Abstract:
An RF pallet ground comprises overhangs disposed at opposite ends. Each overhang includes a downwardly protruding ledge and a notch defined in a bottom surface of the pallet. The ledges and notches extend from one side of the pallet to the other. The ledge provides a positive connection directly to ground. The ledge has a length shorter than the length of the main body of the pallet. Fasteners are positioned on either side of an RF trace on the pallet, allowing positive pressure to be applied to the ground through the ledge.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to RF power amplifiers and mounting systems for such amplifiers. More particularly, the present invention relates to RF power amplifiers mounted on pallet structures. 
     2. Description of Prior Art and Related Information 
     RF power amplifiers require careful design of all RF signal lines to avoid RF energy loss. In particular, RF signal lines require a consistent connection to ground in order to properly propagate RF signals along the signal line. If the signal line is not properly grounded along the entire signal path, RF signals may reflect, radiate or generate heat. In addition, the distance of the path to ground is important. A longer path to ground distance increases the probability of an RF signal radiating or reflecting as opposed to a shorter path. This consideration becomes more significant at both higher power and frequencies, and in particular the frequency ranges common in modern cellular base station applications. 
     Conventionally, RF gain blocks are structurally arranged on a pallet which acts as a substrate and heat sink for the associated RF circuitry. These pallets are mounted in a housing base and electrical connections between the RF and DC lines on the pallet and associated traces on the base must be made. The RF connection point is most critical and is typically referred to as the RF launch. The pallet is designed to keep acceptable tolerances of the over hang of the RF gain blocks and the base to provide a consistent ground across the pallet/base connection. A screw at each RF launch is typically employed to mount the pallet to the base and provide a positive ground connection. Despite these efforts to control the pallet/base connection, existing pallet structures, while providing an RF ground, do not provide a fixed and positive RF ground, which is consistent from amplifier to amplifier and over time and temperatures. 
     One cause of this problem with existing RF pallet structures is that the RF ground is not consistent when moved from a test fixture to a final product. When moving the pallet to a production amplifier assembly, the RF ground changes in distance and location relative to the RF traces and RF launch for various reasons. For example, when the thinned pallet landing is torqued by the machine driving of the screw, the landing may warp. This will change the RF ground location. Also, machine tolerances may be varied in production environments. 
     Another cause of variations in RF ground is due to changes introduced by temperature variations. Such temperature changes will cause relative movement of the pallet and base since the pallet has the heat generating power transistors mounted thereon. Also, the pallet and base may have different thermal coefficients of expansion. Furthermore, over time thermal cycling may cause the screw at the RF launch to loosen. All these factors cause the specific RF ground location relative to the RF traces at the pallet/base connection to change in an unpredictable way. 
     Another important consideration in RF power amplifiers is maintaining correct phase relations between RF signal paths. This requires each path to have a consistent known reactance. A problem with existing RF pallet structures is the inherent existence of unpredictable parasitic reactances. The reactances inherent to the conventional structures are introduced because of variables such as junctions of ground contact locations and changes over temperature. The junctions typically are located in phase dependent (high RF power) locations and are susceptible to phase and amplitude errors, return loss changes and possible coupling issues. As a result, immediate negative effects to tuning are created. 
     Furthermore, some existing RF pallet structures require manual labor in installing pallet to base coupling mechanisms. This leads to increased cost of manufacturing. 
     Thus, the need exists for an RF amplifier assembly having a reliable RF pallet ground which reduces RF signal losses. A need further exists for an RF pallet design where parasitic reactances are substantially eliminated and the immediate negative effects to tuning can be minimized and compensated over temperature while a need further exists for an RF pallet design compatible with low manufacturing costs. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of this invention, an RF amplifier assembly is provided which employs a reliable RF pallet ground which reduces RF signal losses. Furthermore, a fixed and positive RF pallet is provided that minimizes inherent parasitic reactances introduced because of ground contact location and changes over temperature. As a result, susceptibility to phase and amplitude errors, return loss changes and possible coupling issues are decreased and, as a result, the immediate negative effects to tuning can be minimized and compensated over a range of operating temperatures. 
     In one aspect, an RF amplifier assembly is provided. The assembly comprises a base and an RF pallet. The base comprises a recess and a pallet contact portion. The RF pallet is configured in the recess. The pallet has a first end, a second end, and a first overhang disposed at the first end. The overhang comprises a notch and a downwardly protruding ledge. RF amplifier circuitry is configured on the pallet. The pallet contact portion is coupled to the overhang of the pallet. In a preferred embodiment, the RF pallet further comprises a second overhang disposed at the second end. The second overhang is preferably symmetrical to the first overhang. 
     In a preferred embodiment, the pallet contact portion comprises a shelf adapted to support the overhang of the pallet to provide an RF ground connection. The assembly further comprises an RF connector coupled to the pallet and the pallet contact portion. The RF connector comprises first and second end portions. The first end portion of the RF connector is coupled to a top surface of the pallet, and the second end portion is coupled to a top surface of the pallet contact portion. The RF connector preferably comprises a jumper connector, which may be an omega shaped connector. The pallet further comprises an RF trace extending from the connector and a fastener disposed adjacent to the RF trace to maintain positive contact of the ledge with the pallet contact portion. The fastener may, for example, comprise a screw. 
     In a preferred embodiment, the assembly further comprises a housing, wherein the housing comprises the base. The housing may be composed of a same material as the pallet; for example, an aluminum alloy. Alternatively, the pallet may be composed of a material with a higher thermal conductivity, such as copper. 
     In another aspect, a pallet is provided for use in an RF amplifier assembly. The pallet comprises a first end, a second end, a first overhang disposed at the first end, a top major surface, a bottom major surface, and RF amplifier circuitry configured on the top major surface. The first overhang comprises a notch and a downwardly protruding ledge. The ledge comprises a first downward length that is smaller than a second downward length between the top major surface and the bottom major surface. The pallet further comprises a notch surface. The ledge has a bottom ledge surface disposed above the bottom major surface and below the notch surface. The pallet further comprises first and second side edges. The notch and ledge extend from the first side edge to the second side edge. 
     A method is also provided for making an RF amplifier assembly having a reliable connection of RF ground. The method comprises providing a pallet having a downwardly protruding ledge at one end, providing an RF amplifier module on the pallet, providing a pallet contact portion having a shelf adapted to support the ledge of the pallet, coupling the pallet to the pallet contact portion, and coupling an RF connector to the pallet and the pallet contact portion. The coupling the RF connector to the pallet and the pallet contact portion may comprise fastening one end on top of the pallet and another end on top of the pallet contact portion, thus providing full contact with RF traces and positive contact between the ledge and the pallet contact portion. 
     The invention, now having been briefly summarized, may be better appreciated by the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a portion of an RF amplifier assemply having two RF pallets according to a preferred embodiment of the present invention; 
     FIG. 2 is an exploded, perspective view of an RF amplifier assembly according to the invention; 
     FIG. 3 is a bottom perspective view of a housing of the RF amplifier assembly; 
     FIG. 4 is a bottom perspective view of an RF pallet according to the present invention; 
     FIG. 5 is a side view of an RF pallet shown on a heat sink according to an embodiment of the present invention; and 
     FIG. 6 is a cross-sectional side view of the coupling of the RF pallet to the housing base according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the detailed description that follows, it should be appreciated that like element numerals are used to describe like elements illustrated in one or more of the figures. 
     FIG. 1 is a top view of a portion of an RF amplifier assembly and FIG. 2 is an exploded perspective view of the RF power amplifier assembly  40  according to the invention. FIG. 3 is a bottom view of the assembly housing. 
     The illustrated RF amplifier employs two RF gain modules in the form of RF amplifier circuitry, associated components, and traces on two RF pallets, or RF pallet modules,  10  configured in a recess  14  of a base  12 . Although two RF gain modules on two RF pallets are illustrated, it should be appreciated that a single RF gain module and pallet, or more than two, may equally be employed. Conductive traces  22  and electronic components comprising the circuitry of the RF gain module are formed on a top surface  111  of each pallet  10 . Although the illustrated components are RF amplifier IC&#39;s  24  and RF signal splitters  26 , additional components may be provided as known to those skilled in the art. RF launches  28  are disposed adjacent to ends  118  of each pallet  10 . Connectors  142  connect the RF launches  28 . Screws  30  or other suitable mounting elements couple the pallets  10  to the base  12 . Preferably, at least one screw  30  is configured adjacent each RF launch  28 . 
     The assembly  40  preferably includes a housing  42  comprising a plurality of compartments defined by partitions  44 . The portion of the assembly shown in FIG. 1 corresponds to an RF compartment  46 . The recess  14  is defined by a surface  48  adapted to support bottom surfaces of the RF pallets  10 . The compartment  46  further comprises shelves  136  at each end  50  adapted to contact the ledges of the pallets  10  as described in more detail below. The screws are received by cavities formed in the shelves  136  of the base  12 . Although one RF compartment  46  sized to receive two pallets  10  disposed side by side is shown in FIG. 2, additional RF components may be provided. A RF barrier wall  52  is formed in the compartment surface  48  between the areas where the pallets  10  lay. The mounting of wall  52  may employ the teachings of U.S. patent application Ser. No. 09/435,953 filed on Nov. 9, 1999, the disclosure of which is incorporated herein by reference. Alternatively, the RF pallets  10  may be formed in separate recesses in separate RF shielding compartments. 
     The additional compartments may house various RF amplifier control circuitry and such compartments are isolated from the RF compartment by walls  44 . The electronics of the RF amplifier and control circuitry may include the disclosure provided by U.S. Pat. No. 5,796,304 to Gentzler, which is incorporated by reference as though fully set forth herein. 
     FIG. 3 is a bottom perspective view of the housing  42 . Preferably, the housing  42  includes a heat sink  54  for dissipating heat from the pallets. 
     FIG. 4 is a bottom perspective view of a single pallet  10 . The pallet  10  has a top major surface  111 , a bottom major surface  112 , pallet side edges  114 , and end edges  118 . The pallet  10  comprises an overhang  120  at each end edge  118 . Since the edges  118  are symmetrical, only one end edge  118  will be described in detail for brevity purposes. A notch  122  is defined along the width of each end edge  118  such that an outer ledge  124  is formed. The notch  122  includes a notch surface  123 . The notch  122  is disposed interiorly, or medially, with respect to the ledge  124 . 
     Disposed adjacent the end edges  118 , the ledge  124  has a bottom ledge surface  125 . The ledge  124  protrudes, or extends, downwardly such that the bottom ledge surface  125  is disposed on a plane between that of the bottom major surface  112  and the notch surface  123 . Alternatively stated, when the pallet  10  is operatively disposed in the recess  14 , the ledge  124  extends downwardly such that its bottom surface  125  is disposed beneath the notch surface  123  and above the bottom major surface  112 . Thus, the ledge  124  has a downward length “L 1 ” that is shorter than the downward length “L 2 ” of the pallet side edge  114 . Alternatively stated, the ledge  124  has a downward length “L 1 ” smaller than the downward length “L 2 ” defined between the top major surface  111  and the bottom major surface  112 . The notch  122  and ledge  124  extend prismatically from one side edge  114  to the other. The pallet  10  is preferably composed of a metal or metals with sufficient thermal transfer and electrical grounding properties such as copper or an aluminum alloy. Other metal alloys may also be employed. In a preferred embodiment, the pallet  10  is composed of the same material as the housing  42 , shown in FIGS. 2 and 3, such as an aluminum alloy. 
     FIG. 5 is a side view of a portion of an RF power amplifier assembly  40  showing the pallet  10  in an operative configuration on the base. Pallet contact portions  132 , 134  are disposed at both sides of the pallet  10 . In a preferred embodiment, the pallet contact portions  132 ,  134  are formed as an integral part of the housing  42 . Thus, the pallet contact portions  132 ,  134  along with the rest of the housing  42 , may be composed of a metal or metals with sufficient thermal transfer and electrical grounding properties. In a preferred embodiment, the housing  42  is composed of an aluminum alloy. The pallet contact portions  132 , 134  comprise extended portions, or shelves,  136  which are fit to support the ledges  124  of the pallet  10 . The bottom ledge surfaces  125  of the pallet  10  rest on top of the shelves  136  of the pallet contact portions  134 . Thus, the pallet  10  makes a repeatable, reliable connection to RF ground as it sits on the pallet contact portion shelves  136 . This connection is enhanced by fasteners  30 , such as screws, described below in relation to FIG.  6 . 
     FIG. 6 is a detailed view of the coupling between the pallet  10  and one pallet contact portion  132 . An RF connector  142 , which may be an omega shaped jumper connector, provides an electrical connection between the pallet contact portion  132  and the pallet  10 . A similar RF connector may be provided between the opposite pallet contact portion  134  and the pallet  10 . A first end  144  of the RF connector  142  is coupled to a first trace on the surface of the pallet contact portion  132  while the other end  146  of the RF connector  142  is coupled to a second trace on the top surface of the pallet  10 . The RF connector  142  thus provides electrical contact of the RF traces adjacent the RF launches  28 . Fasteners  30 , such as screws, are located at the RF trace to maintain positive contact of the ledges  124  of the pallet  10  with the pallet contact portions  132 ,  134  and to minimize the distance to ground from the traces. The fasteners  30  also securely connect the pallet  10  to the housing  42 , and the contact position is relatively insensitive to temperature changes. It will be appreciated that the RF connector  142  allows an RF signal to propagate therethrough while the fasteners  30  and the ledge  124  provide an adjacent ground connection that minimizes parasitic reactances. 
     Thus, it will be appreciated that the overhangs  120  provide a fixed and positive RF ground that minimizes inherent parasitic reactances introduced as a result of ground contact location and fluctuations in temperature. A more reliable ground connection is provided, in part, by the ledges  124 , each of which also serves as a fulcrum should the pallet  10  torque. As obvious from FIG. 6, should the pallet  10  be torqued or even rotate slightly with respect to the pallet contact portions  132 ,  134  the ledge  124  provides constant contact between pallet  10  and the pallet contact portion  132 . Furthermore, in FIG. 6, the point of ground  150  is defined by contact point between the outer edge  128  of the ledge  124  and the shelf  136 . As a result, a shorter path to ground is provided, thereby making the ground connection even more reliable. 
     The pallet according to the invention decreases susceptibility to phase and amplitude errors, return loss changes and possible coupling issues are decreased. This thus minimizes the potential negative effects to tuning prior to assembly. Also, negative effects over temperature ranges are reduced. Further, the fixed and positive RF ground allows the ground to be consistent while being moved from a fixture to a final product. The ground does not change in distance and location when both the thinned landing is torqued and machine tolerances are varied. Another advantage of the structure is that it costs less because it is adapted for automated assembly reducing costs of manual labor involved in prior art alternatives. 
     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. 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.