FIELD OF INVENTION 
       [0001]    This disclosure relates generally to electromagnetic interference from PCB-chassis structures, and more specifically, regards controlling the level of emissions of electromagnetic interference emitted and received by a PCB-chassis structure. 
       BACKGROUND 
       [0002]    Electromagnetic Interference (EMI) is a disturbance that interrupts, obstructs, degrades, or limits the effective performance of electronics and electrical equipment. It can occur unintentionally as a result of spurious emissions and responses. Electromagnetic compatibility (EMC) tries to ensure that equipment items or systems will not interfere with or prevent the correct operation of other equipment items or systems through emission or absorption of EMI. The damaging effects of EMI pose unacceptable risks in many areas of technology and it is necessary to control EMI and reduce the risks to acceptable levels. Printed circuit boards (PCBs) are often mounted to chassis and may emit and receive EMI. 
       SUMMARY 
       [0003]    Disclosed herein are embodiments of a device for attenuating the propagation and reception of electromagnetic interference for a system, the system comprising a printed circuit board mounted a chassis by at least one fastener, the fastener establishing an electrical connection between the printed circuit board and the chassis. In an embodiment, a device may include a first substantially annular conductive material having a first central opening, the first central opening sufficient to substantially surround the fastener and maintain the electrical connection between the printed circuit board and the chassis. In addition, the device may include a second substantially annular conductive material concentric with the first conductive material, the second conductive material having a second central opening sufficient to substantially surround the fastener and maintain the electrical connection between the printed circuit board and the chassis. Furthermore, the device may include a substantially annular impedance material between and adjacent to the first conductive material and the second conductive material, the impedance material sufficient to attenuate the electromagnetic interference from the system. 
         [0004]    Also disclosed herein are embodiments of a device for attenuating the propagation and reception of electromagnetic interference for a system, the system comprising a printed circuit board mounted a chassis by at least one fastener, the fastener establishing an electrical connection between the printed circuit board and the chassis. In an embodiment, a device may include a first substantially annular conductive material having a first central opening, the first central opening sufficient to substantially surround the fastener and maintain the electrical connection between the printed circuit board and the chassis. Furthermore, the device may include a substantially annular impedance material adjacent to the first conductive material, the impedance material sufficient to attenuate the electromagnetic interference from the system. 
         [0005]    Also disclosed herein are embodiments of a device. In an embodiment, a device may include a first annulus shaped conductive material. In addition, the device may include a second annulus shaped conductive material concentric with the first annulus shaped conductive material. Furthermore, the device may include an impedance material in the shape of a cylinder with a bore between and adjacent to the first annulus shaped conductive material and the second annulus shaped conductive material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  depicts a resistive device  100  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. 
           [0007]      FIG. 2  depicts a capacitive device  200  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. 
           [0008]      FIG. 3  depicts an impedance device  300  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. 
           [0009]      FIG. 4  depicts an inductor device  400  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. 
           [0010]      FIG. 5  depicts a cross-sectional view of the signal path between the PCB and chassis, consistent with embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    EMI can be divided into two categories according to the source and the signal characteristics. First, there is continuous interference, where the source continuously emits at a given range of frequencies. Continuous interference can be further divided into subcategories based on the range of frequencies, the lowest frequencies being audio frequency, then radio frequency interference (RFI), and finally broadband noise. The second category of EMI is pulse or transient interference, where the source emits a short-duration pulse of energy. 
         [0012]    When EMI is emitted, it propagates by the process of coupling. Coupling is the transfer of energy from one medium to another. There are four basic coupling mechanisms: conductive, capacitive, inductive, and radiative. Conductive coupling occurs when the coupling path between the source and the receptor is formed by direct contact with a conducting body, like a transmission line, wire, cable, PCB trace, or metal enclosure. There are two types of conductive coupling, common-mode, where noise appears in phase on two conductors, and differential-mode, where noise appears out of phase on two conductors. Capacitive coupling occurs when a varying electrical field exists between two adjacent conductors, inducing a change in voltage across the gap. Inductive coupling occurs when a varying magnetic field exists between two parallel conductors, inducing a change in voltage along the receiving conductor. Radiative coupling occurs when source and receptor act as radio antennas, that is, the source emits an electromagnetic wave which propagates across open space and is received by the receptor. A coupling path can be broken down into one or more of these coupling mechanisms. 
         [0013]    A printed circuit board (PCB) is used in many electronic products. PCBs are assembled with ground planes or board grounds. The board ground may be at 0 V potential, however, currents flow through the finite impedance of the board ground and produce voltage potential differences across the board. The currents that exist in the PCB and board ground produce radio frequency (RF) fields that are emitted from the circuit components present on the PCB. These fields produce voltage differences between PCB and adjacent metallic members of a chassis, a flat metal surface that supports the PCB and the voltage differences create EMI. When there is impedance between the PCB and chassis, the EMI will couple to other equipment items or systems such as the motherboard and daughter boards. 
         [0014]    One benefit of assembling a board ground to a PCB is the low-impedance of a board ground. When the board ground is fastened to the chassis with low-impedance connections, the voltage differences between the PCB and chassis are shorted. The EMI that could have coupled to other systems is then diminished. Furthermore, by fastening the board ground to the chassis using low-impedance connections, the board ground is connected to the 0 V potential of the chassis. Therefore, the ground plane should be connected to the chassis unless it is prevented by conflicting circumstances. 
         [0015]    A PCB may be fastened directly to a chassis. When a PCB is directly fastened to a chassis, there is a low-impedance connection between the PCB and chassis. To make a direct fasten, mounting pads are placed on the bottom of the PCB and overlap and press against the walls of a metal standoff. The metal standoff, in turn, is wrapped around, in most cases, a screw because of its ability to securely affix one object to another. The metal standoff is also pressed against the chassis and a low-impedance connection is obtained. However, the PCB-chassis structure itself has a frequency at which it resonates. Any circuit currents at this frequency and at higher order multiples of this frequency couple to the structure, causing resonance, and EMI emissions are radiated from the structure. One way to deal with this is to increase the amount of fasteners mounting the PCB to the chassis. This would modify the PCB-chassis structure and increase the frequency at which the PCB-chassis structure resonates. However, this merely causes a shift in the resonant frequency so, although coupling no longer occurs at one frequency, it occurs at another. 
         [0016]    Instead of attempting to create a direct, low-impedance connection, by merely adding mounting fasteners, the connections could be replaced by resistive loading. This should dampen the circuit current frequencies and deter the PCB-chassis structure from resonating. A PCB mounted to a chassis in parallel, with an air dielectric, where the PCB has a length W and the distance between the PCB and chassis is h, should exhibit a characteristic impedance Z 0  given by: 
         [0000]    
       
         
           
             
               Z 
               0 
             
             = 
             
               377 
               · 
               
                 h 
                 W 
               
               · 
               
                 1 
                 
                   ( 
                   
                     1 
                     + 
                     
                       
                         ( 
                         
                           
                             h 
                             / 
                             π 
                           
                           · 
                           W 
                         
                         ) 
                       
                       · 
                       
                         ( 
                         
                           1 
                           + 
                           
                             ln 
                              
                             
                               ( 
                               
                                 2 
                                  
                                 π 
                                  
                                 
                                     
                                 
                                  
                                 
                                   W 
                                   / 
                                   h 
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
         [0017]    If the damping value equals the characteristic impedance Z 0 , the PCB-chassis structure should not resonate. This damping configuration should decrease the EMI generated by the resonation of the PCB-chassis structure, however, it may raise the EMI emissions that are brought about because the PCB no longer has a low-impedance connection with the chassis because the PCB is no longer directly fastened to the chassis. 
         [0018]    However, it is also difficult to directly fasten the PCB to the chassis because screws contain a helical thread and the edge of the screw thread touches the standoff. The standoff must be larger in diameter than the screw diameter in order to allow the screw to be properly inserted. As a result, solid and continuous bonding contact between the threads of the screw and the standoff cannot be guaranteed. Furthermore, when different metals are joined, corrosion may occur between the screw and the standoff, disallowing a low-impedance connection. Therefore, screws cannot be relied upon for a direct, low-impedance connection, and hybrid techniques for fastening a PCB to a chassis may be implemented. 
         [0019]    Anything besides an open or short is considered a hybrid technique of fastening, such as the resistive loading previously discussed. Another hybrid technique for fastening a PCB to a chassis is through a capacitive connection. There are circumstances where a direct conductive connection between the PCB board ground and the chassis may be undesirable, because of functional, safety, or other requirements. For example, there may be instances where high-level lower frequency interference may be present and multipoint conductive fastening allows this to occur. Therefore, by replacing all fastenings of the PCB-chassis structure except one with a capacitive connection of properly chosen value, a lower frequency grounding and higher frequency grounding connection is obtained, decreasing EMI emissions. 
         [0020]    A drawback of the use of hybrid fastening techniques is the series inductance that occurs intrinsically from the device, such as the resistor or capacitor, and the devices connection to the PCB-chassis structure. Excessive series inductance can degrade the effectiveness of the PCB-chassis structure connection, therefore, increasing the EMI emissions. One way to limit the total inductance is to place an inductor in parallel with the series inductance. It is understood that the total inductance of inductors in parallel is equal to the reciprocal of the sum of the reciprocals of their individual inductances: 
         [0000]    
       
         
           
             
               1 
               
                 L 
                 total 
               
             
             = 
             
               
                 1 
                 
                   L 
                   1 
                 
               
               + 
               
                 1 
                 
                   L 
                   2 
                 
               
               + 
               … 
               + 
               
                 
                   1 
                   
                     L 
                     n 
                   
                 
                 . 
               
             
           
         
       
     
         [0021]    Therefore, by placing an inductor in parallel with the series inductance, the total inductance is decreased. By decreasing the total inductance, the degrading of the PCB-chassis structure connection is decreased, and EMI emissions are decreased. 
         [0022]    Adding inductance, resistance, and capacitance between the board ground of the PCB and the chassis is difficult after the PCB has been assembled and unless area has been dedicated on the PCB for these passive components, locations would have to be designated and popped to implement the requirement. A simpler way is to produce a “washer” shaped component that can either slip into place under the head of the mounting screw or under the PCB and provides the connection between the board ground of the PCB and the chassis. 
         [0023]    In this detailed description, reference is made to the accompanying drawings, which illustrate example embodiments. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. In accordance with features of the invention, a device is provided for attenuating the propagation and reception of EMI of a PCB-chassis structure. 
         [0024]    Turning now to the figures,  FIG. 1  depicts a resistive device  100  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. An upper contact  102  and a lower contact  104  may be present at the top and bottom of the device  100  and may be composed of conductive material. Conductive material allows for the moveability of the current carrying charged particles, known as electrons, better than other materials like insulators. The use of a conductive material at the upper contact  102  and the lower contact  104  may maintain a lower impedance connection between the PCB and the chassis than if an insulating material is used, therefore, providing a better connection for the signal to travel between the PCB and the chassis. 
         [0025]    In certain embodiments, a resistor  106  is an impedance material that may be present between the upper contact  102  and the lower contact  104 . A resistor  106  is an electrical component that implements electrical resistance as a circuit element. When a resistor  106  is placed in a circuit, any oscillation induced in the circuit will die away over time if it is not kept going by a source, this effect is called damping. Furthermore, the resistor  106  may also reduce the peak of the resonant frequency. As frequency increases from zero towards resonance, the impedance increases to a maximum value at resonance and then decreases again for frequencies above resonance. PCB-chassis structures display a natural frequency at which they resonate. The geometry of the PCB-chassis structure, the number of fastenings between the PCB and the chassis, the dielectric between the PCB and the chassis, etc. may determine the natural frequency at which the PCB-chassis structure resonates. Circuit signals travel around the PCB and chassis and between the PCB and chassis. Signals with the frequency and higher multiples of the frequency that match the natural frequency at which the PCB-chassis structure resonates, may couple to the PCB-chassis structure, causing the PCB-chassis structure to resonate. This may emit an EMI that may couple to other equipment items or systems and interfere with or prevent their correct operation. However, the PCB-chassis structure also displays a characteristic impedance that may be determined by the geometry of the PCB-chassis structure, the dielectric between the PCB and the chassis, etc. A resistance, that matches the impedance of the characteristic impedance of the PCB-chassis structure, that is placed along the connection between the PCB-chassis structure may reduce the peak of the frequency and higher multiples of the frequency of the signals that match the natural frequency at which the PCB-chassis structure resonates. Therefore, the PCB-chassis structure may not resonate and EMI may not be emitted. 
         [0026]    Consistent with certain embodiments, an inner support  108  may be present between the upper contact  102  and the lower contact  104  and along side the resistor  106 . The inner support  108  may be assembled in such a way as to accept the forces necessary to mount the device under the head of the fastener or under the PCB. The inner support  108  may protect the resistor  106  from damage that may come about from installation or use. Furthermore, the inner support  108  may be composed of an insulating material. Insulating materials do not allow electric charges to flow freely, therefore, they do not conduct an electric current. Thus, stray signals that may have escaped from the PCB-chassis structure, in the form of EMI, that could have propagated and coupled to other equipment items or systems and interfere with or prevent their correct operation, may be decreased. 
         [0027]    In certain embodiments, an outer support  110  may be present between the upper contact  102  and the lower contact  104  and along side the resistor  106 , opposite the inner contact  108 . The outer support  110  may be assembled in such a way as to accept the forces necessary to mount the device under the head of the fastener or under the PCB. The outer support  110  may protect the resistor  106  from damage that may come about from installation or use. Furthermore, the outer support  110  may be composed of an insulating material. Insulating materials do not allow electric charges to flow freely, therefore, they do not conduct an electric current. Thus, stray signals that may have escaped from the PCB-chassis structure, in the form of EMI, that could have propagated and coupled to other equipment items or systems and interfere with or prevent their correct operation, may be decreased. 
         [0028]    Consistent with various embodiments, the device  100  can take any number of different forms. For instance, the device  100  may be circular in shape with an inner circumference and an outer circumference resembling a washer. This may allow the device  100  to be placed underneath the head of a fastener or underneath a PCB. This may also allow the device  100  to dampen the frequency of the signals that may cause the PCB-chassis structure to resonate and area on the PCB may not need to be dedicated to allow for the addition of the resistance. Another embodiment may provide a slot in the device  100 . This slot may allow the device  100  to be placed underneath the head of a fastener or underneath a PCB without removing the fastener from the PCB-chassis structure. This may also allow the device to dampen the frequency of the signals that may cause the PCB-chassis structure to resonate and area on the PCB may not need to be dedicated to allow for the addition of the resistance. 
         [0029]      FIG. 2  depicts a capacitive device  200  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. An upper contact  202  and a lower contact  204  may be present at the top and bottom of the device  100  and may be composed of conductive material. Conductive material allows for the moveability of the current carrying charged particles, known as electrons, better than other materials like insulators. The use of a conductive material at the upper contact  202  and the lower contact  204  may maintain a lower impedance connection between the PCB and the chassis than if an insulating material is used, therefore, providing a better connection for the signal to travel between the PCB and the chassis. 
         [0030]    In certain embodiments, a capacitor  206  may be present between the upper contact  202  and the lower contact  204 . A capacitor  206  is an electrical component used to store energy electrostatically in an electrical field. A PCB may be mounted to a chassis directly at a single point. This works well for grounding low-frequency signals and decreasing the EMI that is emitted. However, a ground loop or unwanted signals at higher frequencies, may remain present throughout a PCB-chassis structure that is mounted at a single point and this may cause excessive EMI to be emitted by the PCB-chassis structure. Multi-point fastening, therefore, may be required for meeting high-frequency EMI emission requirements. To allow for low-frequency and high-frequency signals to both be grounded for a PCB-chassis structure, the PCB may be connected conductively at one fastener and connected capacitively at the other fasteners to the chassis. 
         [0031]    The impedance of a capacitor  206  is inversely proportional to the frequency of the signal. Therefore, a capacitor will have high impedance for low-frequency signals and may act as an open circuit, and will have low impedance for high-frequency signals and may act as a short circuit. Thus, the low-frequency signals may be grounded by the single conductive connection and the high-frequency signals may be grounded by the capacitive connections. To achieve grounding for both low-frequency and high-frequency signals, the proper capacitive connection value may need to be chosen. This may depend on the frequencies of the signals throughout the PCB-chassis structure that may need to be grounded. To obtain the proper capacitive connection, a dielectric  208  may be chosen to be placed between the plates of the capacitor  206 . A dielectric  208  is an electrical insulator that can be polarized by an applied electrical field. The dielectric  208  may enable the capacitor  206  to possess the characteristics necessary to ground the low-frequency and the high-frequency signals that may be present throughout the PCB-chassis structure. This may decrease the ground loops or unwanted signals throughout the PCB-chassis structure, therefore, decreasing the EMI emitted. 
         [0032]    Consistent with certain embodiments, an inner support  210  may be present between the upper contact  202  and the lower contact  204  and along side the capacitor  206 . The inner support  210  may be assembled in such a way as to accept the forces necessary to mount the device under the head of the fastener or under the PCB. The inner support  210  may protect the capacitor  206  from damage that may come about from installation or use. Furthermore, the inner support  210  may be composed of an insulating material. Insulating materials do not allow electric charges to flow freely, therefore, they do not conduct an electric current. Thus, stray signals that may have escaped from the PCB-chassis structure, in the form of EMI, that could have propagated and coupled to other equipment items or systems and interfere with or prevent their correct operation, may be decreased. 
         [0033]    Consistent with various embodiments, the device  200  can take any number of different forms. For instance, the device  200  may be circular in shape with an inner circumference and an outer circumference resembling a washer. This may allow the device  200  to be placed underneath the head of a fastener or underneath a PCB. This may also allow the device  200  to decrease the impedance between the PCB and the chassis, therefore, decreasing the EMI emitted by the PCB-chassis structure and area on the PCB may not need to be dedicated to allow for the addition of the capacitance. Another embodiment may provide a slot in the device  200 . This slot may allow the device  200  to be placed underneath the head of a fastener or underneath a PCB without removing the fastener from the PCB-chassis structure. This may also allow the device  200  to decrease the impedance between the PCB and the chassis, therefore, decreasing the EMI emitted by the PCB-chassis structure and area on the PCB may not need to be dedicated to allow for the addition of the capacitance. 
         [0034]      FIG. 3  depicts an impedance device  300  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. An upper contact  302  and a lower contact  304  may be present at the top and bottom of the device  300  and may be composed of conductive material. Conductive material allows for the moveability of the current carrying charged particles, known as electrons, better than other materials like insulators. The use of a conductive material at the upper contact  302  and the lower contact  304  may maintain a lower impedance connection between the PCB and the chassis than if an insulating material is used, therefore, providing a better connection for the signal to travel between the PCB and the chassis. 
         [0035]    In certain embodiments, a center conductor  308  may be present between the upper contact  302  and the lower contact  304 . The center conductor  308  may also have ferrite material  306  on both of its sides and the ferrite material  306  may also be between the upper contact  302  and the lower contract  304 . Ferrite material  306  is usually made of a non-conductive ceramic compound and is magnetic. When a conductor  308  is surrounded by ferrite material  306  it may form a distributed ferrite bead. At lower frequencies a ferrite bead acts like an inductor and at higher frequencies a ferrite bead acts like a resistor. An inductor acts in the opposite way that a capacitor does. For a capacitor, the impedance is large at low frequencies and small at high frequencies. For an inductor, the impedance is small at low frequencies and large at high frequencies. For signals at lower frequencies the ferrite bead may take on the characteristics of an inductor and be met with little impedance. However, for signals at higher frequencies, the ferrite material  306  may have high resistance. The signal may then be forced to continue through the center conductor  308 . If the signal were to stray from the path through the center conductor  308 , the ferrite material  306  may reflect the stray signal back to the center conductor  308  or the signal may be dissipated as low-level heat. The signal may then be stopped from propagating out of the PCB-chassis structure, in the form of EMI, and coupling to other equipment items or systems and interfere with or prevent their correct operation. 
         [0036]    Consistent with certain embodiments, an inner support  310  may be present between the upper contact  302  and the lower contact  304  and along side the ferrite material  306 . The inner support  310  may be assembled in such a way as to accept the forces necessary to mount the device under the head of the fastener or under the PCB. The inner support  310  may protect the ferrite material  306  and the center conductor  308  from damage that may come about from installation or use. Furthermore, the inner support  310  may be composed of an insulating material. Insulating materials do not allow electric charges to flow freely, therefore, they do not conduct an electric current. Thus, stray signals that may have escaped from the PCB-chassis structure, in the form of EMI, that could have propagated and coupled to other equipment items or systems and interfere with or prevent their correct operation, may be decreased. 
         [0037]    In certain embodiments, an outer support  312  may be present between the upper contact  302  and the lower contact  304  and along side the ferrite material  306 , opposite the inner contact  310 . The outer support  312  may be assembled in such a way as to accept the forces necessary to mount the device under the head of the fastener or under the PCB. The outer support  312  may protect the ferrite material  306  and the center conductor  308  from damage that may come about from installation or use. Furthermore, the outer support  316  may be composed of an insulating material. Insulating materials do not allow electric charges to flow freely, therefore, they do not conduct an electric current. Thus, stray signals that may have escaped from the PCB-chassis structure, in the form of EMI, that could have propagated and coupled to other equipment items or systems and interfere with or prevent their correct operation, may be decreased. 
         [0038]    Consistent with various embodiments, the device  300  can take any number of different forms. For instance, the device  300  may be circular in shape with an inner circumference and an outer circumference resembling a washer. This may allow the device  300  to be placed underneath the head of a fastener or underneath a PCB. This may also allow the device  300  to decrease the amount of stray signals propagating from the PCB and the chassis, therefore, decreasing the EMI emitted by the PCB-chassis structure and area on the PCB may not need to be dedicated to allow for the addition of the impedance material. Another embodiment may provide a slot in the device  300 . This slot may allow the device  300  to be placed underneath the head of a fastener or underneath a PCB without removing the fastener from the PCB-chassis structure. This may also allow the device  300  to decrease the amount of stray signals propagating from the PCB and the chassis, therefore, decreasing the EMI emitted by the PCB-chassis structure and area on the PCB may not need to be dedicated to allow for the addition of the impedance material. 
         [0039]      FIG. 4  depicts an inductor device  400  for attenuating the propagation and reception of EMI of a PCB-chassis structure, consistent with embodiments of the present disclosure. Upper contact  402  and lower contact  404  may be present at the top and bottom of the device  400  and may be composed of conductive material. Conductive material allows for the moveability of the current carrying charged particles, known as electrons, better than other materials like insulators. The use of a conductive material at the upper contact  402  and the lower contact  404  may maintain a lower impedance connection between the PCB and the chassis than if an insulating material is used, therefore, providing a better connection for the signal to travel between the PCB and the chassis. 
         [0040]    In certain embodiments, an inductor  406  may be present between the upper contact  402  and the lower contact  404 . An inductor is an electrical component that resists changes in electric current passing through it and it often consists of a conductor wound into a coil. Fastening a PCB to a chassis should be done with as low impedance as possible. However, when hybrid fastening techniques are applied, the individual devices, such as a resistor or capacitor, have an inherent inductance and create further inductance when connected to the PCB-chassis structure. As discussed, the total inductance of inductors in parallel is equal to the reciprocal of the sum of reciprocals of their individual inductances. By placing an inductor  406  in parallel with the inductance that is inherent to the individual devices and that is created when the individual devices are connected to the PCB-chassis structure, the total inductances may be decreased. An inductor  406  acts in the opposite way that a capacitor does. For a capacitor, the impedance is large at low frequencies and small at high frequencies. For an inductor  406 , the impedance is small at low frequencies and large at high frequencies. When the total inductance is decreased by placing the inductor  406  in parallel with the inductance present, the impedance is decreased for higher frequency signals. Therefore, this may decrease the EMI emitted by the PCB-chassis structure. 
         [0041]    Consistent with certain embodiments, an inner support  408  may be present between the upper contact  202  and the lower contact  204  and along side the inductor  406 . The inner support  408  may be assembled in such a way as to accept the forces necessary to mount the device under the head of the fastener or under the PCB. The inner support  408  may protect the inductor  406  from damage that may come about from installation or use. Furthermore, the inner support  408  may be composed of an insulating material. Insulating materials do not allow electric charges to flow freely, therefore, they do not conduct an electric current. Thus, stray signals that may have escaped from the PCB-chassis structure, in the form of EMI, that could have propagated and coupled to other equipment items or systems and interfere with or prevent their correct operation, may be decreased. 
         [0042]    Consistent with various embodiments, the device  400  can take any number of different forms. For instance, the device  400  may be circular in shape with an inner circumference and an outer circumference resembling a washer. This may allow the device  400  to be placed underneath the head of a fastener or underneath a PCB. This may also allow the device  400  to decrease the impedance between the PCB and the chassis, therefore, decreasing the EMI emitted by the PCB-chassis structure and area on the PCB may not need to be dedicated to allow for the addition of the impedance material. Another embodiment may provide a slot in the device  400 . This slot may allow the device  300  to be placed underneath the head of a fastener or underneath a PCB without removing the fastener from the PCB-chassis structure. This may also allow the device  400  to decrease the impedance between the PCB and the chassis, therefore, decreasing the EMI emitted by the PCB-chassis structure and area on the PCB may not need to be dedicated to allow for the addition of the impedance material. 
         [0043]      FIG. 5  depicts a cross-sectional view of the signal path between the PCB and chassis, consistent with embodiments of the present disclosure. Currents flow through the finite impedance of the board ground  506 . Radio frequency currents that exist within the PCB  504  and the board ground  506  produce radio frequency fields that are then emitted from circuit components and interconnected wiring. These fields may couple efficiently to other metallic structures, through impedance, that generate signals  502  that circulate within the PCB  504  and the board ground  506 . 
         [0044]    The device represented in  FIG. 5  is that of  FIG. 3 , where the device has a center conductor  510  with ferrite material  508  along both sides of the center conductor  510  and may form a distributed ferrite bead. The signal  502  on the left propagates through the lower contact of the device, through the center conductor  510 , through the upper contact of the device, up into the screw  512  that may be used as a fastener that mounts the PCB to the chassis, through the screw  512 , into the standoff  514 , and eventually to the chassis  516 . The standoff  514  may be used to achieve a surface contact with the chassis which may exhibit lower impedance than a point contact, had the standoff not been used. The signal  502  on the left went along a relatively straight path which may mean that the signal  502  had a lower frequency and the ferrite bead took on the characteristics of an inductor, having low-impedance for signals at lower frequencies, or the signal  502  may have a higher frequency, but it did not stray from the center conductor  510 . 
         [0045]    The signal  502  on the right propagates through the lower contact of the device, through the center conductor  510 , is reflected back by the ferrite material  508  to the center conductor  510 , goes through the upper contact of the device, up into the screw  512 , through the screw  512 , into the standoff, and eventually to the chassis  516 . The signal  502  on the right may be reflected back by the ferrite material  508 . This may suggest that the signal  502  on the right had a higher frequency and the ferrite bead took on the characteristics of a resistor, having high-impedance for signals at higher frequencies and when a high-frequency signal strays from the center conductor  510 , the ferrite material  508  may reflect the signal back to the center conductor  510  or absorb the signal and dissipate it as low-level heat. 
         [0046]    While the invention has been described with reference to the specific aspects thereof, those skilled in the art will be able to make various modifications to the described aspects of the invention without departing from the true spirit and scope of the invention. The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that these and other variations are possible within the spirit and scope of the invention as defined in the following claims and their equivalents.
A first substantially annular conductive material has a first central opening, the first central opening is sufficient to substantially surround a fastener and maintain an electrical connection between the printed circuit board and the chassis. A second substantially annular conductive material is concentric with the first conductive material and the second conductive material hays a second central opening which is sufficient to substantially surround the fastener and maintain the electric connection between the printed circuit board and the chassis. A substantially annular impedance material is between and adjacent to the first conductive material and the second conductive material, the impedance material is sufficient to attenuate the electromagnetic interference from the system.