Patent Publication Number: US-6903625-B2

Title: Microstrip RF signal combiner

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention in general relates to microwave power circuits, and more particularly to a device for combining the outputs of a plurality of high power amplifiers and combining them into a unitary signal. 
     2. Description of Related Art 
     In various RF circuits it is often necessary to amplify certain RF signals. However, a single amplifier may not be suitable due to space/weight constraints on a circuit board or due to power limitations of the amplifier. Accordingly, it is common to divide the signal into a plurality of identical signals and provide them to a like plurality of amplifiers. The outputs of these amplifiers are then combined to achieve the desired power rating which is greater than a single amplifier. 
     An RF combiner is a passive RF device used to add together, in equal proportions, two or more of these RF signals. One common type of combiner is the Wilkinson combiner which is easily added to a circuit board but however, takes up too much space on the board. Another type of combiner is the serial stripline coupler. This combiner is smaller than the Wilkinson combiner and utilizes a plurality of layers of dielectric with interposed striplines and ground planes. This structure results in a combiner which is complicated to manufacture and which exhibits undesired losses. The present invention obviates the drawbacks of current combiner circuits. 
     SUMMARY OF THE INVENTION 
     A microwave signal combiner in accordance with the present invention includes a dielectric board having first and second surfaces with a microstrip main line disposed on the first surface. A plurality of cascaded microstrip coupled lines is disposed on the second surface, each for receiving a microwave signal for coupling to the microstrip main line. A ground plane is positioned on the second surface straddling the microstrip coupled lines, while the first surface is devoid of any ground plane. The microstrip main line is substantially coextensive with the microstrip coupled lines. 
     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while disclosing the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings, which are not necessarily to scale, and are given by way of illustration only, and wherein: 
         FIG. 1  represents a circuit in which the present invention may be used. 
         FIG. 2  is a stripline structure serving to illustrate the principle of the present invention. 
         FIG. 3  is a view of a preferred embodiment of the present invention. 
         FIG. 4  illustrates the combiner in relation to an amplifier board. 
         FIGS. 5A and 5B  respectively show the stripline pattern on first and second sides of the combiner dielectric board. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a simplified representation of a transmitter  10  which includes transmitter circuitry  12  such as RF signal generation, modulation and control circuitry. Output signals to be transmitted are provided through antenna switch  14  to an antenna  16 , via a high power amplification stage  18 . 
     The high power amplification stage may be constituted by a single high power amplifier, however a single amplifier with the desired power rating may be either too massive for mounting on a circuit board, or may not be available with that power rating. Accordingly, and as illustrated in  FIG. 1 , a plurality of high power amplifiers, each of a lesser rating, but totaling up to the desired rating, may be used. Thus in  FIG. 1 , the high power amplifier stage  18  includes a plurality of high power amplifiers, for example four,  19 ,  20 ,  21  and  22 . 
     Prior to amplification, the signal from transmitter circuitry  12  is dived into four equal paths by 1:4 RF signal splitter  24  for presentation to respective high power amplifiers  19  to  22 . After amplification, the four outputs from the high power amplifiers are combined for delivery to antenna  16 , in 4:1 signal combiner  26 . The present invention will be described with respect to this signal combiner, although its structure may be used as a signal splitter, with a reversal of the illustrated input and outputs. 
     The 4:1 signal combiner  26  includes a microstrip main line  28  having one end connected to an input  29 , for receiving the output of high power amplifier  19 , and a second end connected to output  30 . The 4:1 combiner  26  additionally includes three microstrip coupled lines  34 ,  35  and  36  having respective inputs  37 , 38  and  39 , for receiving the respective outputs of high power amplifiers  20 ,  21  and  22 . The other ends of these coupled lines  34 ,  35  and  36  are connected to respective grounded resistors  40 ,  41  and  42 , by means of connections  44 ,  45  and  46 , for isolation purposes. 
       FIG. 2  shows a portion of a combiner to illustrate salient features of its construction. The combiner includes a dielectric board  50  having opposed first and second surfaces  51  and  52 . Deposited on the first surface  51  is a microstrip main line  54  and on the second surface, a microstrip coupled line  56 , one of a plurality of such lines. The microstrip coupled line  56  is coplanar with a ground plane  58 , while the surface  51 , containing the microstrip main line  54  is devoid of any ground plane. Although the first surface  51  is devoid of a ground plane, the microstrip main line  54  utilizes the ground plane on the second surface  52  during microwave transmissions. 
     A combiner in accordance with the present invention is illustrated in FIG.  3 . Combiner  26 , corresponding to that described with respect to  FIG. 1 , includes a dielectric board  62  having first and second opposed surfaces  63  and  64 . The dielectric board  62  is of a low dielectric loss material such as a combination of ceramic/polytetrafluoroethylene. 
     The combiner  26  has three cascades stages  66 ,  67  and  68  each including a respective one of the microstrip coupled lines  34 ,  35  and  36 , each having an input at one end and a connection at its other end with reference numerals corresponding to that illustrated in FIG.  1 . A ground plane  70  is deposited on the second surface  64  of the dielectric board  62  and is disposed above the microstrip lines  34 ,  35  and  36  and is interposed between stages  66  and  67 , and  67  and  68 . The ground plane  70  additionally includes ground plane segments  71 ,  72  and  73  disposed below the microstrip coupled lines. Thus the ground plane  70  straddles the plurality of microstrip coupled lines. Grounding connections  74  are electrically connected to the ground plane  70  for grounding purposes, as will be seen. 
     In one embodiment of the invention, in order to save space, the combiner may be mounted at right angles to an amplifier board which contains the high power amplifiers  19 ,  20 ,  21  and  22 , and for this purpose combiner  26  includes L-shaped mounting brackets  76  fastened to respective first and second ends  77  and  78  of the dielectric board  62 . 
       FIG. 4  illustrates the mounting arrangement. High power amplifier board  80  includes 1:4 signal splitter  24 , high power amplifiers  19 ,  20 ,  21  and  22  as well as isolation resistors  40 ,  41  and  42 , all symbolically illustrated. Board  80  has a ground layer  82  on the undersurface and a plurality of conducting vias  84  for making grounding connections with elements of the combiner  26 . 
     More particularly, after combiner  26  is secured to board  80 , by means of fasteners  86 , grounding connections  74  are joined with various vias  84  so that ground plane  58  of combiner  26  is electrically connected to ground layer  82 . Inputs  37 ,  38  and  39  are joined with the respective outputs of high power amplifiers  20 ,  21  and  22  and connections  44 ,  45  and  46  are joined with respective resistors  40 ,  41  and  42 , the other ends of which are grounded through vias  84 . 
       FIG. 5A  is a view of the first surface  63  of combiner  26 , looking along the direction of arrow  88  in  FIG. 4 , and  FIG. 5B  is a view of the second surface  63 , looking along the direction of arrow  89  in FIG.  4 . In  FIG. 5A  the metallization pattern of the second surface  64  is shown dotted and in  FIG. 5B  the metallization pattern of the microstrip main line  28  is shown dotted. It may be seen that the first surface  63  is completely devoid of any ground plane and that the microstrip main line  28  is substantially coextensive with, and overlaps the three microstrip coupled lines  34 ,  35  and  36 . 
     Further, in  FIG. 5B  it may be seen that the microstrip coupled lines  34 ,  35  and  36  progressively decrease in width as a function of its proximity to output  30 . This is a well-known technique to achieve correct coupling. For example, microstrip coupled line  34  may be a 3 dB, or ½ power coupler, microstrip coupled line  35  a 4.8 dB. Or ⅓ power coupler and microstrip coupled line  36  a 6 dB, or ¼ power coupler. 
     During operation, the electromagnetic field in the air, due to the RF signal helps to lower loss, however the wave propagating in the air travels faster than the wave propagating in the dielectric. In order to help match these velocities, and as seen in  FIG. 5B , microstrip main line  28  includes matching stubs  92  and  93  between stages  66  and  67  and between stages  67  and  68 , respectively. 
     The foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.