Abstract:
Very small size true directional couplers have a coupling coefficient that is independent on load VSWR. The coupler uses coupled inductors with a compensation circuit including a resistor and a capacitor, or just a capacitor. Wideband operation is suitable for many portable applications such as power detection and control for GSM, DCS-PCS, CDMA/WCDMA, Bluetooth, and WLAN systems.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority from, and is a divisional of, U.S. patent application Ser. No. 11/530,220 filed on Sep. 8, 2006, the disclosure of which is herein specifically incorporated in its entirety by this reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to directional couplers used in RF applications. 
         [0003]    Directional couplers are used in many different RF applications such as power detection and control, test and measurement, mobile phones, and many others. 
         [0004]    Numerous types of couplers are known in the art. Micro-strip, strip-line, Lange-couplers and other directional couplers require a large size to obtain a reasonable coupling coefficient and low insertion loss over a wide frequency range. Consequently, this type of coupler increases total size and cost of, for example, mobile devices. L-C couplers used in current power detection applications (GSM, CDMA, WLAN, Bluetooth, etc.) are not “true” directional couplers, and the coupling coefficient depends on a standing wave at the output (load VSWR dependent) resulting in incorrect power measurement and control. On-die implementation of micro-strip and other types of directional couplers is limited by high-frequency applications (microwave and millimeter-wave). The geometry of the coupler is directly proportional to the wavelength of the signal to be detected. 
         [0005]    What is desired, therefore, is a true directional coupler for RF applications that can be manufactured with a small geometry and at low cost. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention, a very small size “true” directional coupler has a coupling coefficient that is independent on load VSWR. The coupler uses coupled inductors with a compensation circuit including a resistor and a capacitor, or just a capacitor. The coupler can be made so that its area is more than ten times smaller than that of existing couplers. Wideband operation is suitable for many portable applications such as power detection and control for GSM, DCS-PCS, CDMA/WCDMA, Bluetooth, WLAN systems, and the like. The directional coupler of the present invention is easy and inexpensive to fabricate and can be manufactured using integrated circuits, printed circuit boards, low temperature co-fired ceramic implementations, or micro electromechanical systems (“MEMS”). A wide range of coupling coefficients can be realized by adjustment of the coupled inductance and the compensation circuit parameters. The coupler of the present invention has low insertion loss, and the coupling coefficient and insertion loss are tolerant to process and geometry variations. A small ballast resistor does not require large power dissipation over load VSWR variation. Optimal (minimum coupling between the load port and the coupling port) compensation capacitance is linearly dependent (positive slope) on the inductance coupling coefficient at a fixed inductance value and resistive loss. Optimal compensation capacitance is linearly dependent (negative slope) on inductance resistive loss at a fixed inductance value and coupling coefficient. Optimal compensation capacitance is linearly dependent (positive slope) on coupled inductance at fixed inductance resistive loss value and coupling coefficient. 
         [0007]    A first embodiment of the RF coupler according to the present invention includes an input port, a coupling port, a load port, a ballast resistor port, a compensation circuit coupled between the input port and the coupling port, and a coupled inductor having a first terminal coupled to the input port, a second terminal coupled to the coupling port, a third terminal coupled to the load port, and a fourth terminal coupled to the ballast resistor port. 
         [0008]    A second embodiment of the RF coupler according to the present invention includes a GSM input port, a power detector port, a DCS-PCS input port, a GSM output port, a ballast resistance port, a DCS-PCS output port, a first compensation circuit coupled between the GSM input port and the power detector port, a second compensation circuit coupled between the power detector port and the DCS-PCS input port, and a coupled inductor having a first terminal coupled to the GSM input port, a second terminal coupled to the power detector port, a third terminal coupled to the DCS-PCS input port, a fourth terminal coupled to the GSM output port, a fifth terminal coupled to the ballast resistance port, and a sixth terminal coupled to the DCS-PCS output port. 
         [0009]    A third embodiment of the RF coupler includes an input port, a power detector port, an output port, a ballast resistor port, a compensation circuit coupled between the input port and the power detector port, a coupled inductor having a first terminal coupled to the input port, a second terminal coupled to the power detector port, a third terminal, and a fourth terminal coupled to the ballast resistor port, a shunt resistor coupled between the third terminal of the coupled inductor and ground, and a series resistor coupled between the third terminal of the coupled inductor and the output port. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
           [0011]      FIG. 1  is a schematic diagram of a true directional coupler circuit according to the present invention; 
           [0012]      FIG. 2  is a schematic diagram of a simplified true directional coupler circuit according to the present invention; 
           [0013]      FIG. 3  is a schematic diagram of a quad-band GSM directional coupler circuit according to the present invention; 
           [0014]      FIG. 4  is a schematic diagram of a directional coupler circuit having an enhanced load VSWR tolerant capability according to the present invention; 
           [0015]      FIG. 5  is a schematic diagram of a GSM power amplifier architecture with power control and enhanced load VSWR tolerance according to the present invention; 
           [0016]      FIG. 6  is a schematic diagram of a quad-band GSM directional coupler circuit having an enhanced load VSWR tolerant capability according to the present invention; 
           [0017]      FIG. 7  is a simulation results example for the basic directional coupler parameters over frequency optimized for wideband operation as shown in  FIG. 1 , wherein the impedance of all ports is 50 ohms; 
           [0018]      FIG. 8  is a simulation results example for the simplified directional coupler parameters over frequency optimized for wideband operation as shown in  FIG. 2 , wherein the impedance of all ports is 50 ohms; 
           [0019]      FIG. 9  is a simulation results example for the quad-band GSM directional coupler parameters over frequency optimized for GSM, DCS and PCS band operation as shown in  FIG. 3 , wherein the impedance of all ports is 50 ohms; 
           [0020]      FIG. 10  is a simulation results example for the GSM directional coupler with enhanced load VSWR tolerant capability parameters over frequency optimized for GSM, DCS and PCS band as shown in  FIG. 4 , wherein the impedance of all ports is 50 ohms; 
           [0021]      FIG. 11  is a simulation results example of coupler VSWR tolerance over load VSWR variation for the coupler of  FIG. 4 , wherein the impedance of port  2  is varied; 
           [0022]      FIG. 12  is a layout diagram of an example of a single-layer coupled inductor according to the present invention; 
           [0023]      FIG. 13  is a layout diagram of an example of a dual-layer coupled inductor according to the present invention; and 
           [0024]      FIG. 14  is a layout diagram of an example of a triple-layer coupled inductor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring now to  FIG. 1 , an RF coupler circuit  100  includes an input port  102 , a coupling port  106 , a load port  104 , a ballast resistor port  108 , a compensation circuit  110 ,  112  coupled between the input port and the coupling port, and a coupled inductor  114  having a first terminal coupled to the input port  102 , a second terminal coupled to the coupling port  106 , a third terminal coupled to the load port  104 , and a fourth terminal coupled to the ballast resistor port  108 . The coupled inductor  114  includes first and second inductors  116  and  118 . 
         [0026]    In a typical example optimized for wideband (0.8-6 GHz) operation, the inductance of the first and second inductors  116  and  118  is 0.6 nH. The series resistance for the first and second inductors is 1.2 ohms and the coupling coefficient between inductors  116  and  118  is about 0.7. The compensation circuit for coupler  100  includes a resistor  112  in series with a capacitor  110 . The resistance of resistor  112  is 21.4 ohms. The capacitance of capacitor  110  is 0.164 pF. 
         [0027]    A simplified true directional coupler  200  is shown in  FIG. 2 . Note that the compensation circuit includes only capacitor  110 , whose value is 0.164 pF in a typical example. 
         [0028]    A quad-band GSM directional coupler is shown in  FIG. 3  including a GSM input port  302 , a power detector port  306 , a DCS-PCS input port  310 , a GSM output port  304 , a ballast resistance port  308 , a DCS-PCS output port  312 , a first compensation circuit  314 ,  316  coupled between the GSM input port  302  and the power detector port  306 , a second compensation circuit  318 ,  320  coupled between the power detector port  306  and the DCS-PCS input port  310 , a coupled inductor  322  having a first terminal coupled to the GSM input port  302 , a second terminal coupled to the power detector port  306 , a third terminal coupled to the DCS-PCS input port  310 , a fourth terminal coupled to the GSM output port  304 , a fifth terminal coupled to the ballast resistance port  308 , and a sixth terminal coupled to the DCS-PCS output port  312 . The coupled inductor  322  includes first, second, and third inductors  324 ,  326 , and  328 . 
         [0029]    In a typical example optimized for GSM, DCS and PCS (0.8-1.9 Ghz) operation, the inductance of the first, second, and third inductors  324 ,  326 , and  328  is 1.2 nH. The series resistance for the first, second, and third inductors is two ohms. The coupling coefficient between the first and second inductors is about 0.7. The coupling coefficient between the second and third inductors is about 0.7. The coupling coefficient between the first and third inductors is about 0.5. The first compensation circuit for coupler  300  includes a resistor  314  in series with a capacitor  316 . The resistance of resistor  314  is 21.5 ohms. The capacitance of capacitor  316  is 0.325 pF. The second compensation circuit for coupler  300  includes a resistor  320  in series with a capacitor  318 . The resistance of resistor  320  is 21.5 ohms. The capacitance of capacitor  318  is 0.325 pF. If desired, the compensation circuits can include only a single capacitor. 
         [0030]    Referring now to  FIG. 4 , an RF coupler  400  having enhanced load VSWR tolerant capability includes an input port  102 , a power detector port  106 , an output port  104 , a ballast resistor port  108 , a compensation circuit  110  coupled between the input port  102  and the power detector port  106 , a coupled inductor  114  having a first terminal coupled to the input port  102 , a second terminal coupled to the power detector port  106 , a third terminal, and a fourth terminal coupled to the ballast resistor port  108 , a shunt resistor  120  coupled between the third terminal of the coupled inductor  114  and ground, and a series resistor coupled between the third terminal of the coupled inductor  114  and the output port  104 . The coupled inductor  114  includes first and second inductors  116  and  118 . 
         [0031]    In a typical example optimized for GSM, DCS, and PCS (0.8-1.9 GHz) operation, the inductance of the first and second inductors  116  and  118  is 1.2 nH. The series resistance for the first and second inductors is 2.4 ohms and the coupling coefficient between first and second inductors  116  and  118  is about 0.7. The compensation circuit for coupler  400  includes a capacitor  110  whose value is 0.33 pF. The value of the shunt resistor is 500 ohms, and the value of the series resistor is 3 ohms. 
         [0032]    Referring now to  FIG. 5 , a GSM power amplifier architecture  500  is shown using the directional RF coupler of the present invention. Power amplifier architecture  500  includes a main GSM power amplifier block  508  having an output coupled to an RF coupler  400  as is previously described. RF coupler  400  is also coupled to the RF output port  504 . A power detector and control block  510  is coupled to the control port  506 , to the GSM power amplifier block  508 , and to the RF coupler  400 . Power detector and control block  510  includes an error amplifier  512 , parallel capacitor  514  and resistor  516 , diode  518 , and an attenuator  520 . 
         [0033]    Referring now to  FIG. 6 , a quad-band GSM directional coupler is shown as previously described with respect to  FIG. 3 . In addition to the previously described circuit components, coupler  600  includes a shunt resistor  330  and a series resistor  332  associated with GSM output port  312 . If desired, these resistors can also be associated with DCS-PCS output port  304 , or with both ports. 
         [0034]    The following description refers to a detailed methodology for selecting the component values for the couplers of  FIGS. 1-6  according to the present invention. Following a description of the methodology, the performance of the couplers of the present invention is set forth in  FIGS. 7-11 . 
         [0035]    After an appropriate technology is chosen, the design procedure uses high-frequency EM (electromagnetic) and circuit simulation software—examples of which are AWR Microwave Office, Agilent ADS, and the like. As an initial matter the metal trace width and thickness of the coupled inductors should be chosen, which is dependent on RF power handling capability of the main signal path (input port to output port). This information can be found in appropriate design rules of the particular technology chosen. As a rule, the wider and thicker the metal trace, the lower the insertion loss. The longer the metal trace, the higher the insertion loss. The inductor value is also dependent on proximity to other metal planes (such as ground) and other traces. All of these influences are fairly well developed by existing simulation software tools. 
         [0036]    Next, the inductance of the coupled inductors should be chosen. This inductance directly defines the final coupling coefficient between input port and coupled port of a coupler. As a rule of thumb, the higher the inductance value, the lower (in dB) the coupling coefficient. Coupling between the input port and the coupled port is also dependent on the coupling coefficient between coupled inductors. A higher coupling coefficient between the inductors results in a lower (in dB) coupling between input port and coupled port. The coupling coefficient between inductors is dependent on the distance between the two coils, and as a rule the longer the distance, the lower the coupling. Particular implementation of a coupled inductors&#39; technology defines coupling and can be found in existing EM and circuit simulation software. For instance, simulating by EM software a particular coupled inductors&#39; structure and then comparing with circuit simulation can give the correct value of coupling between the coils. 
         [0037]    Next, the compensation capacitor value should be chosen. The compensation capacitor value can be tuned in a circuit simulation until the minimum coupling between the output port and the coupled port is achieved (all ports should be loaded by 50 ohms). Verify if the coupling coefficient between the input port and the coupled port has the desired value. If not, change the inductor value and repeat the simulation. When the desired coupling coefficient is achieved, add a compensation resistor in the circuit and tune this resistance value in the circuit simulator until the minimum coupling between the output port and the coupled port is achieved. The adding of the compensation resistor almost always does not change the coupling between the input port and the coupled port. 
         [0038]    The difference between the basic coupler with the compensation capacitor and the compensation resistor and the simplified coupler with just the compensation capacitor is in the minimum coupling value between the output port and the coupled port. The basic coupler has an advantage in this coupling (see appropriate simulation results below) although the simplified coupler has an advantage in implementation (compensation resistor is not necessary) and size. 
         [0039]    The placement and positioning of coupled inductors along with compensation capacitors and resistors is very flexible and it can be chosen as appropriate for a particular design. Referring to  FIGS. 1-3 , compensation circuits comprising a capacitor and resistor or just a capacitor can be inserted between the output port and the ballast resistor port instead of as shown in the originally proposed circuits. If desired, the compensation circuits can be inserted at the input and the output simultaneously. In both cases mentioned above a simulation should be done for best results in a particular implementation. Filtering components can be implemented at the input and output if desired as well as between the input and the output. 
         [0040]    Referring to  FIG. 3 , different compensation circuits can be connected to the GSM and the DCS-PCS ports. A simulation should be re-done if using different compensation circuits to assure the best performance for a particular implementation. 
         [0041]    Referring to  FIGS. 1-3 , the coupling port and ballast port resistors power handling capability can be facilitated so that the power dissipation therein is much less than in the main signal path between the input port and the output port. This results in a lower footprint comprising those components. 
         [0042]    Referring to  FIG. 4 , the compensation resistor along with capacitor can be added (as per the basic coupler structure). Series and shunt resistor values can be adjusted if required and they can be placed between input port and inductor as well. It is important to note that the insertion loss between the input port and the output port is higher when the series resistor is higher and the shunt resistor is lower. It should also be noted that the series resistor resistance adds to the series resistance of a coupled inductor, which should be taken into account during simulation. The coupled inductors&#39; parameters can be adjusted, if required. An advantage of this circuit is that when the output port impedance is widely varied, coupling between the input port and the coupling port remains essentially the same. At the same time the impedance variation at the input port is much less than at the output port. The power handling capability of the series resistor should be chosen for maximum RF power transfer between the input port and the output port when the output port is shorted to ground. The power handling capability of the shunt resistor should be chosen for maximum RF power transfer between the input port and the output port when the output port is open (disconnected). 
         [0043]    Referring to  FIG. 5  the directional coupler block can have different component values from those described above, as is true as well for all of the couplers described herein as will be readily apparent to those skilled in the art. The power detector and control block can have a different architecture as per industry well known solutions. 
         [0044]    Referring to  FIG. 6  all of the components and compensation circuitry can be adjusted as described above to suit the requirements of a particular implementation. 
         [0045]    The performance of the couplers of the present invention using the design methodology described above is set forth in  FIGS. 7-11 , wherein:  FIG. 7  is a simulation results example for the basic directional coupler parameters over frequency optimized for wideband operation as shown in  FIG. 1 , wherein the impedance of all ports is 50 ohms;  FIG. 8  is a simulation results example for the simplified directional coupler parameters over frequency optimized for wideband operation as shown in  FIG. 2 , wherein the impedance of all ports is 50 ohms;  FIG. 9  is a simulation results example for the quad-band GSM directional coupler parameters over frequency optimized for GSM, DCS and PCS band operation as shown in  FIG. 3 , wherein the impedance of all ports is 50 ohms;  FIG. 10  is a simulation results example for the GSM directional coupler with enhanced load VSWR tolerant capability parameters over frequency optimized for GSM, DCS and PCS band as shown in  FIG. 4 , wherein the impedance of all ports is 50 ohms; and  FIG. 11  is a simulation results example of coupler VSWR tolerance over load VSWR variation for the coupler of  FIG. 4 , wherein the impedance of port  2  is varied. 
         [0046]    Referring generally to  FIGS. 12-14 , several examples of coupled inductors are shown that are appropriate for achieving the desirably small size for the couplers of the present invention. In general, the small size is possible because the values of the inductance of the coupled inductor, the compensation resistance, and the compensation capacitance are all correspondingly small. Suitable materials for the coupled inductors can include copper, gold, aluminum, and other conductors depending upon the fabrication technology chosen. As described above, the coupled inductors can be fabricated in an integrated circuit, on a printed circuit board, or in low temperature co-fired ceramic implementations, as well as MEMS. 
         [0047]    Referring now to  FIG. 12  a single-layer coupled inductors layout example is shown as per the GSM coupler of  FIG. 4 . Coupled inductor  1200  includes first and second inductors  1202 ,  1204  having four ports  1206 ,  1208 ,  1210  and  1212  corresponding to  FIG. 4 . Metal traces of inductors are made 5 micron wide and 2 micron thick and laying in the same layer on the top of a 100 micron thick substrate, with a dielectric constant of 12.9 and a tangent delta of 0.001 (as per bulk GaAs or similar to Si with high resistivity). Each inductor comprises 3.25 turns of a rectangular shaped coil. Spacing between the metal traces is 5 micron. The internal area is chosen to be 25 by 25 microns. This is the “standard” design rule requirement for low loss and highest inductance (five times metal trace width), although it could be made differently if desired. According to simulation, each inductor  1202 ,  1204  value is about 1.21-1.22 nH, and the coupling coefficient between inductors is about 0.78. This corresponds to the circuit parameters presented in  FIG. 4 . Total external footprint of a coupled inductor is about 165 by 155 microns, which is just a bit higher than a “regular” bond pad dimension on an IC (100 by 100 microns). The long metal traces shown in  FIG. 12  are for simulation purposes only. The ports shown in accordance with  FIG. 4  are positioned at the edges of the rectangular area of 165 by 155 microns. The compensation circuit along with the coupling port and ballast resistance can be designed in a different manner if desired. 
         [0048]    Referring now to  FIG. 13  a dual-layer coupled inductor layout example is shown as per the wide-band coupler shown in  FIGS. 1 and 2 . Coupled inductor  1300  includes first  1302  and second  1304  inductors having four ports  1306 ,  1308 ,  1310 , and  1312  as is shown in  FIGS. 1-2 . Metal traces of the inductors are made to have a 5 micron width and are laying at different layers. The top metal inductor is 2 microns thick and is separated by 4 microns of an air gap from the second layer metal inductor, which is 1 micron thick. The second layer is positioned on a 100 micron thick substrate with a dielectric constant of 12.9 and a tangent delta of 0.001 (as per bulk GaAs or similar to Si with high resistivity). Each inductor comprises 3.25 turns of a rectangular shaped coil. The spacing between the metal traces at the same layer is 5 microns. The internal area is chosen as 25 by 25 microns, although it can be changed if desired. According to simulation, each inductor  1302 ,  1304  value is about 0.57-0.58 nH and the coupling coefficient between inductors is about 0.73. This corresponds to circuit parameters presented in  FIGS. 1 and 2 . The total external footprint of the coupled inductor is 85 by 85 microns, which is less than “regular” bond pad dimension on an IC (100 by 100 microns). The long metal traces shown in  FIG. 13  are for simulation purposes only. The four ports shown in accordance with  FIGS. 1-2  are positioned at the edges of the rectangular area of 85 by 85 microns. The compensation circuit along with the coupling port and ballast resistance can be designed in a different manner if desired. 
         [0049]    Referring now to  FIG. 14  a triple-layer coupled inductors layout example is shown as per the quad-band GSM coupler shown in  FIG. 3 . Coupled inductor  1400  includes first  1402 , second  1404  and third  1406  inductors having six ports  1408 ,  1410 ,  1412 ,  1414 ,  1416 , and  1418 , as shown in  FIG. 3 . Metal traces of inductors are made 5 microns wide and laying at different layers. The top metal inductor is 2 microns thick separated by a 4 micron air gap from the second layer metal inductor of 1 micron thickness, the second layer inductor is separated by a 4 micron air gap from the third layer metal inductor of 0.5 um thickness. The second layer is positioned on a 100 micron thick substrate with a dielectric constant of 12.9 and a tangent delta of 0.001 (as per bulk GaAs or similar to Si with high resistivity). Each inductor comprises 3.25 turns of rectangular shaped coil. Spacing between metal traces is 5 microns. The internal area is chosen as 25 by 15 microns. According to simulation each inductor  1402 ,  1404 ,  1406 , value is about 0.55-0.57 nH and the coupling coefficient between first and second as well as third and second inductors is about 0.7, while the coupling coefficient between first and third inductors is about 0.5. Although this does not fully correspond to the circuit parameters presented in  FIG. 3  (it will result in a lower coupling between input ports and coupling port which can be used in real circuits) this structure presents a clear insight as to the dimensions possible. The total external footprint of the coupled inductor is 95 by 95 microns. The long metal traces shown in  FIG. 14  are for simulation purposes only. The six ports shown in accordance with  FIG. 3  are positioned at the edges of the rectangular area of 95 by 95 microns. Compensation circuits along with the coupling port and the ballast resistance can be designed in a different manner if desired. 
       GLOSSARY 
       [0050]    IC—Integrated Circuit. 
         [0051]    GSM—Global System for Mobile Communications. 
         [0052]    CDMA—Code Division Multiple Access. 
         [0053]    WCDMA—Wideband CDMA. 
         [0054]    WLAN—Wireless Local Area Networks. 
         [0055]    VSWR—Voltage Standing Wave Ratio. 
         [0056]    DCS—Digital Cellular System. 
         [0057]    PCS—Personal Communications Service. 
         [0058]    Bluetooth—Wireless personal area network standard. 
         [0059]    MEMS—Micro Electro-Mechanical System. 
         [0060]    Although the present invention has been described with reference to a preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.