to detect the change; these factors cannot be separated accurately, but the physical phenomena that operate during detection are similar to the reshaping of the electromagnetic space described above. 
     Next, electromagnetic coupling between the conductors, in other words, crosstalk, will be described with reference to  FIGS. 22 and 23 . 
     As shown in  FIG. 22 , the pulse energy input to one of the parallel conductors  11  to  14  causes various combinations of interference on adjacent conductors; the optimal state is ultimately the one in which inverted waveform energy is induced in the proximate conductors by electromagnetic interference as shown in  FIG. 23 , with the crosstalk energy corresponding to the electromagnetic dispersion energy. This is in forward waves. Though backward waves are also induced, they are omitted here. The input induces vertical coupling (coupling between the vertically adjacent conductors  11  and  12  in  FIG. 1 ), and so the upper-left conductor becomes the output of the adjacent vertical coupling. With horizontal coupling (coupling between the horizontally adjacent conductors in  FIG. 1 ), however, the energy becomes the sum of the original energy on one side and the energy of the far end composite wave (upper right); a voltage of 250 mV was obtained experimentally, which is larger than the divided direct current voltage of 155 mV. The difference represents an improvement in the efficiency of impedance conversion. This energy state between parallel conductors is achieved if a relationship corresponding to the one shown in  FIGS. 22 and 23  is formed for even an instant (the time during which interference occurs at the speed of light); the minimum length is thus the length described above. 
     Though the conductors are disposed on the upper surface and lower surface of the dielectric sheet in  FIGS. 1-7 , a structure in which conductors  11  to  14  are all embedded in a dielectric material  21  as shown in  FIG. 24  (a sectional view similar to  FIG. 6 ) is also possible. The first to fourth conductors  11  to  14  may be formed in the same way as two pairs of stacked pair conductors are formed. 
     In the above embodiment, the first to third conductors  11  to  13  have input parts  11   a  and  12   a  and output parts  11   c  and  13   c  as well as central parts  11   b ,  12   b , and  13   b , but the impedance conversion device may comprise only the central parts; the input parts  11   a  and  12   a  and output parts  11   c  and  13   c  may be omitted. 
     Although the first to fourth conductors  11  to  14  extend in straight lines in the above embodiment, they may be curved. The cross-sectional shapes and dimensions of the first to fourth conductors  11  to  14  need not all be the same; some may differ from the others. 
     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.