Abstract:
A power splitter that has a small package size and repeatable electrical characteristics. The power splitter includes a low temperature co-fired ceramic (LTCC) substrate with several layers. Electrical components such as resistors and capacitors are integrated within the LTCC substrate. A transformer is attached to the upper surface of the LTCC substrate and is electrically connected to the resistors and capacitors. The transformer provides impedance matching and dividing functions. The LTCC substrate has 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.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to 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 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]    While power splitters have been used, they have suffered from taking up excessive printed circuit board space and in having difficulty being cascaded. A current unmet need exists for a power splitter that takes up less printed circuit board space and that can be easily assembled.  
         SUMMARY OF THE INVENTION  
         [0006]    It is a feature of the invention to provide a power splitter having a small package size that has repeatable electrical characteristics.  
           [0007]    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 resistors and capacitors are integrated internal within the LTCC substrate. A transformer is attached to the upper layer of the LTCC substrate and is electrically connected to the resistors and capacitors. The transformer 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.  
           [0008]    Another feature of the invention is to provide a power splitter that takes up less printed circuit board space and has improved electrical repeatability.  
           [0009]    A further object of the invention is to provide a method of manufacturing a miniature power splitter.  
           [0010]    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  
       [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 block diagram of a power splitter.  
         [0013]    [0013]FIG. 2 is an exploded perspective view of the preferred embodiment of the present invention.  
         [0014]    [0014]FIG. 3 is an assembled side view of FIG. 2.  
         [0015]    [0015]FIG. 4 is an assembled top view of FIG. 2.  
         [0016]    [0016]FIG. 5 is a graph showing S 1  insertion loss of the power splitter.  
         [0017]    [0017]FIG. 6 is a graph showing S 2  insertion loss of the power splitter.  
         [0018]    [0018]FIG. 7 is a graph showing amplitude unbalance of the power splitter.  
         [0019]    [0019]FIG. 8 is a graph showing phase unbalance of the power splitter.  
         [0020]    [0020]FIG. 9 is a graph showing isolation of the power splitter.  
         [0021]    [0021]FIG. 10 is a graph showing VSWR at port S of the power splitter.  
         [0022]    [0022]FIG. 11 is a graph showing VSWR at port  1  of the power splitter.  
         [0023]    [0023]FIG. 12 is a graph showing VSWR at port  2  of the power splitter.  
         [0024]    [0024]FIG. 13 is a table showing electrical specifications of a power splitter built in accordance with the present invention.  
         [0025]    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 OF THE PREFERRED EMBODIMENT  
       [0026]    Referring to FIG. 1, a block diagram of a power splitter  10  is shown. Power splitter  10  has a RF input port S, an input matching transformer  12 , a divider section  14 , a capacitor  16 , a resistor  18  and a pair of RF output ports  1  and  2 . In a 50 ohm system, the impedance at the input of the divider is close to 25 ohms. The matching transformer converts this to 50 ohms at the RF input to provide a matching impedance. Normally, capacitor  16  is required to match the reactive part of the impedance. The resistor  18  plays a critical role in providing isolation between the two RF output ports  1  and  2 .  
         [0027]    Referring to FIGS. 2, 3 and  4 , power splitter  20  is shown. Power splitter  20  has a transformer  22 . Transformer  22  has a ferrite binocular core  24  with three legs  25 ,  26  and  27 . A winding  30  is wound around leg  27 . Winding  32  is wound around leg  25 . Winding  30  has wires  30 A,  30 B and  30 C. The transformer  30 T is soldered to make continuity with wires  30 B and  30 C Winding  32  has wires  32 A and  32 B. Transformer  22  performs the power splitting and matching functions.  
         [0028]    Transformer  22  is mounted to a low temperature co-fired ceramic (LTCC) structure or substrate  40  using an epoxy  82 . LTCC 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.  
         [0029]    Layer  42  has several circuit features that are patterned on surface  42 A. Surface  42 A has several terminals  55  and a resistor  62 . One of the terminals  55  forms RF input port S. Two of the terminals  55  form RF output ports  1  and  2 . One more terminal  55  forms RF ground. The terminals are electrically connected to vias  60 . The resistor  62  has a protective overglaze  70  to protect the resistor from abrasion and shorting. Layer  44  has an upper capacitor electrode  63  formed on surface  44 A. The upper electrode  63  is connected on two sides to a via  60 . Layer  46  has a ground plane  66  formed on surface  46 A. The ground plane  66  is connected on two sides to a via  60 . Layer  48  has a lower capacitor electrode  64  formed on surface  48 A. The lower electrode  64  is connected on two sides to a via  60 . The upper and lower electrodes and the insulative LTCC layers in between form a capacitor  65 . Layer  50  has a circuit line  68  formed on surface  50 A and conductive pad  69  patterned on the surface  50 B (not shown on FIG. 2) The circuit line  68  is connected at the ends and the middle to vias  60 .  
         [0030]    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 terminals, circuit lines 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 resistive material, fired and trimmed to a desired value. An insulative overglaze is screened over the resistor and fired. Next, the transformer  22  is glued above surface  42 A using an epoxy  82 . Wires  30 A,  30 B,  30 A 1 , and  30 C 1 , and  32 A,  32 B,  32 A 1  and  32 B 1  are welded to terminals  55  using welds  80 .  
         [0031]    The power splitter  22  would be mounted to a printed circuit board. The conductive pads  69  on the bottom of surface  50 B would be attached to the printed circuit board using a reflowed solder paste.  
         [0032]    The present invention has several advantages. Since, the resistor  62  and capacitor  65  are integrated into the LTCC structure, they do not have to be mounted separately on the printed circuit board. This provides a savings of space on the printed circuit board and allows for a faster assembly process at lower cost.  
         [0033]    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. Referring to FIGS. 5 and 6, a graph showing S 1  and S 2  insertion loss for several power splitters is shown for frequencies form 1 to 1000 MHz. Each graph has three curves showing the mean and standard deviation. The middle curve shows the mean average, the top curve shows the mean plus 4.5 sigma and the bottom the mean minus 4.5 sigma. The power splitter  20  has a very small standard deviation (0.02 dB). FIG. 7 shows a graph of amplitude imbalance of the power splitter. Amplitude imbalance is the difference of output power between RF output ports  1  and  2 . The unbalance is typically 0.1 dB with a standard deviation of 0.04 dB. FIG. 8 is a graph showing the phase unbalance of the power splitter. The phase unbalance has a standard deviation of 0.1 degree. FIG. 9 shows a graph of isolation of the power splitter between the RF output ports  1  and  2 . The isolation is about 20 dB up to 1000 MHz. This isolation measurement is very sensitive to parasitic variations due to assembly differences. The standard deviation of the isolation shown in FIG. 9 is about 0.5 dB which is very low when compared to the power splitters of the prior art.  
         [0034]    [0034]FIG. 10 is a graph showing VSWR at the input port S of the power splitter. FIG. 11 is a graph showing VSWR at port  1  of the power splitter. FIG. 12 is a graph showing VSWR at port  2  of the power splitter. The VSWR match is very good with a typical value of 1.15:1. The power splitter  20  is designed to operate over 5-1000 MHz and can be used over the frequency range from 1-1200 MHz. Power splitter  20  can handle 0.5 watt power as a splitter and 0.125 watt power as a combiner. FIG. 13 is a table showing electrical specifications of the power splitter over a frequency range of 5-1000 Mhz and over operating temperature from −40° C. to 85° C.  
         [0035]    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.