Abstract:
A low cost radio frequency interference filter assembly comprises a multiple layer structure including a middle trace layer disposed between an upper ground layer and lower ground layer. Non-conductive insulation layers are disposed between the middle trace layer and the upper and lower ground layers. The upper layer includes input contacts, signal contacts, and capacitors which are coupled to the signal contacts and an upper grounded substrate. The middle trace layer includes a grounded substrate and trace lines which are coupled to the signal contacts of the upper layer by signal vias. The lower layer includes a grounded substrate. Ground vias are formed through the insulation layers to couple the middle grounded substrate to the upper and lower grounded substrates. The filter assembly may be formed as an integral projection of a printed circuit board assembly. A structure for an RF power amplifier assembly is provided along with methods for manufacturing a filter assembly and assembling an electronics assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a radio frequency interference (RFI) filter. More particularly, the present invention relates to an RFI filter feed-through assembly which may be incorporated in an RF power amplifier. 
     2. Description of Related Art 
     Modern wireless communication base stations transmit and receive radio frequency (“RF”) signals through the use of RF power amplifiers. Within the amplifier assemblies are typically plural printed circuit (“PC”) boards on which components that process the RF signals are mounted. RF signals must be transmitted between the various processing components. These intra-device transmissions may be accomplished through the use of microstrip technology, such as coplanar waveguides. 
     Other direct current (“DC”) or low-frequency signals may coexist with the high frequency signals in the amplifier assembly. For some applications, it is necessary to isolate a PC board or a region of a PC board from interference by outside RF signals or intra-device RF signals, while at the same time providing for the communication of DC or low-frequency signals. For example, modern RF power amplifiers typically have control systems which provide monitoring and control through use of microprocessors and/or other control integrated circuits (“ICs”). Therefore, the DC signal may be the control output of an IC. 
     Isolation from RF signals propagating through different sections of an assembly may be provided by a metal enclosure or shield. For instance, referring to FIGS. 1A and 1B (top plan view) which show a conventional construction of an RFI filter, one might may isolate two PC boards  71 ,  72  by placing them on opposite sides of a grounded metal barrier wall or bulkhead  4  within the assembly. The metal bulkhead  4  separates two regions  61 ,  62  lying on opposite sides thereof and shields electromagnetic radiation from being transmitted between the regions  61 ,  62 . Communication of DC signals between the boards  71 ,  72  can be provided by an insulated conductive feed-through  3  which passes through the bulkhead  4 . The feed-throughs  3  are mounted through the bulkhead  4  so that the conductive input and output terminals  20   a ,  20   b  lie on opposite sides of the bulkhead  4 . Attached via ribbon cables  1  near the edges of the two PC boards  71 , 72  are connectors  2  which are adapted to mate with the conductive terminals  20   a ,  20   b  of the feed-through  3 . However, this configuration provides only incomplete isolation because RF signals may still propagate along the feed-through  3 . 
     To prevent such RF propagation, it is conventional to incorporate an RFI filter in the conductive feed-through  3 . In FIG. 2A, a circuit diagram for one embodiment of a. conventional filter is shown. Lying between an input terminal  20   a  and output terminal  20   b  of the filter  3  is a node  21 . Attached to the node  21  is a grounded shunt capacitor  5 . In the normal operation of the filter  3  an input signal containing high frequency (about 0.9 MHz and above) and low frequency components (about 50 kHz and below) is introduced at the input terminal  20   a . The shunt capacitor  5  acts as a low-pass filter, providing a path to ground for undesirable co-propagating RF signals while allowing DC or low frequency signals to pass through the bulkhead  4 . Thus, the output signal presented at the output terminal  20   b  primarily consists of the low frequency components of the input signal. 
     Another variation of an RFI filter is the Pi filter, illustrated in FIG.  2 B. Lying in series between the input terminal  20   a  and output terminal  20   b  of the feed-through  3  is an inductor  7 . On opposite ends of the inductor  7  are nodes  21   a ,  21   b . Attached at the nodes  21   a ,  21   b  are grounded shunt capacitors  5 . In the normal operation of the filter  3 , an input signal containing high frequency and low frequency components is introduced at the input terminal  20   a . The shunt capacitors  5  provide a path to ground which substantially attenuates the high frequency components of the input signal. Moreover, the inductor  7  presents a high impedance to high frequency components and provides additional attenuation of the high frequency components. Thus, the output signal presented at the output terminal  20   b  primarily consists of the low frequency components of the input signal. The feed-through filters described in FIGS. 2A and 2B provide bi-directional filtering of high frequency components, i.e. an input signal may be alternatively introduced at the output. 
     A drawback to conventional feed-through RFI filters is that their use entails added cost. First, the feed-through component  3  itself is costly. Moreover, since the conductive leads  20   a ,  20   b  of the feed-through component  3  are usually not directly coupled to the PC boards  71 ,  72  on either side of the bulkhead  4 , the PC boards  71 ,  72  must somehow be joined to the conductive leads  20   a ,  20   b  of the feed-through  3 . As shown in FIGS. IA and  1 B, this is conventionally done by attaching to each PC board  71 ,  72  a connector  2  which mates with the feed-through leads  20   a ,  20   b  and connects to PC board leads  1 . The use of a separate connector adds to the manufacturing cost of an RF assembly, and may increase the labor cost if the connector on the PC board must be manually mated with the leads of the feed-through component. Also, the feed-through components and separate connectors conventionally used are bulky. 
     Accordingly, a need presently exists for effective prevention of RF interference between sections of an RF assembly that is less expensive than prior art solutions. 
     SUMMARY OF THE INVENTION 
     An RFI filter assembly is provided which overcomes the deficiencies of the prior art. 
     In one aspect, a low cost RFI filter assembly is provided which comprises a multi-layered printed circuit board assembly. In a preferred embodiment, the layered PC board assembly is adapted to pass below or through a shield bulkhead, or partition, for conducting DC or low frequency signals through the shield bulkhead. In a preferred embodiment, three conductive layers are provided, separated by non-conductive insulation layers. A top layer includes a conductive ground plane. A middle trace layer of the three-layered embodiment comprises several narrow conductive paths that traverse the length of the board surrounded by grounded regions. The narrow conductive paths function as an inductor which resists the propagation of high-frequency signals from one end of the board to the other. A bottom layer of the three-layered embodiment comprises a ground plane. The grounded regions of all three layers are electrically coupled by plated ground via holes through all three layers. 
     In a further aspect of the invention, affixed to the ground plane are two groups of capacitive elements spaced across the width of the top ground plane on opposite sides of the bulkhead. Each of the capacitive elements comprises two conductive contacts One contact of each capacitive element in the first group is electrically coupled to the top ground plane while the other contact is electrically coupled to a respective conductive path traversing the middle trace layer of the board assembly. The second group of capacitive elements is similarly coupled at the opposite end of the board. The capacitive elements provide a shunt path for high-frequency signals to ground, and thus attenuate high-frequency signals introduced at the extreme ends of the conductive paths. 
     This configuration functions as a Pi filter providing effective filtering of RF signals without the use of bulky feed-through components. Also, ground layers may be provided on the sides of the narrow conductive paths. These prevent RF radiation propagating through the assembly. Therefore, RF interference between shielded regions is effectively blocked. 
     The invention, now having been briefly summarized, may be better appreciated by the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view of an RF assembly incorporating a prior art feed-through configuration; 
     FIG. 1B is a top plan view of the prior art feed-through configuration; 
     FIG. 2A is a circuit diagram for a prior art low-pass filter arrangement; 
     FIG. 2B is a circuit diagram for a prior art low-pass filter arrangement; 
     FIG. 3 is an exploded perspective view of an RF filter assembly incorporating a three-layer circuit board filter according to a preferred embodiment of the present invention; 
     FIG. 4 is a top plan view of the top layer of the three-layer circuit board filter; 
     FIG. 5A is a top plan view of the middle layer of the three-layer circuit board filter; 
     FIG. 5B is a top plan view of an alternative embodiment of the middle layer of a three-layer circuit board filter; 
     FIG. 6 is a top plan view of the bottom layer of the three-layer circuit board filter, 
     FIG. 7 is a circuit diagram of the filter assembly according to the invention; 
     FIG. 8 is a side elevation view of the filter assembly; 
     FIG. 9 is a perspective view of an RF power amplifier assembly having RF shielded compartments according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention and its various embodiments can now be better understood by turning to the following detailed description wherein illustrated embodiments are described. It is to be expressly understood that the illustrated embodiments are set forth as examples and not by way of limitations on the invention as ultimately defined in the claims. 
     FIG. 3 is a perspective view of an embodiment of an RF filter assembly according to the present invention. In a preferred embodiment, the RF filter assembly  100  is incorporated in a multi-layer PC board assembly. In particular, the filter assembly  100  may be formed as a projection extending integrally from a printed circuit board assembly in an RF power amplifier, such as an amplifier module or a control circuit module. Thus, an electronics module assembly which includes a printed circuit board, such as an amplifier module or a control circuit module, may be formed according to the invention by providing multiple layers with an integral projection containing the filter elements as described below. 
     In the preferred embodiment of FIG. 3, three conductive planar layers  110 ,  130 ,  150  are shown. More specifically, the top layer  110  may comprise a thin conductive layer on an insulating portion, or support,  181 , composed of, for example, any known non-conductive insulating materials conventionally used in the construction of PC boards. Alternatively, the upper insulating portion  181  may comprise a structure separate from the conductive layer  110  which is then mounted thereon. Similarly, the middle layer  130  may comprise a conductive layer formed on an insulation layer  182  while the bottom layer  150  may also comprise an optional insulation layer. The top ground layer  110  comprises a ground plane  113  with RF interference filter elements  115   a ,  115   b  mounted thereon, which in a preferred embodiment are capacitors. The ground plane  113  may be formed by providing a thin conductive layer, e.g. of copper, on insulating support  181  and etching or otherwise removing regions to form a desired pattern. Alternatively, a patterned conductive layer may be formed using a mask. The top layer  110  also includes input terminals, or input contacts,  120   a  and optional signal leads  121   a  which are insulated from the ground plane  113 . It is to be expressly understood that the input contacts  120   a  and RF. interference elements need not be disposed on the top layer  110 . The middle layer  130  is sandwiched between the upper ground layer  110  and the lower layer  150 . The middle layer  130  comprises a ground plane  136  and several internal signal traces  137  insulated from the ground plane  136 . The middle layer  130  may be formed as a thin conductive layer on the insulating support  182  using etching or masking as in the case of the upper layer  110 . Disposed beneath the middle layer  130 , the bottom ground layer  150  comprises a ground plane  154 . 
     In FIG. 4, an array of conductive input terminals  120   a  disposed adjacent to one edge of the top layer  110  may act as input terminals for multiple input signals. For example, input signals may comprise DC power supply lines, low frequency control signals, and monitoring or feedback signals. The input terminals  120   a  are electrically isolated from the ground plane  113  by a surrounding insulation region  123  which is formed by removing a ring of conductive material from around the input terminals  120   a . Disposed near the input terminals  120   a  are capacitive elements, or capacitors,  115   a ,  115   b . Each capacitor  115   a ,  115   b  has two conductive contacts, as shown in FIG.  7 . One conductive contact is electrically coupled to the ground plane  113 . The other contact is electrically coupled to a signal lead  121   a . Since the signal lead  121   a  is optional, the capacitor  115   a  may be electrically connected to the input  120   a . Furthermore, it is to expressly understood that the capacitors  115   a ,  115   b  may be electrically coupled to the ground plane of any other layer. The signal leads  121   a  are electrically isolated from the ground plane  113  by a surrounding insulation region  124  A second set of capacitors  115   b  is spaced away from the first set of capacitors  115   a  at a distance that will place them on the opposite side of the bulkhead  104  when the filer assembly  100  is installed for operation. Each capacitor  115   b  in the second set has two conductive contacts, as shown in FIG.  7 . One conductive contact  125  is electrically coupled to the ground plane  113 . The other contact is electrically coupled to one of a second set of signal leads  121   b . The capacitors  115   a ,  115   b  may comprise the surface mount variety, thus allowing for the mounting of the capacitors  115   a ,  115   b  on the top layer  110  as circuit traces extend into the PC board assembly using conventional surface mount technology. The first and second set of signal leads  121   a ,  121   b  are electrically coupled by shunt via holes to the internal signal traces described below in connection with FIGS. 5A and 5B. 
     Referring now to FIG. 5A, the details of the middle layer  130  of the filter  100  will be described. Traversing the length of the middle layer  130  are internal signal traces  137  formed from an electrically conductive material. Adjacent to the extremities, each internal signal trace  137  is electrically coupled to shunt via holes  138 , 145  plated with an electrically conductive material. As described above, the shunt via holes  138 ,  145  are also electrically coupled to the top layer  110  of the filter assembly  100 . Each internal signal trace  137  may be electrically coupled to optional input via holes  133  near a board edge. The optional input via holes  133  may be plated with an electrically conductive material and electrically coupled to the conductive input terminals  120   a  described above. Encompassing the internal signal traces  137  is a middle ground plane  136 . The middle ground plane  136  is electrically coupled to the top ground plane  113  and the bottom ground plane  154  by way of ground via holes  149  plated with an electrically conductive material. 
     As depicted in FIG. 5B, additional RF shielding may be achieved by forming interstitial ground strips  147  between pairs of internal signal traces  137 . These strips may be desired when very high frequencies are present in the RF assembly, or where harmonics are present which need to be blocked. The interstitial ground strips  147  are electrically coupled to the top ground plane  113  and the bottom ground plane  154  by way of ground via holes  148  plated with an electrically conductive material. The spacing between the interstitial ground strips may be chosen to block propagation of the frequency of RF energy generated in the modules. In particular, the spacing between strips  147  should be less than λ/2 where A is the wavelength propagating in the module. For example, for RF frequencies up to 2 GHz, ground strip spacings will be less than 2.84 inches. The dielectric constant of the board material will affect this dimension and spacings of less than about 1.5 inches may be required depending on the material. Also, harmonics may propagate through spacings blocking the main frequency and spacings between strips  147  of about 0.2 inches or less may be employed to block harmonics. 
     FIG. 6 is a top plan view of the bottom layer  150  of the filter. The bottom layer  150  comprises a ground plane  154  which is electrically coupled to the top ground plane  113  and the middle ground plane  136  by way of ground via holes plated with an electrically conductive material. The bottom layer  150  is substantially similar in structure to the top layer  110  except that the bottom layer  150  is inverted such that the bottom surface of the top layer (not shown) is similar to the top surface of the bottom layer  150  shown in FIG.  6 . Accordingly, the bottom surface of the bottom layer (not shown) is similar to the top surface of the top layer  110  shown in FIG.  4 . The bottom layer  150  may also comprise an insulation support. 
     In practice, the top and bottom ground planes  113 ,  154 , respectively, may also be in electrical contact with the shield bulkhead  104  such that the ground planes  113 ,  154  are held at the same potential as shield bulkhead  104 . The internal, or middle, ground plane  136  and interstitial ground traces  137  would also be held at the same potential as the shield bulkhead  104  due to their electrical connection to the top and bottom ground planes  113 ,  154  by way of the ground via holes. 
     A filter constructed according to the invention electrically functions as a Pi filter, such as described in connection with FIG.  2 B. As will be appreciated by those skilled in the art, narrow conductive strips such as the internal signal traces  137  in FIGS. 5A and 5B have inductive qualities which vary according to their length, width, and thickness. The internal signal traces  137  thus correspond to the inductive element  7  in FIG.  2 B. Moreover, the internal signal traces  137  in FIGS. 5A and 5B are coupled by way of shunt via holes and signal leads  121   a  and  121   b  to one lead of each capacitor  115   a ,  115   b  mounted on the top layer  110  of the filter assembly  100 . The other lead of each capacitor  115   a ,  115   b  is coupled to the top ground plane  113 . 
     Thus, the filter assembly  100  acts as a low-pass filter of a signal introduced at input terminals  120   a . In FIG. 7, high frequency components of an input signal are attenuated by the capacitors  115   a ,  115   b  which provide a path to ground  113 . Moreover, the internal signal traces  137 , due to the impedance generated by their inductive qualities, resist the propagation of the RF components of an input signal. In contrast, low frequency and DC components of an input signal propagate with little resistance along the internal signal traces  137 , and thus can pass between the two regions  161 ,  162  lying on either side of the shield bulkhead  104 , as shown in FIG.  4 . 
     FIG. 8 is a cross-sectional view of the filter assembly  100  of FIG.  3 . It will be appreciated that the inductive trace lines are disposed on the middle layer  130  which is sandwiched between the upper ground layer  110  and the lower ground layer  150 . It will further be appreciated that the capacitors  115   a ,  115   b  are disposed on a different layer than that of the trace lines. As indicated by the break lines, the filter assembly  100  may be formed as an integral projection of a printed circuit board assembly fabricated according to the invention. 
     It will be appreciated that the invention provides a low cost, compact filter assembly that offers improved performance over the prior art. 
     FIG. 9 is a perspective view of an RF power amplifier assembly  200 , such as an amplifier in a cellular base station, incorporating a printed circuit board assembly, or electronics module assembly,  172  having a filter  100  according to the invention. The electronics module assembly  172  may comprise, for example, an amplifier module or control circuit module formed with multiple layers. The multi-layered module  172  formed according to the invention includes an integral filter assembly  100  that extends toward another electronics module assembly, such as control circuit  171 , disposed in a separate region separated by the bulkhead, or partition,  104 . The modules  171 ,  172  may incorporate RF amplifier circuitry and control circuitry such as described in U.S. Pat. No. 5,796,304 to Gentzler, the disclosure of which is incorporated herein by reference. Since the filter assembly  100  may be formed as an integral projection of an electronics module  172 , the filter assembly  100  needs to be connected only once on the other side  162  of the bulkhead  104  as opposed to prior art feed-throughs which need to be twice connected to PC boards on both sides of a partition. Thus, the present invention not only saves labor costs but dispenses with the bulky, costly connectors  2  of the prior art, as shown in FIG.  1 A. Unlike the assembly process for prior art feed-throughs which are not automated, the present invention also provides filters which can be manufactured and assembled in an automated process, thus further saving labor costs. The filter assembly  100  according to the present invention also provides greater performance by more effectively filtering out undesired signals through its multi-layered structure. 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. For example, though the optional interstitial ground traces  147  in FIG. 5B are shown separating pairs of internal signal traces  137 , they could alternatively be formed between each internal signal trace  137  in an alternating pattern. Therefore, it must be understood that the illustrated embodiments have been set forth only for the purposes of examples and that they should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification the generic structure, material or acts of which they represent a single species. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to not only include the combination of elements which are literally set forth. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what incorporates the essential idea of the invention.