Abstract:
Provided are common mode filtering method and device for use with a defected ground structure, the device including a substrate, coupled microstrip lines formed on the substrate and a ground plane formed underneath the substrate, the common mode filtering method being characterized by forming at least a defected ground structure on the ground plane and making dual mode signals pass through the coupled microstrip lines, thereby using the defected ground structure to suppress dual model noises within a specific frequency band and prevent signal distortion.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to common mode filtering methods and devices, and more particularly to a method and device for use with different defected ground structures to suppress common mode noises. 
         [0003]    2. Description of Related Art 
         [0004]    Along with wide application of various kinds of electronic equipments, television networks, switches, mobile communication equipments and office automation equipments, electromagnetic environment of electronic systems is becoming more complicated and EMI (Electromagnetic interference) is becoming an increasingly severe problem, which adversely affects operation of the electronic systems. 
         [0005]    Generally, differential signals are used for high-speed data transmission, such as USB 3.0 and IEEE 1394, so as to reduce noise interference. Since differential signals generate much less noise and suppress common mode noise, signal distortion can be avoided. However, coupling noise interference often leads to currents of same phase in two signals, i.e., common mode currents, which are the main source of EMI and should be considered in circuit wiring and design. 
         [0006]    EMI is divided into radiated interference and conducted interference according to energy transmission methods. The radiated interference is preferably eliminated by shielding technique and the conducted interference is preferably eliminated and suppressed by magnetic filtering elements. An anti-EMI element can be disposed as close as possible to an interference source so as to efficiently prevent generation of radiated interference. 
         [0007]    The most commonly used method for suppressing common mode current is to externally add a common mode choke. The common mode choke comprises two separate coils having a same number of turns and wound on a common magnet, which is equivalent in structure to a magnetic core coil. Such a filter suppresses noise through impedance and frequency characteristics of the magnetic material. The impedance of the magnetic core coil at high frequency is far greater than the impedance at low frequency, and in order to obtain a preferred interference filtering effect, the filter has a maximum impedance at the center frequency of the interference. Combination of self inductance and mutual inductance in a choke leads to a high impedance so as to eliminate common mode noise. However, frequency characteristic and parasitic effect of the ferromagnetic material prevents the common mode choke from operating at frequencies of GHz. 
         [0008]    In addition, with the recent progress of multi-layer board fabrication process, a method of controlling the EMI radiation through PCB stack-up is proposed. Although this method has a design concept similar to the common mode choke, its fabrication process is rather complicated and costly. 
         [0009]    Therefore, how to provide a common mode filtering method and device with simplified fabrication process, low fabrication cost and good suppression effect has become urgent. 
       SUMMARY OF THE INVENTION 
       [0010]    According to the above drawbacks, the present invention provides a filtering device for use with a defected ground structure, which comprises: a substrate; coupled microstrip lines disposed on the substrate for passing through dual mode signals; and a ground plane disposed underneath the substrate and having at least one defected ground structure for suppressing common mode signals within a specific frequency band that pass through the coupled microstrip lines, wherein the defected ground structure comprises: a first rectangular region, a second rectangular region which has a same size as the first rectangular region and is parallel with the first rectangular region, and a third rectangular region with two sides thereof connecting the first and second rectangular regions respectively, the sides of the first and second rectangular regions contacting the third rectangular region having a length greater than said two sides of the third rectangular region. 
         [0011]    According to another embodiment, the defected ground structure further comprises: a first line segment formed at one side of the first and second rectangular regions, with its projection crossing the coupled microstrip lines; and a second line segment formed at the other side of the first and second rectangular regions opposed to the first line segment, with its projection crossing the coupled microstrip lines, wherein the first line segment comprises a first sub line segment, a second sub line segment and a third sub line segment, the second sub line segment is parallel with the third rectangular region, the first sub line segment and the third sub line segment face toward the second line segment and form an angle with the second sub line segment; and the second line segment comprises a fourth sub line segment, a fifth sub line segment and a sixth sub line segment, the fifth sub line segment is parallel with the third rectangular region, the fourth sub line segment and the sixth sub line segment face toward the first line segment and form an angle with the fifth sub line segment. Preferably, the angle is 90 degree. 
         [0012]    According to another embodiment, the defected ground structure further comprises: a fourth rectangular region connected to one side of the first rectangular region facing the second rectangular region, one side of the fourth rectangular region being flush with an upper side of the first rectangular region; a fifth rectangular region connected to one side of the first rectangular region facing the second rectangular region, one side of the fifth rectangular region being flush with a lower side of the first rectangular region; a sixth rectangular region connected to one side of the second rectangular region facing the first rectangular region, one side of the sixth rectangular region being flush with an upper side of the second rectangular region; and a seventh rectangular region connected to one side of the second rectangular region facing the first rectangular region, one side of the seventh rectangular region being flush with a lower side of the second rectangular region. 
         [0013]    Preferably, the above-described defected ground structure further comprises: a third line segment formed at one side of the first and second rectangular regions, with its projection crossing the coupled microstrip lines; and a fourth line segment formed at the other side of the first and second rectangular regions opposed to the third line segment, with its projection crossing the coupled microstrip lines, wherein the third line segment comprises a seventh sub line segment, an eighth sub line segment and a ninth sub line segment, the eighth sub line segment is parallel with the third rectangular region, the seventh sub line segment and the ninth sub line segment face toward the fourth line segment and form an angle of 90 degree with the eighth sub line segment; and the fourth line segment comprises a tenth sub line segment, an eleventh sub line segment, and a twelfth sub line segment, the eleventh sub line segment is parallel with the third rectangular region, the tenth sub line segment and the twelfth sub line segment face toward the third line segment and form an angle of 90 degree with the eleventh sub line segment. 
         [0014]    The present invention further provides a common mode filtering method applied in a common mode filtering device with a defected ground structure, wherein the common mode filtering device comprises a substrate, coupled microstrip lines formed on the substrate and a ground plane formed underneath the substrate, the common mode filtering method comprising: (1) forming at least one defected ground structure on the ground plane; and (2) making dual mode signals pass through the coupled microstrip lines, wherein the defected ground structure comprises: a first rectangular region, a second rectangular region which has a same size as the first rectangular region and is parallel with the first rectangular region, and a third rectangular region with two sides thereof connecting the first and second rectangular regions respectively, the sides of the first and second rectangular regions contacting the third rectangular region having a length greater than said two sides of the third rectangular region. 
         [0015]    In the present invention, since differential mode signals and common mode signals passing through the coupled microstrip lines have different reference return path, a defected ground structure equivalent to a LC resonator can be formed on the return ground path of the common mode signals so as to suppress the common mode signals without affecting the differential mode signals. Through different defected ground structures, the present invention not only suppresses common mode noises over a wider frequency band and increases the insertion loss, but also reduces etching area of the ground plane, thereby simplifying the fabrication process, saving the fabrication cost and reducing the circuit volume. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  shows the structure of a common mode filtering device for use with a defected ground structure according to the present invention; 
           [0017]      FIG. 2  shows a common mode filtering device for with a defected ground structure according to a first embodiment of the present invention; 
           [0018]      FIG. 3  shows a common mode filtering device for with a defected ground structure according to a second embodiment of the present invention; 
           [0019]      FIG. 4  shows a common mode filtering device for with a defected ground structure according to a third embodiment of the present invention; 
           [0020]      FIG. 5  shows a common mode filtering device for with a defected ground structure according to a fourth embodiment of the present invention; 
           [0021]      FIG. 6  shows a common mode filtering device for with a defected ground structure according to a fifth embodiment of the present invention; and 
           [0022]      FIG. 7  is a process view of a common mode filtering method for use with a defected ground structure. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparent to those skilled in the art after reading the disclosure of this specification. 
         [0024]      FIG. 1  shows a common mode filtering device for use with a defected ground structure according to the present invention. As shown in the drawing, the common mode filtering device comprises a substrate  10 , coupled microstrip lines  11 , a ground plane  12  and a defected ground structure  13 . 
         [0025]    The substrate  10  is the core of a printed circuit board. The substrate  10  is made up of resin, reinforcing material and/or metal foil. The most common substrate is a copper clad laminate (CCL) substrate, which is formed by adhering a copper foil to single or double surfaces of a base material at high temperature and high pressure and using a polymer resin such as an epoxy resin, phenolic resin, polyamine-formaldehyde resin, silicone resin or Teflon as an adhesive. The copper foil is formed by depositing copper on a rolling wheel immersed in sulfuric acid electrolyte. During the electroplating process, the copper surface tends to become rough such that it is easy to be adhered to the substrate  10 . However, it should be noted that the substrate is not limited to the above-described material. 
         [0026]    The coupled microstrip lines  11  comprise two microstrip lines, which are a kind of planar transmission line. The microstrip lines are metal line segments formed on the substrate and have predetermined length and width corresponding to desired frequency and impedance characteristics. When the two unshielded microstrip lines are close to each other, their electromagnetic fields interact with each other so as to form the coupled microstrip lines  11 . 
         [0027]    The ground plane  12  is a metal contact layer of the substrate  10 . The ground plane  12  is etched in different shapes so as to obtain a defected ground structure  13  which can change transmission characteristic of the couple microstrip lines  11 . The defected ground structure  13  can be equivalent to a parallel LC resonant circuit such that energy of a part of signals at resonant frequency can be absorbed by the ground layer, thereby forming a band stop effect at specific frequency. 
         [0028]      FIG. 2  shows a common mode filtering device for use with a defected ground structure according to a first embodiment of the present invention. The defected ground structure  13  of the common mode filtering device comprises a first rectangular region  21 , a second rectangular region  22  and a third rectangular region  23 , wherein the first rectangular region  21  has a size same as that of the second rectangular region  22  and is parallel with the second rectangular region  22 , two sides of the third rectangular region  23  connect the first and second rectangular regions  21 ,  22  respectively, and sides of the first and second rectangular regions  21 ,  22  contacting the third rectangular region  21  have a length longer than said sides of the third rectangular region  21 . 
         [0029]    In practical application, dual mode signals are passed through the coupled microstrip lines  11 . The dual mode signals comprise differential mode signals and common mode signals, wherein the reference return path of the common mode signals passes through the ground plane  12  and accordingly the defected ground structure  13  influences the common mode signals by increasing insertion loss at specific frequency band so as to suppress passing through of the common mode signals. 
         [0030]    In a preferred embodiment, an axis parallel with the horizontal direction of the third rectangular region  23  and passing through the centroid of the third rectangular region  23  passes through the centroids of the first rectangular region  21  and the second rectangular region  22  respectively. Accordingly, a H-shaped defected ground structure  13  which is up-down symmetric and left-right symmetric is formed. 
         [0031]    According to another preferred embodiment, the center of the coupled microstrip lines  11  is aligned with the center of the defected ground structure  13 , and the distance between the coupled microstrip lines  11  is less than the maximum range of the defected ground structure  13  on the substrate. 
         [0032]      FIG. 3  shows a common mode filtering device for use with a defected ground structure according to a second embodiment of the present invention. The defected ground structure  13  is periodically formed underneath the substrate  10 . Compared with the first embodiment, the present embodiment can suppress common mode signals at a much wider frequency band and increase more insertion loss so as to obtain a better suppressing effect. However, a larger area requires to be etched in the present embodiment. 
         [0033]      FIG. 4  shows a common mode filtering device for use with a defected ground structure according to a third embodiment of the present invention. Compared with the first embodiment, the defected ground structure  13  of the present embodiment further comprises a first line segment  40  and a second line segment  41 . 
         [0034]    The first line segment  40  is formed at one side of the first and second rectangular regions  21 ,  22  and preferably spaced from the first and second rectangular regions  21 ,  22 , and the projection of the first line segment  40  crosses the coupled microstrip lines  11 . 
         [0035]    The second line segment  41  is formed at the other side of the first and second rectangular regions  21 ,  22  opposed to the first line segment  40  and preferably spaced from the first and second rectangular regions  21 ,  22 , and the projection of the second line segment  41  crosses the coupled microstrip lines  11 . 
         [0036]    The first line segment  40  comprises a first sub line segment  401 , a second sub line segment  402  and a third sub line segment  403 . The second sub line segment  402  is parallel with the third rectangular region  23 , the first sub line segment  401  and the third sub line segment  403  face toward the second line segment  41  and form an angle with the second sub line segment  402 . Preferably, the angle is 90 degree. The second line segment  41  comprises a fourth sub line segment  411 , a fifth sub line segment  412 , and a sixth sub line segment  413 . The fifth sub line segment  412  is parallel with the third rectangular region  23 , the fourth sub line segment  411  and the sixth sub line segment  413  face toward the first line segment  40  and form an angle with the fifth sub line segment  412 . Preferably, the angle is 90 degree. 
         [0037]    In a preferred embodiment, the first line segment  40  and the second line segment  41  have same size and do not overlap with each other. The first sub line segment  401  and the fourth sub line segment  411  have same size, the second sub line segment  402  and the fifth sub line segment  412  have same size, and the third sub line segment  403  and the sixth sub line segment  413  have same size. 
         [0038]    According to another preferred embodiment, a first distance  42  is formed between the first line segment  40  and the third rectangular region  23 , and a second distance  43  is formed between the second line segment  41  and the third rectangular region  23 , wherein the first distance  42  is equal to the second distance  43 . 
         [0039]    In practical application, the ground plane  12  has a H-shaped defected ground structure formed at the central portion thereof, and a          -shaped defected ground structure and a          -shaped defected ground structure respectively formed on the upper and lower portions thereof. Compared with the first embodiment, the          -shaped defected ground structure and          -shaped defected ground structure of the present embodiment can suppress the common mode signals at a much wider frequency band and increase more insertion loss so as to obtain a better suppression effect. Compared with the second embodiment, the present embodiment requires less etching area, thereby efficiently reducing size of electronic elements and saving the fabrication cost. 
         [0040]      FIG. 5  shows a common mode filtering device for use with a defected ground structure according to a fourth embodiment of the present invention. Compared with the first embodiment, the defected ground structure  13 ′ of the present embodiment further comprises a fourth rectangular region  50 , a fifth rectangular region  51 , a sixth rectangular region  52  and a fourth rectangular region  53 , wherein the fourth rectangular region  50  is connected to one side of the first rectangular region  21  facing the second rectangular region  22  and one side of the fourth rectangular region  50  is flush with an upper side of the first rectangular region  21 , the fifth rectangular region  51  is connected to one side of the first rectangular region  21  facing the second rectangular region  22  and one side of the fifth rectangular region  51  is flush with a lower side of the first rectangular region  21 , the sixth rectangular region  52  is connected to one side of the second rectangular region  22  facing the first rectangular region  21  and one side of the sixth rectangular region  52  is flush with an upper side of the second rectangular region  22 , and the seventh rectangular region  53  is connected to one side of the second rectangular region  22  facing the first rectangular region  21  and one side of the seventh rectangular region  53  is flush with a lower side of the second rectangular region  22 . The fourth rectangular region  50  is spaced from the sixth rectangular region  52 , the fifth rectangular region  51  is spaced from the seventh rectangular region  53 , and the fourth to seventh rectangular regions are respectively spaced from the third rectangular region  23 . 
         [0041]    In a preferred embodiment, the fourth to seventh rectangular regions have same size. 
         [0042]    According to another preferred embodiment, the fourth rectangular region  50  and the sixth rectangular region  52  are parallel with each other, and the fifth rectangular region  51  and the seventh rectangular region  53  are parallel with each other. 
         [0043]    Compared with the first embodiment, the defected ground structure  13 ′ of the present embodiment comprising four additional rectangular ground structures causes the operating frequency of the common mode filter in the first embodiment to move towards low frequency, i.e., causes the frequency band of the suppressed common mode signal to shift toward low frequency, thus meeting the demand of the current products with a frequency between 1 GHz and 2 GHz. In the case the defected ground structure  13 ′ has a same area as that of the defected ground structure  13 , the defected ground structure  13 ′ can suppress common mode signals at a lower frequency. In order to suppress common mode signals at higher frequency as in the first embodiment, the present embodiment only needs to reduce the etching area of the defected ground structure. Therefore, through application of the defected ground structure  13 ′ of  FIG. 5 , size of electronic elements can be efficiently reduced and the fabrication cost can be saved. 
         [0044]      FIG. 6  shows a common mode filtering device for use with a defected ground structure according to a fifth embodiment of the present invention. Compared with the fourth embodiment, the defected ground structure  13 ′ of the present embodiment further comprises a third line segment  60  and a fourth line segment  61 , wherein the third line segment  60  is formed at one side of the first and second rectangular regions  21 ,  22  and preferably spaced from the first, second, fourth and sixth rectangular regions  21 ,  22 ,  50 ,  52 , and the projection of the third line segment  60  crosses the coupled microstrip lines  11 , the fourth line segment  61  is formed at the other side of the first and second rectangular regions  21 ,  22  opposed to the third line segment  60  and preferably spaced from the first, second, fifth and seventh rectangular regions  21 ,  22 ,  51 ,  53 , and the projection of the fourth line segment  61  crosses the coupled microstrip lines  11 . The third line segment  60  comprises a seventh sub line segment  601 , an eighth sub line segment  602  and a ninth sub line segment  603 . The eighth sub line segment  602  is parallel with the third rectangular region  23 , the seventh sub line segment  601  and the ninth sub line segment  603  face toward the fourth line segment  61  and form an angle with the eighth sub line segment  602 . Preferably, the angle is 90 degree. The fourth line segment  61  comprises a tenth sub line segment  611 , an eleventh sub line segment  612 , and a twelfth sub line segment  613 . The eleventh sub line segment  612  is parallel with the third rectangular region  23 , the tenth sub line segment  611  and the twelfth sub line segment  613  face toward the third line segment  60  and form an angle with the eleventh sub line segment  612 . Preferably, the angle is 90 degree. 
         [0045]    In a preferred embodiment, the third line segment  60  and the fourth line segment  61  have same size and do not overlap with each other. The seventh sub line segment  601  and the tenth sub line segment  611  have same size, the eighth sub line segment  602  and the eleventh sub line segment  612  have same size, and the ninth sub line segment  603  and the twelfth sub line segment  613  have same size. 
         [0046]    According to another preferred embodiment, a third distance  62  is formed between the third line segment  60  and the third rectangular region  23 , and a fourth distance  63  is formed between the fourth line segment  61  and the third rectangular region  23 , wherein the third distance  62  is equal to the fourth distance  63 . 
         [0047]    In practical application, as shown in  FIG. 6 , the ground plane  12  has a H-shaped defected ground structure  13 ′ formed at the central portion thereof, and a          -shaped defected ground structure and a          -shaped defected ground structure respectively formed on the upper and lower portions thereof. As disclosed in the third embodiment, the          -shaped defected ground structure and          -shaped defected ground structure of the present embodiment can suppress the common mode signals at a wider frequency band and increase more insertion loss so as to obtain a better suppression effect. Therefore, the filter using the defected ground structure  13 ′ plus the          -shaped and          -shaped ground structures not only obtain a preferred common mode signal suppression effect, but also efficiently reduce size of electronic elements and save the fabrication cost. 
         [0048]      FIG. 7  shows a flow process of a common mode filtering method for use with a defected ground structure according to the present invention. The common mode filtering method is applied in a common mode filtering device with a defected ground structure, wherein the common mode filtering device comprises a substrate, coupled microstrip lines formed on the substrate and a ground plane formed underneath the substrate. The common mode filtering method according to the present invention comprises the following steps. 
         [0049]    At step S 1 , at least one defected ground structure is formed in the ground plane underneath the substrate by etching or a similar method. The defected ground structure has a pattern of one of the first to fifth embodiments. Then, the process goes to step S 2 . 
         [0050]    At step S 2 , dual mode signals are passed through the coupled microstrip lines. The dual mode signals comprise differential mode signals and common mode signals. Due to different reference return path of the differential mode signals and common mode signals passing through the coupled microstrip lines, a defected ground structure equivalent to a LC resonator is formed on the return ground path of the common mode signals so as to suppress the common mode signals without affecting the differential mode signals. 
         [0051]    Through the above-described common mode filtering method and device for use with a defected ground structure, the present invention at least achieves the following effects:
       (1) simplified fabrication process. Since the defected ground structure is formed on the known printed circuit boards, it does not require complicated fabrication processes of multi-layer boards or additional filtering elements such as common mode chokes.   (2) suppression over a wider frequency band and better suppression effect. The defected ground structure according to the present invention strengthens electromagnetic coupling relationship between LC resonant circuits so as to achieve suppression over a wider frequency band and better suppression effect.   (3) cost-saving. Compared with a conventional defected ground structure, the present invention reduces the etching area on a printed circuit board and saves the fabrication cost.       
 
         [0055]    Therefore, the present invention not only achieves suppression over a wider frequency band and better suppression effect, but also provides a simplified fabrication process at lower fabrication cost, thereby increasing the competitiveness and industrial application value of the present invention. 
         [0056]    The above-described descriptions of the detailed embodiments are only to illustrate the preferred implementation according to the present invention, and it is not to limit the scope of the present invention, Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present invention defined by the appended claims.