Abstract:
A directional coupler that has improved coupling flatness. The directional coupler includes a first, second and third coupler. Each of the couplers has an input port, an output port, a forward coupled port and a reverse coupled port. The forward coupled port of the first coupler is connected to the input port of the second coupler. The reverse coupled port of the first coupler is connected to ground. The output port of the second coupler is connected to the input port of the third coupler. The output port of the third coupler forms a second forward coupled port. The second and third couplers reduce the variation in coupling over a frequency range.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to directional couplers in general and more particularly to a directional coupler that has an improved variation in coupling over the frequency range.  
         [0003]     2. Description of Related Art p Directional couplers are used in a variety of applications in the RF and microwave frequency range.  FIG. 1  shows a schematic diagram of a prior art directional coupler  20  including a pair of coupled circuit lines  22  and  24 . Circuit lines  22  and  24  would typically be formed in a stripline configuration. The directional coupler  20  has four ports, an input port  25 , an output port  26 , a forward coupled port  27  and a reverse coupled port  28 . An input signal or power applied to the input port  25  will go mainly to the output port  26 . A portion of the input signal will be electromagnetically coupled to circuit line  24  and appear mostly at forward coupled port  27 . A very small portion of the signal will go to the reverse coupled port  28 . The electrical signal coupled to the forward and reverse ports depends upon the coupled circuit line characteristic impedance and the coupling between the lines.  
         [0004]     Several measurements are used to quantify the electrical performance of a coupler. Insertion loss is defined as the ratio of power at the input port to the power appearing at the output port. Coupling coefficient is defined as the ratio of power at the input port to the power appearing at the coupled port. Coupling flatness is a measure of the peak to peak variation in the coupling coefficient over a particular frequency range. Directivity is a measure of the coupler differentiation. Directivity is defined as the difference in dB of power at the coupled port when power is transmitted in one direction to the power at the same port when power is transmitted in the opposite direction. Return loss is a measure of the reflected signal to the incident signal.  
         [0005]     Referring to  FIGS. 2-4 , the electrical performance of directional coupler  20  is shown.  FIG. 2  is a graph of insertion loss versus frequency.  FIG. 3  is a graph of coupling and directivity versus frequency.  FIG. 4  is a graph of return loss versus frequency for the directional coupler of  FIG. 1 .  
         [0006]     The coupling flatness of a coupler is a limiting parameter in the application of the coupler. Various compensation circuits can be used to increase the bandwidth while maintaining an acceptable flatness. An example of a compensation circuit to improve flatness is a multi-section TEM mode synthesis circuit. Unfortunately, this circuit needs a large printed circuit board to accommodate the quarter wavelength circuit lines needed for each section.  
         [0007]     A continuing need exists for a directional coupler that has improved electrical performance and in particular a directional coupler that has improved coupling flatness.  
       SUMMARY  
       [0008]     It is a feature of the invention to provide a directional coupler that has a small size with good electrical performance.  
         [0009]     A further feature of the invention is to provide a directional coupler that has improved coupling flatness.  
         [0010]     Another feature of the invention is to provide a directional coupler that includes a a first, second and third coupler. Each of the couplers has an input port, an output port, a forward coupled port and a reverse coupled port. The forward coupled port of the first coupler is connected to the input port of the second coupler. The reverse coupled port of the first coupler is connected to ground. The output port of the second coupler is connected to the input port of the third coupler. The output port of the third coupler forms a second forward coupled port. The second and third couplers reduce the variation in coupling over a frequency range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic drawing of a conventional directional coupler of the prior art.  
         [0012]      FIG. 2  is a graph of insertion loss versus frequency for the directional coupler of  FIG. 1 .  
         [0013]      FIG. 3  is a graph of coupling and directivity versus frequency for the directional coupler of  FIG. 1 .  
         [0014]      FIG. 4  is a graph of return loss versus frequency for the directional coupler of  FIG. 1 .  
         [0015]      FIG. 5  is a schematic drawing of a directional coupler in accordance with the present invention.  
         [0016]      FIG. 6  is a top view of the directional coupler of  FIG. 5  packaged on a circuit board and housing.  
         [0017]      FIG. 7  is a top view of the circuit board of  FIG. 6 .  
         [0018]      FIG. 8  is a graph of insertion loss versus frequency for the directional coupler of  FIG. 5 .  
         [0019]      FIG. 9  is a graph of coupling and directivity versus frequency for the directional coupler of  FIG. 5 .  
         [0020]      FIG. 10  is a graph of return loss versus frequency for the directional coupler of  FIG. 5 . 
     
    
       [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]      FIG. 5  shows a schematic drawing of a coupling flatness compensated directional coupler  30  in accordance with the present invention. Directional coupler  30  has three interconnected couplers  40 ,  60  and  80 . Coupler  40  has a pair of coupled circuit lines  42  and  44 . Circuit lines  42  and  44  can be formed in a stripline configuration. Circuit line  42  has ends  42 A and  42 B. Circuit line  44  has ends  44 A and  44 B. Coupler  40  has four ports, an input port  45 , an output port  46 , a forward coupled port  47  and a reverse coupled port  48 . Input port  45  is connected to end  42 A. Output port  46  is connected to end  42 B. A reverse coupled port  48  can be located at end  44 B making directional coupler  30  a bidirectional coupler. In  FIG. 5 , end  44 B is shown connected to a 50 ohm terminating resistor  50  making the directional coupler a uni-directional coupler. Resistor  50  is connected between end  44 B and ground.  
         [0023]     3 dB Coupler  60  has a pair of coupled circuit lines  62  and  64 . Circuit lines  62  and  64  can be formed in a stripline configuration. Circuit line  62  has ends  62 A and  62 B. Circuit line  64  has ends  64 A and  64 B. End  62 A is connected to end  44 A. End  64 A is connected through a 50 ohm resistor  66  to ground. End  64 B is connected through a 50 ohm resistor  67  to ground. Coupler  60  also has four ports, end  62 A is the input port, end  62 B is the output port, end  64 A is the forward coupled port and end  64 B can be the reverse coupled port.  
         [0024]     3 dB Coupler  80  has a pair of coupled circuit lines  82  and  84 . Circuit lines  82  and  84  can be formed in a stripline configuration. Circuit line  82  has ends  82 A and  82 B. Circuit line  84  has ends  84 A and  84 B. End  82 A is connected to end  62 B. End  84 A is connected through a 50 ohm resistor  86  to ground. End  84 B is connected through a 50 ohm resistor  87  to ground. End  82 B is connected to forward coupled port  88 .  
         [0025]     Coupler  80  also has four ports, end  82 A is the input port, end  82 B is the output port, end  84 A is the forward coupled port and end  84 B can be the reverse coupled port. The use of the 3 dB couplers  60  and  80  reduces the variation in coupling coefficient or coupling flatness.  
         [0026]     Referring to  FIGS. 6 and 7 , a top view of directional coupler assembly  100  is shown.  FIG. 6  shows the directional coupler  30  of  FIG. 5  realized in a physical package.  
         [0027]     Directional coupler assembly  100  has a housing  102  with a cavity  103 , sides  104  and screw holes  105 . Apertures  106  extend through sides  104 . Housing  102  would typically be made of metal. A metal cover (not shown) would typically go over cavity  103  and be attached with screws into holes  105 .  
         [0028]     Several coaxial connectors  170  are threaded into apertures  106 . Coaxial connectors  170  have threaded ends  171  and  172  and a pin  174 . Coaxial connectors  170  serve as input port  45 , output port  46  and forward coupled port  47 . Coaxial connectors  170  can be an SMA type coaxial connector. Housing  102  would typically be grounded.  
         [0029]     A printed circuit board  120  is mounted inside cavity  103 . Printed circuit board  120  has a top surface  121  and a bottom surface (not shown). The bottom surface can have a ground plane that is soldered to a connector. Printed circuit board  120  would typically have several layers that are connected by plated through holes (not shown). The printed circuit board has overall dimensions of 1.38 inches by 1.23 inches. Printed circuit board  120  has several conductive lines and conductive pads patterned on top surface  121 . The lines and pads are typically formed from etched copper. Conductive printed circuit lines  124 ,  125  and  126  are located on surface  121 . The circuit lines can have a typical line width of 0.064 inches. Conductive printed circuit pads  127 ,  128 ,  129 ,  130 ,  131 ,  132 ,  133 ,  134 ,  135  and  136  are also located on surface  121 .  
         [0030]     Coupler  40  and 3 dB couplers  60  and  80  are mounted on top surface  121 . Coupler  40  and 3 dB couplers  60  and  80  are commercially available from Mini-Circuits Corporation of Brooklyn, N.Y. Coupler  40  and 3 dB couplers  60  and  80  can be formed from low temperature co-fired ceramic (LTCC) material with internal coupled lines. The couplers have metal leads extending away from the coupler bodies  40 A,  60 A and  80 A. The leads are soldered to the printed circuit lines and pads.  
         [0031]     Coupler  40  has leads A, B, C, D, E, F, G, H, I and J. Lead A is connected to circuit line  126 . Leads B, C, D, G, H and I are connected to circuit pad  134 . Lead F is connected to circuit pad  133 . Lead J is connected to circuit line  125 .  
         [0032]     Coupler  60  has leads K, L, M, N, O, P, Q, R, S and T. Lead K is connected to circuit pad  132 . Leads L, M, N, Q, R and S are connected to circuit pad  128 . Lead O is connected to circuit pad  133 . Lead P is connected to circuit pad  129 . Lead T is connected to circuit pad  131 .  
         [0033]     Coupler  80  has leads U, V, W, X, Y, Z, M, AB, AC and AD. Lead U is connected to circuit pad  129 . Leads V, W, X, AA, AB, and AC are connected to circuit pad  128 . Lead Y is connected to circuit pad  130 . Lead Z is connected to circuit pad  127 . Lead AD is connected to circuit line  124 .  
         [0034]     Resistors  50 ,  66 ,  67 ,  86  and  87  are soldered to the lines and pads on top surface  121 . Resistors  50 ,  67  and  87  are made up to two resistors  50 A,  50 B,  67 A,  67 B,  87 A and  87 B, respectively that are connected in parallel. The resistors can be conventional surface mount electronic components. Conventionally, a solder paste is screened onto selected portions of the circuit lines and pads and the components placed with a pick and place machine and the solder paste is then reflowed.  
         [0035]     Resistors  50 A and  50 B are connected between circuit pads  135  and  136 . Resistor  66  is connected between circuit pads  128  and  132 . Resistors  67 A and  67 B are connected between circuit pads  128  and  131 . Resistor  86  is connected between circuit pads  128  and  130 . Resistors  87 A and  87 B are connected between circuit pads  128  and  127 .  
         [0036]     The pins  174  of coaxial connectors  170  are connected to circuit lines  124 ,  125  and  126 , respectively by solder  180 .  
         [0037]     A directional coupler assembly.  100  was designed, fabricated and tested for electrical performance. Directional coupler assembly  100  was tested over the frequency range of 400 to 2400 MHz.  
         [0038]      FIGS. 8, 9  and  10  show the electrical performance of directional coupler assembly  100 .  FIG. 8  shows a graph of insertion loss versus frequency for directional coupler assembly  100 .  FIG. 9  shows a graph of coupling and directivity versus frequency for directional coupler assembly  100 . In  FIG. 9 , the coupling varied 1.98 dB over the frequency range of 400 to 2400 MHz. In comparison,  FIG. 3  shows the coupling variation in a directional coupler of the prior art. The coupling variation for directional coupler  20  was 4.38 dB over the frequency range of 400 to 2400 MHz. Therefore, by the use of the present invention the coupling flatness was improved from 4.38 dB to 1.98 dB.  FIG. 10  is a graph of return loss versus frequency for directional coupler assembly  100 .  
         [0039]     The present invention has several advantages. The present invention provides an improvement in coupling flatness measurements over directional couplers of the prior art.  
         [0040]     The use of 3 dB couplers  60  and  80  results in a smaller package size since quarter wavelength circuit lines are not needed.  
         [0041]     The use of 3 dB couplers  60  and  80  results in a directional coupler that can handle a wider frequency range.  
         [0042]     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.