Patent Publication Number: US-7218186-B2

Title: Directional coupler

Description:
BACKGROUND 
   1. Field of the Invention 
   This invention relates to directional couplers in general and more particularly to a directional coupler for low frequencies that has good power handling and a small package size. 
   2. Description of Related Art 
   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 coupled reverse 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. Directivity is a measure of the bi-directional coupler differentiation. 
   Directional couplers using stripline configurations have been applied to higher frequency applications, typically above 600 MHz. The length of the coupled lines is typically set at one quarter of the wavelength at the center frequency. The directional coupler  20  of  FIG. 1  is impractical for higher frequency applications. Directional couplers operating at lower frequencies are often faced with size and space constraints, which require the use of transformers to handle the power levels. The use of transformers add higher costs to the product and result in a larger overall package. 
   A current unmet need exists for a directional coupler that can operate at low frequencies, with minimal size and improved electrical performance. 
   SUMMARY 
   It is a feature of the invention to provide a directional coupler that has a small size with good electrical performance. 
   It is a feature of the invention to provide a directional coupler that can be used for low frequencies with high power. 
   Another feature of the invention is to provide a directional coupler that includes a first circuit line that has a first end and a second end. An input port is connected to the first end and an output port is connected to the second end. The second circuit line has a third end and a fourth end. The circuit lines are located proximate to each other such that they are electromagnetically coupled. A forward coupled port is connected to the third end and a reverse coupled port is connected to the fourth end. A low pass filter is connected to the forward coupled port. The low pass filter shifts the operating frequency of the directional coupler. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic drawing of a conventional directional coupler. 
       FIG. 2  is a schematic drawing of a directional coupler in accordance with the present invention. 
       FIG. 3  is a schematic drawing of another embodiment of a directional coupler in accordance with the present invention. 
       FIG. 4  is a top view of the directional coupler of  FIG. 3  packaged in a circuit board, an LTCC substrate and a housing. 
       FIG. 5  is an exploded perspective view of the LTCC substrate of  FIG. 4  showing the inner layers. 
       FIG. 6  is a graph of insertion loss versus frequency for a directional coupler. 
       FIG. 7  is a graph of coupling versus frequency for a directional coupler. 
       FIG. 8  is a graph of return loss versus frequency for a directional coupler. 
       FIG. 9  is a graph of insertion loss versus frequency for the directional coupler of  FIG. 3 . 
       FIG. 10  is a graph of coupling versus frequency for the directional coupler of  FIG. 3 . 
       FIG. 11  is a graph of return loss versus frequency for the directional coupler of  FIG. 3 . 
   

   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 
     FIG. 2  shows a schematic drawing of a directional coupler  30  in accordance with the present invention. Directional coupler  30  has a pair of coupled circuit lines  32  and  34 . Circuit lines  32  and  34  are typically formed in a stripline configuration as will be discussed later. Circuit line  32  has ends  32 A and  32 B. Circuit line  34  has ends  34 A and  34 B. The directional coupler  30  has four ports, an input port  35 , an output port  36 , a forward coupled port  37  and a reverse coupled port  38 . Input port  35  is connected to end  32 A. Output port  36  is connected to end  32 B. 
   A low pass filter  40  is connected between end  34 A and forward coupled port  37 . Similarly, another low pass filter  42  is connected between end  34 B and reverse coupled port  38 . 
   Low pass filter  40  has an inductor L 1  with ends L 1 A and L 1 B. End L 1 A is connected to forward coupled port  37  and end L 1 B is connected to circuit line end  34 A. Resistor R 1  has ends R 1 A and R 1 B. End R 1 A is connected to the junction of circuit line end  34 A and inductor end L 1 B. Resistor R 2  has ends R 2 A and R 2 B. End R 2 A is connected to the junction of forward coupled port  37  and inductor end L 1 A. A capacitor C 1  has ends C 1 A and C 1 B. Capacitor end C 1 A is commoned with resistor ends R 1 B and R 2 B. Capacitor end C 1 B is connected to ground. 
   Low pass filter  42  has an inductor L 2  with ends L 2 A and L 2 B. End L 2 A is connected to reverse coupled port  38  and end L 2 B is connected to circuit line end  34 B. Resistor R 3  has ends R 3 A and R 3 B. End R 3 A is connected to the junction of circuit line end  34 B and inductor end L 2 B. Resistor R 4  has ends R 4 A and R 4 B. End R 4 A is connected to the junction of reverse coupled port  38  and inductor end L 2 A. A capacitor C 2  has ends C 2 A and C 2 B. Capacitor end C 2 A is commoned with resistor ends R 3 B and R 4 B. Capacitor end C 2 B is connected to ground. 
   Directional coupler  30  can be implemented with circuit lines  32  and  34  having an impedance of 50 ohms. Typical values of resistor R 1 , R 2 , R 3  and R 4  is 50 ohms, capacitor C 1  and C 2  is 2.4 picofarads and for inductors L 1  and L 2  is 10 nanohenries. 
   Directional coupler  30  is a bi-directional coupler. Low pass filters  40  and  42  are constant impedance filters. The use of low pass filters  40  and  42  causes the center operating frequency of the directional coupler to be shifted to a lower frequency. 
   If desired, only one of the low pass filters can be used with the same effect. If low pass filter  40  or  42  was omitted, the center operating frequency would be shifted higher. 
   Referring to  FIG. 3 , a schematic drawing of another embodiment of a directional coupler is shown. Directional coupler  50  has a substrate  52  containing a pair of coupled circuit lines  32  and  34 . Circuit lines  32  and  34  are typically formed in a stripline configuration as will be discussed later. Circuit line  32  has ends  32 A and  32 B. Circuit line  34  has ends  34 A and  34 B. Directional coupler  50  has three ports, an input port  35 , an output port  36  and a forward coupled port  37 . Input port  35  is connected to end  32 A. Output port  36  is connected end  32 B. 
   A low pass filter  54  is connected between end  34 A and forward coupled port  37 . A resistor network  56  is connected between end  34 B and ground. 
   Low pass filter  54  has an inductor L 3  with ends L 3 A and L 3 B. End L 3 A is connected to forward coupled port  37  and end L 3 B is connected to circuit line end  34 A. Resistor R 6  has ends R 6 A and R 6 B. Resistor R 7  has ends R 7 A and R 7 B. Resistors R 6  and R 7  are connected in parallel. Resistor ends R 6 A and R 7 A are connected together and are also connected to inductor end L 3 B and circuit line end  34 A. Resistor R 5  has ends R 5 A and R 5 B. End R 5 A is connected to the junction of forward coupled port  37  and inductor end L 3 A. A capacitor C 3  has ends C 3 A and C 3 B. Capacitor end C 3 A is commoned with resistor ends R 5 B, R 6 B and R 7 B. Capacitor end C 3 B is connected to ground. 
   Resistor network  56  has a pair of resistors R 8  and R 9  connected in parallel. Resistor R 8  has ends R 8 A and R 8 B. Resistor R 9  has ends R 9 A and R 9 B. Resistor ends R 8 A and R 9 A are connected together and are also connected to circuit line end  34 B. Resistor ends R 8 B and R 9 B are connected to ground. 
   Directional coupler  30  can be implemented with circuit lines  32  and  34  having an impedance of 50 ohms. Typical values of resistor R 5  is 50 ohms, R 6 , R 7 , R 8  and R 9  are 100 ohms, capacitor C 3  and C 4  are 2.4 picofarads and inductor L 3  is 10 nanohenries. The 1 dB point of low pass filter  54  is 400 MHz. The 3 dB point of low pass filter  54  is 800 MHz. 
   The use of low pass filter  54  causes the center operating frequency of the directional coupler to be shifted to a lower frequency. Resistor network  56  is an impedance matching termination. 
   Referring to  FIG. 4 , a top view of directional coupler assembly  60  is shown.  FIG. 4  shows the directional coupler  50  of  FIG. 3  realized in a physical package. 
   Directional coupler assembly  60  has a housing  62  with a cavity  63 , sides  64  and screw holes  65 . Apertures  66  extend through sides  64 . Housing  62  would typically be made of metal. A metal cover (not shown) would typically go over cavity  63  and be attached with screws into holes  65 . 
   Several coaxial connectors  70  are threaded into apertures  66 . Coaxial connectors  70  have threaded ends  71  and  72  and a pin  74 . Coaxial connectors  70  serve as input port  35 , output port  36  and forward coupled port  37 . Coaxial connectors  70  can be an SMA type coaxial connector. The reverse coupled port is terminated in a matching impedance created by resistor network  56 . Housing  62  would be grounded. Directional coupler assembly  60  is therefore a 3 port device. 
   A printed circuit board  80  is mounted inside cavity  63 . Printed circuit board  80  has a top surface  81  and a bottom surface  82 . Bottom surface  82  would typically be glued or soldered into cavity  63 . Printed circuit board  80  would typically have several layers that are connected by plated through holes (not shown). Printed circuit board  80  has several conductive lines and conductive pads patterned on top surface  81 . Conductive lines  84 ,  85 ,  86  and  87  are located on top surface  81 . Conductive pads P 1 , P 2 , P 3 , P 4 , P 5 , P 6 , P 7 , P 8 , P 9  and P 10  are located on top surface  81 . 
   Substrate  52 , low pass filter  54  and resistor network  56  are mounted on top surface  81 . An inductor L 3 , resistors R 5 , R 6 , R 7 , R 8 , R 9  and capacitor C 3  are soldered to the conductive lines and conductive pads on top surface  81 . The inductor capacitor and resistors can be conventional surface mount electronic components. Conventionally, a solder paste is screened onto selected lines and pads and the components placed with a pick and place machine and the solder paste is then reflowed. 
   Inductor end L 3 A is soldered to conductive line  85 . Inductor end L 3 B is soldered to conductive line  87 . Resistor ends R 6 A and R 7 A are soldered to conductive line  87 . Resistor ends R 6 B and R 7 B are soldered to conductive pad P 8 . Resistor end R 5 A is soldered to conductive line  85 . Resistor end R 5 B is soldered to conductive pad P 9 . Capacitor end C 3 A is soldered to conductive pad P 8  and end C 3 B is soldered to conductive pad P 10 . Resistor ends R 8 A and R 9 A are soldered to conductive pad P 2 . Resistor ends R 8 B and R 9 B are soldered to conductive pad P 7 . An end of conductive lines  84 ,  85  and  86  are soldered to connector pins  74 . 
   Referring now to  FIG. 5 , an exploded perspective view of substrate  52  is shown. Substrate  52  is a multi-layered dielectric substrate  52  formed from layers of low temperature co-fired ceramic (LTCC) material. Substrate  52  is comprised of multiple layers  90 ,  91 ,  92 ,  93  and  94  of LTCC material. There are 5 LTCC layers in total. Substrate  52  has a top surface  90 A and bottom surface  94 B. Various circuit features are patterned on the layers. 
   Several conductive terminals are located on bottom surface  94 B. The terminals are formed from a solderable metal. Terminals T 1 , T 2 , T 3  and T 4  are located on bottom surface  94 B. Ground shield or plane G 1  is located on bottom surface  94 B. Ground shield or plane G 2  is located on top surface  90 A. The ground shields would be connected to a source of ground potential. 
   The terminals and ground plane G 1  are used to electrically connect substrate  52  to printed circuit board  80 . The terminals and a portion of ground plane G 1  would be soldered to printed circuit board  80 . An orientation mark M 1  is placed on top surface  90 A in order to prevent incorrect installation on the printed circuit board  80 . Terminal T 1  is soldered to conductive pad P 1 . Terminal T 2  is soldered to conductive pad P 2 . Terminal T 3  is soldered to conductive pad P 3 . Terminal T 4  is soldered to conductive pad P 4 . Ground plane G 1  is soldered to conductive pads P 5  and P 6 . 
   Planar layers  90 ,  91 ,  92 ,  93 , and  94  are all stacked on top of each other and form a unitary structure  52  after firing in an oven. Layer  90  is the top layer, layer  94  is the bottom layer and layers  91 ,  92  and  93  form inner layers. The layers are commercially available in the form of an unfired tape. Each of the layers has a top surface  90 A,  91 A,  92 A,  93 A and  94 A. Similarly, each of the layers has a bottom surface  90 B,  91 B,  92 B,  93 B and  94 B. The layers have several circuit features that are patterned on the surfaces. Multiple vias  100  extend through each of the layers. Vias  100  are formed from an electrically conductive material and electrically connect the circuit features on one layer to the circuit features on another layer. 
   Coupled circuit line  32  is formed on surface  93 A. Coupled circuit line  34  is formed on surface  92 A. Coupled circuit line  32  has ends  32 A and  32 B. Coupled circuit line  34  has ends  34 A and  34 B. Circuit lines  32  and  34  have a snake like or sinuous shape and are located directly above each other on different planes. Circuit lines  32  and  34  are separated by layer  92 . Circuit lines  32  and  34  are electromagnetically coupled through the dielectric medium of layer  92 . The circuit lines are formed from a conductive metal material. Circuit lines  32  and  34  are referred to as striplines because they are sandwiched between ground or reference planes G 1  and G 2 . 
   A mesh ground shield or plane G 2  is formed on surface  90 A. Another mesh ground shield or plane G 1  is formed on surface  94 B. Lines  102  connect several of the grounded vias together on layers  91 ,  92  and  93 . 
   The circuit features such as the vias, circuit lines, terminals and ground planes are formed by screening a thick film paste material and firing in an oven. This process is well known in the art. First, layers of low temperature co-fired ceramic have via holes punched, the vias are then filled with a conductive material. Next, the circuit features are screened onto the layers. The terminals, lines and ground planes are formed with a conductive material. The layers are then aligned and stacked on top of each other to form substrate  52 . The substrate  52  is then fired in an oven at approximately 900 degrees centigrade to form a single unitary piece. 
   A directional coupler in the form of substrate  52  and directional coupler assembly  60  were designed, fabricated and tested for electrical performance. Substrate  52  was designed with an 1800 MHz center operating frequency. Directional coupler assembly  60  with substrate  52  and low pass filter  54  operates at a 900 MHz center frequency. 
   Substrate  52  as built and tested had an overall substrate size of 0.3 inches by 0.25 inches by 0.27 inches. The circuit lines  32  and  34  had a line width of 0.005 inches and a line thickness of 0.0003 inches. 
   Directional coupler assembly  60 , used the following component values: resistor R 5  50 ohms; resistors R 6 , R 7 , R 8  and R 9  100 ohms; capacitor C 3 , C 4  2.4 picofarads and inductor L 3  10 nanohenries. 
     FIGS. 6–8  show the electrical performance of the coupled circuit lines of substrate  52  without the use of the low pass filter.  FIGS. 9–11  show the electrical performance of substrate  52  mounted in assembly  60  with the use of the low pass filter  54  and resistor network  56 . 
   Turning now to  FIGS. 6–8 , a graph of insertion loss versus frequency for substrate  52  is shown in  FIG. 6 .  FIG. 7  shows a graph of coupling versus frequency for substrate  52 .  FIG. 8  is a graph of return loss versus frequency for substrate  52 . The operating frequency is centered at 1800 MHz. 
   Turning now to  FIGS. 9–11 , a graph of insertion loss versus frequency for directional coupler assembly  60  is shown in  FIG. 9 .  FIG. 10  shows a graph of coupling versus frequency for directional coupler assembly  60 .  FIG. 11  is a graph of return loss versus frequency for directional coupler assembly  60 . The operating frequency is centered at 900 MHz. 
   The present invention has several advantages. The present invention allows for flexibility in designing directional couplers for differing frequencies. The same substrate  52  can be used for many different center frequencies just by changing the component values in the low pass filters. This allows for a fast design cycle for prototype parts and production. The present invention provides an improvement over previous directional coupler designs. 
   The use of substrate  52  over a range of frequencies results in lower costs as the same part is used for several design applications. 
   The use of a high frequency part for lower frequencies results in a smaller size component. 
   The directivity of the directional coupler is improved. 
   Since, high frequency couplers have good power handling, directional coupler  60  also has good power handling capabilities at low frequencies of operation. 
   Fabricating the substrate  52  using a low temperature co-fired ceramic process results in more uniform electrical characteristics. 
   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.