Abstract:
A PCB is provided that is suitable for use in applications where EMI control is of interest. The PCB includes circuitry that communicates with an edge connector having edge traces located on its surface. Additionally, embedded traces are disposed within the dielectric material of the PCB, and each embedded trace electrically connects an edge trace with a corresponding median trace located on a surface of the PCB. An embedded ground layer substantially disposed within the dielectric material defines an area within the dielectric material through which the embedded traces pass. Finally, one or more vias are provided that extend through the dielectric material of the PCB and are filled with a conductive material. The vias are electrically connected to the embedded ground layer and configured to electrically communicate with an associated module. In this way, a structure is implemented that facilitates control of electromagnetic radiation emitted by the PCB circuitry.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a division, and claims the benefit, of U.S. patent application Ser. No. 10/425,090, entitled ELECTROMAGNETIC INTERFERENCE CONTAINMENT TRANSCEIVER MODULE, filed Apr. 28, 2003, which, in turn, claims the benefit of U.S. Provisional Patent Application No. 60/419,444, filed Oct. 17, 2002. Both of the aforementioned applications are incorporated herein in their respective entireties by this reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to optical modules. More particularly, exemplary embodiments of the invention concern an optical module that includes  
           [0004]    2. Related Technology  
           [0005]    Fiber optics are increasingly used for transmitting voice and data signals. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that would otherwise interfere with electrical signals. Light also provides a more secure signal because it doesn&#39;t allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.  
           [0006]    While optical communications provide a number of advantages, the use of light as a transmission medium presents a number of implementation challenges. In particular, the data carried by a light signal must be converted to an electrical format when received by a device, such as a network switch. Conversely, when data is transmitted to the optical network, it must be converted from an electronic signal to a light signal. A number of protocols define the conversion of electrical signals to optical signals and transmission of those optical, including the ANSI Fibre Channel (FC) protocol. The FC protocol is typically implemented using a transceiver module at both ends of a fiber optic cable. Each transceiver module typically contains a laser transmitter circuit capable of converting electrical signals to optical signals, and an optical receiver capable of converting received optical signals back into electrical signals.  
           [0007]    Typically, a transceiver module is electrically interfaced with a host device—such as a host computer, switching hub, network router, switch box, computer I/O and the like—via a compatible connection port. Moreover, in some applications it is desirable to miniaturize the physical size of the transceiver module to increase the port density, and therefore accommodate a higher number of network connections within a given physical space. In addition, in many applications, it is desirable for the module to be hot-pluggable, which permits the module to be inserted and removed from the host system without removing electrical power. To accomplish many of these objectives, international and industry standards have been adopted that define the physical size and shape of optical transceiver modules to insure compatibility between different manufacturers. For example, in 2000, a group of optical manufacturers developed a set of standards for optical transceiver modules called the Small Form-factor Pluggable (“SFP”) Transceiver Multi-Source Agreement (“MSA”), incorporated herein by reference. In addition to the details of the electrical interface, this standard defines the physical size and shape for the SFP transceiver modules, and the corresponding host port, so as to insure interoperability between different manufacturers&#39; products. There have been several subsequent standards, and proposals for new standards, including the XFP MSA for 10 Gigabit per second modules using a serial electrical interface, that also define the form factors and connection standards for pluggable optoelectronic modules, such as the published draft version 0.92 (XFP MSA), incorporated herein by reference.  
           [0008]    As optical transmission speed provided by electronic modules increases, additional problems arise. For example, electronic devices and components operating at high frequencies typically emit signals referred to as electromagnetic interference. This electromagnetic interference, referred to as “EMI”, is electrical noise in the form of an electromagnetic wave. The phenomenon is undesirable because EMI can interfere with the proper operation of other electrical components. Optical transceiver packages, especially those operating at high transmission speeds, are especially susceptible to emitting EMI. In particular, the physical configuration of existing transceiver modules does a poor job of containing EMI—especially as the generating speed of the module increases. For example, as is shown in FIGS. 7A through 8C, a transceiver module  8  typically includes a housing  5  that contains a printed circuit board  10  and associated electrical and optical components. However, the housing  5  does not completely enclose the printed circuit board  10 . Instead, a portion of the printed circuit board  10  is formed as an edge connector  12 . The edge connector  12  includes a number of high speed traces for communicating signals to and from the electrical contacts on the edge connector  12 . In operation, the edge connector  12  is capable of electrically and physically interfacing with a corresponding host connector  702  that is positioned on a host board  700 .  
           [0009]    Thus, in order for the edge connector  12  to be exposed externally to the module, the module housing  5  must provide an opening, shown at  20  in FIG. 6B. Moreover, insofar as the housing  5  is typically constructed of a conductive material, the opening  20  typically provides a minimum clearance area (the diagonal dimension of which is represented as “X” in FIG. 6B), so as to not electrically interfere with the high speed traces on the edge connector portion of the board  10 . Unfortunately, this opening  20  also allows for the emission of an unacceptable amount if EMI; the emission is especially problematic as transmission speeds increase.  
           [0010]    Therefore, there is a need in the industry for a pluggable module, such as an optoelectronic transceiver module, that is configured so as to minimize the emission of EMI. Preferably, the module configuration could be used in environments having high frequency data signal transmissions. Moreover, the module configuration should not affect the data signal integrity or the speed capabilities of the module. In addition, the electronic module should be implemented in a manner that meets existing standard form factors. Preferably, the module should maintain the ability to properly dissipate heat from the components inside the module.  
         BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION  
         [0011]    Briefly summarized, exemplary embodiments of the present invention are directed to a PCB suitable for use in connection with electronic modules, such as optical transceiver modules for example. An exemplary PCB includes a variety of electronic components disposed on its surface, as well as a connector portion formed at one end. The connector portion includes a plurality of conductive edge traces that interconnect with traces of a host system, such as a computer, signal router, or other input/output device, when an electronic module wherein the PCB is disposed is interfaced with the host system.  
           [0012]    The PCB also includes embedded traces disposed within the dielectric material of the PCB and electrically connecting an edge trace with a corresponding median trace located on a surface of the PCB. An embedded ground layer substantially disposed within the dielectric material defines an area within the dielectric material through which the embedded traces pass. Finally, one or more vias are provided that extend through the dielectric material of the PCB and are filled with a conductive material. The vias are electrically connected to the embedded ground layer and configured to electrically communicate with an associated electronic module.  
           [0013]    In this way, a structure is implemented that facilitates control of electromagnetic radiation emitted by the PCB circuitry. These, and other, aspects of the present invention will become more fully apparent from the following description and appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    In order that the manner in which the above recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be given by making reference to a specific embodiment that is illustrated in the appended drawings. These drawings depict only a few embodiments of the invention and are not to be considered limiting of its scope:  
         [0015]    [0015]FIG. 1A is an exploded perspective view of a transceiver module configured to contain EMI waves in accordance with one embodiment of the present invention;  
         [0016]    [0016]FIG. 1B is an exploded perspective view of the rear end of a transceiver module configured to contain EMI waves in accordance with one embodiment of the present invention;  
         [0017]    [0017]FIG. 1C is an exploded perspective view of the bottom side of a transceiver module configured to contain EMI waves in accordance with one embodiment of the present invention;  
         [0018]    [0018]FIG. 1D is an exploded perspective view of the bottom side of the rear end of a transceiver module configured to contain EMI waves in accordance with one embodiment of the present invention;  
         [0019]    [0019]FIG. 2A is another perspective view of a transceiver module configured to contain EMI waves in accordance with one embodiment of the present invention;  
         [0020]    [0020]FIG. 2B is a rear view of the transceiver module of FIG. 2A;  
         [0021]    [0021]FIG. 2C is a side view of the transceiver module of FIG. 2A;  
         [0022]    [0022]FIG. 2D illustrates a cutaway rear view taken along lines  2 D- 2 D of FIG. 2C;  
         [0023]    [0023]FIG. 2E illustrates a close up cutaway rear view taken along lines  2 E in FIG. 2D;  
         [0024]    [0024]FIG. 3A illustrates an exploded view of another embodiment of a transceiver module;  
         [0025]    [0025]FIG. 3B is an exploded view showing additional details of the rear end of the transceiver module of FIG. 3A;  
         [0026]    [0026]FIG. 3C is an exploded perspective view of the bottom of the transceiver module of FIG. 3A;  
         [0027]    [0027]FIG. 3D is an exploded view of the rear end of the transceiver module of FIG. 3A;  
         [0028]    [0028]FIG. 4A is another perspective view of the transceiver module of FIG. 3A;  
         [0029]    [0029]FIG. 4B is a rear view of the transceiver module of FIG. 4A;  
         [0030]    [0030]FIG. 4C is a side view of the transceiver module of FIG. 4A;  
         [0031]    [0031]FIG. 4D is a cutaway rear view of the transceiver module taken along lines  4 D- 4 D in FIG. 4C;  
         [0032]    [0032]FIG. 4E is a close up cutaway view of the transceiver module taken along lines  4 E in FIG. 4D;  
         [0033]    [0033]FIG. 5A is an end view of a transceiver module configured in accordance with another embodiment of the present invention;  
         [0034]    [0034]FIG. 5B is a close-up cut away view of the transceiver module taken along lines  5 B in FIG. 5A;  
         [0035]    [0035]FIG. 6A illustrates a perspective view of a printed circuit board portion in accordance with yet another alternative embodiment of the present invention;  
         [0036]    [0036]FIG. 6B illustrates a cutaway rear view of a printed circuit board taken along lines  6 B in FIG. 6A; and  
         [0037]    [0037]FIGS. 7A-8C show various exemplary views of prior art modules.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0038]    Reference will now be made to the drawings to describe exemplary embodiments of the invention. It is to be understood that the drawings are diagrammatic and schematic representations of such exemplary embodiments, and are not limiting of the present invention, nor are they necessarily drawn to scale.  
         [0039]    In general, the present invention relates to an electronic pluggable module that is structured in a manner that minimizes the emission of potentially harmful EMI waves. In preferred embodiments, the module maintains a low profile, and conforms with the physical dimensions set forth by existing industry standards. In addition, EMI shielding is provided in a manner that does not interfere with the electronic performance of the module. Likewise, the module is constructed so as to dissipate heat efficiently and thereby avoid overheating of the electrical or optical components. Although the preferred embodiments are described in the context of an optoelectronic transceiver module, it will be appreciated that teachings of the present invention can be used in the context of other environments, including other electrical pluggable modules.  
         [0040]    Reference is initially made to FIGS. 1A-1D and FIGS. 2A-2E, which together illustrate one presently preferred embodiment of a transceiver module, designated generally at  100 . As shown in FIGS. 1A and 1C, the transceiver module  100  includes a top housing portion  105 , a printed circuit board (“PCB”)  110 , and a bottom housing portion  115 . The top and bottom housing portions  105 ,  115  are designed to fit together and form an interior portion containing a PCB  110  and associated electronic and optical components. A set of screws  116  (or any other appropriate fastening mechanism) are used to fasten the two housing portions  105 ,  115  together to form the outer shell, or outer housing  103 , of the transceiver module  100 . When joined together, the top and bottom housing portions  105 ,  115  also form a front opening  117  and a rear opening  118 . The front opening  117  is designed to accept a modular plug (not shown) that is connected to two optical waveguides, one input waveguide and one output waveguide (not shown), the structure and implementation of which are well known in the art of optical communications. The rear opening  118  is designed to expose an electrical edge connector, denoted at  112 , formed along one end of the PCB  110 . The edge connector  112  is capable of being electrically and physically received within a corresponding connector (such as is shown in FIG. 8A at  702 ) that is typically mounted on a host board ( 700  in FIG. 8A) of an appropriate host device (not shown). The housing portions  105 ,  115  can include multiple holes or gaps that allow heat to escape from inside the transceiver module  100  during operation.  
         [0041]    In a preferred embodiment, the top and bottom housing portions  105 ,  115  are at least partially composed of a conductive material so that when the housing portions are joined together, a shell of conductive material is formed about the periphery of the transceiver module  100 . The conductive material on the housing portions  105 ,  115  form what is known as the chassis ground. A ground is an electrical pathway or drop through which voltage can pass. As discussed below, a ground can also have electromagnetic effects. The chassis ground is electrically isolated from all circuitry on the PCB  110 .  
         [0042]    As noted, the PCB  110  is substantially positioned within the interior portion formed between the top and bottom housing portions  105 ,  115 . The top and bottom housing portions  105 ,  115  contain various support structures to securely support the PCB  110  when the two housing portions are joined together. FIGS. 2A-2C illustrate a completed transceiver module  100  in which the PCB  110  is securely positioned between the top and bottom housing portions  105 ,  115 . The PCB  110  includes a top surface  110 A oriented toward the top housing portion  105 , and a bottom surface  110 B oriented toward the bottom housing portion  115 .  
         [0043]    The PCB  100  further includes a plurality of high speed traces  140  that electrically transfer data from one location to another. Specifically, the PCB  100  includes edge traces  140 A that are located on both the top and bottom surfaces  110 A and  110 B of the edge connector  112 , and connecting traces  140 B located on the surface of the top PCB surface. Because data are being transmitted at a very high frequency in an electrical form, potentially harmful EMI waves are generated within the transceiver module  100 . The type of EMI waves that are allowed to leak out of the transceiver module  100  is dependent on the size and position of any opening present in the housing. Moreover, if the housing portions  105 ,  115  were not grounded, EMI would not be efficiently contained.  
         [0044]    The largest opening present in the housing of a typical prior art transceiver module is located at the rear end of the housing, designated at  119 , where the edge connector of the PCB  110  is exposed so as to electrically connect with a corresponding connector. As noted, the rear opening  118  located at the rear housing end  119  must provide a minimum clearance for the PCB  110  so as to not interfere with the electrical signals present on the high speed traces  140 A and  140 B communicating with the edge connector  112 . In known devices, the largest opening distance is generally measured diagonally across an opening (denoted as the dimension “X” in FIG. 7C) because this is the longest one dimensional length of space available. The length of the largest opening is mathematically related to the frequencies of EMI waves that are allowed to leak out of the transceiver module. As data are transferred faster, the operating frequencies of the electrical components increase; thus the frequency of emitted EMI increases. Because frequency is inversely proportional to wavelength, the higher the frequency, the shorter the relative wavelength of the EMI waves that are generated by the high speed data. Therefore, in order to reduce the leakage of EMI waves generated from higher frequency data transmissions, the largest opening distance must be decreased. Unfortunately, the opening at the rear of the transceiver module cannot be entirely eliminated because of the high speed traces that are located on the surface of the PCB. As mentioned, there must be some space between the housing portions, which are grounded at chassis ground, and the conductive traces that pass through rear opening of the transceiver module in order to avoid signal degradation of the signals passing through the traces.  
         [0045]    Continuing reference is made to FIGS. 1A-2E. Embodiments of the present invention provide a means for reducing the size of the opening, thereby minimizing the amount of EMI that can escape therethrough. For example, in the illustrated embodiment, the PCB  110  includes at least two holes that extend through the entire thickness of the board designated at  110 . The holes are lined with a conductive material to form electrically conductive vias  120 . FIG. 2E illustrates how the holes are lined to create the vias  120 . The vias  120  are formed in the board  110  in a manner as to be electrically isolated from the remainder of the circuitry on the PCB  110 . As will be seen, the conductive vias  120  enable a “chassis ground fence” to be created adjacent the rear housing end  119  in order to reduce the emission of EMI from the interior of the transceiver module  100 .  
         [0046]    In the illustrated embodiment, the top housing portion  105  includes a raised structure  104  located at the rear housing end  119  adjacent the rear opening  118 . As best seen in FIG. 1D, the raised structure  104  includes two posts  125  oriented toward the bottom housing portion  115 . The posts  125  are positioned to be received within the holes defining the conductive vias  120  when the top and bottom housing portions  105 ,  115  are joined together. Thus, the posts  125  serve to align the PCB  110  with respect to the top housing  105 . With the posts  125  aligned with and partially received into the conductive vias  120 , two plates  122  formed on the raised structure  104  of the top housing portion  105  are brought into contact with conductive portions on the outer periphery of the conductive vias  120 . The plates  122  are formed of an electrically conductive material that enables them to electrically connect with the conductive vias  120  and to contribute in establishing chassis ground when the top and bottom housings  105 ,  115  are joined together, as will be seen. This arrangement is best seen in FIG. 2E, which shows the plates  122  in electrical contact with the conductive plating of vias  120 , thereby establishing electrical contact between vias  120  and top housing  105 .  
         [0047]    As best seen in FIGS. 1B and 2E (in cross section), the bottom housing  115  includes two plates  130  that are positioned on a ridge  132  of the bottom housing as to be aligned with the conductive vias. In this alignment, the plates  130  are positioned such that they physically contact the exterior periphery of the conductive vias  120  when the top and bottom housing portions  105  and  115  are joined. The plates  130  are also composed of a conductive material and are electrically connected to the bottom housing  115 . This enables the plates  130  to participate in conducting chassis ground between the top and bottom housing portions  105  and  115  when the housing portions are joined, as shown in FIG. 2A. The plates  122  of the top housing portion  105  and the plates  130  of the bottom housing portion  115  are positioned such they only electrically connect with the vias  120  and are electrically isolated from all other circuitry on the PCB  110 .  
         [0048]    In greater detail, because the plates  122  of the top housing  105  are electrically connected to the conductive vias  120  defined in the PCB  110 , and the vias are electrically connected to the plates  130 , the top housing  105  is in indirectly and electrically connected to the bottom housing  115  by way of a conductive pathway that extends through the conductive vias and each set of plates when the housing portions are clamped into contact with one another. Thus, a chassis ground present at one or both of the housing portions  105 ,  115  is extended through the conductive pathway, as just described. The extension of chassis ground through this conductive pathway creates what is referred to herein as a “chassis ground fence.” As explained below, this chassis ground fence reduces the escape of EMI from the transceiver module, thereby improving the performance of the transceiver.  
         [0049]    [0049]FIG. 2E illustrates a close-up cross sectional view of a portion of the chassis ground fence. Accordingly, this figure shows one post  125 , one plate  122 , one complete conductive via  120 , and one plate  130  as arranged when the top and bottom housing portions  105  and  115  are joined to form the transceiver module housing. The physical connection between these components is evident in the figure, thereby giving rise to the electrical connection between the top housing portion  105  and the bottom housing portion  115 .  
         [0050]    As can be seen from FIGS. 2A-2E, the rear opening  118  of the rear housing end  119  is divided and reduced in dimension as a result of the presence of the chassis ground fence defined by the posts  125 , plates  122 , conductive vias  120 , and plates  130 . Specifically, these structures introduce portions of the chassis ground through formerly open area defined by the rear opening  118  (see, in comparison, dimension “X” in FIG. 6B) to define two reduced dimension openings  136  and  138 . Because of the chassis ground fence and the corresponding reduction in the overall dimension of the rear opening  118 , EMI is unable to effectively penetrate the rear housing end  119  through the reduced dimension openings  136 . This in turn reduces EMI emission from the transceiver module  100  and prevents interference with operation of either the transceiver or other nearby components. The reduced dimension openings  136  and  138  remain sufficiently sized to allow the connecting traces  140 B to pass through without affecting the quality of the signals they carry.  
         [0051]    It will be appreciated that the specific configuration of the chassis ground fence defined by the above components can be varied while still providing the desired EMI protection. For instance, the number of plate-via-post-plate combinations can be varied to increase or decrease both the number and size of the reduced-dimension openings at the rear housing end  119 . Additionally, the presence, particular shape, and configuration of the raised structure  104  of the top housing  105 , as well the ridge  132  supporting the plates  103  of the bottom housing  115  can also be modified as desired to achieve optimum function. In one embodiment, for instance, the height of the raised structure  104  can be altered in order to vary the clearance provided between the top housing  105  and the surface of the PCB  110  on which the connecting traces  140 B are located. These and other changes to the chassis ground fence are complicated.  
         [0052]    As seen in FIG. 2E, though they align with and may be in electrical contact with the conductive vias  120 , the posts  125  in the illustrated embodiment do not completely extend through the vias to directly contact any portion of bottom housing  115 . Indeed, direct contact between the posts  125  and the bottom housing  115  is not necessary to extend chassis ground between the top and bottom housing portions  105  and  115 . Rather, chassis ground in the illustrated embodiment extends along multiple paths comprising one each of the plates  122 , conductive vias  120  and plates  130 . If desired, however, each post  125  can be configured in one embodiment to electrically connect with the respective conductive via  120  into which it is received, thereby contributing to the provision of chassis ground between the top and bottom housings  105 ,  115 .  
         [0053]    In view of the above discussion, it is seen that the combination of the plates  122 , the conductive vias  120 , the posts  125 , and the plates  130  serve as one means for electrically connecting the top and bottom housing portions  105  and  115  while reducing the area of EMI emission at an end of the transceiver module  100  proximate the edge connector  112 . However, it is appreciated that other means can be also employed to achieve this same functionality. For example, alternative structures, such as conductive foams and springs, could be utilized in establishing an electrical chassis ground connection between the top housing  105  and the bottom housing  115 . These alternative embodiments, in addition to other embodiments to be explicitly described below, are therefore contemplated within the claims of the present invention.  
         [0054]    Reference is next made to FIGS. 3A-3D and FIGS. 4A-4E, which illustrate a transceiver module, designated generally at  200 , configured in accordance with another embodiment of the present invention. The transceiver module  200  includes many of the same components as the embodiment described in detail with reference to FIGS. 1A-1D and FIGS. 2A-2E, and to the extent that common features are shared between them, some of these features will not be discussed. As shown in the figures, the module  200  includes a top housing portion  205 , a bottom housing portion  215 , and a PCB  210 . As described above, the top and bottom housing portions  205 ,  215  fit together to form an outer housing  203  that at least partially encloses the PCB  210 . In addition, this outer housing  203  carries an electrical chassis ground around the PCB  210  in the same manner as described above. Extending from a rear housing end  219  of the module  200  is an edge connector  212  portion of the PCB  210 . The edge connector  212  includes a plurality of conductive edge traces  240 A in communication with connecting traces  240 B for transferring electrical data signals between the module  200  and a host device (not shown) that interfaces with the edge connector.  
         [0055]    In the illustrated embodiment, the PCB  210  further includes two holes  220  formed through the PCB. Unlike the embodiment described above, the holes  220  are not lined with any conductive material but are simply formed through the dielectric material comprising the PCB  210 . The holes  220  are located substantially adjacent the edge connector  212  portion of the PCB  210 .  
         [0056]    The top housing portion  205  includes two bosses  225 , best seen in FIGS. 3C and 3D, that are positioned on a raised structure  204  of the top housing  205  so as to be received within the corresponding holes  220  when the top and bottom housing portions  205  and  215  are joined together. Specifically, FIGS. 3D and 4E illustrate how the sockets  225  are received within the holes  220 . Each socket  225  is composed of an electrically conductive material and is electrically connected to the top housing  205 . In this way, the sockets  225  are connected to chassis ground when the top housing portion  205  is chassis grounded.  
         [0057]    The bottom housing portion  215  includes two pins  230  positioned on a ridge  232  that are to be partially received within the holes  220  defined in the PCB  210 . In particular, the pins  230  are configured as to be received within corresponding sockets formed in the ends of the bosses  225  when the top and bottom housing portions  205 ,  215  are joined together. This is best shown in FIG. 4E. Like the bosses  225 , the pins  230  are also composed of an electrically conductive material and are electrically connected to the bottom housing portion  215  so that the pins are connected to chassis ground when the bottom housing portion  215  is chassis grounded.  
         [0058]    The alignment described above enables the holes  220 , bosses  225 , and pins  230  to form electrically conductive paths between the top and bottom housings  205  and  215  when the housing portions are joined together. In particular, when the housing portions  205  and  215  are joined, each pin  230  partially passes through the corresponding hole  220  and is received into the socket formed in the corresponding boss  225  (the boss also being partially received by the hole) such that the pin and boss are electrically connected. Because it is not plated with a conductive material, the hole  220  does not contribute to the electrical connection between the pin  230  and the boss  225  in contrast to the previous embodiment, but rather merely provides space for them to connect. As such, the PCB  210  and any circuitry located thereon are electrically isolated from any of the bosses  225  or pins  230 .  
         [0059]    As was the case with the previous embodiment, when the top and bottom housing portions  205 ,  215  are joined together, the electrically conductive paths established therebetween via the boss-hole-pin configuration form multiple chassis ground paths between the housing portions when one or both housing portions are chassis grounded. As before, the chassis ground passes through the PCB  210  between the top and bottom housings  205  and  215  via the boss-hole-pin configuration to form a chassis ground fence. Again, as with the previous embodiment, the chassis ground fence reduces the overall size of the opening (i.e., rear opening  218 ) at the rear housing end  219  by sub-dividing it into smaller-dimensioned openings, designated at  236  and  238 . The openings  236  and  238  are sufficiently sized as to enable the connecting traces  240 B to pass therethrough without impairing the signals they carry. FIG. 4E illustrates the maximum dimension of one of the openings, opening  238 , as comprising a distance  235 . As can be seen in comparison with the opening  20  of the prior art transceiver shown in FIG. 6B, the dimension  235  of the opening  238  is substantially smaller than dimension X of the opening  20 . As before, this relatively reduced dimension of the openings  236  and  238  at the rear housing end  219  results in reduced EMI emissions escaping from the rear housing end of the transceiver module  200 . Again, absent the two sets of holes  220 , sockets  225 , and pins  230 , the opening distance would span an area much greater than that defined by the two reduced dimension openings  236 ,  238 , thereby undesirably increasing the area of escape for EMI. As before, it should be remembered that the particular hole-boss-pin configuration shown in the present embodiment is merely exemplary of the structure that can be utilized in providing a chassis ground fence for containing EMI in a transceiver module. Other configurations that preserve this function are also contemplated. Additionally, it is appreciated that the chassis ground fence concept can be extended to areas of the transceiver module other than the rear housing end, if desired.  
         [0060]    Reference is now made to FIGS. 5A-5B, which illustrate yet another embodiment of the present invention. Again, to the extent that common features are shared between this and previous embodiments, some of these features will not be discussed. FIGS. 5A and 5B include views of a portion of an optical transceiver module  300  having top and bottom housing portions  305  and  315 , respectively. The top and bottom housing portions  305  and  315  together form the transceiver housing, which contains a PCB  310 . Similar to the previous embodiment, the PCB  310  includes two holes  320  defined therethrough near an end thereof, the end being located substantially adjacent a rear housing end  319  of the transceiver module  300 . The holes  320  are not lined with a conductive material and are aligned to each receive a post  325  extending from and electrically connected to a portion of the top housing portion  305 . Each post  325  extends through the respective hole  320  and contacts a portion of the bottom housing portion  315 , in this case, one of two plates  330 , which are electrically connected with the bottom housing portion.  
         [0061]    The engagement of each post  325  with the respective plate  330  creates a conductive path that electrically connects the top housing portion  305  to the bottom housing portion  315 . Further, this connection enables chassis ground to be extended through the conductive path, thereby forming a chassis ground fence at the rear housing end  319  of the transceiver module  300 , as in previous embodiments. In contrast to previous embodiments, however, each post  325  completely extends through the respective hole  320  of the PCB  310  to contact the respective plate  330  of the bottom housing portion  315 . In other embodiments, the post  325  can extend through the hole  325  and directly contact a flat surface of the bottom housing portion  315 , thereby obviating the need for the plate  330 . Or, in yet another embodiment, the post-hole-plate configuration can be modified by plating each hole  320  with a conductive plating similar to that found in the embodiment illustrated in FIGS. 1A-2E, further enhancing the electrical contact between the housing portions. The present embodiment, in addition to the previous and following embodiments, therefore serves as another example of a means for electrically connecting the top and bottom housing portions with chassis ground as to reduce the area of emission of electromagnetic interference from an end of the transceiver module.  
         [0062]    Reference is next made to FIGS. 6A-6B, which illustrate a printed circuit board, designated generally at  500 , configured in accordance with yet another embodiment of the present invention. The printed circuit board  500  in this embodiment is designed to fit within the housing of a transceiver module. However, the EMI reduction modifications are implemented on the printed circuit board  500  alone. Therefore, the printed circuit board  500  described in this embodiment can be utilized with existing unmodified housings of the same form factor to form a complete transceiver module. The printed circuit board  500  includes high speed edge traces  520  positioned on the edge connector portion  512  of the printed circuit board  500 . The high speed edge traces  520  are electrically connected to embedded traces  525  that tunnel through the dielectric material within the printed circuit board and are then electrically connected to median traces  530 . The median traces  530  are positioned on the surface of the printed circuit board  500  like the high speed edge traces  520  as illustrated in FIG. 6A. The printed circuit board  500  further includes two electrical vias  535 . The electrical vias  535  are narrow holes that extend through the entire printed circuit board  500  that are filled with an electrically conductive material. The vias  535  are electrically isolated from all other circuitry on or within the printed circuit board  500 . These vias  535  provide a connection point to the module housing such that a chassis ground can extend through the printed circuit board. Only one side of the vias  535  need be electrically connected to the housing in order to encircle the embedded traces  525  with the chassis ground because of an embedded ground layer  540 , described below.  
         [0063]    The cut-away view illustrated in FIG. 6B shows how the printed circuit board further includes an embedded ground layer  540  that encircles the embedded traces  525 . The embedded ground layer  540  is electrically connected to the vias  535  in order to carry the chassis ground from the housing. The embedded ground layer  540  is a conductive material that is embedded within the printed circuit board in the manner shown in FIG. 6B. By incorporating this embedded ground layer  540  that carries the chassis ground, particular EMI waves generated within the housing are effectively prevented from leaking out. The printed circuit board  500  in this embodiment is designed to fit within a housing that completely eliminates a rear opening by actually touching the printed circuit board  500  at a lateral location between the high speed edge traces  520  and the vias  535 . Since all electrical data within this region is transferred through the embedded traces  525 , it is possible for the housing to physically contact the printed circuit board (completely eliminating the open space that commonly allows EMI waves to leak out) without electrically interfering with the transference of data. As described above, the vias  535  can be equipped with conductive devices known in the art to ensure that an electrical connection between the vias  535  and the housing (not shown) is established, such as a conductive foam or spring. In addition, the embedded ground of this embodiment can be combined with the interconnection schemes of the previous embodiments to provide a different electrical connection between the printed circuit board  500  and the housing (not shown).  
         [0064]    In one embodiment, in order to further facilitate reliable electrical connection between the top housing and the vias  535  and between the bottom housing and the vias  535 , a conductive band  550  can be formed around the surface of the PCB  500 , as illustrated in FIGS. 6A and 6B. The thickness of the conductive band  550  is exaggerated in FIG. 6B for purposes of visibility in the illustration. The conductive band  550  both establishes a conductive ground path around the PCB and also enhances the electrical connection with the top and bottom housings. This conductive band or strip  550  is particularly useful in establishing a reliable electrical connection with springs or foam EMI gaskets that can be used with the top and bottom housing.  
         [0065]    In summary, the present invention relates to a module design that reduces the leakage of particular EMI waves by passing a ground through or piercing a PCB that is within a grounded housing. The piercing ground does not affect any of the circuitry on the PCB but has the effect of minimizing the size of any openings through which EMI can escape. The teachings of the present invention are applicable to any electrical module that potentially generates high frequency data that causes potentially harmful EMI waves.  
         [0066]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.