Abstract:
A printed circuit board with reduced noise effects and without the need to increase the distance between a noise source and a wireless communication board. The circuit board includes multilayer structural conductive layers having a first conductive plane connected to power supply potential and a second conductive plane connected to ground potential. The first and second conductive planes are formed such that an electric field generated by a potential difference between the first conductive plane and the second conductive plane is concentrated on one side of one of the first conductive plane and the second conductive plane. The conductive plane associated with the concentrated electric field and the wireless communication board are on different sides relative to the conductive plane that is not associated with the concentrated electric field.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a technique of suppressing electromagnetic waves generated within an apparatus provided with a units of wireless communication, and in particular to a technique of reducing electric field noise generated within a printed circuit board or an information processing apparatus. 
   In an information processing apparatus, as LSIs used in the apparatus becomes high-speed in their clocks and high-density in their mounting, noise generated within the apparatus increases. In an information processing apparatus or the like having a units of wireless communication, noise generated within the apparatus causes to deteriorate the wireless communication performance such as reduction of the data throughput or of the communication distance between the information processing apparatus and an external source of the communication data (ex. a radio antenna). 
     FIG. 16  shows an example of results of investigation on the effects of noise within an apparatus on the communication performance. The horizontal axis indicates a communication distance (m) between two communication apparatuses, and the vertical axis indicates a bit error rate (%) of the data. This figure shows a case (a dotted line a) where there exists interference by noise of a CPU or the like, and a case (a solid line b) where there does not exist interference by noise, in a PC mounted with a wireless unit using a 2.4 GHz band. In the case where no interference exists, the communication distance at the error rate 0.1% is about 100 m, while, in the case where interference exists, the communication distance is only about 20 m, i.e., 20% of the former case. Thus, when there is a noise source within an information processing apparatus having a wireless communication function, the noise source deteriorates the wireless communication performance of the apparatus. 
   As a method of reducing the effects of a noise source on a wireless communication function, there is a proposed mounting method in which a noise source on a circuit board is distant to a wireless communication board, as disclosed in Japanese Application Patent Laid-Open No. 10-268988, “Portable Information Apparatus with Wireless”. 
   However, in this method, a board having a wireless function should be located in a sufficient distance from a noise source on a circuit board, and sometimes this becomes an obstacle to miniaturization of an information processing apparatus. Thus, it is difficult to apply this method, for example, to a cellular phone having a small-sized chassis. 
   The present invention has been taken the above situation into consideration. One advantage of the present invention is to provide a circuit board that can reduce effects of noise on a wireless communication board mounted on the circuit board, even when a noise source is not distant to the wireless communication board. 
   Another advantage of the present invention is to apply the above-mentioned circuit board to an information processing apparatus having a wireless communication function, and to provide a small-sized information processing apparatus having an improved wireless communication throughput and an increased communication distance. 
   SUMMARY OF THE INVENTION 
   To attain the above advantages, a first mode of the present invention provides a printed circuit board mounted with a wireless communication board, comprising multi-layered conductive layers including a first conductive plane connected to power supply potential and a second conductive plane connected to ground potential. The first and second conductive planes are formed such that: one conductive plane interposes between the other conductive plane and a surface of the printed circuit board on the side where the wireless communication board is mounted, and an electric field generated by a potential difference between the power supply potential and the ground potential is concentrated on the side of the above-mentioned the other conductive plane rather than the side of the above-mentioned one conductive plane. 
   To attain the above advantages, another mode of the present invention provides the printed circuit board according to the above-mentioned first mode, wherein: said printed circuit board comprises a conductive belt that is formed in a conductive layer in which said the other conductive plane lies, and located adjacent to said the other conductive plane with slits interposing between said conductive belt and said the other conductive plane; and said conductive belt is electrically connected to said one conductive plane. 
   To attain the above advantages, another mode of the present invention provides the printed circuit board according to the above-mentioned first mode, wherein: said printed circuit board further comprises a U-shaped conductive member that encloses a part of said printed circuit board to cover the surface of said printed circuit board in an area over which said wireless communication board is mounted, and said conductive member is electrically connected to said one conductive layer. 
   To attain the above advantages, another mode of the present invention provides the printed circuit board according to the above-mentioned first mode, wherein: said the other conductive plane is formed in a smaller size than said one conductive plane, and located within an area of said one conductive plane. 
   To attain the above advantages, another mode of the present invention provides the printed circuit board according to the above-mentioned first mode, wherein: said printed circuit board comprises: a conductive belt that is formed in a conductive layer in which said the other conductive plane lies, and located adjacent to said the other conductive plane with slits interposing between said conductive belt and said the other conductive plane; and a third conductive plane formed in a conductive layer that is located on an opposite side to said one conductive plane with respect to said the other conductive plane; wherein, said conductive belt and said third conductive plane are electrically connected to said one conductive plane. 
   To attain the above advantages, another mode of the present invention provides a wireless communication apparatus comprising: one of the above-mentioned printed circuit boards; and a chassis that houses said printed circuit board. 
   To attain the above advantages, another mode of the present invention provides the above-mentioned wireless communication apparatus further comprising a radio wave absorption member that is located inside said chassis, and absorbs an electric field generated by a potential difference between said power supply potential and said ground potential. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A to 1C  are schematic diagrams showing a printed circuit board according to a first embodiment of the present invention; 
       FIG. 2  is a diagram showing a relation between a wave impedance of an electromagnetic wave and distance from a wave source; 
       FIG. 3  is a diagram showing an interference relation between a type of radio antenna and a noise source; 
       FIGS. 4A and 4B  are diagrams explaining generation of an electric field; 
       FIGS. 5A and 5B  are schematic diagrams showing a board as a measuring object (measuring object board), which is similar to the first embodiment; 
       FIGS. 6A and 6B  are schematic diagrams showing a measuring object board similar to a conventional board; 
       FIG. 7  shows a simulation result of an electric field inside a measuring object board; 
       FIGS. 8A to 8C  show measurement results of an electric field around a measuring object board; 
       FIGS. 9A and 9B  are schematic diagrams showing a printed circuit board according to a second embodiment of the present invention; 
       FIGS. 10A and 10B  are schematic diagrams showing a printed circuit board according to a third embodiment of the present invention; 
       FIG. 11  is a diagram explaining generation of electric fields in the third embodiment of the present invention; 
       FIG. 12  is a schematic diagram showing a printed circuit board according to a fourth embodiment of the present invention; 
       FIG. 13  is an exploded view of a cellular phone according to a fifth embodiment of the present invention; 
       FIGS. 14A and 14B  are schematic diagrams showing a printed circuit board according to the fifth embodiment of the present invention; and 
       FIG. 15  is a schematic diagram showing a printed circuit board and a chassis according to a sixth embodiment of the present invention. 
       FIG. 16  is a graphic representation of a relation of a communication distance and an error rate. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention will be described. 
   A first embodiment of the present invention will be described. 
     FIGS. 1A to 1C  are schematic diagrams showing a printed circuit board to which a first embodiment of the present invention is applied.  FIG. 1A  is a plan view,  FIG. 1B  is a cross section taken along the A–A′ line of  FIG. 1A , and  FIG. 1C  is a back view. 
   The printed circuit board of the present embodiment comprises: a main circuit board  30  mounted with digital circuits; an RF circuit board  40  mounted with a wireless communication circuit; an antenna  41 ; and a connector  27 . As shown in  FIGS. 1A and 1B , the antenna  41  is located on the RF circuit board  40 . The connector  27  supports the antenna  41  and the RF circuit board  40  on the main circuit board  30 , and connects the RF circuit board  40  electrically to the main circuit board  30 . 
   The main circuit board  30  is a printed circuit board comprising signal layers  31 ,  33 ,  34  and  36 , a power plane  32  and a conductive layer  35 , i.e., six layers in total ( FIG. 1B ). As shown in  FIGS. 1B and 1C , the conductive layer  35  is provided with slits  39 . The slits  39  separate the conductive layer  35  into a ground plane  75  and a power belt  37  surrounding the ground plane  75 . A plurality of vias  38  connect the power belt  37  with the power plane  32  so that the power belt  37  has the same potential as the power plane  32  with respect to high frequency. 
   The RF circuit board  40  is mounted with the antenna  41  on its surface opposite to the surface facing to the main circuit board  30 . The RF circuit board  40  is connected to a surface of the main circuit board  30  through the connector  27 . The surface is on the opposite side to the conductive layer  35  having the slits  39  (i.e., the layer arranged with the pair of the power belt  37  and the ground plane  75 ), with reference to the power plane  32 . And, the RF circuit board  40  is electrically connected to the main circuit board  30  through the connector  27 . 
   Next, will be described the principle of electric field suppression by the printed circuit board of the present embodiment. 
   First, referring to  FIG. 2 , will be generally described noise interference between a radio antenna and a digital circuit inside a chassis, and conditions that cause the interference. 
     FIG. 2  is a diagram showing a relation between a wave impedance of an electromagnetic wave (H: magnetic field type, E: electric field type) and a distance D from a wave source. The horizontal axis indicates distance from a noise source (a wave source) being normalized with a wave length λ, and the vertical axis indicates wave impedance (in Ω). 
   In the example shown in  FIG. 2 , when the noise source is a magnetic field type (the solid line d), then, in the very close neighborhood of the wave source (the normalized distance from the wave source=0.01), an magnetic field type wave impedance (H) is 1.E+00&lt;H&lt;1.E+01, and H increases in proportion to the third power of the distance r, until the normalized distance from the wave source becomes nearly 1. When the distance is more than 1, the value of the magnetic field type wave impedance (H) becomes almost constant at 377 Ω. 
   Further, when the noise source is an electric field type (the dotted line c), then, in the very close neighborhood of the wave source (the normalized distance from the wave source=0.01), an electric field type wave impedance (E) is 1.E+04&lt;E&lt;1.E+05, and H increases in inverse proportion to the third power of the distance r, until the normalized distance becomes nearly 1. When the distance is more than 1, the value of the electric field type wave impedance (E) becomes almost constant at 377 Ω. 
   In the case of the interference inside a chassis, a noise radiation pattern differs whether a noise source is an electric field type or a magnetic field type, because of the above-described characteristics of the impedances E and H. In the case of frequencies of 0.8 to 5.5 GHz used in a cellular phone or a wireless LAN, the normalized distance 1 becomes about 50 mm at most. Sizes of chassis of notebook-sized computers and cellular phones belong to this category. Accordingly, when electromagnetic noise inside a chassis of a notebook-sized computer or a cellular phone is to be considered, the main noise is an electric field noise. 
   Next, referring to  FIG. 3 , will be described a relation between a type of radio antenna as an object of interference and a type of noise source. 
   As a radio antenna, may be mentioned a magnetic field type antenna such as a loop antenna and an electric field type antenna such as an F antenna, a helical antenna and a patch antenna. Generally, an electric field type antenna is used as a radio antenna. As a result, a radio antenna is affected strongly when a noise radiation source is an electric field type. 
   The antenna  41  in the present embodiment is an electric field type antenna. Accordingly, the antenna  41  is affected strongly when a noise radiation source is an electric field type. As an electric field type noise radiation source, may be mentioned, for example, a patch antenna structure formed by a power plane and a ground plane arranged in opposition to each other. 
   Next, referring to  FIGS. 4A and 4B , will be described a principle of suppressing electric field noise that reaches from a printed circuit board of the patch antenna structure to an electric field type antenna. 
     FIG. 4A  illustrates the effect of electric fields (E 1  and E 2 ) on an electric field type antenna. Here, the electric fields E 1  and E 2  are generated at the resonance frequency of a conventional printed circuit board of the patch antenna structure. 
   A main circuit board  230  of the conventional printed circuit board having the patch antenna structure comprises a power plane  232  and a ground plane  235  arranged in opposition to each other, and each plane is wired up to its end portion. As shown in  FIG. 4A , an RF board mounted with a wireless communication circuit is arranged over and facing to the power plane  232 , on the other side of the power plane  232  from the ground plane  235 . An antenna  241  is fixed on the RF board  240 . 
   Owing to the potential difference between the power plane  232  and the ground plane  235 , voltage amplitudes becomes maximum at the end portions of the planes  232  and  235 . As a result, strong electric fields E 1  and E 2  (shown by the dotted lines and arrows in the figure) are generated at the end portions of the planes  232  and  235 , and the electric field E 1  is coupled with the antenna  241  and the RF circuit  240  to cause interference (noise) with the antenna  241 . 
   On the other hand,  FIG. 4B  illustrates the effect of electric fields (E 3  and E 4 ) on an electric field type antenna. Here, the electric fields E 3  and E 4  are generated at the resonance frequency of the printed circuit board of the present embodiment shown in  FIGS. 1A to 1C . 
   In the printed circuit board of the present embodiment, the electric fields E 3  and E 4  are generated between the power belt  37  and the ground plane  75 , namely, at the slits  39 . As described above, the slits  39  between the power belt  37  in the end portion of the board and the ground plane  75  are arranged on the back side of the power plane  32 , seen from the antenna  41 . 
   Accordingly, coupling of the generated electric fields E 3  and E 4  with the antenna  41  and the RF circuit  40  is weak, and noise interference with the antenna  41  is suppressed and performance of wireless communication (an error rate and communication distance) can be improved. 
   Next, effects of the present embodiment will be described. 
     FIG. 5A  is a plan view showing a board (board as an object of measuring an electric field) whose structure is similar to the main circuit board  30  constituting the printed circuit board shown in  FIG. 4B .  FIG. 5B  is a cross section taken along the A–A′ line of  FIG. 5A . In addition,  FIGS. 5A and 5B  show a board which inspects an effect of the present embodiment. In  FIGS. 1A ,  1 B,  1 C,  4 A and  4 B, the positions of a power plane and a grand plane are replaced. However, the effect dose not change. 
   In  FIGS. 5A and 5B , the board as the measuring object (measuring object board) is a square board, one side of which is 87 mm in length, and the dielectric ratio εr of the board is 4.5. A power plane  301  is stuck to one surface of the board, and a ground plane (which is also an electric field measurement surface)  302  is stuck to the other surface. Further, a distance between the power plane  301  and the ground plane  302  (i.e., the thickness of the board) is 1.55 mm. 
   Similarly to the main circuit board  30  shown in  FIG. 1 , the measuring object board shown in  FIG. 5  is provided with a ground belt  337  on the periphery of the power plane  301 , while slits  339  (slit width 0.5 mm) interpose between the ground belt  337  and the power plane  301 . The ground belt  337  is electrically connected with the ground plane  302  through vias  338 . 
   In the center of the power plane  301 , an SMA connector  305  is fixed. This connector  305  is connected to the ground plane  302  through a signal line  306 . The connector  305  is connected to a signal generator  310  through a wire  308 . Noise of a 2.4 GHz band generated by the signal generator  310  is put into the ground plane  302  at a point  309  through the wire  308  and the signal line  306 . This printed circuit board is designed such that resonance in the TE22 mode occurs in the 2.45 GHz band. Further, an electric field display range  341  is a range (shown in  FIG. 8  referred to later) of displaying an electric field measured in the ground plane  30  and its neighborhood. 
     FIG. 6A  is a plan view of a board (a board as an object of measuring an electric field) whose structure is similar to the main circuit board  230  constituting the conventional printed circuit board shown in  FIG. 4A .  FIG. 6B  is a cross section taken along the one-dot chain line B–B′ of  FIG. 6A . 
   Differently from the measuring object board shown in  FIG. 5 , the measuring object board shown in  FIGS. 6A and 6B  do not have a ground belt  337  interposed by slits  339  in the periphery of a power plane  301 . Except that, the board is similar to the measuring object board shown in  FIGS. 5A and 5B , and like numerals refer to like parts. 
     FIG. 7  shows a result of simulating a resonance mode of an electric field of the measuring object board shown in  FIGS. 6A and 6B . A contour map of an electric field E 5  in  FIG. 7  shows behavior of generating the electric field E 5  in the 2.4 GHz band inside the printed board  300 . This map is obtained by calculation assuming a dielectric ratio εr 4.5 for a square part that is 87 mm in its side length and located between the ground plane  302  and the power plane  301 . This result shows generation of a train of standing waves having peaks of strong electric fields regularly distributed on the surface at the vertexes (i.e., corners) of the measuring object board, the centers of the sides (i.e., outside edges  340  of the board), and the center of the measuring object board. 
   Next, with respect to the measuring object boards shown in  FIGS. 5A ,  5 B,  6 A and  6 B,  FIGS. 8A to 8C  shows the results of the measurement of the electric fields (E 6  and E 7 ) in the ground planes  302  and their neighborhood. 
     FIG. 8A  is a contour map showing the measurements of the electric field E 6  in the electric field display range  341  for the power plane  302  of the measuring object board (the board having the structure similar to the main circuit board of the present embodiment) shown in  FIGS. 5A and 5B .  FIG. 8B  is a contour map showing the measurements of the electric fields E 7  in the electric field display range  341  for the ground plane  302  of the measuring object board (the board having the structure similar to the conventional main circuit board) shown in  FIGS. 6A and 6B .  FIG. 8C  shows hatching patterns for indicating the field strengths in the  FIGS. 8A and 8B . Although the line patterns are used in the figures to distinguish the field strengths, color patterns may be used instead of the line patterns. 
   In  FIGS. 8A and 8C , only the left half (the electric field display range  341 ) of the electric field is shown. Since the ground plane  302  has a symmetrical shape, the electric field of the right half is also symmetrical to that of the left half. 
   In the case of the electric field E 7  of the measuring object board having the structure ( FIG. 6 ) similar to the conventional main circuit board,  FIG. 8B  shows that the electric field radiates strongly at the vertexes (corners) and the centers of the outside edges  340  of the board of the ground plane  302 . The distribution of the electric field E 7  is similar to that of the electric field E 5  inside the board shown in the simulation result of  FIG. 7 . 
   On the other hand, in the case of the electric field E 6  of the measuring object board having the structure ( FIG. 5 ) similar to the main circuit board of the present embodiment,  FIG. 8A  shows the weaker field strengths than the result of  FIG. 8B  by about 7 to 10 dB. 
   In the above two cases, the measurements of the electric fields were commonly done on and around the side of the ground plane  302  rather than that of the power plane  301 .  FIG. 8  illustrates that the electric fields on the ground plane side in the construction in  FIG. 5  (with the slits  339 ) are weaker than that in the construction in  FIG. 6  (without slits). Thus, setting the RF circuit board  40  with the antenna  41  over the ground plane  302  (as the relation between the the circuit board  40  and the power plane  32  in  FIG. 4  B) allows to suppress the effects of the electric field on the circuit board  40  and the antenna  41 . 
   As described above, in the present embodiment, the power belt  37  connected to the power plane  32  is provided in the periphery of the ground plane  75 , while the slits  39  interpose between the power belt  37  and the ground plane  75 . The slits  39  from which the electric fields radiate are located on the opposite side to that mounted on the antenna  41 , with reference to the power plane  32 . This construction, then, enables to emit the electric field noises in wireless communication on the ground-plane side on which the antenna  41  is not mounted. Thereby, interference of electric field noise with the antenna  41  and the RF circuit  40  can be suppressed. In the present embodiment, the slits  39  between the power belt  37  and the ground plane  75  form the structure for causing an electric field. However, it is possible to exchange the ground potential with the power supply potential. In other words, in  FIG. 1 , a ground belt  37  may be provided in the periphery of a power plane  75 , while slits  39  interpose between the ground belt  37  and the power plane  75 . The ground belt  37  is connected to a ground plane  32 , and an antenna  41  is arranged on the opposite side to the power plane  35 , with reference to the ground plane  32 . In other words, in  FIG. 1 , the numerals  32  and  37  refer to the ground, and the numeral  75  refers to the power supply potential. 
   Next, a second embodiment of the present invention will be described. 
     FIGS. 9A and 9B  are schematic diagrams showing a printed circuit board to which the second embodiment of the present invention is applied.  FIG. 9A  is a plan view, and  FIG. 9B  is a cross section taken along the A–A′ line of  FIG. 9A . 
   As shown in the figure, the printed circuit board of the present embodiment comprises a main circuit board  1130  mounted with digital circuits, an RF circuit board  1140  mounted with a wireless communication circuit, an antenna  1141 , a connector  1127 , a U-shaped metal part  1137 , and setscrews  1138 . 
   A main circuit board  1130  is a printed circuit board comprising signal layers  1131 ,  1133 ,  1134  and  1136 , a ground plane  1132  and a power plane  1135 , i.e., six layers in total. The RF circuit board  1140  is mounted with an antenna  1141 , and electrically connected to the main circuit board  1130  through the connector  1127 . 
   As shown in the figure, the U-shaped metal part  1137  is fixed just below the antenna  1141  such that the U-shaped metal part  1137  partly encloses the main circuit board  1130 . An insulating sheet  1139  is attached to this U-shaped metal part  1137 , at the side nearest to the conductive layer  1136 . 
   The setscrews  1138  are used to fix the U-shaped metal part  1137  such that the U-shaped metal part  1137  partly encloses the main circuit board  1130 . To that end, the setscrews  1138  penetrate the U-shaped metal part  1137  and the main circuit board  1130 . The setscrews  1138  are electrically connected to the U-shaped metal part  1137  and the ground plane  1132 . On the other hand, the setscrews  1138  are not electrically connected to the power plane  1135 , and pass through a hole portion opened in the power plane  1135 . Further, the setscrews  1138  penetrate the insulating sheet  1139  to reach the U-shaped metal part  1137  again. 
   According to the thus-described construction, the U-shaped metal part  1137  has the same ground potential as the ground plane  1132 . As a result, as shown in  FIG. 9B , an electric field E 8  is generated between the base of the U shape of the U-shaped metal part  1137  and the power plane  1135 , and thus the base of the U-shaped metal part  1137  and the power plane  1135  are coupled in their capacities. 
   According to the present embodiment in  FIG. 9 , the antenna  1141  on the RF circuit board  1140  is arranged on the side of the ground plane  1132  that has the same potential as the U-shaped metal part  1134 . In other words, the antenna  1141  is arranged over the surface opposite to the surface where the electric field E 8  is generated. As a result, similar to the first embodiment, coupling between the electric field E 8  and the antenna  1141  can be suppressed, noise interference can be reduced, and performance of wireless communication (an error rate and communication distance) can be improved. 
   Next, a third embodiment of the present invention will be described. 
     FIGS. 10A and 10B  are schematic diagrams showing a printed circuit board to which a third embodiment of the present invention is applied.  FIG. 10A  is a plan view of the printed circuit board, and  FIG. 10B  is a cross section taken along the A–A′ line of  FIG. 10(A) . 
   As shown in the figure, the printed circuit board of the present embodiment comprises a main circuit board  1230  mounted with digital circuits, an RF circuit board  1240  mounted with a wireless communication circuit, an antenna  1241  and a connector  1227 . The main circuit board  1230  is a printed circuit board comprising signal layers  1231 ,  1233 ,  1234  and  1236 , a ground plane  1232  and a power plane  1235 , i.e., six layers in total. 
   The RF circuit board  1240  is mounted with the antenna  1241 , and connected electrically to the main circuit board  1230  through the connector  1227 . The power plane  1235  is of a size contained within an area of the ground plane  1232 , seen from the top surface of the printed circuit board. Namely, four sides of the power plane  1235  are smaller than the ones of the ground plane  1232 . In  FIG. 10A , a dotted line  1239  shows the outside edges of the power plane  1235 . 
     FIG. 11  illustrates the effect of electric fields E 9  and E 10  generated in the printed circuit board of  FIGS. 10A and 10B , on an electric field type antenna. The electric fields E 9  and E 10  are generated at the resonance frequency of the printed circuit board of the present embodiment shown in  FIGS. 10A and 10B . As the present embodiment shown in  FIG. 10 , the projection of the power plane  1235  onto the ground plane  1232  is larger than the area of the ground plane  1232 . The electric fields E 9  and E 10 , then, occur between the end portions of the power plane  1235  and those of the ground plane  1232 . Those electric fields E 9  and E 10  can be distributed in the opposite side to that mounted with the antenna  1241 , with reference to the ground plane  1232 . Accordingly, the electric field noises from the electric fields E 9  and E 10  that interferes with the antenna  1241  can be reduced. 
   Next, a fourth embodiment of the present invention will be described. 
     FIG. 12  shows a schematic cross section of a printed circuit board to which the fourth embodiment of the present invention is applied. 
   As shown in  FIG. 12 , the printed circuit board of the present embodiment comprises a main circuit board  1530  mounted with digital circuits, an RF circuit board  1540  mounted with a wireless communication circuit, an antenna  1541  and a connector  1527 . 
   The main circuit board  1530  comprises signal layers  1531 ,  1533  and  1536 , ground planes  1532  and  1535  and a conductive layer  1534 , i.e., six layers in total. 
   The conductive layer  1534  is provided with slits  1539 . The slits  1539  separate the conductive layer  1534  into a power plane  1575  and a ground belt  1537  at the end portion of the board. A plurality of vias  1538  connect the ground belt  1537  with the ground planes  1532  and  1535  so that the ground belt  1537  has the same potential as the ground plane  1535  with respect to high frequency. 
   One side of the RF circuit board  1540  is mounted with the antenna  1541 . The other side of the RF circuit board  1540  connected electrically to a surface  1531  of the main circuit board  1530  through the connector  1527 . 
   According to the present embodiment shown in  FIG. 12 , the power plane  1575  is located between the two ground planes  1532  and  1535 . The ground belt  1537  is located on the same conductive layer  1534  as the power plane  1575  lies on, while the slits  1539  interpose between the ground belt  1537  and the power plane  1575 . Therefore, the slits  1539 , that is to say, the sources of the electric field noise, are contained inside of the main board  1530  by the two ground planes  1532  and  1535 . In addition, the ground belt  1537  is a member of the closed end of the main circuit board  1530 , which end is shielded by ground with vias  1538 . And the closed end of the main circuit board  1530  is closer to the RF circuit board  1540  with the antenna  1541  than the open end of the board  1530 . As a result, the electric field generated at the closed end portion of the main circuit board  1530  can be suppressed to a lower level, and electric field noise to the antenna  1541  can be reduced. Moreover, the electric field noise to the antenna  1541  from the open end of the main circuit board is smaller due to the longer distance between the antenna  1541  and the open end. 
   Here, the electric field is generated between the power plane  1575  and the ground belt  1537  (which are adjacent to each other with the slits  1539  interposing therebetween) and in the neighborhood. This electric field is enclosed by the ground planes  1532  and  1535  and the plurality of vias  1538  having the ground potential, and accordingly, the effect of this electric field on the antenna  1541  is suppressed. 
   Next, a fifth embodiment of the present invention will be described. 
     FIG. 13  is an exploded view showing schematic construction of a cellular phone mounted with a printed circuit board to which the fifth embodiment of the present invention is applied. 
   As shown in the figure, the cellular phone  100  of the present embodiment comprises a back cmore than  150 , an RF circuit board  140  for wireless communication, a main circuit board  130  and a front cmore than  110 . 
   The main circuit board  130  is mounted with digital circuits that include a CPU  122 , memories  123 - a  and  123 - b , a baseband chip  124 , a liquid crystal controller  125 , a liquid crystal panel  127 , a connector  128 , an external connector/charging terminal  129 , and the like. On the back side (i.e., a surface on which the liquid crystal panel  127  is located) of the main circuit board  130 , the slits  139  are provided. 
   Further, the RF circuit board  140  for wireless communication is mounted with a chip antenna  141 , a rod antenna connector  142 , and an RF module  143 . This RF circuit board  140  is electrically connected to the main circuit board  130  through the connector  128 . 
   The back cmore than  150  is fixed with a rod antenna  152 . This rod antenna  152  is connected to the RF circuit board  140  through the rod antenna connector  142 . 
   This cellular phone  100  is mounted with two antennas, i.e., the chip antenna  141  and the rod antenna  152 . The chip antenna  141  functions as a main antenna, and the rod antenna  152  as an auxiliary antenna. 
     FIGS. 14A and 14B  are schematic diagrams showing the main circuit board attached with the RF circuit board  140 .  FIG. 14A  is a plan view showing the main circuit board  130  attached with the RF circuit board  140 , and  FIG. 14B  is a cross section taken along the one-dot chain line A–A′ of the  FIG. 14A . 
   The main circuit board  130  is a printed circuit board comprising signal layers  131 ,  133 ,  134  and  136 , a ground plane  132 , and a conductive layer  135 , i.e., six layers in total. The conductive layer  135  is provided with slits  139 . The slits  139  separate the conductive layer  135  into a power plane  735  and a ground belt  137 . In the present embodiment, a part (i.e., an upper half in  FIG. 14A ) of the power plane  735  is enclosed by the ground belt  137  with the slits interposing between that part and the ground belt  137 . The ground belt  137  is connected to the ground plane  132  through a plurality of vias  138  so that the ground belt  137  has the same potential as the ground plane  132  with respect to high frequency. The power plane  735  is connected to the power supply potential. The RF circuit board  140  is electrically connected to the main circuit board  130  through the connector  128  provided on the surface on the opposite side to the conductive layer  135 , with reference to the ground plane  132 . Further, the RF circuit board  140  is located on the side (i.e., the upper half in  FIG. 14A ) where the slits  139  are provided, and the antenna  141  is located on the RF circuit board  140 . Thus, the slits  139  between the ground belt  137  and the power plane  735  are located on the opposite side to the the antenna  141  on the RF circuit board  140  with reference to the ground plane  132 . 
   According to the present embodiment, similarly to the above-described first embodiment, an electric field is generated in the neighborhood of the slits  139  between the ground belt  137  and the power plane  735 . As a result, in the neighborhood of the antenna  141  and the rod antenna  152 , effect of the electric field can be suppressed. 
   In the present embodiment, as the main circuit board mounted, may be used one of the main circuit boards shown in the above-described first through fourth embodiments. 
   Next, a sixth embodiment of the present invention will be described. 
     FIG. 15  is a schematic diagram showing a wireless communication apparatus to which a sixth embodiment of the present invention is applied. As shown in the figure, the wireless communication apparatus of the present embodiment is obtained by mounting the printed circuit board shown in  FIG. 1  into a chassis  1490 . A radio wave absorption belt  1480  is attached to the chassis  1490  at a part under the slits  39  provided in the main circuit board  30 . 
   According to the present embodiment, similarly to the above-described first embodiment, interference of electric field noise with the wireless communication antenna  41  can be suppressed. Further, in the present embodiment, a part of the electric field is absorbed by the radio wave absorption belt  1480  attached to the chassis  1490 . Accordingly, it is possible to prevent reflection of electric field noise in the chassis  1490  and stray noise owing to coupling with the chassis  1490 . 
   In the present embodiment, as the main circuit board mounted, may be used one of the main circuit boards shown in the above-described second through fifth embodiments. 
   Hereinabove, the embodiments of the present invention have been described. 
   The above-described embodiments, except for the second embodiment, employ the construction in which shapes or locations of the ground plane and the power plane are changed. The second embodiment employs the construction in which the U-shaped metal part is attached to the printed circuit board. According to such construction, the location of the electric field noise source between the ground plane and the power plane is moved to a location far from the antenna. Thus, in each of the embodiments, noise to the antenna can be reduced without changing a positional relation between the antenna and the printed circuit board including the ground plane and the power plane. 
   As a result, it is not necessary to keep a large distance between a wireless circuit board and a CPU as a source of electromagnetic noise. It is possible to provide a smaller-sized printed circuit board mounted with a wireless circuit, and a smaller-sized portable information processing apparatus provided with that printed circuit board. 
   The present invention is not limited to the above-described embodiments, and can be changed variously within the scope of the invention. For example, in the second embodiment shown in  FIG. 9 , an additional U-shaped metal part  1137  may be attached to the open end of the printed circuit board  1130  on the opposite side to the already-provided U-shaped metal part  1137 . The newly-attached U-shaped metal part  1137  can further reduce the electric field noise from the printed circuit board  1130  to the antenna  1141 . 
   In the sixth embodiment shown in  FIG. 15 , it is possible to employ construction in which the RF board  40  and the antenna  41  are located outside the chassis  1490  while maintaining the electric connection with the printed circuit board  30  through the connector  27 . According to this construction, the electric field noise from the printed circuit board  30  to the antenna  41  can be reduced further. 
   As described above, the present invention can provide a printed circuit board with reduced effect of noise, without having a large distance between a noise source and a board having a wireless communication function. When the above-described printed circuit board is applied to an information processing apparatus having a wireless communication function, it is possible to provide a small-sized information processing apparatus having improved throughput of wireless communication and increased communication distance.