Abstract:
A waveguide shield for containing electromagnetic interference (EMI) is disclosed. The waveguide shield includes an array of waveguide cells. Each waveguide cell has a contiguous inner surface coated with an absorber layer that absorbs EMI over a select frequency range. Each waveguide cell also has an aperture. The waveguide shield can be combined with a metallic chassis covering portions of a computer that generate the EMI and heat. The absorber layer allows the waveguide cells to have apertures of a size that can contain the EMI within the chassis while also allowing the heat trapped within the chassis to escape.

Description:
RELATED APPLICATIONS  
       [0001]    This patent application is related to U.S. patent application Ser. No. ______, entitled “Method and apparatus for reducing electromagnetic leakage through chassis apertures,” filed on Jun. 26, 2001, and commonly assigned to the Assignee of the present application. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to computers, and in particular relates to ventilation and electromagnetic interference (EMI) containment.  
         BACKGROUND OF THE INVENTION  
         [0003]    Modem computers include different types of circuits, including microprocessors and memory arrays, enclosed in a chassis. The microprocessors each include a central processing unit (CPU) that performs arithmetic and logic operations, and that controls the operation of the computer by decoding and executing sets of instructions.  
           [0004]    The speed of the computer is dictated by the speed of CPU as determined by its “clock.” A clock is an oscillator circuit that generates a series of evenly spaced electrical pulses. The typical frequency of the clock pulses for present-day CPUs ranges from the megahertz (MHz) to gigahertz (GHz). Even higher frequencies are anticipated as integrated circuit technology advances.  
           [0005]    The periodic emission of electrical signals by the clock results in the generation of electromagnetic radiation. If the metal chassis could be made without any apertures, the electromagnetic radiation generated by CPUs would be contained within the chassis. However, a significant amount of heat is generated by the flow of current through the numerous circuits, requiring ventilation apertures in the chassis. Unfortunately, typical ventilation apertures are large enough to allow electromagnetic radiation to escape the chassis. This radiation can detrimentally interact with electronic objects or humans residing near the computer, and is generally referred to as electromagnetic interference, or EMI. Accordingly, the Federal Communication Commission (FCC) places limits on the amount of EMI that can escape from a computer chassis.  
           [0006]    As CPU clock speeds increase, the amounts of heat and EMI generated by the computer also increases. Further, the EMI frequencies include not only to the fundamental clock speed frequency, but also include high-frequency harmonics (e.g., 5×to 10×) of the fundamental. Consequently, EMI leakage can occur from increasingly smaller apertures. This in turn requires that the ventilation apertures in the computer chassis be made increasingly smaller to contain the EMI. However, the smaller apertures reduce ventilation capability, which can lead to overheating of the internal components of the computer.  
           [0007]    To address this problem, vents in the form of metallic waveguide shields have been used to provide both ventilation and EMI protection. The waveguide shields are formed from an array of individual waveguide cells. The EMI leaving the chassis passes through the waveguide cells and interacts with the waveguide cell walls, which are made of metal and sometimes coated with a zinc-based paint for aesthetics. The EMI drives a surface current in the walls, which re-radiate at an attenuated level, thereby reducing the amount of outputted EMI. The waveguide apertures also allow heated air trapped in the chassis to escape.  
           [0008]    [0008]FIG. 1 is a plot of the absolute radiation level Emax in decibel-microvolts/meter (dB-μV/m) versus EMI frequency in gigahertz (GHz) for a conventional metal waveguide shield coated with zinc paint, based on computer simulation. The plot illustrates that the conventional metal waveguide shield does not provide adequate EMI shielding above 4.5 GHz. With the advent of CPUs that operate in the GHz range and beyond, conventional waveguide shields will not be able to provide adequate protection from EMI without significantly reducing the size of the waveguide apertures. Unfortunately, since the faster CPUs generate more heat than slower CPUs, decreasing the size of the waveguide apertures to contain the EMI is not a viable option.  
           [0009]    What is needed is a cost-effective EMI waveguide shield having apertures sized to provide adequate containment of high-frequency EMI within the computer chassis, but that also provide for adequate ventilation of the computer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a plot based on computer simulation of the absolute radiation level Emax in decibel-microvolts/meter (dB-μV/m) versus EMI frequency in gigahertz (GHz) for a conventional metal waveguide shield with square waveguide cells coated with zinc paint, based on computer simulation;  
         [0011]    [0011]FIG. 2 is a perspective view of a waveguide shield of the present invention having a rectangular body with rectangular waveguide cells;  
         [0012]    [0012]FIG. 3A is a face-on view of a waveguide shield of the present invention having a circular body and circular waveguide cells;  
         [0013]    [0013]FIG. 3B is a side view of the waveguide shield of FIG. 3A, showing three of the waveguide cells within the body;  
         [0014]    [0014]FIG. 4A is a face-on view of a waveguide shield of the present invention having a rectangular body and circular waveguide cells;  
         [0015]    [0015]FIG. 4B is a side view of the waveguide shield of FIG. 4A, showing three of the waveguide cells within the body;  
         [0016]    [0016]FIG. 5A is a face-on view of a waveguide shield of the present invention having a triangular body and triangular waveguide cells;  
         [0017]    [0017]FIG. 5B is a side view of the waveguide shield of FIG. 5A, showing three of the waveguide cells within the body;  
         [0018]    [0018]FIG. 6 is a close-up side view of a portion of a waveguide cell of the waveguide shield of the present invention, showing the EMI absorber layer formed on the waveguide cell inner surface;  
         [0019]    [0019]FIG. 7 is a plot illustrating the relative maximum electric field Emax (dB) for the same waveguide as for FIG. 1, but with an EMI absorber layer with a resistivity of 900 Ohms/square covering the inner surface of each waveguide cell (line with squares), and wherein the baseline of 0 dB (line with circles) is that for the zinc-painted waveguide of FIG. 1;  
         [0020]    [0020]FIG. 8A is a partial cut-away perspective view of a computer chassis housing the central processing units (CPUs) of a computer, showing the waveguide shield of FIG. 2 attached to the chassis; and  
         [0021]    [0021]FIG. 8B is a cross-sectional view of the apparatus of FIG. 8A, showing the waveguide shield blocking the EMI while allowing heat to escape from the chassis. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0023]    FIGS.  2 ,  3 A,B,  4 A,B and  5 A,B illustrate different embodiments of a waveguide shield  10 . Waveguide shield  10  generally has a body  20  that includes an array of waveguide cells  30 . Each waveguide cell  30  has a contiguous inner surface  32  and an associated aperture  40 . Body  20  and waveguide cells  30  can each have any one of a number of cross-sectional shapes, such as circular or polygonal. The waveguide cells can be assembled together to form the body. Alternatively, the body can be molded to form the waveguide cells. Further, the waveguide cells can be machined from or drilled out of the body. Thus, in one embodiment the waveguide body defines the waveguide cells, while in another embodiment the waveguide cells define the waveguide body.  
         [0024]    In one example embodiment, body  20  is metal, such as aluminum. In another example embodiment, body  20  is an insulator, such as molded plastic, sheet plastic, rigid polymer, composite material, ceramic, glass or wood. An insulating body is advantageous because it does not support the re-radiating surface currents that occur in a metal body. An insulating body is also advantageous because it can be more lightweight and inexpensive than a metal body.  
         [0025]    In the example embodiment of waveguide shield  10  illustrated in FIG. 2, body  20  is a rectangular cylinder of height H1, width W1 and depth D1. Waveguide cells  30  are also rectangular, with each cell having a height H2, a width W2 and a depth D2=D1. In another similar example embodiment, body  20  is square and waveguide cells  30  are square. In a specific example, body  20  has a width W1=37 cm, height H1=36 cm and depth D1=2.0 cm, while each waveguide cell has dimensions W2=H2=2.5 cm and D2=2.0 cm.  
         [0026]    In another example embodiment illustrated in FIGS. 3A and 3B, body  20  is a circular cylinder of depth D1 and radius R1, with circular cylinder waveguides  30  of radius R2 and depth D1. In another example embodiment illustrated in FIGS. 4A and 4B, waveguide shield  10  has a rectangular body with circular cylinder waveguide cells. In yet another example embodiment illustrated in FIGS. 5A and 5B, waveguide shield  10  has a triangular body with triangular cylinder waveguide cells. These are just a few of the possible geometries of waveguide shield  10 , and it will apparent to one skilled in the art that the waveguide shield of the present invention is not limited by the particular shapes of the waveguide cells and waveguide body.  
         [0027]    Regardless of the waveguide shield geometry, waveguide cells  30  are sized to ensure that apertures  40  provide both adequate blockage of EMI as well as adequate ventilation when the waveguide shield is attached to a computer chassis, as described below in connection with FIGS. 8A and 8B.  
         [0028]    [0028]FIG. 6 is a close-up side view of a portion of a typical waveguide cell  20 . An EMI absorber layer  60  of thickness T covers at least a portion of each inner surface  32  of each waveguide cell. In an example embodiment, absorber layer  60  covers the entire inner surface. Absorber layer  60  operates to absorb electromagnetic radiation in the select frequency range of EMI. In an example embodiment, the select frequency range includes MHz and GHz frequencies. Absorber layer  60  may be a single layer, or may include multiple layers of different EMI absorbing materials. In an example embodiment, EMI absorber layer is an epoxy resin filled with particles having a high magnetic loss tangent in the EMI frequency range. A suitable material for absorber layer  60  is called C-RAM and is available from Cuming Microwave Corporation, 225 Bodwell Street, Avon, Mass. 02332.  
         [0029]    Absorber layer  60  may be sprayed on inner surface  32  to form a thin layer and to ensure adhesion. Absorber layer  60  may also be brushed on. Alternatively, body  20  may be masked except for some or all of inner surfaces  32  and then dipped into a bath of absorber layer material to simultaneously coat some or all of the inner surfaces. Dipping may require diluting the absorber material so that the select thickness T is obtained. Multiple dippings may be used to build up layers to achieve the select thickness T. Absorber layer  60  may also be in the form of a sheet fixed to inner surface  32 .  
         [0030]    In an example embodiment, the absorber layer has a thickness T in the range from about 1 to about 10 mils, i.e., about 0.025 mm to about 0.25 mm. Generally, the higher the frequency of the EMI, the thinner EMI absorber layer  60  can be. The precise thickness T required to sufficiently absorb radiation over a given frequency range can be readily determined empirically or by simulation. In another example embodiment, absorber layer  60  has a resistivity in the range from about 200 Ohms/square to about 1200 Ohms/square.  
         [0031]    [0031]FIG. 7 plots the relative maximum electric field Emax in decibels (dB) versus the EMI frequency in GHz for the same waveguide shield considered in FIG. 1, except that the zinc coating was replaced with an absorber layer with a resistivity of 900 Ohms/square. The waveguide shield has a rectangular body dimensions H1=37 cm, W1=36 cm, D1=2.0 cm and square waveguide cell dimensions of H2=W2=2.5 cm and D2=D1=2.0 cm.  
         [0032]    It is seen in FIG. 7 that the waveguide shield with the absorber layer has significant EMI benefits over a relatively large frequency range (i.e., at least from 4.5 GHz to 10 GHz). This is advantageous because the waveguide cell apertures  40  do not need to be made smaller to maintain EMI shielding effectiveness as the EMI frequency increases. Accordingly, adequate ventilation of heat generated by the computer can be realized without comprising EMI containment.  
         [0033]    [0033]FIGS. 8A and 8B show a computer  100  with a motherboard  110  to which is fixed CPUs chips  116 , which emit heat  120  and EMI  122 . A chassis  130  defining an interior  132  covers the motherboard and includes a main aperture  140  for ventilation. Chassis  130  is preferably metal so that it acts as a natural shield to EMI. A waveguide shield  10  is then attached to the chassis at aperture  40  to provide for substantial containment of EMI. In an example embodiment, waveguide shield  10  is attached to chassis  130  by screws  150 . The waveguide shield can be fixed to the outside of the chassis (as shown), to the inside of the chassis, or directly within the main aperture.  
         [0034]    The combination of chassis  130  and waveguide shield  10  of the present invention serves to substantially contain EMI  122  over a wide range of select EMI frequencies. Further, use of waveguide shield  10  provides for effective ventilation of heat  120  trapped in interior  132  through main aperture  140  via the waveguide cell apertures  40 . This is because the waveguide cell apertures do not need to be reduced in size to shield the EMI as compared to the apertures of conventional waveguide shields. In addition, because body  20  of waveguide  10  need not be metal, waveguide shield  10  can be cost-effective and lightweight and insusceptable to surface currents that can re-radiate the EMI.  
         [0035]    While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.