Abstract:
A low EMI digitizer tablet is covered with a shielding layer on an antenna circuit thereof to shield the digitizer tablet from invasion of electromagnetic interference (EMI) and thereby enhance the immunity of the digitizer tablet against noise.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to a digitizer tablet, particularly to a low EMI (Electromagnetic Interference) digitizer tablet.  
         BACKGROUND OF THE INVENTION  
         [0002]    In an electromagnetic induction digitizer tablet shown in FIGS. 1 and 2, an antenna circuit  1  usually comprises an X-axis antenna  11 , a Y-axis antenna  12 , a dielectric layer  13 , and a metallic isolation layer  14 .  
           [0003]    Both the X-axis antenna  11  and the Y-axis antenna  12  are equidistantly spaced in an array so as to induce a stylus  15  and calculate the absolute coordinates thereof. The related principle works on the basis of magnetic flux variation caused by change of electromagnetic field of the stylus  15  to thereby have voltage variation induced in the digitizer tablet for calculating position of the stylus  15 .  
           [0004]    When the stylus  15  is placed adjacent to some antenna circuit of the digitizer tablet, the farther an antenna circuit is located from the stylus  15 , a weaker voltage is induced in the antenna circuit and vice versa because the field intensity is inversely proportional to square of distance. A microprocessor in the digitizer tablet is programmed to scan all the antennas and analyze magnitude of the scanned voltages, the absolute coordinates of the stylus  15  are obtainable.  
           [0005]    As a digitizer tablet is usually accompanied by some computer peripherals or information products operated in high frequencies, EMI is deemed inevitable that would considerably affect the accuracy in positioning the stylus, therefore, “ways to lessen or eliminate EMI” has already become a focus issue in recent days.  
         SUMMARY OF THE INVENTION  
         [0006]    The primary object of this invention is to provide a low EMI digitizer tablet, which is realized by coating a shielding layer on an antenna circuit to thereby shield the antenna circuit from invasion of any external electromagnetic interference.  
           [0007]    For more detailed information regarding advantages or features of this invention, at least an example of preferred embodiment will be elucidated below with reference to the annexed drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 shows the antenna circuit structure of a conventional digitizer tablet.  
         [0009]    [0009]FIG. 2 shows the X- and Y-axis antenna of a conventional digitizer tablet.  
         [0010]    [0010]FIG. 3 shows an antenna circuit structure of a digitizer tablet of this invention.  
         [0011]    [0011]FIG. 4 shows the grid structure of a shielding layer on an antenna circuit of the digitizer tablet of this invention.  
         [0012]    [0012]FIG. 5 shows the circuitry arrangement of the X-axis (Y-axis) antenna of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    As illustrated in FIG. 1, a metallic isolation layer  14  (a metallic plate for example) is disposed under an antenna circuit  1  of a conventional digitizer tablet for grounding, therefore, the magnetic field created by a stylus  15  is inductively received by the top surface of the digitizer tablet. Similarly, the conventional digitizer tablet is interfered on its top surface by electromagnetic waves.  
         [0014]    A low Electromagnetic interference (EMI) digitizer tablet of this invention shown in FIG. 3 is made by laying a dielectric layer  13  and a shielding layer  16  on the surface of an antenna circuit  1 , wherein the shielding layer  16  is a conductor in net structure to shield the antenna circuit of the digitizer tablet from invasion of EMI.  
         [0015]    According to a preferred embodiment of this invention shown in FIG. 4, the shielding layer  16  in net structure is an antenna circuit composed of an X-axis and a Y-axis antenna  11 ,  12  for shielding the digitizer tablet from EMI without affecting the detection operation for magnetic flux variation caused by the stylus  15 . Besides, the shielding layer  16  may be substituted with a uniformly laid conductor layer, which can pierce through the magnetic field without deteriorating detection of the magnetic field variation caused by the stylus  15 .  
         [0016]    In this embodiment, the X-axis and the Y-axis antenna  11 ,  12  are composed of a plurality of          -shaped segmental antennas respectively. Taking the X-axis antenna for example, it comprises a plurality of component antenna sets X1˜X12, totally in 12 sets, as shown in FIG. 5, wherein each component antenna set Xn (n=1˜12) further comprises a plurality of          -shaped segmental antennas  31 , wherein each segmental antenna  31  belongs to a component antenna set Xn different from that of its immediate neighbors for discrimination of a          -shaped segmental antenna  31  where the stylus is located. For instance, two immediate neighboring          -shaped segmental antennas  31  of two          -shaped segmental antennas  31  (A, B) of the component antenna set X 5  belong to the component antenna set X 4 , X 6  and X 12 , X 2  respectively. If the stylus  15  is located at the          -shaped segmental antenna  31  (A), this position can be confirmed by checking reference signal created by its immediate neighboring component antenna set X 4 , X 6 .  
         [0017]    In the above described, at least one preferred embodiment has been described in detail with reference to the drawings annexed, and it is apparent that numerous variations or modifications may be made without departing from the true spirit and scope thereof, as set forth in the claims below.