Abstract:
An ultra-high-frequency notch filter ( 100 ) comprises a capacitor ( 102 ) defining a conductive trace ( 106 ) on its body ( 103 ) and extending between its terminals ( 104 ). The trace has an inductance that forms a parallel LC circuit with the capacitance of the capacitor. When mounted on a printed circuit board ( 120 ) to connect two segments of a signal line ( 124 ), the notch filter and a ground plane ( 122 ) of the PCB form a virtual conductive loop having an inductance and a capacitance whose product is the center frequency of the notch of the notch filter. The center frequency is tuned by varying the width of the trace.

Description:
TECHNICAL FIELD 
     This invention relates to suppression of electromagnetic interference (EMI). 
     BACKGROUND OF THE INVENTION 
     Use of high-bandwidth transmission lines to implement local area networks (LANs) is becoming increasingly common. An example thereof is the Gigabit Ethernet LAN. The high-frequency transmission affected by such transmission lines make suppression of their radiated emissions a significant challenge, on account of the fact that radiated emissions, and the crosstalk to other signal lines caused thereby, increase as transmission frequency increases. 
     A notch filter is designed to reject a band of frequencies while passing through all other frequencies. Although the use of notch filters to filter out EMI is known (see, e.g., U.S. Pat. No. 6,539,253), a technical challenge in developing a notch filter for EMI suppression is how to effectively deal with parasitic inductance and capacitance, which can deleteriously affect the intended performance of the filter. At ultra-high transmission frequencies, even small parasitic effects can cause significant problems and therefore must be accounted for in the notch filter design. 
     SUMMARY OF THE INVENTION 
     This invention is directed to solving these and other problems and disadvantages of the prior art. According to one aspect of the invention, an apparatus comprises a capacitor having a body and a pair of terminals attached to the body, and a conductor defined on the body and connecting the terminals, the conductor having an inductance defining together with a capacitance of the capacitor a parallel LC circuit. The circuit is tuned by varying the width of the traces. The apparatus is illustratively suited for use as a notch filter. According to another aspect of the invention, a notch filter having a notch center frequency comprises a capacitor that has a body and a pair of terminals attached to the body and that has a self-resonant frequency equal to or greater than the notch center frequency, and further comprises a conductive trace that has an inductance and that extends along the body and connects the terminals. Illustratively, when mounted on a printed circuit board (PCB) in a signal line proximate to a ground plane, the notch filter and the ground plane form a virtual conductive loop the product of whose inductance and capacitance is the notch center frequency. According to yet another aspect of the invention, a PCB comprises a signal conductor comprising a pair of discrete conductor segments defined by the PCB, a ground plane defined by the PCB, a capacitor having a body and a pair of terminals on the body that connect the capacitor between the segments, and a conductor defined on the body and connecting the pair of terminals. The conductor has an inductance and forms with the capacitor a notch filter for the signal conductor such that the product of the inductance and the capacitance of a virtual conductive loop formed by the notch filter and the ground plane equals a center frequency of the notch of the notch filter. 
     Advantages of the invention include a notch filter that is effective at ultra-high frequencies, that is easy to construct, that is tuneable, that minimizes the number of parts used in its construction, that is compact so that it takes up little real estate on a printed circuit board, and that is compatible with surface-mount circuit-assembly techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       These and other features and advantages of the invention will become more apparent from the following description of an illustrative embodiment of the invention considered together with the drawing in which: 
         FIG. 1  is a perspective diagram of a printed-circuit-board-mounted notch filter that includes an illustrative embodiment of the invention; 
         FIG. 2  is a graph of load lines of capacitors illustratively used to implement a 1 GHz notch filter; 
         FIG. 3  is a graph of load lines of capacitors illustratively used to implement a 4.8 GHz notch filter; and 
         FIG. 4  is a graph of load lines of capacitors illustratively used to implement a 6.25 GHz notch filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an illustrative embodiment of a notch filter  100  mounted on a printed-circuit board (PCB)  120 . Notch filter  100  spans two segments  124   a  and  124   b  of a printed-circuit conductor  124  carrying signals that are to be filtered for EMI. Each segment of conductor  124  terminates in a solder pad  126  to which notch filter  100  is electrically connected, e.g., by a component surface-mounting process. 
     Notch filter  100  consists of a capacitor  102 , preferably a surface-mount capacitor, and a conductive trace  106  of width w and length l defined by (e.g., plated or printed on) and extending the length of body  103  of capacitor  102 . Capacitor  102  is electrically connected to solder pads  126  by conductive terminals  104  that extend from opposite ends of body  103  of capacitor  102 . Trace  106  is electrically connected to terminals  104 , and acts as an inductor there between. Capacitor  102  and trace  106  together form a parallel inductive-capacitive (LC) circuit between the segments of conductor  124 . PCB  120  has a ground plane  122  as one of its layers, which serves as a return path for signals conducted by conductor  124 . Ground plane  122 , capacitor  102 , and trace  106  together form a virtual conductive loop  130  at the resonant frequency of the structure that is formed by them. Loop  130  has a height h l  which is the distance between trace  106  and ground plane  122 . h l  consists of the height h c  of capacitor  102  and depth h g  at which ground plane  122  is buried in PCB  120 . A standard thickness of PCB  120  is 62 mils; consequently, h g  is normally anywhere between 1 mil and 61 mils. The product of the capacitance (C) and inductance (L) of loop  130  define the center frequency f n  of the notch implemented by filter  100  that will be filtered out of the signals on conductor  124 . 
     As is known, capacitors have an individual self-resonant frequency f c  below which they behave capacitively and above which they behave inductively. Typically, the smaller is the capacitance of a capacitor, the smaller is its physical package, and the higher is its self-resonant frequency f c . For ease of design, it is desirable that self-resonant frequency f c  of capacitor  102  equal or exceed f n . At this self-resonant frequency f c , the capacitance C of loop  130  is effectively the capacitance of capacitor  102 . Consequently, the required inductance L of loop  130  is L=1/(4π 2 f n   2  C). Inductance L is provided by loop  130 . Inductance L is related to loop height h l  as follows: 
               L   =     5   ⁢     (     10     -   3       )     ⁢           ⁢   ln   ⁢           ⁢     (       4   ⁢     h   l       d     )     ⁢   l       ,         
where L is measured in μH, h l  is measured in mils, l is the length of trace  106  in inches, and d is the diameter in mils of an equivalent circular cross-section having a circumference πd equal to twice the sum of the width w and thickness t of trace  106 . L is set by selecting the width w of trace  106 . It is assumed that the thickness t of trace  106  is a standard and unvarying approximately 1 mil (.˜7 to ˜1.4 mil) of copper, aluminum, or other conductor; i.e., the standard thickness of a printed circuit trace. Given the dimensions of conventional surface-mountable capacitors, values of L that are reasonably achievable by varying the width w of trace  106  are between about 0.2 nH and about 1.5 nH.
 
     In this illustrative example, it is assumed that conductor  124  suffers from EMI or crosstalk from a Gigabit Ethernet, i.e., f n =1 GHz. Given f n  and the reasonably-achievable values of L, an available suitable capacitor  102  is selected. In this example, an illustrative commercially-available capacitor is a surface-mountable 0603-type capacitor (length of 60 mils, width and height of 30 mils) of 27 pF. The selection of capacitor  102  determines height h l  of loop  130  (h g  being fixed by PCB  120 ) and length l of trace  106 . The inductance L of loop  130  therefore must be set to produce the desired value of f n  by selecting the width w of trace  106 . 
     The proper width w of trace  106  is determined from the following formulas. 
                 L   ⁡     (       h   g     ,   w   ,   t   ,   l     )       =     5.0   ⁢       (     10     -   6       )     ·   l   ·   ln     ⁢     {       2   ⁢     (       h   l     +     h   g       )     ⁢   π       (     w   +   t     )       }         ,         
where
 
     L=inductance (in μH) of loop  130   
     h g =vertical distance from bottom surface of capacitor  102  to the return reference plane  122  (in mils) 
     h l =height of capacitor  102  (in mils) 
     w=width of trace  106  (in mils) 
     t=thickness (height) of trace  106  (in mils) 
     l=length of trace  106  (in mils) 
                   f   n     ⁡     (       h   g     ,   w   ,   t   ,   l   ,   C     )       =     1     2   ⁢   π   ⁢         L   ⁡     (       h   g     ,   y   ,   t   ,   l     )       ·   C             ,   where         
where
 
     f n =center frequency of the notch filter, and 
     C=capacitance (in farads) of loop  130   
     The Procedure for Determining w, and h g  for Fixed t, l, and C Values is as Follows: 
     
         
         (1) Plot f n (h g ,w,t,l,C) for 1≦h g ≦h pcb  (total thickness of PCB  120  in mils) and 
       
    
                 h   l     5     ≤   w   ≤     h   l           
in mils as a surface plot, with h g  as the x-axis and w as the y-axis. The vertical z-axis is then the resonant frequency for a given (h g , w) pair.
     (2) Superimpose a “reference” surface plot on top of the surface plot generated from step (1) that represents the desired resonant frequency f n . This surface plot will necessarily be a planar surface and should cover the entire (h g , w) range of values as stated in step (1).   (3) The intersection of the surface plot from (1) and the planar surface plot from (2) represents the full range of (h g , w) pairs that will produce the desired resonant frequency. This intersecting contour will be a line, referred to as a load line. Implement the solution by fabricating an electroplated copper trace  106  of length l (mils), and width w (mils).   (4) If no intersection results from step (3), alter the value of the capacitance C until an intersecting contour is generated from the two surface plots. Make sure to select C such that this capacitor behaves capacitively slightly beyond the desired resonant frequency. In other words, the selected capacitor must have a resonant frequency f c  that exceeds the desired resonant frequency f n  of the notch filter.   (5) If the variable h g  is known a-priori, then select the (h g , w) pair that lies on the load line determined from step (3). Implement the solution by fabricating trace  106  of length l, and width w. Usually h g  is known a-priori, since the layer stackup of printed circuit board  120  is known before designing the notch filter.   
       FIG. 2  shows a load line  204  that defines the value of w as a function of h g  at f n =1 GHz for a 27 pF 0603-type capacitor. As described above load line  204  is derived by superimposing two surface plots, with their intersection being the load line for a given notch filter center frequency fn._One of the surface plots is a plot of the achievable resonant frequencies as a function of the width w of trace  106  and the depth h g  of the reference return path. This surface plot is for a given fixed capacitance of 27 pF in this example. Also, in this example, h l =(30+h g ) mils. Next, a reference plane is superimposed onto the aforementioned first surface plot. This reference plane is the desired notch filter resonant frequency f n  of 1 GHz in this example. The intersection of these two surfaces is line  204  that highlights the needed width of trace  106  as a function of the depth h g  of ground plane  122  within printed circuit board  120 . The 27 pF 0603-type capacitor is currently believed to be the only capacitor that will provide a 1 GHz notch filter for any depth of ground plane  122  within a conventional 62 mil thick printed circuit board  120 . There are other capacitor values that can provide a 1 GHz notch filter; however, these other values will prevent the depth h g  of ground plane  122  from covering the entire 62 mil thickness of PCB  120 . In these cases, the depth h g  of ground plane  122  must be greater than some minimal depth, or will only work within some subset of the entire 62 mil PCB thickness. These constraints are restrictive and limit the practicality of using anything but an 0603-type 27 pF capacitor. 
     Computer simulations indicate that notch filter  100  constructed as described above produces an attenuation better than 7 dB of the 1 GHz EMI. 
     Instead of using one capacitor  102  and trace  106  to implement notch filter  100 , a plurality of capacitors can be connected in parallel to form capacitor  102 , and one or more of those capacitors can carry traces that together, in parallel, form trace  106 . If capacitors of slightly-different values are used in parallel, the result is a plurality of slightly-different notch filters—or, equivalently, a notch filter having a wider notch—resulting in improved EMI attenuation. One of the advantages of a notch filter  100  constructed in the illustrative manner is that it occupies a very small amount of PCB real estate. To preserve this advantage in the case of a notch filter constructed from a plurality of capacitors, the capacitors may be vertically stacked, illustratively as described in U.S. patent application Ser. No. 10/292,670, filed on Nov. 12, 2002, now abandoned, and assigned to the same assignee as this application. In this illustrative example of a 1 GHz notch filter, a 23 pF 0603-type capacitor may be used in parallel with the 27 pF capacitor. The load line for the parallel combination of the 23 pF and 27 pF capacitors is shown as load line  202  in  FIG. 2 . 
     Of course, the invention may be used to implement notch filters at frequencies other than 1 GHz. Illustratively,  FIG. 3  shows a load line  304  for a surface-mountable 0402-type capacitor (length of 40 mils, width and height of 20 mils) of 1.7 pF used to implement a 4.8 GHz notch filter. The dimension h g  is the depth at which a ground plane is buried in a PCB, and the dimension w is the width of a trace. Correspondingly to the example  FIG. 2 , the 1.7 pF capacitor may advantageously be used in parallel with a 0402-type capacitor of 1.508 pF to implement the 4.8 GHz notch filter. The load line for the parallel combination of the two capacitors is shown as load line  302  in  FIG. 3 . Also illustratively,  FIG. 4  shows load line  404  for a surface-mountable 0402-type capacitor of 1.023 pF used to implement a 6.1 GHz notch filter. Again, this capacitor may advantageously be used in parallel with an 0402-type capacitor of 0.9 pF to implement the 6.1 GHz notch filter. The load line for the parallel combination of the two capacitors is shown as load line  402  in  FIG. 4 . The dimension h g  is the depth at which a ground plane is buried in a PCB, and the dimension w is the width of a trace. 
     Of course, various changes and modifications to the illustrative embodiment described above will be apparent to those skilled in the art. These changes and modifications can be made without departing from the spirit and the scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the following claims except insofar as limited by the prior art.