Abstract:
A filter board for filtering electromagnetic emissions from an electronic device couples to a conductive housing in the device. The filter board includes a plurality of filter components connected to a substrate, which is coupled to a continuous ground plane. Signal lines from the electronic device are coupled to the filter components and pass through the substrate and ground plane before being connected to other circuits. The filter board prevents high frequency noise and excessive emissions from exceeding required limits.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to electromagnetic filtering, and more particularly to filtering high-frequency noise signals occurring in an electronic device. 
     2. Background of the Invention 
     Recent developments in mobile multimedia products for vehicles incorporate many diverse electronic products, such as computer boards, DVD players, video displays, modems, digital signal processors, into conventional product designs ordinarily having a simple radio. The high frequency and fast rise time of these additional circuits makes meeting electromagnetic compatibility (EMC) standards, especially with respect to radiated emissions, more difficult. 
     In product designs incorporating one or two devices, such as a radio and/or CD player, the device typically has a conductive housing. Control and display signals are transmitted from the device&#39;s main processor board through its conductive housing to a keyboard and display via a harness cable, flex circuit or a direct board-to-board connector. Any filtering of the signal transmission lines is done on the processor board. With optimized circuits, such as devices with typical clock frequencies of around 4 MHZ, the radiated emissions from the device can meet acceptable EMC levels. 
     Mobile multimedia devices, by contrast, often employ multiple circuits and modules with clock rates of 100 MHz or more, sub-nanosecond rise times, expanded mode processors and video signals of 25 MHz or more. These circuits possess both differential signals and common mode noise with harmonics extending on the order of GHz. Board-to-board cables connect multiple modules within the device, and the cables conduct noise throughout the device. Also, electromagnetic fields from the IC&#39;s and circuitry on the boards couple to the board-to-board cables, exacerbating noise and radiated emissions. 
     The higher noise and radiated emissions levels generated by mobile multimedia devices makes it more difficult to keep emissions within EMC standards. Using conventional methods to connect the display and keyboard in mobile multimedia devices will still allow signals and noise to be conducted through the conductive housing and onto the display and/or keyboard. The resulting energy radiation usually exceeds EMC standards, but the processor board in mobile multimedia devices is insufficient for filtering the signals due to the common mode and coupled noise as explained above. 
     There is a need for a structure that can filter signals emitted from devices, such as mobile multimedia devices, that generate higher noise levels. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a filtering device for controlling emissions from an electronic device having a conductive housing, comprising at least one filter component, a substrate supporting said at least one filter component, and a ground plane supporting said substrate, wherein the ground plane is to be connected to the conductive housing such that noise from the electronic device is grounded to the conductive housing via at least one of the ground plane and said at least one filter component. 
     The invention is also directed to an electronic device comprising a conductive housing, at least one device circuit, and a filter board for controlling emissions from said at least one device circuit, the filter board including at least one filter component, a substrate supporting said at least one filter component, and a ground plane supporting said substrate, wherein the ground plane is connected to the conductive housing such that noise from the device travels through said at least one filter component and is grounded to the conductive housing via the ground plane. 
     The invention is further directed to a method for filtering signal emissions from an electronic device having at least one device circuit, comprising the acts of placing a filter board having at least one filter component and a ground plane and connecting the ground plane to the conductive housing such that noise from the electronic device is grounded to the conductive housing via at least one of the ground plane and said at least one filter component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a representative side sectional view of one embodiment of a filtering device according to the present invention; 
     FIG. 2 is a representative front view of a conductive housing to be connected with the inventive filtering device; 
     FIG. 3A is a representative front view of the conductive housing shown in FIG. 2 after being connected with the inventive filtering device; 
     FIG. 3B is a representative side view of the conductive housing and inventive filtering device of FIG. 3A; 
     FIGS. 4A and 4B are plan views of one embodiment of the inventive filter board; 
     FIGS. 5A,  5 B, and  5 C are representative side sectional views illustrating two embodiments of the inventive filter board coupled to an electronic device housing. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a representative diagram of a filtering device, such as a filter board  100 , according to the present invention. The filter board  100  includes a circuit board formed by mounting one or more filter components  102  onto a substrate  104 . The specific material used for the substrate  104  can be epoxy or fiberglass if the filter board  100  to form a conventional two-layer circuit board structure. Alternatively, the filter board  100  can be made from Mylar™ or Kapton™ to form a flexible circuit. Regardless of the specific substrate material used, conductive traces (shown in FIG. 5A) are disposed on one or both sides of the substrate  104 . The filter components  102  are attached to the conductive traces on one side of the substrate  104 , while a ground plane  106  is attached to the other side of the substrate  104 . The conductive traces create a path for noisy signals from an input signal line to the filter components  102 . 
     The filter components  102  themselves can be any component, such as a shunt element, that filters high frequency noise and can be disposed on either side of the substrate  104 . The filter components  102  can be, for example, a grounded capacitor, an L-filter, a pi-filter, or any other filter than can pass a desired signal and reject high frequency noise. In one embodiment, the power, control and signal lines exiting an electronic device coupled to the filter pass through the filter components  102  so that the desired signal reaches its destination (e.g., keyboard, display) while any high frequency noise in the signal is grounded. 
     The ground plane  106  provides the actual grounding function in the filter board  100 . Because of this function, the ground plane  106  should be made of an electrically conductive material, such as copper. The ground plane  106  should be continuous for as large a portion as possible to minimize impedance at the desired frequencies. 
     FIGS. 2,  3 A and  3 B illustrate possible ways in which the inventive filter board  100  can be coupled to an electronic device. In this example, the filter board  100  is connected to a conductive housing  302  to ground the conductive housing  302  and thereby ground signal noise. The actual connection structure between the conductive housing  302  and the filter board  100  can couple the conductive housing  302  to the filtering devices  102  in the filter board  100  and/or directly connect the filter board  100  to the conductive housing  302  . Other possible connector options include but are not limited to cables, ribbons, wire cables, and flex circuits. These connection structures will be described in greater detail below. 
     FIG. 2 illustrates one possible conductive housing  302  configuration for accommodating the inventive filter board structure  100  to ensure optimum filtering. The conductive housing  302  in this embodiment has embossed portions, such as a plurality of emboss points  303 , that are raised from the conductive housing  302  surface. The emboss points  303  provide low impedance paths for RF energy. In one embodiment, the emboss points  303  are spaced apart less than {fraction (1/10)} of a wavelength apart for optimum emission reduction. The conductive housing  302  also has one or more openings  304  for accommodating connectors, such as screws and cables. Some of the openings  304  allow filtered signals to leave the conductive housing through the conductive housing  302 . Regardless of the opening&#39;s purpose, the size of all openings  304  should be minimized to avoid RF leakage through the conductive housing  304 . One possible guideline is to keep the opening dimensions around {fraction (1/10)} to {fraction (1/20)} of the wavelength of the highest frequency to be filtered to prevent radiation from escaping the housing. 
     FIGS. 3A and 3B illustrate the conductive housing  302  of FIG. 2 after the inventive filter board  100  has been attached to the conductive housing  302  with fasteners  350 , such as screws, pegs, or any other similar fastening structure. The emboss points  304  contact the ground plane  106  of the filter board  100  to create a Faraday shield, which effectively reduces emissions. A board-to-board connector  306  having flexible tabs  307  may also be attached to the conductive housing  304  and the filter board  100  with conductive adhesive and grounded. In one embodiment, the connector  306  faces the outside of the conductive housing  302  for connection to a display or another electronic sub-assembly (not shown). 
     Cables, such as wire bundles or flexible circuits can act as signal lines  308  connecting modules (not shown) inside the conductive housing  302  through filter board  100  to a desired circuit on the outside of the conductive housing  302 . The pass-through connection can be, for example, a wire conductor or a plated via. Note that the arrows entering and exiting the signal lines  308  shown in FIG. 3A represent the signal flow through the signal lines  308 . Regardless of the specific pass-through connection used, any openings in the ground plane  104  allowing the signal line  306  to connect to components outside the conductive housing should be minimized. In this way, any noise voltage either conducted directly from a noisy PCB or coupled onto the cable is shunted to a low impedance ground with the return current conducted back to the source via the conductive housing&#39;s own ground. Thus, the noise voltages on any lines exiting the housing are reduced as much as possible, reducing radiated emissions. 
     The ground plane  106  of the filter board  100  may be designed to cover the openings  304  on the conductive housing  302 . This reduces aperture radiation, which can otherwise result from currents flowing around the openings  304  due to any electromagnetic fields inside the housing  302  from signal lines  308  or other sources. Note that in the embodiments shown in FIGS. 5A and 5B, the filter board  100  has two ground planes  106  disposed on either side of the substrate  104 . In this embodiment, the filtering devices  102  are attached to one of the ground planes  106  to be supported by the substrate  104 . 
     To provide good coupling, the ground plane  106  can be bonded to the conductive housing  302  at selected points, such as the emboss points  303  described above, or continuously with a conductive gasket material or similar material. The filter board  100  may include conductive vias  400  that couple the ground plane to the emboss points  303  on the conductive housing  302 . Regardless of the specific connection between the ground plane  106  of the filter board  100  and the conductive housing  302 , good connection between the ground plane  106  and the conductive housing  302  provides an effective Faraday shield at the frequencies of interest to reduce emissions. 
     Once the filter board  100  is connected to the conductive housing  302 , noisy signals inside the conductive housing  302  travel through signal lines  308  through the filter board  100  and terminate at the board-to-board connector  306 . Once the signals reach the board-to-board connector  306 , any RF emissions in the signal are greatly reduced. 
     FIGS. 4A and 4B illustrate the filter board  100  in more detail. As can be seen in the Figure, the filter board  100  includes one or more output vias  500  for allowing the signal lines  308  to pass through the filter board  100 , additional vias  400  to maintain the Faraday shield formed by mating the ground plane  106  and the conductive housing  302 , one or more filter components  102 , and a circuit trace  406  that directs signals from input signal lines  600  through the filter components  102  to the output vias  500 . In one embodiment, the entrance and exit portions of the output vias  500  may be larger than the via portion extending through the filter board  100  itself. 
     FIGS. 5A,  5 B, and  5 C illustrate three possible ways in which the inventive filter board  100  can be connected to the conductive housing  302 . FIGS. 5A and 5C illustrate one embodiment where the filter board  100  is attached to an inside surface of the conductive housing  302 , while FIG. 5B illustrates an embodiment where the ground plane  106  of the, filter board  100  is attached to an outside surface of the conductive housing  302 . In each embodiment, the filter board  100  is attached to the housing  302  with the fastener  350 . The emboss points  303  on the conductive housing  302  contact the ground plane  106  of the filter board  100 . A plurality of vias  400  provide interconnection of the upper and lower segments of the ground plane  106 . Further, as shown in the Figures, noisy signals from an input signal line  308  travel through the circuit trace  406  to the filter components  102 , which shunt the noise to the ground plane  106 , allowing a clean, filtered signal to leave the filter board through an output signal line  602 . 
     Note that the filter board  100  in the embodiment shown in FIG. 5C includes two inner layers  603 ,  604  disposed between the ground planes  106 . In one embodiment, circuit traces (not shown) may be routed on the inner layers  603 ,  604 . One of the inner layers  604  may be constructed to contact one or more of the output vias  500  and act as an inner ground plane to maintain a Faraday shield. Although the embodiment in FIG. 5C shows a filter board  100  with two inner layers, the filter board  100  may be constructed with any number of inner layers and/or without any inner ground plane, if desired. In such a case, the inner layers simply act as supports for circuit traces and do not need to contact any of the output vias  500  to maintain a Faraday shield. Additional circuitry (not shown) may also be interconnected using the inner layers  603 ,  604  in the filter board, if needed. 
     Further, in the embodiment shown in FIG. 5C, the output trace  602  travels through a solder pad  605  and a connector  606 . The connector  606  may have a large aperture that would ordinarily allow electromagnetic fields to escape. The inner ground plane  604  helps re-establish a Faraday shield to block emissions. Those of skill in the art will understand that elements of the different embodiments shown in FIGS. 5A,  5 B, and  5 C can be combined in different ways without departing from the scope of the invention. 
     The inventive filter board  100  therefore effectively filters even high levels of noise and electromagnetic emissions while maintaining a simple, easily connectable structure. The filter board  100  allows efficient routing of signal lines, which in turn leads to a higher quality and more reliable electronic device overall. Note that although the description above focuses on reducing radiated emissions, the inventive structure can also reduce conductive emissions as well, such as those encountered on device harness connectors. 
     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.