Abstract:
A power splitter that has a small package size and repeatable electrical characteristics. The power splitter includes a low temperature co-fired ceramic (LTTC) substrate with several layers. Electrical components such as resistors, capacitors and ground planes are integrated within the LTTC substrate. A binocular core transformer is attached to the upper surface of the LTCC substrate and is electrically connected to the resistors, capacitors and ground plane. The transformer provides impedance matching and dividing functions. The LTCC substrate has electrically conductive vias extending through the layers. The vias are used to connect the power splitter to terminals on outer surfaces of the substrate and to make electrical connections between the layers of the LTCC substrate. The power splitter circuit requires only one transformer.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to power splitters in general and more particularly to a two way 180 degree power splitter having a small package size.  
           [0003]    2. Description of the Prior Art  
           [0004]    Power splitters operating at frequencies below 1 GHz have been made with ferrite transformers along with appropriate resistors and capacitors arranged around the ferrite transformer. These splitters provide multi-decade bandwidth. The power splitter components are packaged on a printed circuit board. In some applications, printed circuit board space can be extremely limited with additional space just not available. In some applications, it is desirable to build multiple output-port splitters such as 4-way and 8-way by cascading the splitters. Unfortunately, placing resistors and capacitors beside each transformer complicates the assembly program followed by the automated pick and place surface mount assembly equipment. This leads to lower production by the assembly machinery.  
           [0005]    [0005]FIG. 1 shows a schematic of a prior art 2 way 180 degree power splitter circuit for frequencies of 1-750 MHz. Circuit  20  has transformers T 1 , T 2 , T 3  and T 4 . Transformer T 1  is connected between ground and an input terminal IN. Transformer T 2  has one end connected to winding W 1  of transformer T 3  and the other end connected to winding W 3  of transformer T 4 . The mid-points of transformers T 1  and T 2  are connected to ground through a capacitor C 1 . A resistor R 1  is connected across transformer T 2 . Transformer T 3  has a windings W 1  and W 2 . Winding W 1  is connected between transformer T 2  and output terminal OUT 2 . Winding W 2  has both ends connected to ground. Transformer T 4  has windings W 3  and W 4 . Winding W 3  is connected between transformer T 2  and ground. Winding W 4  has one end connected to ground and the other end connected to output terminal OUT 1 . The circuit of FIG. 1 is a 2 way power splitter that provides an output signal that has 0 degrees of phase shift at terminal OUT 2  and 180 degrees of phase shift at terminal OUT 1 . The circuit of FIG. 1 requires four transformers to perform the power splitting function.  
           [0006]    While power splitters have been used, they have suffered from taking up excessive printed circuit board space and in having multiple transformers that add expense and complexity to manufacturing. A current unmet need exists for a power splitter that takes up less printed circuit board space with fewer components that can be easily assembled.  
         SUMMARY  
         [0007]    It is a feature of the invention to provide a power splitter having a small package size that has repeatable electrical characteristics.  
           [0008]    Another feature of the invention is to provide a power splitter that includes a substrate having several layers. The substrate has a first and a second outer surface. A resistor is formed on the first outer surface. A ground plane is formed on one of the layers. One or more capacitor electrodes are located on one of the layers. The capacitor electrodes form a capacitor. Several terminals are located on the first and second outer surfaces. A transformer is attached to the first outer surface and is electrically connected to the terminals, the resistor, the capacitor and the ground plane. The transformer provides impedance matching and dividing. Several vias extend between the layers to provide an electrical connection between the resistor, the capacitor, the terminals, the ground plane and the transformer.  
           [0009]    Another feature of the invention is to provide a power splitter that takes up less printed circuit board space and has improved electrical repeatability.  
           [0010]    A further object of the invention is to provide a method of manufacturing a 180 degree power splitter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    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:  
         [0012]    [0012]FIG. 1 is a schematic of a prior art power splitter.  
         [0013]    [0013]FIG. 2 a schematic of the power splitter of the present invention.  
         [0014]    [0014]FIG. 3 is an exploded perspective view of the present invention.  
         [0015]    [0015]FIG. 4 is an assembled top view of FIG. 3.  
         [0016]    [0016]FIG. 5 is an assembled side view of FIG. 3.  
         [0017]    [0017]FIG. 6 is a graph showing insertion loss of the power splitter of FIG. 3.  
         [0018]    [0018]FIG. 7 is a graph showing phase unbalance of the power splitter of FIG. 3.  
         [0019]    [0019]FIG. 8 is a graph showing isolation of the power splitter of FIG. 3.  
         [0020]    [0020]FIG. 9 is a graph showing VSWR of the power splitter of FIG. 3. 
     
    
       [0021]    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  
       [0022]    Referring to FIG. 2, a schematic of a 2-way 180 degree power splitter  21  in accordance with the present invention is shown. Power splitter  21  has a transformer T 5 . Transformer T 5  has windings W 5  and W 6  wound on the center leg  26  of a binocular ferrite core. One end of winding W 5  is connected to terminals P 4 , P 4 A and the other end is connected to input terminals P 6 , P 6 A. One end of winding W 6  is connected to terminals P 3 , P 3 A and to ground through a capacitor C 3 . The other end is connected to output terminals P 1 , P 1 A. The mid-point of winding W 6  at node N 1  is connected to a parallel combination of resistor R 2  and capacitor C 2 . Node N 1  is also connected to terminal P 5 . A node N 2  joins the other end of resistor R 2  and capacitor C 2 . Node N 2  is connected to terminals P 4 , P 4 A, P 2 , P 2 A and to ground.  
         [0023]    Power splitter  21  provides an output signal that has 0 degrees of phase shift at terminal P 3 , P 3 A and 180 degrees of phase shift at terminal P 1 , P 1 A. Transformer T 5  is a 1:2 transformer and matches the impedance at the input of the splitter to two outputs. In a 50 ohm system, resistor R 2  has a value of 25 ohms. Resistor R 2  provides isolation and impedance matching between output terminals P 1 , P 1 A and P 3 , P 3 A. The coupling of primary winding W 5  to secondary winding W 6  in transformer T 5  is important for isolation and return loss. Capacitor C 2  improves isolation and return losses at high frequencies. Capacitor C 3  improves phase unbalance at high frequencies. Power splitter  21  is a 2-way power splitter since the input signal is split into two output signals. Power splitter  21  uses fewer transformers than the prior art circuits.  
         [0024]    Referring to FIGS. 3, 4 and  5 , the physical implementation of power splitter  21  is shown. Power splitter  21  has a transformer T 5 . Transformer T 5  has a ferrite binocular core  24  with three legs  25 ,  26  and  27  and a pair of holes  28 . Windings W 5  and W 6  are wound around center leg  26 . Winding W 5  has wires  30 A and  30 B. Winding W 6  has wires  32 A,  32 B and  32 C. Transformer T 5  performs the power splitting and matching functions.  
         [0025]    Transformer T 5  is mounted to a low temperature co-fired ceramic (LTCC) structure or substrate  40  using an epoxy  82 . LTTC substrate  40  is comprised of multiple layers of LTCC material. Planar layers  42 ,  44 ,  46 ,  48  and  50  are all stacked on top of each other and form a unitary structure  40  after firing in an oven. LTCC layers  42 - 50  are commercially available in the form of a green unfired tape from Dupont Corporation. Each of the layers has a top surface,  42 A,  44 A,  46 A,  48 A and  50 A. Similarly, each of the layers has a bottom surface,  42 B,  44 B,  46 B,  48 B and  50 B. The layers have several circuit features that are patterned on the top surfaces. Multiple vias  60  extend through each of the layers. Vias  60  are formed from an electrically conductive material and electrically connect one layer to another layer.  
         [0026]    Layer  42  has several circuit features that are patterned on surface  42 A. Surface  42 A has terminals P 1 , P 2 , P 3 , P 4 , P 5  and P 6  and a resistor R 2 . Terminals P 4 , P 4 A, P 2  and P 2 A are connected to ground. The terminals are electrically connected to vias  60 . The resistor R 2  has a protective overglaze  70  to protect the resistor from abrasion and shorting. Layer  44  has a ground plane  63  formed on surface  44 A. Ground plane  63  is connected on two sides to a via  60 . Layer  48  has a ground plane  66  formed on surface  48 A. The ground plane  66  is connected on two sides to a via  60 . Layer  46  has a pair of capacitor electrodes  64  and  65  formed on surface  46 A. The capacitor electrodes  64  and  65  are each connected to separate vias. The capacitor electrodes  64  and  65 , ground planes  63  and  66  and the insulative LTCC layers  44  and  46  in between form capacitors C 2  and C 3 . Layer  50  has terminals P 1 A, P 2 A, P 3 A, P 4 A, and P 6 A located on bottom surface  50 B.  
         [0027]    The circuit features such as resistors, capacitor electrodes, vias and ground planes are formed by screening a thick film paste material and firing in an oven as is 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 terminals, ground planes and capacitor electrodes are formed with a conductive material. The layers are then aligned and stacked on top of each other to form LTCC substrate  40 . The LTCC structure  40  is then fired in an oven at approximately 900 degrees centigrade to form a unitary piece. The resistor is formed with a resistor material, fired, and trimmed to a desired value. An insulative overglaze is screened over the resistor and fired. Next, the transformer T 5  is glued above overglaze  70  using an epoxy  82 . Wires  30 A,  30 B,  32 A,  32 B and  32 C are welded to their respective terminals using welds  80 .  
         [0028]    The power splitter  21  would be mounted to a printed circuit board (not shown). Terminals P 1 A, P 2 A, P 3 A, P 4 A, and P 6 A on the bottom of surface  50 B would be attached to the printed circuit board using a reflowed solder paste. The paste could be screened onto the terminals and then the splitter is set on the printed circuit board and reflowed in an oven.  
         [0029]    The present invention has several advantages. Since, the resistor R 2  and capacitors C 2  and C 3  are integrated into the LTCC structure, they do not have to be mounted separately on the printed circuit board. Further, the mounting of transformer T 5  above the resistor and capacitor provides a compact, small package, that saves space on the printed circuit board and allows for a faster assembly process at lower cost.  
         [0030]    Repeatability of electrical performance is a prime concern for electrical design engineers. Fabricating the power splitter using an LTCC process with the circuit of FIG. 2 results in a more uniform electrical performance in the resulting power splitter. Referring to FIG. 6, a graph showing S 1  and S 2  insertion loss for power splitter  21  is shown for frequencies from 0 to 750 MHz.  
         [0031]    [0031]FIG. 7 is a graph showing the phase unbalance of the power splitter. FIG. 8 shows a graph of isolation of the power splitter between the output terminals P 3  and P 1  for 0 to 750 MHz. FIG. 9 is a graph showing VSWR at the input terminal P 6  and output terminals P 1  and P 3  of the power splitter. The VSWR match is very good with a typical value of 1.25:1. The power splitter  21  is designed to operate over the frequency range of 1-750 MHz.  
         [0032]    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.