Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a directional coupler including an impedance matching and impedance transforming attenuator, in particular, a directional coupler for power monitoring, RF circuits or RF front-end circuits. 
     BACKGROUND OF THE INVENTION 
     In recent times, wireless handsets and terminals have evolved to have a high level of functionality while also becoming extremely compact. Wireless handsets and terminals often include a range of personal media functions, and are capable of operating on multiple systems such as the Global System for Mobile Communications (GSM) and the Universal Mobile Telephone System (UMTS). The components of the various systems in a contemporary wireless handset are required to offer high performance while the physical dimensions are required to become progressively smaller. 
     In the RF front-end circuit of a wireless handset, a power monitoring circuit is usually employed to control the transmitted power, for example, to ensure that the handset conforms with emission regulations pertaining to the system of operation and in the region of operation and in order to conserve battery life. A prior art block diagram of a conventional power monitoring circuit of an RF front-end circuit is shown in  FIG. 1 . 
     The directional coupler is a well known RF device which is used for monitoring the level of power traveling along a signal line in a particular direction. A directional coupler comprises a pair of transmission lines which are in close physical proximity to each other so that they become electromagnetically coupled to each other. A single transmission line can be characterized primarily by its electrical length and its characteristic impedance, thus a pair of transmission lines has a pair of electrical lengths and a pair of characteristic impedances. A coupled pair of transmission lines, such as those of a directional coupler, are more commonly characterized by the even mode impedance and the odd mode impedance and the even mode phase length and the odd mode phase length of the coupled transmission lines. 
       FIG. 2A  shows a diagram of a prior art directional coupler. The directional coupler of  FIG. 2A  comprises a pair of transmission lines  25  which are electromagnetically coupled to each other. Both of the transmission lines have input/output (I/O) ports at each end, so that the pair of coupled transmission lines  25  comprise four I/O ports. The I/O ports are labeled as an input port  21 , a direct port  22 , a coupled port  23  and an isolated port  24 . The pair of transmission lines of the directional coupler of  FIG. 2A  are formed so as to be embedded inside or on the surface of an insulating substrate and the transmission lines may be arranged to provide broadside coupling i.e. where respective broadsides of each line are adjacent to each other or to provide edge coupling i.e. where respective edges of each line are adjacent to each other. When a signal is fed to the input port  21  of the directional coupler of  FIG. 2A , inevitably, some of the signal is fed to the output port  22 ; however, the electromagnetic coupling between the transmission lines is such that a signal on one line induces a corresponding signal on the other line so that some of the input signal is also fed to the coupled port  23 , and under certain (non-ideal) conditions some of the input signal may also be fed to the isolated port  24 . 
     The structure depicted in  FIG. 2A  has at least one axis of symmetry  20 , and may have further axes of symmetry (not shown), so the designation of labels to the ports is somewhat arbitrary; for example, an input could be fed to port  22 , so that the direct port would become port  21 , the coupled port would become port  24  and so that the isolated port would become port  23 . 
     Directional couplers can be broadly categorized as either equal coupling or weakly coupled. Directional couplers offering roughly equal power splitting between the direct port and the coupled port—known as 3 dB couplers—typically comprise transmission lines having an electrical length equal to one quarter of the wavelength of the operating frequency of the coupler. Weakly coupled directional couplers, i.e. those which pass most of the input power to the direct port, and which couple only a small percentage thereof to the coupled port, may also comprise lines with an electrical length equal to one quarter of one wavelength; alternatively, such couplers can be fabricated using lines which are much shorter than one quarter of one wavelength. The choice of the electrical length depends on the required operating bandwidth, the required coupling ratio and the physical limitations of the fabrication process. 
     For couplers comprising short transmission lines (i.e. where the electrical length of the transmission lines is substantially less than one quarter of one wavelength at the frequency of operation of the directional coupler) and lines of equal length, the even mode phase length and odd mode phase length are approximately equal. Hence, such couplers can be characterized by three main parameters: the even mode impedance, the odd mode impedance, and the electrical length. 
     The operating performance of a directional coupler is usually given in terms of four electrical specifications: the coupling ratio, the insertion loss, the isolation and the return loss. These specifications can be determined analytically from the characterizing parameters of the directional coupler, or by direct measurement. The first specification, the coupling ratio, is a measure of the RF power which is emitted at the coupled port for a given level of power fed to the input port. Typically, this value is expressed as a ratio measured in decibels. Practical coupling ratios can vary from as low as −40 dB (corresponding to very weakly coupled lines) to −3 dB (strongly coupled lines providing equal power splitting between the direct port and the coupled port). The second specification for the performance of a directional coupler is the insertion loss for signals passing between the input port and the direct port. For couplers offering weak coupling between the input port and the coupled port, the insertion loss should be very low; for example, a coupling ratio of 1:10 (−10 dB at the coupled port) will give rise to a theoretical minimum insertion loss of 0.45 dB. Table 1 gives the relationship between the coupling ratios (in decibels) and the minimum insertion loss for a matched RF coupler. The third specification of the directional coupler is the isolation. A well designed directional coupler will feed power from the input port to the direct port and to the coupled port only. Thus, there should be no power at the isolated port so that an ideal coupler would have infinite isolation. In practice, some power is always passed to the isolated port, and the isolation of the coupler gives the relative level of this power. The final specification of a directional coupler, the return loss, can be measured at each port. Typically, a directional coupler is designed to be terminated into 50Ω loads at each port, and the return loss is a measure of how closely matched the impedance presented by the coupler at a given port is to the impedance terminating the same port. 
     An alternative measure of the isolation of a directional coupler is the directivity, which is the isolation in decibels minus the coupling ratio in decibels. In this context, a coupler can be described as a high directivity coupler if there is a very low ratio of the power fed to the isolated port from the input port compared with the power fed to the coupled port from the input port. 
     It is well known in the design of a directional coupler, that a critical requirement for high isolation and high directivity is that the product of the even mode impedance Z 0E  of the coupled transmission lines with the odd mode impedance Z 0O  of the coupled transmission lines should be equal to the square of the reference terminating impedance Z 0  on the four ports of the directional coupler—see EQUATION 1 below. For example, see Mongia, R; Bahl, I; Bhartia, P; “RF and Microwave Coupled Line Circuits” ISBN: 0-89006-830-5; Artech House 1999; pp 137. The standard reference impedance Z 0  in most RF applications is 50 Ohms.
 
 Z   0O   ×Z   0E   =Z   O   2   EQUATION 1
 
     Generally speaking, the even mode impedance is determined by the physical dimensions of the coupled transmission lines the properties of the material surrounding them and the proximity of the coupled transmission lines to RF ground. On the other hand, the odd mode impedance is a function of the physical dimensions of the coupled transmission lines the properties of the material between the two transmission lines and the proximity of the coupled transmission lines to each other. Thus, both parameters are independent of each other, and the criteria of EQUATION 1 can be met provided that there are no limitations in the fabrication process of the coupled transmission lines. 
       FIG. 2B  shows an alternative prior art directional coupler which includes resistive attenuators  26 ,  28  connected at the coupled and isolated ports of  FIG. 2A  respectively. Resistive attenuators  26 ,  28  are both two terminal devices, a first terminal of resistive attenuator  26  is connected to coupled port  23 , and a second terminal of attenuator  26  provides a matched coupled port  27  of the directional coupler; similarly, a first terminal of resistive attenuator  28  is connected to isolated port  24 , and a second terminal of attenuator  28  provides a matched isolated port  28  of the directional coupler. Resistive attenuator  26 , connected at coupled port  23 , is provided to reduce the effect of a mismatch from a connection at matched coupled port  27  of the directional coupler. A mismatch would occur, for example, if the impedance connected at matched coupled port  27  of the directional coupler were not exactly equal to 50 Ohms, and in typical applications, this can often be the case. As an example, a 5 dB attenuator connected at coupled port  23  would improve the return loss at matched coupled port  27  of the directional coupler by 10 dB. Conversely, the use of an attenuator in the manner shown in  FIG. 2B  ensures that, regardless of the termination at matched coupled port  27 , the impedance presented to the coupled pair of transmission lines  25  is close to the required reference impedance and thus that the conditions of EQUATION 1 are met. 
     The attenuator  28  at the isolated port  24  of  FIG. 2B  is provided for symmetry, i.e. if the directional coupler is to be used in reverse, with power being fed to direct port  22  and power being coupled to isolated port  24 . The attenuator  28  at isolated port  24  will minimize the effect of any mismatch which may be connected at isolated port  24 . Attenuators  26 ,  28  do not significantly affect the insertion loss of the directional coupler of  FIG. 2B . Attenuator  26  gives rise to a reduction in the coupling ratio; however, compensation for this effect is possible by re-design of the pair of coupled transmission lines for higher coupling. As an example, it can be seen from TABLE 1 that for directional couplers providing coupling ratios of less than −15 dB, compensation for the addition of a 5 dB attenuator at the coupled port will produce a degradation of 0.32 dB or less in the insertion loss of the directional coupler. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Theoretical Minimum Insertion Loss of a Directional Coupler for a given 
               
               
                 Coupling Ratio. 
               
             
          
           
               
                 Percentage of Input Power 
                 Relative Power fed 
                 Theoretical Minimum 
               
               
                 fed to Coupled Port 
                 to Coupled Port/dB 
                 Insertion Loss 
               
               
                   
               
               
                 50% 
                 −3.0 dB  
                  −3.0 dB 
               
               
                 25% 
                 −6.0 dB  
                 −1.25 dB 
               
               
                 10% 
                 −10 dB 
                 −0.46 dB 
               
               
                  3% 
                 −15 dB 
                 −0.14 dB 
               
               
                  1% 
                 −20 dB 
                 −0.04 dB 
               
               
                 0.3%  
                 −25 dB 
                 −0.01 dB 
               
               
                   
               
             
          
         
       
     
       FIG. 3  shows a block diagram of part of the TX section of a prior art RF front-end circuit which includes a directional coupler and other components to monitor power levels emitted from the power amplification stage and to monitor power levels reflected from the antenna. A percentage of the RF power emitted by the power amplifier (PA) is fed via the directional coupler to the first power detector so that the level of power emitted by the PA can be monitored. Similarly, a percentage of the RF power reflected back into the circuit by the antenna is fed via the directional coupler to the second power detector. Hence, the directional coupler in the circuit of  FIG. 3  facilitates independent monitoring of the RF power emitted by the PA and the RF power reflected by the antenna. However, independent monitoring of these two power levels requires that the isolation of the directional coupler is sufficiently high to prevent a significant percentage of the signal emitted from the PA being fed directly to the 2 nd  power detector. Specifically, for a capability to measure two substantially different power levels emitted by the PA and reflected by the antenna (say a difference of 20 dB), the directional coupler is typically required to have a very high directivity E.G. 25 dB or higher. 
     From the description of the prior art provided above, it is clear that for RF power monitoring applications, a directional coupler is required to be compact, and to offer high directivity. 
     Significant problems in the design and fabrication of directional couplers arise from the limitations in the accuracy and control over the fabrication of transmission lines with the required physical dimensions. Similar problems arise due to the limitations in the consistency of the material properties of the substrate on which the transmission lines are fabricated and batch variations in the thickness of the substrate. These limitations influence the capability to fabricate a coupler which meets the conditions of EQUATION 1. Furthermore, in the design of a directional coupler, the choice of available substrates is also limited to a few materials and a few discrete substrate thicknesses. 
     The drive for greater miniaturization is another limiting factor: the realization of a directional coupler with sufficiently small outer dimensions typically demands transmission lines that have physical dimensions which may be outside the capability of the fabrication process. For example, fabrication of a directional coupler on a thin substrate allows a reduction in the height of the coupler, and the use of a substrate with a high dielectric constant allows for reduction in the length of the coupled transmission lines of the coupler for a given coupling ratio. However, the use of a thin substrate will lower the even mode impedance of the coupled transmission lines, and the use of a substrate with a high dielectric constant will lower both the even mode impedance and the odd mode impedances of the coupled lines. 
     It is possible to compensate for the reduction in the even mode impedance by using narrower transmission lines; however the design rules of the production process typically sets a lower limit on the dimensions of lines. On the other hand, it is possible to compensate for a low odd mode impedance arising from the use of a substrate with a high dielectric constant by designing a coupler with transmission lines which are spaced further apart; unfortunately, increasing the spacing between the transmission lines lowers the coupling ratio of the directional coupler, and the only way to compensate for a lower coupling ratio is to use longer transmission lines thereby canceling any the benefit of selecting a high dielectric substrate for miniaturization. 
     In summary, the designer of a miniaturized directional coupler is faced with the dilemma that dimensions of the coupled transmission lines, and the electrical properties of the material of the substrate determine the even mode impedance and the odd mode impedance of the directional coupler, but that the product of the even mode impedance and the odd mode impedance of the directional coupler must equal the square of the reference impedance according to EQUATION 1—2500Ω 2  for conventional RF applications. Hence, the designer is presented with a limited range of options to produce a directional coupler of the required size with the required performance and which can be fabricated to the required precision. 
     To overcome these problems, the designer needs an additional degree of freedom when selecting line widths and line spacing for producing a miniaturised directional coupler. 
     As mentioned previously, it has been well established in the design of a directional coupler, where high directivity is a goal, that the product of the even mode impedance and the odd mode impedance should be equal to the square of the reference impedance—see EQUATION 1. This condition, while valid, does not provide the most general requirement. 
     Referring once again to  FIG. 2A , the most general requirement for the design of a directional coupler with high directivity is that the product of the impedance terminating the direct port  22  and the impedance terminating the coupled port  23  should be equal to the product of the even mode impedance Z 0E  and the odd mode impedance Z 0O  of the coupled transmission lines. This relationship is given by EQUATION 2
 
 Z   P2   ×Z   P3   =Z   OO   ×Z   OE   EQUATION 2
 
where Z P2  is the value of the impedance terminating the direct port  22  and where Z P3  is the value of the impedance terminating the coupled port  23 .
 
     In practical use, the impedance terminating the direct port of a directional coupler Z P2  will invariably be the reference impedance. In fact, the assumption that the reference impedance terminates all ports of a directional coupler is the starting point in most technical analyses on the subject. However, it is possible to transform the impedance terminating the coupled port using an impedance transformation circuit. One example of a circuit which can provide impedance transformation is a resistive attenuator, such as a PI-type resistive attenuator. Conveniently, as described above and as illustrated in  FIG. 2B  a resistive attenuator can be used advantageously in a directional coupler to provide matching of a poor or unknown termination at the coupled port. However, a resistive attenuator may also be used to provide impedance transformation of a reference impedance to some other value. 
       FIG. 4  shows an exemplary drawing of a prior art PI-type attenuator circuit which can provide both impedance matching and attenuation and which can also provide impedance transformation. The level of attenuation and the impedance transformation ratio of the circuit of  FIG. 4  is determined by the values of resistors R 41 , R 42 , and R 43 . 
     SUMMARY OF THE INVENTION 
     The present invention to provide a directional coupler according to claim  1 . 
     Preferably, the directional coupler includes an impedance matching and impedance transforming attenuator connected at the third RF port which provides a level of attenuation and, moreover, which transforms a reference impedance value Z 0  to a transformed impedance value Z P3  not equal to Z 0  and given by the following equation: 
     
       
         
           
             
               
                 
                   
                     Z 
                     
                       P 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       3 
                     
                   
                   = 
                   
                     
                       
                         Z 
                         00 
                       
                       × 
                       
                         Z 
                         
                           0 
                           ⁢ 
                           E 
                         
                       
                     
                     
                       Z 
                       0 
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     Preferably the product of the even mode impedance and odd mode impedance of the pair coupled transmission lines of the present invention has a value that is less than the square of the standard reference impedance for RF devices—i.e. less than 2500Ω 2 , so that transformed impedance value Z P3  is less than a reference impedance value Z 0 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a block diagram of a prior art RF front-end circuit employing a directional coupler for power monitoring. 
         FIG. 2A  shows a diagram of a prior art directional coupler comprising a pair of electromagnetically coupled transmission lines and 4 input/output ports. 
         FIG. 2B  shows a circuit diagram of a prior art directional coupler with attenuators added at the coupled port and at the isolated port. 
         FIG. 3  shows a block diagram of a prior art RF front-end circuit employing a directional coupler with separate monitoring of PA output power and reflected power. 
         FIG. 4  shows a circuit diagram of a prior art PI-type resistive attenuator, which can provide impedance transformation. 
         FIG. 5  shows a circuit diagram of a directional coupler according to the present invention including an impedance matching and impedance transforming attenuator according to a first embodiment of the present invention. 
         FIG. 6A  shows a 3 dimensional drawing of a prior art structure comprising a pair of broadside coupled transmission lines. 
         FIG. 6B  shows a 3 dimensional drawing of a prior art structure comprising a pair of edge coupled transmission lines. 
         FIG. 7  shows a circuit diagram of a symmetrical directional coupler according to the present invention including a pair of impedance matching and impedance transforming attenuators according to a second embodiment of the present invention. 
         FIG. 8  shows a circuit diagram of a directional coupler according to the present invention including an impedance matching and impedance transforming attenuator according to a third embodiment of the present invention. 
         FIG. 9  shows a cross section drawing of a thin-film structure to be used in the fabrication of a directional coupler with integrated matching and impedance transformation. 
         FIG. 10  shows an example layout of a directional coupler according to the present invention and implemented using thin-film technology as shown in  FIG. 9 . 
         FIG. 11  shows a cross section drawing of a multilayer chip component suitable for the fabrication of a directional coupler according to the present invention. 
         FIG. 12  shows a comparison of four performance plots of a directional coupler according to  FIG. 7  for two values of the impedance Z P3 . and showing that the isolation of the directional coupler is optimum when Z P3 .=40Ω. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 5  shows a circuit diagram of a directional coupler according to a first embodiment of the present invention comprising a pair of coupled transmission lines  55 , the pair of transmission lines  55  being located in close proximity to each other so that they are electromagnetically coupled to each other. The pair of coupled transmission lines  55  comprises a first transmission line  55 A and a second transmission line  55 B where the first transmission line  55 A comprises a first end, to which a first RF port  51  is connected, and a second end, to which a second RF port  52  is connected, and where the second transmission line  55 B comprises a first end, to which a third RF port  53  is connected, and a second end, to which a fourth RF port  54  is connected. An input electrical signal that is fed to first RF port  51  will produce a direct electrical signal at second RF port  52 , and a coupled RF signal at third RF port  53 . Under ideal operating conditions, the same input signal will produce no signal (or a negligibly small signal) at fourth RF port  54 . The pair of coupled transmission lines can be characterized by an even mode impedance and odd mode impedance of the coupled transmission lines, where the values of the even mode impedance and the odd mode impedance are determined by the physical dimensions of the pair of coupled transmission lines  55  and the electrical properties of the materials between and surrounding the pair of coupled transmission lines  55 . The material of the pair coupled transmission lines also has an effect on the impedances but this effect is small provided that the pair of coupled transmission lines are fabricated from a material that is a good electrical conductor at the frequency of operation of the directional coupler. Preferably, the dimensions of the pair of coupled transmission lines and properties of the materials between and surrounding them are selected to enable easy fabrication and miniaturization of the directional coupler. 
     The directional coupler of  FIG. 5  of the present invention further includes a two terminal impedance matching and impedance transforming attenuator  56  with one terminal thereof connected to third RF port  53  and with another terminal thereof forming a fifth RF port  57 . Impedance matching and impedance transforming attenuator  56  provides a level of attenuation and, moreover, transforms the reference impedance value Z 0  (typically 50 Ohms) to a transformed impedance value Z P3  given by EQUATION 3. 
     Preferably the product of the even mode impedance and odd mode impedance of the pair coupled transmission lines  55  of the present invention has a value that is less than the square of the standard reference impedance for RF devices—ie less than 2500Ω 2 —so that the transformed impedance value Z P3  is less than 50 Ohms, and preferably less than 45 Ohms, or 10% less than the reference impedance, so enabling a commensurate increase in the width of one or both of the transmission lines  55 A,  55 B. 
     The directional coupler of the present invention has 4 input/output ports as follows: first RF port  51 , which can be labeled as the input port of the directional coupler; second RF port  52 , which can be labeled as the direct port of the directional coupler; fifth RF port  57 , which can be labeled as the coupled port of the directional coupler; and fourth RF port  54  which can be labeled as the isolated port of the directional coupler. 
     In  FIG. 5 , the impedance matching and impedance transforming attenuator  56  comprises a PI network, however, as will be described later, it could equally comprise a T network. 
     Preferably the impedance matching and impedance transforming attenuator  56  comprises three resistors, a first shunt resistor R 51  connected to input/output port  57  of the directional coupler, a second shunt resistor R 52  connected to third RF port  53  and a series resistor R 53  with one terminal connected to third RF port  53  and another terminal connected to input/output port  57  of the directional coupler. 
     The respective values of resistors R 51 , R 52 , and R 53  are given by EQUATIONS 4a, 4b, 4c and 4d below, where ATT is the attenuation of impedance matching and impedance transforming attenuator  56 . 
     
       
         
           
             
               
                 
                   
                     R 
                     53 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           k 
                           - 
                           1 
                         
                         ) 
                       
                       2 
                     
                     ⁢ 
                     
                       
                         
                           
                             Z 
                             0 
                           
                           ⁢ 
                           
                             Z 
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               3 
                             
                           
                         
                         k 
                       
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                   ⁢ 
                   a 
                 
               
             
             
               
                 
                   
                     1 
                     
                       R 
                       52 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           k 
                           + 
                           1 
                         
                         ) 
                       
                       
                         
                           Z 
                           
                             P 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             k 
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       1 
                       
                         R 
                         53 
                       
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                   ⁢ 
                   b 
                 
               
             
             
               
                 
                   
                     1 
                     
                       R 
                       51 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           k 
                           + 
                           1 
                         
                         ) 
                       
                       
                         
                           Z 
                           0 
                         
                         ⁡ 
                         
                           ( 
                           
                             k 
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       1 
                       
                         R 
                         53 
                       
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                   ⁢ 
                   c 
                 
               
             
             
               
                 
                   k 
                   = 
                   
                     10 
                     
                       ATT 
                       10 
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                   ⁢ 
                   d 
                 
               
             
           
         
       
     
     The arrangement of the pair of coupled transmission lines  55 , with impedance matching and impedance transforming attenuator  56  in the present invention is such that the directional coupler is matched to the reference impedance Z 0  at all input/output ports  51 ,  52 ,  57  and  54 , while, at the same time, the designer has the option to choose a low value for the product of the even mode impedance Z 0E  and the odd mode impedance Z 0O  of the pair of coupled transmission lines  55  so as to facilitate easy fabrication and miniaturization. 
     Specifically, the arrangement of the pair of coupled transmission lines  55 , with impedance matching and impedance transforming attenuator  56  in the present invention is such that the designer has the option to select a pair of coupled transmission lines  55 , where the constituent lines  55 A and  55 B are wider than would be required in order that the criteria of EQUATION 1 be met. 
     The use of wider lines reduces the product of the even mode impedance and the odd mode impedance of the pair of coupled transmission lines  55 , however the designer can correct for this effect by a suitable choice of the impedance Z P3 , and corresponding suitable values of resistors R 51 , R 52  and R 53  in order that the criteria of EQUATION 3 be met. The use of wider transmission lines  55 A and  55 B for the directional coupler of the present invention has a number of benefits for mass production: wider lines are easier to fabricate, which may enable the process or result in a lower cost process; wider lines are less affected by variations in mass production process; wider lines are less affected by misalignment of layers for broadside coupled lines. Moreover, wider lines offer higher coupling, which can be of benefit to the designer when trying to produce a lineup of directional couplers offering a range of coupling ratios. 
       FIG. 6A  shows a 3 dimensional drawing of a pair of broadside coupled transmission lines  62  comprising first transmission line  62 A and second transmission line  62 B, where first and second transmission lines  62 A and  62 B are fabricated in an insulating substrate  60 . Substrate  60  may, for example, be constructed of several insulating layers which are stacked and which are formed into a block as part of the production process. Electromagnetic coupling between the pair of coupled transmission lines  62  takes place primarily between the adjacent faces of the first and second transmission lines  62 A and  62 B. Metal ground planes  64  and  66  are typically (but not necessarily) fabricated above and below the pair of coupled transmission lines or a single ground plane may be provided above  64  or below  66  the pair of coupled transmission lines  62 . The distance H 1  from the pair of coupled transmission lines  62  to the nearest ground plane and the widths W of the coupled transmission lines are critical parameters in determining the even mode impedance of the pair of coupled transmission lines. The gap G between the adjacent faces of the pair of coupled transmission lines  62  and the widths W of the coupled transmission lines are critical parameters in determining the coupling between the lines and the odd mode impedance of the pair of coupled transmission lines. The coupled transmission lines  55  of the embodiment of the present invention depicted in  FIG. 5  may, for example, be formed as a pair of broadside coupled transmission lines, such as is shown in  FIG. 6A . 
     For a directional coupler comprising a pair of broad side coupled transmission lines as depicted in  FIG. 6A , it is often preferable for the designer to use a pair of coupled transmission lines  62 , where the first transmission line  62 A is wider than the second transmission line  62 B (or vice versa). This design choice reduces the effects of misalignment error in the mass production of the directional coupler, but also has the effect of lowering the product of the even-mode impedance and the odd mode impedance of the pair of coupled transmission lines. Nonetheless, according to present invention, the effect of the lowered impedance product can be corrected by a suitable choice of the impedance matching and impedance transforming attenuator so that the product of the even mode impedance Z 0E  and odd mode impedance Z 0O  of the coupled transmission lines, the value of the reference impedance Z 0 , and the value of the transformed impedance Z P3 , are in agreement with EQUATION 3. 
       FIG. 6B  shows a 3 dimensional drawing of a pair of edge coupled transmission lines  63  comprising first metal transmission line  63 A and second metal transmission line  63 B, where first and second transmission lines  63 A and  63 B are fabricated in an insulating substrate  61 . The electromagnetic coupling between the pair of transmission lines takes place primarily between the two adjacent edges of the pair of coupled transmission lines. Metal ground planes  65  and  67  may be fabricated above and/or below the pair of coupled transmission lines  63 . The distance H 2  from the pair of coupled transmission lines  63  to the nearest ground plane and the widths W of the coupled transmission lines are critical parameters in determining the even mode impedance of the pair of coupled transmission lines. The spacing S between the first and second transmission lines  63 A and  63 B is a critical parameter in determining the coupling between the lines, and similarly the odd mode impedance of the coupled transmission lines. The pair of coupled transmission lines  55  of the embodiment of the present invention depicted in  FIG. 5  may, for example, be formed as a pair of edge coupled transmission lines, such as is shown in  FIG. 6B . 
       FIG. 7  shows a circuit diagram of a directional coupler according to a second embodiment of the present invention comprising a pair of coupled transmission lines  75 , the pair of transmission lines  75  being located in close proximity to each other so that they are electromagnetically coupled to each other. The pair of coupled transmission lines  75  comprises a first transmission line  75 A and a second transmission line  75 B where the first transmission line  75 A comprises a first end, to which a first RF port  71  is connected, and a second end, to which a second RF port  72  is connected, and where the second transmission line  75 B comprises a first end, to which a third RF port  73  is connected, and a second end, to which a fourth RF port  74  is connected. An input electrical signal that is fed to first RF port  71  will produce a direct electrical signal at second RF port  72 , and a coupled RF signal at third RF port  73 ; under ideal operating conditions, the same input signal will produce no signal (or a negligibly small signal) at fourth RF port  74 . As for the first embodiment depicted in  FIG. 5 , the pair of coupled transmission lines can be characterized by an even mode impedance and odd mode impedance of the coupled transmission lines, where the values of the even mode impedance and the odd mode impedance are determined by the physical dimensions of the pair of coupled transmission lines  75  and the electrical properties of the materials between and surrounding the pair of coupled transmission lines  75 . Preferably, these dimensions and properties are selected to enable easy fabrication and miniaturization of the directional coupler. 
     The directional coupler of  FIG. 7  of the present invention further includes a pair of two terminal impedance matching and impedance transforming attenuators  76 ,  78  with one terminal of impedance transformation attenuator  76  connected to third RF port  73  and with another terminal thereof forming a fifth RF port  77 ; similarly, one terminal of impedance transformation attenuator  78  is connected to fourth RF port  74  and another terminal thereof forms a sixth RF port  79  of the directional coupler. Impedance matching and impedance transforming attenuator  76  provides a level of attenuation and, moreover, transforms the reference impedance value Z 0  (typically 50 Ohms) to a transformed impedance value Z P3  given by EQUATION 3. Similarly, impedance transforming attenuator  78  provides a level of attenuation and, moreover, transforms the reference impedance value Z 0  (typically 50 Ohms) to a transformed impedance value Z P4 . Preferably, Z P4  is equal to Z P3 . 
     Preferably the product of the even mode impedance and odd mode impedance of the pair coupled transmission lines  75  of the present invention has a value that is less than the square of the standard reference impedance for RF devices—ie less than 2500Ω 2 —so that the transformed impedance value Z P3  is less than 50 Ohms, and preferably less than 45 Ohms, or 10% less than the reference impedance, so enabling a commensurate increase in the width of one or both of the transmission lines  75 A,  75 B. 
     The directional coupler of  FIG. 7  has 4 input/output ports as follows: first RF port  71 , which can be labeled as the input port of the directional coupler; second RF port  72 , which can be labeled as the direct port of the directional coupler; fifth RF port  77 , which can be labeled as the coupled port of the directional coupler; and sixth RF port  79  which can be labeled as the isolated port of the directional coupler. 
     In  FIG. 7 , the impedance matching and impedance transforming attenuators  76  and  78  comprise respective PI networks, however, as will be described later, they could equally comprise a T network. 
     Preferably impedance matching and impedance transforming attenuator  76  comprises three resistors, a first shunt resistor R 71  connected to input/output port  77  of the directional coupler, a second shunt resistor R 72  connected to third RF port  73  and a series resistor R 73  with one terminal connected to third RF port  73  and another terminal connected to input/output port  77  of the directional coupler. 
     The respective values of resistors R 71 , R 72 , and R 73  are given by EQUATIONS 4a, 4b, 4c and 4d above. 
     A similar arrangement describes impedance matching and impedance transforming attenuator  78 . 
       FIG. 12  shows a comparison of four performance plots of a manufactured directional coupler according to  FIG. 7  where the widths of the pair of coupled transmission lines  75  were selected to suit the tolerances of the manufacturing process and with increased widths compared with a directional coupler designed to satisfy the criteria of EQUATION 1. Two alternative versions of this directional coupler were produced: one with a conventional impedance matching attenuator connected at third RF port  73 , thus providing an impedance of 50Ω at third RF port  73  and a second with an impedance matching and impedance transforming attenuator  76  connected at third RF port  73  which transforms an impedance of 50Ω at fifth RF port  77  providing a transformed impedance Z P3  of 40Ω at third RF port  73 . It can be seen that the isolation and directivity of the second directional coupler (i.e. when Z P3 .=40Ω) are both improved when compared with the first. 
       FIG. 8  shows a circuit diagram of a directional coupler according to a third embodiment of the present invention comprising a pair of coupled transmission lines  85 , the pair of transmission lines  85  being located in close proximity to each other so that they are electromagnetically coupled to each other. The pair of coupled transmission lines  85  comprises a first transmission line  85 A and a second transmission line  85 B where first transmission line  85 A comprises a first end, to which a first RF port  81  is connected, and a second end, to which a second RF port  82  is connected, and where second transmission line  85 B comprises a first end, to which a third RF port  83  is connected, and a second end, to which a fourth RF port  84  is connected. An input electrical signal that is fed to first RF port  81  will produce a direct electrical signal at second RF port  82 , and a coupled RF signal at third RF port  83 ; under ideal operating conditions, the same input signal will produce no signal (or a negligibly small signal) at fourth RF port  84 . The pair of coupled transmission lines can be characterized by an even mode impedance and odd mode impedance of the coupled transmission lines, where the values of the even mode impedance and the odd mode impedance are determined by the physical dimensions of the pair of coupled transmission lines  85  and the electrical properties of the materials surrounding and between coupled transmission lines  95 . Preferably, these dimensions and properties are selected to enable easy fabrication and miniaturization of the directional coupler. 
     The directional coupler of  FIG. 8  of the present invention further includes a two terminal impedance matching and impedance transforming attenuator  86  with one terminal thereof connected to third RF port  83  and with another terminal thereof forming a fifth RF port  87  of the directional coupler. Impedance matching and impedance transforming attenuator  86  provides a level of attenuation and, moreover, transforms the reference impedance value Z 0  (typically 50 Ohms) to a transformed impedance value Z P3  given by EQUATION 3. 
     Preferably the product of the even mode impedance and odd mode impedance of the pair coupled transmission lines  85  of the present invention has a value that is less than the square of the standard reference impedance for RF devices—ie less than 2500Ω 2 —so that the transformed impedance value Z P3  is less than 50 Ohms, and preferably less than 45 Ohms, or 10% less than the reference impedance, so enabling a commensurate increase in the width of one or both of the transmission lines  85 A,  85 B. 
     The directional coupler of  FIG. 8  of the present invention has 4 input/output ports as follows: first RF port  81 , which can be labeled as the input port of the directional coupler; second RF port  82 , which can be labeled as the direct port of the directional coupler; fifth RF port  87 , which can be labeled as the coupled port of the directional coupler; and fourth RF port  84  which can be labeled as the isolated port of the directional coupler. 
     Impedance matching and impedance transforming attenuator  86  of  FIG. 8  comprises a T network in this case comprising three resistors, a first series resistor R 81  with a first terminal thereof connected to input/output port  87  of the directional coupler, a second series resistor R 82  with a first terminal thereof connected to third RF port  83  where the second terminals of said first and second series resistors R 81  and R 82  are connected together at a common node N. Impedance matching and impedance transforming attenuator  86  further comprising a shunt resistor R 83  which is connected to common node N. 
     The values of first series resistor R 81 , second series resistor R 82  and shunt resistor R 83  are given by equations 5a-5d, and preferably the value of Z P3  is less than that of Z 0 . 
     
       
         
           
             
               
                 
                   
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       FIG. 9  shows a cross section of thin-film structure which is, for example, suitable for a physical implementation of the embodiments of the directional couplers of the present invention described herein. The structure comprises a thin-film chip  90  with a first surface including multiple thin layers fabricated thereon where thin-film chip  90  is mounted on a carrier PCB  99 , comprising a substrate layer  97  sandwiched between two metal or electrically conductive layers  96 A,  96 B. In the exemplary drawing of  FIG. 9 , thin-film chip  90  is mounted so that the first surface of the chip faces carrier PCB  99 —i.e. faces downwards in  FIG. 9 . Thin-film chip  90  comprises a base substrate  91  formed of an insulating material with high Q at RF frequencies (E.G. Alumina or high Q Silicon). Thin layers are fabricated on the first surface of thin film chip  90  as follows: first insulation layer  92 A fabricated firstly on the first surface of thin-film chip  90 ; first metal layer  93 A fabricated secondly on the first surface of thin-film chip  90 ; resistive film layer  94  fabricated thirdly on the first surface of thin-film chip  90 ; second insulation layer  92 B fabricated fourthly on the first surface of thin-film chip  91 ; second metal layer  93 B fabricated fifthly on the first surface of thin-film chip  91 ; third insulation layer  92 C fabricated sixthly on the first surface of thin-film chip  90 . First insulation layer  92 A is provided as a barrier to protect base substrate  91  from the effects of the fabrication of the subsequent layers. During the fabrication process each of first metal layer  93 A, resistive film layer  94 , second insulation layer  92 B, second metal layer  93 B and third insulation layer  92 C are patterned to provide the required electrical properties of a directional coupler according to the present invention. Electrically conducting pads  98  protrude from the top of thin-film chip  90  so as to provide electrical contact between carrier PCB  99  and thin-film chip  90 . Electrically conducting pads  98  are fabricated so as to produce a specific gap between thin-film chip  90  and carrier PCB  99  after mounting and assembly. 
     The metal layer  96 B of carrier PCB  99  which is furthest from thin-film chip  90  typically is connected to electrical ground, and hence provides a ground plane of thin-film chip  90 . A back-side metal layer  95  may optionally be fabricated on the other face of thin-film chip  90 . 
       FIG. 10  shows an example layout of a directional coupler according to the present invention and implemented using thin-film technology as described above. Layer  01  of  FIG. 10  shows a suitable pattern for first metal layer  93 A superimposed with resistive film layer  94 , and Layer  02  of  FIG. 10  shows a suitable pattern for second metal layer  93 B. As mentioned in the description of  FIG. 9 , patterned insulating layers would typically be formed above, below and between Layer  01  and Layer  02 , but these layers are not shown in  FIG. 10 . 
     The layout shown in  FIG. 10  is based on the circuit diagram of a symmetrical directional coupler according to the present invention shown in  FIG. 7  herein. Resistors R 71 , R 72  and R 73  are shown as R 1 , R 2  and R 3  respectively in  FIG. 10 , where R 1 , R 2 , and R 3  each are rectangles of resistive film left behind after the process of patterning layer  94  has been completed. Similarly, resistors R 74 , R 75  and R 76  are shown as resistive film rectangles R 4 , R 5  and R 5  respectively in  FIG. 10 . The resistance of a rectangle of resistive film is easily calculated by counting the number of squares contained in the rectangle and by multiplying that number by a given constant for the resistive film; thus, it can be seen that the patterned rectangles of resistive film R 1  R 2  and R 3  of  FIG. 10  each have different resistances as would be predicted by equation 3 and equations 4 above. 
     The directional coupler layout of  FIG. 10  comprises coupled transmission lines T 1  and T 2 , corresponding to coupled transmission lines  75  of  FIG. 7 . Coupled transmission lines T 1  and T 2  of  FIG. 10  are fabricated on separate layers, with an insulating layer between, and consequently the directional coupler of  FIG. 10  comprises a pair of broadside coupled transmission lines, as depicted in  FIG. 6A  above. 
     Input/output ports of the directional coupler of  FIG. 10  are labeled as follows: input port  101 , direct port  102 , coupled port  107  and isolated port  109 . It should be noted that by symmetry, the input/output ports of the directional coupler of  FIG. 10  might just as easily be labeled as input port  102 , direct port  101 , coupled port  109  and isolated port  107 . 
     Electrical connection between Layer  01  and Layer  02  of  FIG. 10  would typically be achieved by fabricating holes in the insulating layer separating Layer  01  and Layer  02 . For example, holes would be formed in the insulating layer to permit electrical connection between the pads shown in  FIG. 10 . 
     The directional coupler of the present invention might alternatively be formed as a multilayer chip component comprising a plurality of electrically insulating layers where the insulating layers are stacked on top of each other and where patterned metallic circuit layers, and patterned metallic ground layers are interspersed between the insulating layers. In this case, the pair of coupled transmission lines is formed within the multilayer chip component, and the at least one impedance matching and impedance transforming attenuator is formed externally to the multilayer chip component. 
       FIG. 11  shows a cross section view of a multilayer chip component  110  suitable for the fabrication of a directional coupler according to the present invention. 
     Multilayer chip component  110  comprises a plurality of electrically insulating layers  111 A,  111 B,  111 C,  111 D,  111 E, where the layers are stacked on top of each other. Electrically insulating layers  111 A,  111 B,  111 C,  111 D,  111 E, are formed of a suitable insulating material, for example ceramic, or a composite material, where the material is suitable for a stacking and curing process, and where the material provides a high electrical Q or a low loss factor at RF and microwave frequencies—for example from 500 MHz to 60 GHz. 
     Interspersed between insulating layers  111 A,  111 B,  111 C,  111 D,  111 E are patterned metallic circuit layers,  113 A  113 B, and patterned metallic ground layers  115 A,  115 B. The patterning of metallic circuit layers  113 A,  113 B and metallic ground layers  115 A,  115 B takes place during the fabrication process of multilayer chip component  110 . 
     Patterned metallic circuit layers  113 A and  113 B, form a pair of coupled transmission lines, either broadside coupled—as shown in  FIG. 6A  or edge coupled—as shown to  FIG. 6B . Patterned metallic ground layers  115 A,  115 B provide respective upper and lower ground planes for the pair of coupled transmission line. However, as noted herein, either or both of upper and lower ground planes  115 A,  115 B can be omitted from the chip structure, for example, in the case where there is a ground plane provided on by the carrier PCB on which multilayer chip component  110  is mounted. 
     Multilayer chip component  110  comprises metallic terminals  117 A,  117 B for electrical connection between multilayer chip component  110  and an external circuit (not shown). Metallic terminals  117 A,  117 B are preferably located on a reverse face of multilayer chip component  110 . Multilayer chip component  110  may also include metallic SMT pads  119 A,  119 B for electrical connection between multilayer chip component and one or more SMT components to be mounted on an obverse face of multilayer chip component  110 . Electrical connection between SMT pads  119 A,  119 B (if present), patterned metallic ground layers  115 A,  115 B, patterned metallic circuit layers  113 A,  113 B and metallic terminals  117 A and  117 B are provided by a plurality of electrically conducive through holes TH, which penetrate insulating layers  111 A,  111 B,  111 C,  111 D,  111 E. Through holes TH are rendered electrically conductive during the fabrication of multilayer chip  110  by a process of filling each through hole TH with electrically conducive paste, or by a process of electroplating the inner surface of each through hole TH. 
     The pair of coupled transmission lines of the directional coupler of the present inventions may be formed on a pair of adjacent metallic circuit layers  113 A,  113 B of multilayer chip component  110  as shown in  FIG. 11 , or they may be formed on a single metallic circuit layer, for example in the case where the pair of coupled transmission lines are of the edge-coupled type, as shown in  FIG. 6B . Alternatively, the pair of coupled transmission lines may be formed over several metallic circuit layers of multilayer chip component  110  (not shown). 
     The impedance matching and impedance transforming attenuator of the directional coupler of present invention may be formed externally to chip component  110 , E.G. by a set of three SMT resistors mounted adjacent to the coupled port of the directional coupler, with appropriate values given by EQUATIONS 4a, 4b, 4c, 4d or 5a, 5b, 5c, 5d above. Alternatively, the impedance matching and impedance transforming attenuator may be formed on the surface of chip component  110 , E.G. by mounting a set of three SMT resistors on a surface of multilayer chip component and where electrical contact between the SMT resistors and the other circuit elements (pair of coupled transmission lines, patterned metallic ground layers  115 A,  115 B etc.) is made by means of SMT pads  119 A,  119 B and through holes TH. 
     The present invention is not limited to the embodiments described herein.

Technology Category: 5