Abstract:
A power splitter that has a small package size and low cross-talk noise. The power splitter includes a low temperature co-fired ceramic (LTTC) substrate with several layers. Electrical components such as transmission lines and resistors are integrated onto and within the LTCC substrate. The power splitter provides impedance matching and dividing functions. The LTCC substrate has counter rotating spiral shaped circuit lines and electrically conductive vias extending therethrough. The vias are used to connect the power splitter to an external printed circuit board. The vias are also used to make electrical connections between the layers of the LTCC substrate. The counter rotating circuit lines allow the power splitter to have a small package size and low cross-talk noise.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to microwave power splitters in general and more particularly to a power splitter having a small package size.  
           [0003]    2. Description of the Prior Art  
           [0004]    Power splitters have been made by forming transmission lines on microstrip structures using printed circuit boards. Power splitters have also been fabricated on ceramic substrates using screened on thick film conductors and dielectrics. In some applications, printed circuit board space is extremely limited with additional space just not available. It is desirable that the splitter be as small as possible while still having the proper impedance and not having excessive cross-talk noise. Printed circuit boards have a problem in power splitter applications in that the desired transmission line impedance can be hard to achieve in a small package size due to the low dielectric constant of the printed circuit board material. Ceramic materials have a higher dielectric constant and can achieve the same impedance transmission lines in a smaller size. Unfortunately, using a thick film process to fabricate a multilayered structure is difficult to manufacture on a repeatable and cost effective basis. Further, if the circuit lines are placed to close to each other in the ceramic package, excessive cross-talk noise can result.  
           [0005]    While power splitters have been used, they have suffered from taking up excessive space, being difficult to manufacture and having excessive cross-talk noise. A current unmet need exists for a power splitter that is smaller, has low cross-talk noise and that can be easily fabricated.  
         SUMMARY  
         [0006]    It is a feature of the invention to provide a power splitter having a small package size that has repeatable electrical characteristics and low cross-talk noise.  
           [0007]    Another feature of the invention is to provide a power splitter that includes a substrate having several layers. A resistor is formed on an outer layer. A first transmission line is formed by a first spiral shaped circuit line formed on an inner layer. A second transmission line is formed by a second spiral shaped circuit line formed on the inner layer. A ground plane is formed on another inner layer. Several vias extend between the layers and provide an electrical connection between the resistor, the ground plane and the circuit lines.  
           [0008]    Another feature of the invention is to provide a power splitter that includes a low temperature co-fired ceramic (LTCC) substrate. The LTCC substrate has several layers. Electrical components such as transmission lines and resistors are integrated internal within the LTTC substrate. A pair of counter rotating circuit lines in a spiral are formed on a layer. The circuit lines are joined to input and output pads on layers above and below by vias. A resistor is connected between the output pads. The power splitter provides impedance matching and dividing functions. The LTCC substrate has electrically conductive vias extending therethrough. The vias are used to make electrical connections between layers of the LTCC substrate.  
           [0009]    Another feature of the invention is to provide a power splitter that takes up less space and has improved electrical repeatability.  
           [0010]    A further feature of the invention is to provide a method of manufacturing a miniature power splitter.  
           [0011]    Another feature of the invention is to provide a power splitter with low cross-talk noise.  
           [0012]    The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    In order that the invention may be more fully understood, it will now be described, by way of example, with reference to the accompanying drawings in which:  
         [0014]    [0014]FIG. 1 is a schematic diagram of a microstrip power splitter.  
         [0015]    [0015]FIG. 2 is a perspective view of a prior art microstrip power splitter.  
         [0016]    [0016]FIG. 3 is a perspective view of the preferred embodiment of the power splitter having counter rotating circuit lines in accordance with the present invention.  
         [0017]    [0017]FIG. 4 shows the power splitter of FIG. 3 with the addition of the vias and input and output pads.  
         [0018]    [0018]FIG. 5 is a cross-sectional view of FIG. 3. 
     
    
       [0019]    It is noted that the drawings of the invention are not to scale. In the drawings, like numbering represents like elements between the drawings.  
       DETAILED DESCRIPTION  
       [0020]    Referring to FIG. 1, a schematic diagram of a microstrip power splitter or divider  20  is shown. Power splitter  20  has an input port  22  that splits to connect with a parallel pair of transmission lines  24  and  26 . Transmission line  24  is connected to output port  28  and transmission line  26  is connected to output port  30 . An isolation resistor  32  is connected between output ports  28  and  30 . For a microstrip power splitter designed to operate around 2 GHz, the transmission lines would have impedances of 70.7 ohms and the resistor  32  would have a value of 100 ohms. The transmission lines are fabricated to be 90 degrees in length to a signal traveling on the line.  
         [0021]    Referring to FIG. 2, a prior art implementation of the schematic power splitter  20  is shown as microstrip power splitter  40 . Power splitter  40  has a ceramic or fiberglass substrate  42  with an input port  42  that splits to connect with a parallel pair of transmission lines  44  and  46 . Transmission line  44  is connected to output port  48  and transmission line  46  is connected to output port  50 . An isolation resistor  52  is connected between output ports  48  and  50 . Transmission lines  44  and  46  are formed by screening and firing a conductive paste onto a ceramic substrate or by etched copper circuit lines on a printed circuit board. The impedance of the circuit lines is a function of the line width, line height, thickness of the substrate and dielectric constant of the substrate. For a microstrip power splitter designed to operate around 2 GHz, the transmission lines would be 10 mils wide by 474 mils long. Substrate  42  would be approximately 0.5 inches long by 0.2 inches wide for an area of 0.1 square inches.  
         [0022]    Referring now to FIGS. 3, 4 and  5 , the preferred embodiment of the power splitter having counter rotating circuit lines in accordance with the present invention is shown. Power splitter  100  has a low temperature co-fired ceramic (LTCC) structure or substrate  102 . LTTC substrate  102  is comprised of multiple layers of LTCC material. There are seven LTCC layers in total. Planar layers  111 ,  112 ,  113 ,  114 ,  115 ,  116  and  117  are all stacked on top of each other and form a unitary structure  102  after firing in an oven. LTCC layers  111 - 117  are commercially available in the form of a green unfired tape from Dupont Corporation. Each of the layers has a top surface,  111 A,  112 A,  113 A,  114 A,  115 A and  116 A. Similarly, each of the layers has a bottom surface,  111 B,  112 B,  113 B,  114 B,  115 B and  116 B. The layers have several circuit features that are patterned on the top surfaces. Multiple vias  150  extend through each of the layers. Vias  150  are formed from an electrically conductive material and electrically connect one layer to another layer. A via pad  155  extends around each via  150  on the top and bottom surfaces and allows the vias to electrically connect with each other.  
         [0023]    Layer  111  has several circuit features that are patterned on surface  111 A. Surface  111 A has output pads  144 , ground pads  146 , resistors  132 , resistor pads  136  and probe pad  134 . Output pads  144  form output ports  28  and  30 . Placing the resistors  132  on the outer surface allows for laser trimming and for lower capacitance to ground. Forming the resistor as two resistors  132  allows the resistors to be measured in parallel. A lead frame  160  is shown soldered to pad  146  using solder  162 . Several lead frames would be soldered to the pads in order to connect the power splitter to other electrical components. Splitter  100  is usually mounted to a printed circuit board. Layer  112  has a pair of circuit lines  128  and  130  that are patterned on surface  111 A. Vias  150  connect the circuit lines  128  and  130  to output pads  144  on layer  111 . Layer  113  has no patterning. Vias  150  only pass through layer  113 . Layer  114  has a pair of spiral shaped counter rotating circuit lines  124  and  126  that are patterned on surface  114 A. Layer  114  also has a T-junction  122  where the circuit lines  124  and  126  join. The spiral circuit lines  124  and  126  terminate in the middle of the spiral and connect to a vias  150  which connects with the circuit lines  128  and  130  on layer  112 . It is noted that circuit line  124  spirals clockwise going toward the center. Circuit line  126  spirals counter-clockwise going toward the center. T-junction  122  is connected to input pad  140  by vias  150 . Circuit line  124  forms transmission line  24  and circuit line  126  forms transmission lines  26 . Input pad  140  forms input port  22 . Input pad  140  preferably has a lower impedance to provide a better impedance match. Spiraling the circuit lines  124  and  126  raises the impedance of the lines allowing the circuit lines to be closer to the ground plane for a given line width and impedance value.  
         [0024]    Layers  115  and  116  have no patterning. Vias  150  only pass through these layers. Layer  117  has a mesh ground plane  180  that is patterned on surface  117 A. input pad  140 , output pad  144  and ground pads  146  are patterned on surface  117 B. Vias  150  connect the mesh ground plane  180  to ground pads  146  through layer  117 . The mesh ground plane  180  helps to prevent warping of the LTCC structure during fabrication and also acts as an impedance reference plane and reduces cross-talk noise.  
         [0025]    The circuit features are formed by screening a thick film paste material and firing in an oven. This process is well known in the art. First, the LTCC layers have via holes punched, the vias are then filled with a conductive material. Next, the circuit features are screened onto the layers. The resistors are formed with a resistor material. The pads and circuit lines are formed with a conductive material. An insulative overglaze (not shown) can be screened over the resistor. The layers are then aligned and stacked on top of each other to form LTCC substrate  102 . The LTCC structure  102  is then fired in an oven at approximately 900 degrees centigrade to form a unitary piece. The resistors  132  can then be laser trimmed to adjust their resistance value using pads  134  and  136  to probe the resistor during laser trimming. The power splitter  100  would be mounted to a printed circuit board by soldering lead frames  160 .  
         [0026]    The present invention has several advantages. Since, the circuit lines  124  and  126  are coiled, they take up less space, resulting in a smaller package. A power splitter  100  operating at 2 GHz would have a package size of 0.2 inches by 0.2 inches. This is 0.04 square inches which is 60 percent less area than the prior art design. This provides a savings of space on the printed circuit board and allows for a faster assembly process at lower cost. The frequency of operation of the power splitter can be adjusted by scaling the size of the coiled lines  124  and  126 . The line width and spacing is held constant, while the line length is varied.  
         [0027]    Repeatability of electrical performance is a prime concern for electrical design engineers. Fabricating the power splitter using an LTCC process results in a more uniform electrical performance in the resulting power splitter. The LTCC layers have tightly controlled tolerances that provide well defined RF characteristics. The mesh ground plane provides for lower noise.  
         [0028]    While the invention was shown using seven LTCC layers, it is possible to use more or fewer LTCC layers. Also, several power splitters could be combined into one package.  
         [0029]    While the invention was shown applied to a power splitter, it is contemplated to use the same packaging methodology to fabricate other devices such as filters and microwave components.  
         [0030]    While the invention has been taught with specific reference to these embodiments, someone skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.