Abstract:
The present invention discloses a pair of differential microstrip lines with low cross-talk for high-frequency signal transmission. The pair of microstrip lines comprises two microstrip lines. The first microstrip line is used to transmit the first transmission signal. The second microstrip line is parallel to the first microstrip line and used to transmit the second transmission signal. The first transmission signal is the complementary signal of the second transmission signal and has a 180° phase difference from the second transmission signal. Particularly, there are a plurality of slots periodically arranged on the outer sides of the first and the second microstrip lines to form a subwavelength configuration. The subwavelength configuration is to make the periodical arrangement length of these slots shorter than the wavelengths of the first and the second transmission signals. These slots can provide subwavelength confinement for enhancing electromagnetic wave.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Taiwan Patent Application No. 103104920, filed on Feb. 14, 2014, in the Taiwan Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a transmission line, in particular to low cross-talk differential microstrip lines for high-frequency signal transmission. 
         [0004]    2. Description of the Related Art 
         [0005]    In recently years, since signal transmission rate becomes higher, and the size of electronic products becomes smaller, electronic circuits tend to be designed to be more intensive. Due to the high integrity, the cross-talk between electronic circuits becomes more serious than before. When signals are transmitted via transmission lines, the adjacent transmission lines will be interfered by each other due to the electromagnetic wave coupling phenomenon; as a result, the interfered transmission lines may generate coupling voltage and current, which is so-called cross-talk. Excessive cross-talk may influence the efficiency of the system, even result in the false trigger of the circuit to damage the system. Besides, when designing a bent electronic circuit, engineers usually increase the interval between the adjacent microstrip lines, or increase the rising and falling time of the digital signals to reduce the cross-talk; however, the cross-talk still cannot be completely eliminated. 
         [0006]    As the conventional methods cannot effectively eliminate the cross-talk occurring between transmission lines, it is necessary to propose novel differential microstrip lines with low cross-talk for high-frequency signal transmission for the purpose of suppressing the cross-talk and reducing the mode conversion effect between differential mode and common mode. 
       SUMMARY OF THE INVENTION 
       [0007]    When signals are transmitted through microstrip lines, most of the surface current will be distributed at the edges of the microstrip lines. In other words, the edges of the lines have very high current density. The invention is to etch periodical subwavelength corrugations along the edges of the microstrip lines to introduce the current at the edges into the slots to form an approximately closed loop. This subwavelength structure is favorable to increase the self-inductance of the circuit and confine the magnetic field around the lines, which could greatly reduce thecross-talk due to the mutual inductance between the adjacent circuits. The confinement effect of the magnetic field could be controlled by the configuration and depth of the slots. 
         [0008]    A periodical structure in the conventional microstrip circuits is usually for band stop filters; however, it is not so practical because of its long length. Besides, another purpose of the periodical structure in the conventional microstrip circuits is to serve as a proper R-L structure for coupling adjacent circuits. Therefore, the concept of the invention is different from the above two conventional circuits. As the purposes of the periodical structures in the conventional microstrip circuits are deeply rooted in those skilled in the art, it is impossible for them to use a periodical structure as a main signal transmission body. Additionally, the circuit design softwares used by them usually do not support this kinds of circuits; therefore, it is inconceivable for them to use this kinds of circuits as a signal line. Currently, there are two common methods to suppress cross-talk. First one is to bend the differential pair and the single-ended line many turns to reduce cross-talk; however, it may generate the common mode signal in the differential pair, which is unfavorable to the operation of the whole circuit. The other one is to install additional guard trace grounded lines between adjacent loops; however, it could result in two obvious shortcomings First, the areas of the loops cannot be effectively deceased. Second, the guard trace grounded lines can block out the electrical field between the two loops, but cannot effectively suppress the mutual inductance between lines. According to the present invention, corrugate paths are sculptured on the surface of conductors, and the current distributed over the edge of the line will flow into the corrugate paths to form a quasi closed loop, which can effectively confine the magnetic field and suppress the cross-talk resulting from the mutual inductance. The confinement will become stronger if the frequency of the signal is higher. As the periodical length is much shorter than the wavelength; therefore, its working frequency is away from the band gap of the periodic structure, and its main function is to transmit signal rather than reflect signal. Accordingly, the application of the invention is different from filter. The present invention is applicable to high-frequency microwave circuit and high-speed circuit; in particular, the present invention can effectively isolate the mutual interference in high density circuits. The differential pair is mainly used to transmit complementary signals; and, it has better anti-jamming performance, but needs more signal lines than the single-ended transmission line; therefore, the circuit area of the differential pair is bigger. So as to reduce the circuit area of the differential pair, the distances between the differential pair and other transmission lines need to be further decreased, which will bring about serious cross-talk and mode conversion effect between the differential signal and common mode signal. Thus, it is necessary to use an innovative transmission line to replace the conventional differential microstrip lines. The differential pair includes two transmission lines, and both of them are used to transmit signals; however, there is a 180° phase difference between the signals transmitted by them, which is different from the single-ended transmission line. 
         [0009]    One of the primary objects of the present invention is to provide a pair of differential microstrip lines with low cross-talk for high-frequency signal transmission. The differential microstrip lines are comprised of two microstrip lines. The first microstrip line is used for transmitting the first transmission signal, and has a plurality of slots arranged periodically. The second microstrip line is used for transmitting the second transmission signal, which is parallel to the first microstrip line and has a plurality of slots arranged periodically. The second transmission signal is a complementary signal of the first transmission signal and has a 180° phase difference from the first transmission signal. In particular, the slots are periodically arranged along outer sides of the first microstrip line and the second microstrip line to form a subwavelength configuration making the periodical length of these arranged slots shorter than the wavelengths of the first and the second transmission signals, so that the slots is capable of enhancing subwavelength confinement for the electromagnetic wave. 
         [0010]    In a preferred embodiment, the differential microstrip lines further comprise two terminals. The first terminal respectively receives the complementary signals from the first microstrip line and the second microstrip line; the second terminal respectively outputs the complementary signals from the first microstrip line and the second microstrip line, wherein the slot are arranged along the edges of the microstrip lines. Whereby, the slots reduce the energy cross-talk effect from the adjacent microstrip lines or differential pair, or reduce the mode conversion effect between the differential mode and the common mode when the first terminal transmits the complementary signal to the second terminal. 
         [0011]    In a preferred embodiment, the differential microstrip lines further comprise a plurality of slots periodically arranged along the inner side of the first microstrip line opposite the outer side of the first microstrip line; and a plurality of slots periodically arranged along the inner side of the second microstrip line opposite the outer side of the second microstrip line. 
         [0012]    The low cross-talk differential microstrip lines for high-frequency signal transmission according to the present invention have the following advantages: 
         [0013]    The low cross-talk differential microstrip lines for high-frequency signal transmission according to the present invention can effectively solve the cross-talk and mode conversion effect to improve signal transmission quality and reduce the size of the circuit board. 
         [0014]    The low cross-talk differential microstrip lines for high-frequency signal transmission according to the present invention have periodical slots with subwavelength size, and the shape and the size of the slots can be adjusted with the actual design in order to restrain the electromagnetic wave above the slots by artificial surface plasmon polaritons. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The detailed structure, operating principle and effects of the present invention will now be described in more details hereinafter with reference to the accompanying drawings that show various embodiments of the invention as follows. 
           [0016]      FIG. 1  is a schematic view of a differential pair with opening-type periodical subwavelength configuration in accordance with the present invention. 
           [0017]      FIG. 2  is a schematic view of a coupling circuit of a conventional differential pair and the differential pair with opening-type periodical subwavelength configuration in accordance with the present invention. 
           [0018]      FIG. 3  is a schematic view of the cross-talk between the conventional differential pair and the differential pair with the opening-type periodical subwavelength configuration in accordance with the present invention, where S dd21  stands for signal transmission ability, and S dd41  stands for cross-talk. 
           [0019]      FIG. 4  is a schematic view of the mode conversion effect between differential mode and common mode of the differential pair with opening-type periodical subwavelength configuration in accordance with the present invention, where S cd21  stands for mode conversion effect. 
           [0020]      FIG. 5  is a schematic view of a differential pair with hairpin-type periodical subwavelength configuration in accordance with the present invention. 
           [0021]      FIG. 6  is a schematic view of the detailed structure of the differential pair with hairpin-type periodical subwavelength configuration in accordance with the present invention. 
           [0022]      FIG. 7  is a schematic view of a coupling circuit of a conventional differential pair and the differential pair with hairpin-type periodical subwavelength configuration in accordance with the present invention. 
           [0023]      FIG. 8  is a schematic view of the cross-talk between the conventional differential pair and the differential pair with the hairpin-type periodical subwavelength configuration in accordance with the present invention, where S dd21  stands for signal transmission ability, and S dd41  stands for cross-talk. 
           [0024]      FIG. 9  is a schematic view of the mode conversion effect between differential mode and common mode of the differential pair with hairpin-type periodical subwavelength configuration in accordance with the present invention, where S cd21  stands for mode conversion effect. 
           [0025]      FIG. 10  is a schematic view of a differential pair with slot-type periodical subwavelength configuration in accordance with the present invention. 
           [0026]      FIG. 11  is a schematic view of a coupling circuit of a conventional differential pair and the differential pair with slot-type periodical subwavelength configuration in accordance with the present invention. 
           [0027]      FIG. 12  is a schematic view of the cross-talk between the conventional differential pair and the differential pair with the slot-type periodical subwavelength configuration in accordance with the present invention, where S dd21  stands for signal transmission ability, and S dd41  stands for cross-talk. 
           [0028]      FIG. 13  is a schematic view of the mode conversion effect between differential mode and common mode of the differential pair with slot-type periodical subwavelength configuration in accordance with the present invention, where S cd21  stands for mode conversion effect. 
           [0029]      FIG. 14  is a schematic view of a differential pair with double sided opening-type periodical subwavelength configuration in accordance with the present invention. 
           [0030]      FIG. 15  is a schematic view of a coupling circuit of a conventional differential pair and the differential pair with double sided opening-type periodical subwavelength configuration in accordance with the present invention. 
           [0031]      FIG. 16  is a schematic view of the cross-talk between the conventional differential pair and the differential pair with the double sided opening-type periodical subwavelength configuration in accordance with the present invention, where S dd21  stands for signal transmission ability, and S dd41  stands for cross-talk. 
           [0032]      FIG. 17  is a schematic view of the mode conversion effect between differential mode and common mode of the differential pair with double sided opening-type periodical subwavelength configuration in accordance with the present invention, where S cd21  stands for mode conversion effect. 
           [0033]      FIG. 18  is a schematic view of a differential pair with double sided slot-type periodical subwavelength configuration in accordance with the present invention. 
           [0034]      FIG. 19  is a schematic view of a coupling circuit of a single-ended microstrip line and the differential pair with double sided slot-type periodical subwavelength configuration in accordance with the present invention. 
           [0035]      FIG. 20  is a schematic view of the cross-talk between the single-ended microstrip line and the differential pair with the double sided slot-type periodical subwavelength configuration in accordance with the present invention, where S dd21  stands for signal transmission ability, and S dd41  stands for cross-talk. 
           [0036]      FIG. 21  is a schematic view of the mode conversion effect between differential mode and common mode of the differential pair with double sided slot-type periodical subwavelength configuration in accordance with the present invention, where S cd21  stands for mode conversion effect. 
           [0037]      FIG. 22  is a schematic view of a differential pair with double sided hairpin-type periodical subwavelength configuration in accordance with the present invention. 
           [0038]      FIG. 23  is a schematic view of the detailed structure of the differential pair with double sided hairpin-type periodical subwavelength configuration in accordance with the present invention. 
           [0039]      FIG. 24  is a schematic view of a coupling circuit of a single-ended microstrip line and the differential pair with double sided hairpin-type periodical subwavelength configuration in accordance with the present invention. 
           [0040]      FIG. 25  is a schematic view of the cross-talk between the single-ended microstrip line and the differential pair with the double sided hairpin-type periodical subwavelength configuration in accordance with the present invention, where S dd21  stands for signal transmission ability, and S dd41  stands for cross-talk. 
           [0041]      FIG. 26  is a schematic view of the mode conversion effect between differential mode and common mode of the differential pair with double sided hairpin-type periodical subwavelength configuration in accordance with the present invention, where S cd21  stands for mode conversion effect. 
           [0042]      FIG. 27  is a schematic view of a differential pair with slot-type periodical subwavelength configuration in accordance with the present invention. 
           [0043]      FIG. 28  is a schematic view of a differential pair with slot-type periodical subwavelength configuration in accordance with the present invention. 
           [0044]      FIG. 29  is a schematic view of a differential pair with opening-type periodical subwavelength configuration in accordance with the present invention. 
           [0045]      FIG. 30  is a schematic view of a differential pair with opening-type periodical subwavelength configuration in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    The technical content of the present invention will become apparent by the detailed description of the following embodiments and the illustration of related drawings as follows. 
         [0047]    The first embodiment of the present invention is, as shown in  FIG. 1 , a differential pair of microstrip lines with the opening-type periodical subwavelength configuration, wherein the differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with periodical subwavelength configurations. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 . Two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and each of the rectangular convex bodies  16  comprises two first extended portions  17  parallel extend to centers of the adjacent slots respectively. 
         [0048]    The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, the dielectric constant of the medium of the substrate  21  is ε r , and the thickness of the first extended portion  17  is a 1 . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or differential pairs. The numerical analysis of the coupling circuit configuration shown in  FIG. 2  can prove that the differential pair with the periodical subwavelength configuration can dramatically suppress the cross-talk between adjacent microstrip lines and reduce the mode conversion effect between the differential mode and common mode. 
         [0049]      FIG. 2  shows a coupling circuit which consists of two microstrip lines with periodically arranged subwavelength slots and a conventional differential pair (the distance between the microstrip lines is W 4 .). S dd21  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the second terminal  14 , which clearly shows the transmission ability of the differential pairs. S dd41  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the fourth terminal  23 , which clearly shows the cross-talk between the differential pair ( 11 ,  12 ) and the conventional differential pair. The distance between the two differential pairs is W 2 . The S parameter, S dd21 , stands for the transmission ability of the differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14 . The S parameter, S dd41 , stands for the generated cross-talk effect when differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the conventional differential pair. The S parameter, S cd21 , stands for the generated mode conversion effect from the differential mode to the common mode when differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The “conventional” in the figures stands for the transmission and cross-talk effects of the differential pair without the periodical subwavelength configuration, which are illustrated by solid lines. The transmission and cross-talk effects of the differential pair with the periodical subwavelength configuration are illustrated by dashed lines. The simulation parameters of  FIG. 3  and  FIG. 4  are as follows: W=W 1 =W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is  10  cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. In  FIG. 2 , the first terminal  13  receives the differential signals complementary to each other, the second terminal  14  is the receiver of the differential pair, the third terminal  22  is the near end of the conventional differential pair and the fourth terminal  23  is the remote end of the conventional differential pair. In  FIG. 3 , S dd21  stands for the signal transmission ability of the differential pairs and S dd41  stands for the cross-talk between the adjacent differential pairs. In  FIG. 4 , S cd21  stands for the mode conversion effect from the differential signal to the common mode signal. 
         [0050]    In the first embodiment, if both differential pairs are conventional differential pairs, the simulation result can be illustrated by the solid lines of  FIG. 3  and  FIG. 4 . As shown in  FIG. 3 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the transmission ability of the conventional differential pair is expressed by the S parameter, S dd21 : S dd21 =−0.08821 dB when the signal frequency is 200 MHz, S dd21 =−2.32492 dB when the signal frequency is 12 GHz. The cross-talk of the conventional differential pair (i.e. the differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23 ) is expressed by the S parameter, S dd41 : S dd41 =−48.55245 dB when the signal frequency is 200 MHz, S dd41 =−9.38157 dB when the signal frequency is 12 GHz. As shown in  FIG. 4 , the mode conversion effect between the differential mode to common mode when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S cd21 : S cd21 =−12.37439 dB when the signal frequency is 12 GHz. 
         [0051]    In the first embodiment, if one of the differential pairs is a conventional differential pair and the other one is a differential pair with the periodical subwavelength configuration, the simulation result can be illustrated by the dashed lines of  FIG. 3  and  FIG. 4 . As shown in  FIG. 3 , the transmission ability of the differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S dd21 : S dd21 =−0.07573 dB when the signal frequency is 200 MHz, and S dd21 =−1.21404 dB when the signal frequency is 12 GHz. As shown in  FIG. 3 , the cross-talk of the differential signals which are inputted into the first terminal  13  and outputted from the fourth terminal  23  is expressed by the S parameter, S dd41 : S dd41 =−60.6408 dB when the signal frequency is 200 MHz, S dd41 =−29.62501 dB when the signal frequency is 12 GHz and the maximum of the cross-talk between 1 GHz-10 GHz is S dd41 =−34.538 dB when the signal frequency is 5.1 GHz. As shown in  FIG. 4 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to common mode is expressed by the S parameter, S cd21 : S cd2 =−27.66008 dB when the signal frequency is 12 GHz. 
         [0052]    In the first embodiment, the overall comparison results between the conventional differential pair and the differential pair with the periodical subwavelength configuration are shown in  FIG. 3  and  FIG. 4 . As shown in  FIG. 3 , the transmission ability of the conventional differential pair is S dd21 =−2.32492 dB when the signal frequency is 12 GHz; the transmission ability of the differential pair with the periodical subwavelength configuration is S dd21 =−1.21404 dB when the signal frequency is 12 GHz. The transmission ability is obviously improved at high signal frequency. As shown in  FIG. 3 , when the signal frequency is 12G Hz, the cross-talk of the conventional differential pair is S dd41 =−9.38157 dB and that of the differential pair with the periodical subwavelength configuration is S dd41 =−29.62501 dB; obviously, the cross-talk is significantly suppressed. As shown in  FIG. 4 , when the signal frequency is 12 GHz, the mode conversion effect between the differential mode and the common mode of the conventional differential pair is S cd21 =−12.37439 dB and that of the differential pair with the periodical subwavelength configuration is S cd21 =−27.66008 dB; obviously, the mode conversion effect is significantly decreased. PS:  FIG. 3  is the S parameter calculation result of the coupling circuit of  FIG. 2 . Please refer to  FIG. 3 , S dd21  of the conventional differential pair is illustrated by a solid line, which shows its transmission ability is −0.08821 dB when the signal frequency is 200 MHz, and is −2.32492 dB when the signal frequency is 12 GHz. S dd21  of the differential pair with the periodical subwavelength configuration is illustrated by a dashed line, which shows its transmission ability is −0.07573 dB when the signal frequency is 200 MHz, and is −1.21404 dB when the signal frequency is 12 GHz. Apparently, the differential pair with the periodical subwavelength configuration has better transmission ability and confinement of the electromagnetic field. With the strong confinement of the electromagnetic field, the differential pair with the periodical subwavelength configuration will not result in serious interference to adjacent microstrip lines. With the increase of the frequency, the cross-talk will become more obvious. The cross-talk between the conventional differential pairs is S dd41 =−9.38157 dB when the signal frequency is 12 GHz; and the cross-talk between the conventional differential pair and the differential pair with periodical subwavelength configuration is S dd41 =−29.62501 dB when the signal frequency is 12 GHz, which effectively reduces the cross-talk.  FIG. 4  shows the relation between the mode conversion effect and the frequency. With the increase of the frequency, the mode conversion effect will become more obvious. However, the differential pair with periodical subwavelength configuration can effectively suppress the mode conversion effect. The mode conversion effect of the conventional differential pair is S cd21 =−12.37439 dB when the signal frequency is 12 GHz, and the mode conversion effect of the differential pair with periodical subwavelength configuration is S cd21 =−27.66008 dB; therefore, the periodical subwavelength configuration can greatly suppress the mode conversion effect between the differential mode and common mode. 
         [0053]    The second embodiment of the present invention is, as shown in  FIG. 5 , a differential pair with the hairpin-type periodical subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with periodical subwavelength configurations. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of Z-shaped convex bodies  20  arranged continuously and periodically, and each of the Z-shaped convex bodies  20  comprises two extended portions  17 ,  18 , wherein the first extended portion  17  parallel extends from the opening of one slot to the center of the adjacent slot; and the second extended portion  18  parallel extends from the middle of the Z-shaped convex body  20  to the center of another adjacent slot. The direction where the first extended portion  17  extends is inverse to the direction where the second extended portion  18  extends. 
         [0054]    The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . There are still other structure parameters, such as a 1 , a 2  (the widths of the outer openings), a 3  (the width of the inner openings), b 1  (the width of the thin metal bars) and b 2  (the interval of the thin metal bars). If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. The numerical analysis of the coupling circuit configuration shown in  FIG. 7  can prove that the differential pair with the hairpin-type periodical subwavelength configuration can dramatically suppress the cross-talk between the adjacent microstrip lines and reduce the mode conversion effect between the differential mode and the common mode.  FIG. 7  shows a coupling circuit composed of a conventional differential pair and a differential pair with the hairpin-type periodical subwavelength configuration. S dd21  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the second terminal  14 , which clearly shows the transmission ability of the differential pairs. The cross-talk between the conventional differential pair and differential pair ( 11 ,  12 ) can be acquired by analyzing the outputs from the fourth terminal  23 . The distance between the two differential pairs is W 2 . The S parameter, S dd21 , stands for the transmission ability of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14 . The S parameter, S dd41 , stands for the generated cross-talk effect when differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the conventional differential pair. The S parameter, S cd21 , stands for the generated mode conversion effect from the differential mode to common mode when differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The “conventional” in the figures stands for the transmission and cross-talk effects of the differential pair without the periodical subwavelength configuration, which are illustrated by solid lines. The transmission and cross-talk effects of the differential pair with the periodical subwavelength configuration are illustrated by dashed lines. The simulation parameters are as follows: W=W 1 =W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. In  FIG. 7 , the first terminal  13  receives the differential signals complementary to each other, the second terminal  14  is the receiver of the differential pair, the third terminal  22  is the near end of the conventional differential pair and the fourth terminal  23  is the remote end of the conventional differential pair. The simulation parameters of  FIG. 8  are as follows: W=W 1 =W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is  10  cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The S parameter, S dd21 , stands for the signal transmission ability. The S parameter, S dd41 , stands for the cross-talk effect between the differential pair with the periodical subwavelength configuration and the conventional differential pair. The simulation parameters of  FIG. 8  are as follows: W=W 1 =W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is  10  cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The S parameter, S cd21 , stands for the mode conversion effect from the differential signal to thecommon mode signal. 
         [0055]    In the second embodiment, if both differential pairs are conventional differential pairs, the simulation result can be illustrated by the solid lines of  FIG. 8  and  FIG. 9 . As shown in  FIG. 8 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the transmission ability is expressed by the S parameter, S dd21 : S dd21 =−0.08821 dB when the signal frequency is 200 MHz, S dd21 =−2.32492 dB when the signal frequency is 12 GHz. As shown in  FIG. 8 , the cross-talk of the conventional differential pair (i.e. the differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23 ) is expressed by the S parameter, S dd41 : S dd41 =−48.55245 dB when the signal frequency is 200 MHz, S dd41 =−9.38157 dB when the signal frequency is 12 GHz. As shown in  FIG. 9 , the mode conversion effect between the differential mode to common mode when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S cd21 : S cd21 =−12.37439 dB when the signal frequency is 12 GHz. 
         [0056]    In the second embodiment, the simulation result of the coupling circuit of the conventional differential pair and the differential pair with the hairpin-type periodical subwavelength configuration can be illustrated by the dashed lines of  FIG. 8  and  FIG. 9 . 
         [0057]    As shown by the dashed lines of  FIG. 8 , the transmission ability of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S dd21 : S dd21 =−0.09344 dB when the signal frequency is 200 MHz, and Sdd 21 =−1.20989 dB when the signal frequency is 12 GHz. As shown in  FIG. 8 , the cross-talk of the differential signals which are inputted into the first terminal  13  and outputted from the fourth terminal  23  is expressed by the S parameter, S dd41 : S dd41 =−63.57423 dB when the signal frequency is 200 MHz, and S dd41 =−33.33179 dB when the signal frequency is 12 GHz. As shown in  FIG. 9 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode is expressed by the S parameter, S cd21 : S cd21 =−35.91338 dB when the signal frequency is 12 GHz. 
         [0058]    In the second embodiment, the overall comparison results between the conventional differential pair and the differential pair with the hairpin-type periodical subwavelength configuration are shown in  FIG. 8  and  FIG. 9 . As shown in  FIG. 8 , the transmission ability that the both differential pairs are conventional differential pairs is S dd21 =−2.32492 dB when the signal frequency is 12 GHz; the transmission ability of the differential pair with the periodical subwavelength configuration is S dd21 =−1.20989 dB when the signal frequency is 12 GHz. The transmission ability is obviously improved at high signal frequency. As shown in  FIG. 8 , when the signal frequency is 12 GHz, the cross-talk between the two conventional differential pairs is S dd41 =−9.38157 dB and the cross-talk between the conventional differential pair and the differential pair with the periodical subwavelength configuration is S dd41 =−33.33179 dB; obviously, the cross-talk is significantly suppressed. As shown in  FIG. 9 , when the signal frequency is 12 GHz, the mode conversion effect between the conventional differential pairs is S cd21 =−12.37439 dB and that of the differential pair with the periodical subwavelength configuration is S cd21 =−35.91338 dB; obviously, the mode conversion effect is significantly decreased. PS:  FIG. 8  is the S parameter calculation result of the coupling circuit of  FIG. 7 . Please refer to  FIG. 8 , S dd21  of the conventional differential pair is illustrated by a solid line, which shows its transmission ability is −0.08821 dB when the signal frequency is 200 MHz, and is −2.32492 dB when the signal frequency is 12 GHz. S dd21  of the differential pair with the hairpin-type periodical subwavelength configuration is illustrated by a dashed line, which shows its transmission ability is −0.09344 dB when the signal frequency is 200 MHz, and is −1.20989 dB when the signal frequency is 12 GHz. The conventional differential pair has a little bit better transmission ability at low frequency. With the increase of the frequency, the differential pair with periodical subwavelength configuration will have better transmission ability and confinement of the electromagnetic field. With the strong confinement of the electromagnetic field, the differential pair with the hairpin-type periodical subwavelength configuration will not result in serious interference to adjacent microstrip lines. With the increase of the frequency, the cross-talk will become more obvious. The cross-talk between the conventional differential pairs is S dd41 =−9.38157 dB when the signal frequency is 12 GHz 
         [0059]    However, the cross-talk between the conventional differential pair and the differential pair with the hairpin-type periodical subwavelength configuration is S dd41 32 −33.33179 dB, which effectively reduces the cross-talk.  FIG. 9  shows the relation between the mode conversion effect and the frequency. With the increase of the frequency, the mode conversion effect will become more obvious. However, the differential pair with hairpin-type periodical subwavelength configuration can effectively suppress the mode conversion effect. The mode conversion effect of the conventional differential pair is S cd21 =−12.37439 dB when the signal frequency is 12 GHz; and the mode conversion effect of the differential pair with the hairpin-type periodical subwavelength configuration is S cd21 =−35.91338 dB; therefore, the periodical subwavelength configuration can greatly suppress the mode conversion effect between the differential mode and common mode. 
         [0060]    The third embodiment of the present invention is, as shown in  FIG. 10 , a differential pair with the slot-type periodical subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with the slot-type periodical subwavelength configuration. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and the interval of the two adjacent rectangular convex bodies  16  is the periodical arrangement length of these slots. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. 
         [0061]    The numerical analysis of the coupling circuit configuration shown in  FIG. 11  can prove that the differential pair with the periodical subwavelength configuration can dramatically suppress the cross-talk between the adjacent microstrip lines and reduce the mode conversion effect between the differential mode and the common mode.  FIG. 11  shows a coupling circuit composed of a conventional differential pair and a differential pair with the slot-type periodical subwavelength configuration. S dd21  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the second terminal  14 , which clearly shows the transmission ability of the differential pairs. S dd41  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the fourth terminal  23  of the conventional differential pair, which clearly shows the cross-talk of the differential pairs. The distance between the two differential pairs is W 2 . The S parameter, S dd21 , stands for the transmission ability of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14 . The S parameter, S dd41 , stands for the generated cross-talk effect when differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the conventional differential pair. The S parameter, S cd21 , stands for the generated mode conversion effect from the differential mode to common mode when differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The “conventional” in the figures stands for the transmission and cross-talk effects of the differential pair without the periodical subwavelength configuration, which are illustrated by solid lines. The transmission and cross-talk effects of the differential pair with the periodical subwavelength configuration are illustrated by dashed lines. The simulation parameters are as follows: W=W 1 =W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The first terminal  13  receives the differential signals complementary to each other, the second terminal  14  is the receiver of the differential pair, the third terminal  22  is the near end of the conventional differential pair and the fourth terminal  23  is the remote end of the conventional differential pair. As shown in  FIG. 12 , the simulation parameters are as follows: W=W=W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The S parameter, S dd21 , stands for the signal transmission ability. The S parameter, S dd41 , stands for the cross-talk effect between the differential pair with the periodical subwavelength configuration and the conventional differential pair. As shown in  FIG. 13 , the simulation parameters are as follows: W=W 1 =W 2 =W 3 =W 4 =1. 2  mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.6W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The S parameter, S cd21 , stands for the mode conversion effect from the differential signal to the common mode signal. 
         [0062]    In the third embodiment, if both differential pairs are conventional differential pairs, the simulation result can be illustrated by the solid lines of  FIG. 12  and  FIG. 13 . As shown in  FIG. 12 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the transmission ability is expressed by the S parameter, S dd21 : S dd21 =−0.08821 dB when the signal frequency is 200 MHz, S dd21 =−2.32492 dB when the signal frequency is 12 GHz. As shown in  FIG. 12 , the cross-talk of the conventional differential pair (i.e. differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23 ) is expressed by the S parameter, S dd41 : S dd41 =−48.55245 dB when the signal frequency is 200 MHz, S dd41 =−9.38157 dB when the signal frequency is 12 GHz. As shown in  FIG. 13 , the mode conversion effect between the differential mode to common mode when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S cd21 : S cd21 =−12.37439 dB when the signal frequency is 12 GHz. 
         [0063]    In the third embodiment, the simulation result of the coupling circuit of the conventional differential pair and the differential pair with the slot-type periodical subwavelength configuration can be illustrated by the dashed lines of  FIG. 12  and  FIG. 13 . As shown by the dashed lines of  FIG. 12 , the transmission ability of the differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S dd21 : S dd21 =−0.07265 dB when the signal frequency is 200 MHz, and S dd21 =−1.14271 dB when the signal frequency is 12 GHz. As shown in  FIG. 12 , the cross-talk of the differential signals which are inputted into the first terminal  13  and outputted from the fourth terminal  23  is expressed by the S parameter, S dd41 : S dd41 =−61.53771 dB when the signal frequency is 200 MHz, and S dd41 =−36.11641 dB when the signal frequency is 12 GHz and the maximum of the cross-talk between 1 GHz-10 GHz is S dd41 =−32.2849 dB when the signal frequency is 5.36 GHz. As shown in  FIG. 13 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode expressed by the S parameter, S cd21 : S cd21 =−19.69095 dB when the signal frequency is 12 GHz. 
         [0064]    In the third embodiment, the overall comparison results between the conventional differential pair and the differential pair with the slot-type periodical subwavelength configuration are shown in  FIG. 12  and  FIG. 13 . As shown in  FIG. 12 , the transmission ability of the conventional differential pair is S dd21 =−2.32492 dB when the signal frequency is 12 GHz; the transmission ability of the differential pair with the periodical subwavelength configuration is S dd21 =−1.14271 dB when the signal frequency is 12 GHz. The transmission ability is obviously improved at high signal frequency. As shown in  FIG. 12 , when the signal frequency is 12 GHz, the cross-talk between the two conventional differential pairs is S dd41 =−9.38157 dB and the cross-talk between the conventional differential pair and the differential pair with the periodical subwavelength configuration is S dd41 =−36.11641 dB; obviously, the cross-talk is significantly suppressed. As shown in  FIG. 13 , when the signal frequency is 12 GHz, the mode conversion effect between the conventional differential pairs is S cd21 =−12.37439 dB and that of the differential pair with the periodical subwavelength configuration is S cd21 =−19.69095 dB; obviously, the mode conversion effect is significantly decreased. PS:  FIG. 12  is the S parameter calculation result of the coupling circuit of  FIG. 11 . Please refer to  FIG. 12 , S dd21  of the conventional differential pair is illustrated by a solid line, which shows its transmission ability is −0.08821 dB when the signal frequency is 200 MHz, and is −2.32492 dB when the signal frequency is 12 GHz. S dd21  of the differential pair with the slot-type periodical subwavelength configuration is illustrated by a dashed line, which shows its transmission ability is −0.07265 dB when the signal frequency is 200 MHz, and is −1.14271 dB when the signal frequency is 12 GHz. The differential pair with the periodical subwavelength configuration has just a little bit better transmission ability at low frequency. With the increase of the frequency, the differential pair with periodical subwavelength configuration will have better transmission ability and confinement of the electromagnetic field. With the strong confinement of the electromagnetic field, the differential pair with the periodical subwavelength configuration will not result in serious interference to adjacent microstrip lines or conventional differential pair. With the increase of the frequency, the cross-talk will become more obvious. The cross-talk between the conventional differential pairs is S dd41 =−9.38157 dB when the signal frequency is 12 GHz, and the cross-talk between the conventional differential pair and the differential pair with the periodical subwavelength configuration is S dd41 =−36.11641 dB when the signal frequency is 12 GHz, which clearly shows that the differential pair with the periodical subwavelength configuration can effectively reduce the cross-talk.  FIG. 13  shows the relation between the mode conversion effect and the frequency in the coupling circuit. With the increase of the frequency, the mode conversion effect will become more obvious. However, the differential pair with periodical subwavelength configuration can effectively suppress the mode conversion effect. The mode conversion effect of the conventional differential pair is S cd21 =−12.37439 dB when the signal frequency is 12 GHz; and the mode conversion effect of the differential pair with the slot-type periodical subwavelength configuration is S cd21 =−19.69095 dB; therefore, the periodical subwavelength configuration can greatly suppress the mode conversion effect between the differential mode and common mode. 
         [0065]    The fourth embodiment of the present invention is, as shown in  FIG. 14  and  FIG. 15 , a differential pair with the double sided opening-type periodical subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with periodical subwavelength configurations. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and each of the rectangular convex bodies comprises two first extended portions  17  parallel extend to centers of the adjacent slots respectively. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r , and the thickness of the first extended portion  17  is a 1 . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. The numerical analysis of the coupling circuit configuration shown in  FIG. 15  can prove that the differential pair with the periodical subwavelength configuration can dramatically suppress the cross-talk between the adjacent microstrip lines and reduce the mode conversion effect between the differential mode and the common mode. 
         [0066]      FIG. 15  shows a coupling circuit composed of a conventional differential pair and a differential pair with the double sided opening-type periodical subwavelength configurations. S dd21  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the second terminal  14 , which clearly shows the transmission ability of the differential pairs. S dd41  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the fourth terminal  23  of the conventional differential pair, which clearly shows the cross-talk of the differential pairs. The distance between the two differential pairs is W 2 . The S parameter, S dd21 , stands for the transmission ability of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14 . The S parameter, S dd41 , stands for the generated cross-talk effect when differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the conventional differential pair. The S parameter, S cd21 , stands for the generated mode conversion effect from the differential mode to common mode when differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The “conventional” in the figures stands for the transmission and cross-talk effects of the differential pair without the periodical subwavelength configuration, which are illustrated by solid lines. The transmission and cross-talk effects of the differential pair with the periodical subwavelength configuration are illustrated by dashed lines. In  FIG. 16  and  FIG. 17 , the simulation parameters are as follows: W=W 1 =W 2 =W 3 =W 4 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth (b) of the slots at both sides is 0.3W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. As shown in  FIG. 15 , the first terminal  13  receives the differential signals complementary to each other, the second terminal  14  is the receiver of the differential pair, the third terminal  22  is the near end of the conventional differential pair and the fourth terminal  23  is the remote end of the conventional differential pair. S dd21  in  FIG. 17  shows the mode conversion effect between the differential signal and the common mode signal. 
         [0067]    In the fourth embodiment, if both differential pairs are conventional differential pairs, the simulation result can be illustrated by the solid lines of  FIG. 16  and  FIG. 17 . As shown in  FIG. 16 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the transmission ability is expressed by the S parameter, S dd21 : S dd21 =−0.08821 dB when the signal frequency is 200 MHz, S dd21 =−2.32492 dB when the signal frequency is 12 GHz. As shown in  FIG. 16 , the cross-talk of the conventional differential pair (i.e. differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23 ) is expressed by the S parameter, S dd41 : S dd41 =−48.55245 dB when the signal frequency is 200 MHz, S dd41 =−9.38157 dB when the signal frequency is 12 GHz. As shown in  FIG. 17 , the mode conversion effect between the differential mode to common mode when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S cd21 : S cd21 =−12.37439 dB when the signal frequency is 12 GHz. 
         [0068]    In the fourth embodiment, the simulation result of the coupling circuit of the conventional differential pair and the differential pair with the double sided opening-type periodical subwavelength configuration can be illustrated by the dashed lines of  FIG. 16  and  FIG. 17 . As shown in  FIG. 16 , the transmission ability of the differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, and S dd2 S dd2 =−0.07977 dB when the signal frequency is 200 MHz, S dd21 =−1.0001 dB when the signal frequency is 12 GHz. As shown in  FIG. 16 , the cross-talk of the differential signals which are inputted into the first terminal  13  and outputted from the fourth terminal  23  is expressed by the S parameter, S dd41  S dd41 =−49.2638 dB when the signal frequency is 200 MHz, S dd41 =−30.72547 dB when the signal frequency is 12 GHz and the maximum of the cross-talk between 1 GHz-10 GHz is S dd41 =−24.5046 dB when the signal frequency is 5.26 GHz. As shown in  FIG. 17 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode expressed by the S parameter, S cd21 : S cd21 =−28.37445 dB when the signal frequency is 12 GHz. 
         [0069]    In the fourth embodiment, the overall comparison results between the conventional differential pair and the differential pair with the double sided opening-type periodical subwavelength configuration are shown in  FIG. 16  and  FIG. 17 . As shown in  FIG. 16 , the transmission ability of the conventional differential pair is S dd21 =−2.32492 dB when the signal frequency is 12 GHz; the transmission ability of the differential pair with the periodical subwavelength configuration is S dd21 =−1.0001 dB when the signal frequency is 12 GHz. The transmission ability is obviously improved at high signal frequency. As shown in  FIG. 16 , when the signal frequency is 12 GHz, the cross-talk between the two conventional differential pairs is S dd41 =−9.38157 dB and the cross-talk between the conventional differential pair and the differential pair with the periodical subwavelength configuration is S dd41 =−30.72547 dB; obviously, the cross-talk is significantly suppressed. As shown in  FIG. 17 , when the signal frequency is 12 GHz, the mode conversion effect between the conventional differential pairs is S cd21 =−12.37439 dB and that of the differential pair with the periodical subwavelength configuration is S cd21 =−28.37445 dB; obviously, the mode conversion effect is significantly decreased. PS:  FIG. 16  is the S parameter calculation result of the coupling circuit of  FIG. 15 . Please refer to  FIG. 16 , S dd21  of the conventional differential pair is illustrated by a solid line, which shows its transmission ability is −0.08821 dB when the signal frequency is 200 MHz, and is −2.32492 dB when the signal frequency is 12 GHz. S dd21  of the differential pair with the double sided opening-type periodical subwavelength configuration is illustrated by a dashed line, which shows its transmission ability is −0.07977 dB when the signal frequency is 200 MHz, and is −1.0001 dB when the signal frequency is 12 GHz. Obviously, the differential pair with periodical subwavelength configuration will have better transmission ability and confinement of the electromagnetic field. With the strong confinement of the electromagnetic field, the differential pair with the periodical subwavelength configuration will not result in serious interference to adjacent microstrip lines or conventional differential pair. With the increase of the frequency, the cross-talk will become more obvious. The cross-talk between the conventional differential pairs is S dd41 =−9.38157 dB when the signal frequency is 12 GHz, and the cross-talk between the conventional differential pair and the differential pair with the double sided opening-type periodical subwavelength configuration is S dd41 =−30.72547 dB when the signal frequency is 12 GHz, which clearly shows that the differential pair with the double sided opening-type periodical subwavelength configuration can effectively reduce the cross-talk.  FIG. 17  shows the relation between the mode conversion effect and the frequency in the coupling circuit. With the increase of the frequency, the mode conversion effect will become more obvious. However, the differential pair with double sided opening-type periodical subwavelength configuration can effectively suppress the mode conversion effect. The mode conversion effect of the conventional differential pair is S cd21 =−12.37439 dB when the signal frequency is 12 GHz; and the mode conversion effect of the differential pair with the double sided opening-type periodical subwavelength configuration is S cd21 =−28.37445 dB; therefore, the periodical subwavelength configuration can greatly suppress the mode conversion effect between the differential mode and common mode. 
         [0070]    The fifth embodiment of the present invention is, as shown in  FIG. 18 , a differential pair with the double sided slot-type periodical subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with periodical subwavelength configurations. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and the interval of two adjacent rectangular convex bodies  16  is the periodical arrangement length of these slots. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r  and the width of the slots is a. If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. The numerical analysis of the coupling circuit configuration shown in  FIG. 19  can prove that the differential pair with the periodical subwavelength configuration can dramatically suppress the cross-talk between adjacent microstrip lines and reduce the mode conversion effect between the differential mode and common mode.  FIG. 19  shows a coupling circuit composed of a differential pair with the double sided slot-type periodical subwavelength configurations and a single-ended microstrip line. S dd21  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the second terminal  14 , which clearly shows the transmission ability of the differential pairs. Inputting differential signals into the first terminal  13  and analyzing the output signals from the fourth terminal  23  of the conventional differential pair can obtain the cross-talk of the differential pair and adjacent single-ended microstrip line. As shown in  FIG. 19 , the distance between the two differential pairs is W 2 . The S parameter, S dd21 , stands for the transmission ability of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14 . The S parameter, S dd41 , stands for the generated cross-talk effect when differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the conventional differential pair. The S parameter, S cd21 , stands for the generated mode conversion effect from the differential mode to common mode when differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The “conventional” in the figures stands for the transmission and cross-talk effects of the differential pair without the periodical subwavelength configuration, which are illustrated by solid lines. The transmission and cross-talk effects of the differential pair with the periodical subwavelength configuration are illustrated by dashed lines. The simulation parameters are as follows: W=W 1 =W 2 =W 3 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth (b) of the slots is 0.3W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The first terminal  13  receives the differential signals complementary to each other, the second terminal  14  is the receiver of the differential pair, the third terminal  22  is the near end of the conventional differential pair and the fourth terminal  23  is the remote end of the conventional differential pair. As shown in  FIG. 20 , the simulation parameters are as follows: W=W 1 =W 2 =W 3 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth (b) of the slots is 0.3W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. S dd21  stands for signal transmission ability and S dd41  stands for the cross-talk between the differential pair and the adjacent single-ended microstrip line. As shown in  FIG. 21 , the simulation parameters are as follows: W=W 1 =W 2 =W 3 =1.2 mm, the total length of the microstrip line is  10  cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth (b) of the slots is 0.3W, the periodical arrangement length (d) is 1.0 mm, and d=2a, and the analysis range is from 200 MHz to 12 GHz. S cd21  stands for the mode conversion effect from the differential signal to the common mode signal. 
         [0071]    In the fifth embodiment, the simulation result of the coupling circuit of the conventional differential pair and the single-ended microstrip line can be illustrated by the solid lines of  FIG. 20  and  FIG. 21 . As shown in  FIG. 20 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the transmission ability is expressed by the S parameter, S dd21 : S dd21 =−0.0679 dB when the signal frequency is 200 MHz, S dd21 =−2.36253 dB when the signal frequency is 12 GHz. As shown in  FIG. 20 , the cross-talk of the conventional differential pair (i.e. differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the conventional differential pair) is expressed by the S parameter, S sd41  S sd41 =−42.63854 dB when the signal frequency is 200 MHz, S sd41 =−6.55742 dB when the signal frequency is 12 GHz. As shown in  FIG. 21 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode is expressed by the S parameter, S cd21 : S cd21 =−12.96263 dB when the signal frequency is 12 GHz. 
         [0072]    In the fifth embodiment, the simulation result of the coupling circuit of the conventional differential pair and the single-ended microstrip line can be illustrated by the dashed lines of  FIG. 20  and  FIG. 21 . As shown in  FIG. 20 , the transmission ability of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S dd21 , S dd21 =−0.10201 dB when the signal frequency is 200 MHz, and S dd21 =−1.18541 dB when the signal frequency is 12 GHz. As shown in  FIG. 20 , the cross-talk of the differential signals which are inputted into the first terminal  13  and outputted from the fourth terminal  23  is expressed by the S parameter, S sd41  S sd41 =−42.82679 dB when the signal frequency is 200 MHz, and S sd41 =−13.93195 dB when the signal frequency is 12 GHz. As shown in  FIG. 21 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode is expressed by the S parameter, S cd21 : S cd21 =−23.28997 dB when the signal frequency is 12 GHz. 
         [0073]    In the fifth embodiment, the overall comparison results between the conventional differential pair and the differential pair with the double sided slot-type periodical subwavelength configuration are shown in  FIG. 20  and  FIG. 21 . As shown in  FIG. 20 , the transmission ability of the conventional differential pair is S dd21 =−2.36253 dB when the signal frequency is 12 GHz; the transmission ability of the differential pair with the periodical subwavelength configuration is S dd21 =−1.18541 dB when the signal frequency is 12 GHz. The transmission ability is obviously improved at high signal frequency. As shown in  FIG. 20 , when the signal frequency is 12 GHz, the cross-talk between the conventional differential pair and the single-ended microstrip line is S sd41 =−6.55742 dB and the cross-talk between the conventional differential pair and the differential pair with the periodical subwavelength configuration is S sd41 =−13.93195 dB; obviously, the cross-talk is significantly suppressed. As shown in  FIG. 21 , when the signal frequency is 12 GHz, the mode conversion effect of the conventional differential pair is S cd21 =−12.96263 dB and that of the differential pair with the periodical subwavelength configuration is S cd21 =−23.28997 dB; obviously, the mode conversion effect is significantly decreased. PS:  FIG. 20  is the S parameter calculation result of the coupling circuit of  FIG. 19 . Please refer to  FIG. 20 , S dd21  of the conventional differential pair is illustrated by a solid line, which shows its transmission ability is −0.0679 dB when the signal frequency is 200 MHz, and is −2.36253 dB when the signal frequency is 12 GHz. S dd21  of the differential pair with the double sided slot-type periodical subwavelength configuration is illustrated by a dashed line, which shows its transmission ability is −0.10201 dB when the signal frequency is 200 MHz, and is −1.18541 dB when the signal frequency is 12 GHz. Obviously, the conventional differential pair has a little bit better transmission ability. With the increase of the frequency, the differential pair with the periodical subwavelength configuration will have better transmission ability and confinement of the electromagnetic field. With the strong confinement of the electromagnetic field, the differential pair with the double sided slot-type periodical subwavelength configuration will not result in serious interference to adjacent microstrip lines. With the increase of the frequency, the cross-talk will become more obvious. The cross-talk between the conventional differential pair and the single-ended microstrip line is S sd41 =−6.55742 dB when the signal frequency is 12 GHz, and the cross-talk between the differential pair with the double sided slot-type periodical subwavelength configuration and the single-ended microstrip line is S sd41 =−13.93195 dB when the signal frequency is 12 GHz, which clearly shows that the differential pair with the double sided slot-type periodical subwavelength configuration can effectively reduce the cross-talk. The mode conversion effect of the conventional differential pair is S cd21 =−12.96263 dB when the signal frequency is 12 GHz; and the mode conversion effect of the differential pair with the double sided slot-type periodical subwavelength configuration is S cd21 =−23.28997 dB; therefore, the periodical subwavelength configuration can greatly suppress the mode conversion effect between the differential mode and common mode. 
         [0074]    The sixth embodiment of the present invention is, as shown in  FIG. 22 , a differential pair with the double sided hairpin-type periodical subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with the periodical subwavelength configuration. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of Z-shaped convex bodies  20  arranged continuously and periodically, and each of the Z-shaped convex bodies  20  comprises two extended portions  17 ,  18 , wherein the first extended portion  17  parallel extends from the opening of one slot to the center of the adjacent slot; and the second extended portion  18  parallel extends from the middle of the Z-shaped convex body  20  to the center of another adjacent slot. The direction where the first extended portion  17  extends is inverse to the direction where the second extended portion  18  extends. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . There are still other structure parameters, such as a 1 , a 2  (the widths of the outer openings), a 3  (the width of the inner openings), b 1  (the width of the thin metal bars) and b 2  (the interval of the thin metal bars). If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. The numerical analysis of the coupling circuit configuration shown in  FIG. 24  can prove the differential pair with the double sided hairpin-type periodical subwavelength configuration can dramatically suppress the cross-talk between the adjacent microstrip lines and reduce the mode conversion effect between the differential mode and common mode.  FIG. 24  shows a coupling circuit composed of a differential pair with double sided hairpin-type periodical subwavelength configuration and a single-ended microstrip line. S dd21  can be obtained by inputting differential signals into the first terminal  13  and analyzing the output signals from the second terminal  14 , which clearly shows the transmission ability of the differential pairs. The cross-talk between the differential pair and the single-ended microstrip line can be acquired by analyzing the outputs from the fourth terminal  23 . The distance between the two differential pairs is W 2 . The S parameter, S dd21 , stands for the transmission ability of the differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14 . The S parameter, S sd41 , stands for the generated cross-talk effect when differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23  of the single-ended microstrip line. The S parameter, S cd21 , stands for the generated mode conversion effect from the differential mode to common mode when differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The “conventional” in the figures stands for the transmission and cross-talk effects of the differential pair without the periodical subwavelength configuration, which are illustrated by solid lines. The transmission and cross-talk effects of the differential pair with the periodical subwavelength configuration are illustrated by dashed lines. The simulation parameters are as follows: W=W 1 =W 2 =W 3 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.3W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. In  FIG. 24 , the first terminal  13  receives the differential signals complementary to each other, the second terminal  14  is the receiver of the differential pair, the third terminal  22  is the near end of the conventional differential pair and the fourth terminal  23  is the remote end of the conventional differential pair. The simulation parameters of  FIG. 25  are as follows: W=W 1 =W 2 =W 3 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.3W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The S parameter, S dd21 , stands for the signal transmission ability. The S parameter, S sd41 , stands for the cross-talk effect between the differential pair with the periodical subwavelength configuration and the single-ended microstrip line. Other parameters are as follows: a 1 =0.1d, a 2 =0.2d, a 3 =0.7d, b 1 =b 2 =0.25b. The simulation parameters of  FIG. 26  are as follows: W=W 1 =W 2 =W 3 =1.2 mm, the total length of the microstrip line is 10 cm, the material of the substrate  21  is RO4003, the thickness of the metal film (t) is 0.0175 mm, the thickness of the substrate (h) is 0.508 mm, the depth of the slot (b) is 0.3W and the periodical arrangement length (d) is 1.0 mm, and the analysis range is from 200 MHz to 12 GHz. The S parameter, S cd21 , stands for the mode conversion effect from the differential signal to the common mode signal. 
         [0075]    In the sixth embodiment, the simulation result of the coupling circuit of the conventional differential pair and the single-ended microstrip line can be illustrated by the solid lines of  FIG. 25  and  FIG. 26 . As shown in  FIG. 25 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the transmission ability is expressed by the S parameter, S dd21 : S dd2 =−0.0679 dB when the signal frequency is 200 MHz, S dd21 =−2.36253 dB when the signal frequency is 12 GHz. As shown in  FIG. 25 , the cross-talk of the conventional differential pair (i.e. differential signals are inputted into the first terminal  13  and outputted from the fourth terminal  23 ) is expressed by the S parameter, S sd41 : S sd41 =−42.63854 dB when the signal frequency is 200 MHz, S sd41 =−6.55742 dB when the signal frequency is 12 GHz. As shown in  FIG. 26 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode is expressed by the S parameter, S cd21 : S cd21 =−12.96263 dB when the signal frequency is 12 GHz. 
         [0076]    In the sixth embodiment, the simulation result of the coupling circuit of the differential pair with the double sided hairpin-type periodical subwavelength configuration and the single-ended microstrip line can be illustrated by the dashed lines of  FIG. 25  and  FIG. 26 . As shown in  FIG. 25 , the transmission ability that of differential signals which are inputted into the first terminal  13  and outputted from the second terminal  14  is expressed by the S parameter, S dd21 : S dd21 =−0.11412 dB when the signal frequency is 200 MHz, and S dd21 =−1.1716 dB when the signal frequency is 12 GHz. As shown in  FIG. 25 , the cross-talk of differential signals which are inputted into the first terminal  13  and outputted from the fourth terminal  23  is expressed by the S parameter, S sd41 : S sd41 =−43.8893 dB when the signal frequency is 200 MHz, and S sd41 =−23.45903 dB when the signal frequency is 12 GHz. As shown in  FIG. 26 , when the differential signals are inputted into the first terminal  13  and outputted from the second terminal  14 , the mode conversion effect between the differential mode to the common mode is expressed by the S parameter, S cd21 : S cd21 =−36.05781 dB when the signal frequency is 12 GHz. 
         [0077]    In the sixth embodiment, the overall comparison results between the conventional differential pair and the differential pair with the double sided hairpin-type periodical subwavelength configuration are shown in  FIG. 25  and  FIG. 26 . As shown in  FIG. 25 , the transmission ability of the conventional differential pair is S dd21 =−2.36253 dB when the signal frequency is 12 GHz; the transmission ability of the differential pair with the periodical subwavelength configuration is S dd21 =−1.1716 dB when the signal frequency is 12 GHz. The transmission ability is obviously improved at high signal frequency. As shown in  FIG. 25 , when the signal frequency is 12 GHz, the cross-talk between the conventional differential pair and the single-ended microstrip line is S sd41 =−6.55742 dB and the cross-talk between the single-ended microstrip line and the differential pair with the periodical subwavelength configuration is S sd41 =−23.45903 dB; obviously, the cross-talk is significantly suppressed. As shown in  FIG. 26 , when the signal frequency is 12 GHz, the mode conversion effect of the conventional differential pair is S cd21 =−12.96263 dB and that of the differential pair with the periodical subwavelength configuration is S cd21 =−36.05781 dB; obviously, the mode conversion effect is significantly decreased. PS:  FIG. 25  is the S parameter calculation result of the coupling circuit of  FIG. 24 . Please refer to  FIG. 25 , S dd21  of the conventional differential pair is illustrated by a solid line, which shows its transmission ability is −0.0679 dB when the signal frequency is 200 MHz, and is −2.36253 dB when the signal frequency is 12 GHz. S dd21  of the differential pair with the double sided hairpin-type periodical subwavelength configuration is illustrated by a dashed line, which shows its transmission ability is −0.11412 dB when the signal frequency is 200 MHz, and is −1.1716 dB when the signal frequency is 12 GHz. The conventional differential pair has a little bit better transmission ability at low frequency. With the increase of the frequency, the differential pair with periodical subwavelength configuration will have better transmission ability and confinement of the electromagnetic field. With the strong confinement of the electromagnetic field, the differential pair with the double sided hairpin-type periodical subwavelength configuration will not result in serious interference to adjacent microstrip lines. With the increase of the frequency, the cross-talk will become more obvious. The cross-talk between the conventional differential pair and the single-ended microstrip line is S sd41 =−6.55742 dB when the signal frequency is 12 GHz; however, the cross-talk between the differential pair with the double sided hairpin-type periodical subwavelength configuration and the single-ended microstrip line is S sd41 =−23.45903 dB when the signal frequency is 12 GHz, which effectively represses the cross-talk.  FIG. 26  shows the relation between the mode conversion effect and the frequency. With the increase of the frequency, the mode conversion effect will become more obvious. However, the differential pair with double sided hairpin-type periodical subwavelength configuration can effectively suppress the mode conversion effect. The mode conversion effect of the conventional differential pair is S cd21 32 −12.96263 dB when the signal frequency is 12 GHz; and the mode conversion effect of the differential pair with the double sided hairpin-type periodical subwavelength configuration is S cd21 =−36.05781 dB; therefore, the periodical subwavelength configuration can greatly suppress the mode conversion effect between the differential mode and common mode. 
         [0078]    One of the primary objects of the present invention is to provide low cross-talk differential microstrip lines for high-frequency signal transmission, such like the fourth embodiment, the fifth embodiment and the sixth embodiment. These slots are periodically arranged to form a subwavelength configuration, such as  FIG. 14 ,  FIG. 18  and  FIG. 22 . The present invention further comprises a plurality of slots periodically arranged along the inner side of the first microstrip line  11  opposite the outer side of the first microstrip line  11 ; and a plurality of slots periodically arranged along the inner side of the second microstrip line  12  opposite the outer side of the second microstrip line  12 , wherein the distance between the inner side of the first microstrip line  11  and the inner side of the second microstrip line  12  is W 1 , as shown in  FIG. 14 ,  FIG. 18  and  FIG. 22 . Thus, in the fourth embodiment, the fifth embodiment and the sixth embodiment, these slots along the edges of the both sides of the first microstrip line  11  and the second microstrip line  12  are arranged to form the periodical subwavelength configuration. 
         [0079]    The Seventh embodiment of the present invention is, as shown in  FIG. 27 , a differential pair with the slot-type periodical subwavelength configuration, wherein the slots are periodically arranged along the inner sides of the first microstrip line and the second microstrip line to form the subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with the slot-type periodical subwavelength configuration. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and the interval of the two adjacent rectangular convex bodies  16  is the periodical arrangement length of these slots. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. Similarly, as the previous embodiments, the differential pair with the slot-type periodical subwavelength configuration of the embodiment can also effectively reduce both of the mode conversion effect and the cross-talk, whereby the above problems can be solved. 
         [0080]    The eighth embodiment of the present invention is, as shown in  FIG. 28 , a differential pair with the slot-type periodical subwavelength configuration, wherein the slots are periodically arranged along the inner side of the first microstrip line and the outer side of the second microstrip line to form the subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with the slot-type periodical subwavelength configuration. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and the interval of the two adjacent rectangular convex bodies  16  is the periodical arrangement length of these slots. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and the common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or the differential pairs. Similarly, as the previous embodiments, the differential pair with the slot-type periodical subwavelength configuration of the embodiment can also effectively reduce both of the mode conversion effect and the cross-talk, whereby the above problems can be solved. 
         [0081]    The ninth embodiment of the present invention is, as shown in  FIG. 29 , a differential pair of microstrip lines with the opening-type periodical subwavelength configuration, wherein the slots are periodically arranged along the inner side of the first microstrip line and the outer side of the second microstrip line to form the subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with periodical subwavelength configurations. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 . Two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and each of the rectangular convex bodies  16  comprises two first extended portions  17  parallel extend to centers of the adjacent slots respectively. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or differential pairs. Similarly, as the previous embodiments, the differential pair with the opening-type periodical subwavelength configuration of the embodiment can also effectively reduce both of the mode conversion effect and the cross-talk, whereby the above problems can be solved. 
         [0082]    The tenth embodiment of the present invention is, as shown in  FIG. 30 , a differential pair of microstrip lines with the opening-type periodical subwavelength configuration, wherein the slots are periodically arranged along the inner sides of the first microstrip line and the second microstrip line to form the subwavelength configuration. The differential pair is composed of two microstrip lines, the first microstrip line  11  and the second microstrip line  12 , with periodical subwavelength configurations. The signals are inputted into the first terminal  13  and outputted from the second terminal  14 . The signal transmitted via the first microstrip line  11  has a 180° phase difference from the signal transmitted via the second microstrip line  12  (i.e.: the signal transmitted via the first microstrip line  11  is the complementary signal of that transmitted via the second microstrip line  12 .). The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 . Two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and each of the rectangular convex bodies  16  comprises two first extended portions  17  parallel extend to centers of the adjacent slots respectively. The width of the two microstrip lines is W, the distance between the two microstrip lines is W 1 , the thickness of the substrate  21  is h, the periodical arrangement length of the two microstrip lines is d, the depth of the slots is b, and the dielectric constant of the medium of the substrate  21  is ε r . If there is a single-ended microstrip line or another differential pair beside the conventional differential pair without slots, two obvious phenomena will take place. The first phenomenon is that an obvious mode conversion effect between the differential mode and common mode will occur between the first terminal  13  and the second terminal  14 . The second phenomenon is that the complementary signals inputted into the first terminal  13  will bring about cross-talk to the adjacent microstrip lines or differential pairs. Similarly, as the previous embodiments, the differential pair with the opening-type periodical subwavelength configuration of the embodiment can also effectively reduce both of the mode conversion effect and the cross-talk, whereby the above problems can be solved. 
         [0083]    One of the primary objects of the present invention is, as shown in  FIG. 10  and  FIG. 18 , to provide a differential pair with slot-type periodical subwavelength configuration, such like the third embodiment and the fifth embodiment. The configuration of the slots comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and the interval of the two adjacent rectangular convex bodies  16  is the periodical arrangement length of these slots. 
         [0084]    One of the primary objects of the present invention is, as shown in  FIG. 1  and  FIG. 14 , to provide a differential pair with opening-type periodical subwavelength configuration, such like the first embodiment and the fourth embodiment. The configuration of the slots in these embodiments comprises a plurality of rectangular convex bodies  16  continuously and periodically combined with a plurality of rectangular concave bodies  15 , and the two adjacent rectangular convex bodies  16  are divided by one rectangular concave body  15 , and each of the rectangular convex bodies  16  comprises two first extended portions  17  parallel extend to centers of the adjacent slots respectively. 
         [0085]    One of the primary objects of the present invention is, as shown in  FIG. 5  and  FIG. 22 , to provide a differential pair with hairpin-type periodical subwavelength configuration, such like the second embodiment and the sixth embodiment. The configuration of the slots in these embodiments comprises a plurality of Z-shaped convex bodies  20  arranged periodically and continuously, and each of the Z-shaped convex bodies comprises two extended portions  17 ,  18 . The first extended portion  17  parallel extends from the opening of one slot to the center of the adjacent slot. The second extended portion  18  parallel extends from the middle of the Z-shaped convex body to the center of another adjacent slot. In particular, the direction where the first extended portion  17  extends is inverse to the direction where the second extended portion  18  extends. 
         [0086]    While the means of specific embodiments in present invention has been described by reference drawings, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims. The modifications and variations should be limited by the specification of the present invention in a range.