Abstract:
A differential reflection bridge is provided for a 100 Ohm load, the bridge not being compromised by a translation to a 50 Ohm system. The reflection bridge uses two transmission line baluns. The first traditional balun T 1  connects the input signal source to a resistor bridge. The second balun T 2  connects between a central node of the resistor bridge and an output OUT as well as a second test port that eliminates a path to ground. With no ground path the bridge is immune to common mode impedance disturbances.

Description:
BACKGROUND 
   1. Technical Field 
   The present invention relates to making differential impedance measurements of a resistive device using a reflection bridge. More particularly, the present invention relates to a reflection bridge providing for differential measurements of both 50 Ohm and 100 Ohm complex impedances. 
   2. Related Art 
   Conventional differential measurement systems for measuring 100 Ohm components included balanced 100 Ohm transmission lines. Characterization of components was provided using multiple single port measurements. The balanced parameters were extracted from at least two single ended measurements. 
     FIG. 1  shows a conventional differential bridge used to make differential measurements by obtaining two single ended measurements. The test system includes a signal source  2  connected through a 50 Ohm line  4  to provide an input. A resistive load  8 , labeled R 1 , and resistive load  9 , labeled R 2 , are each connected to one of two test ports TEST PORT  1  and TEST PORT  2  to be measured. A measurement output OUT is provided from one terminal of a resistor  6 , with a second end of resistor  6  connected to ground. Two couplers  10  and  12  individually connect the test ports with the signal source  2  to one of the test ports, and return a reflected signal to the output OUT. The couplers  10  and  12  are individually connected with switches, the first coupler  10  being connected with the switches in the “1” position, and the second coupler  12  being connected with the switches in the “2” position. 
   The two separate measurements at the output OUT determine θ1 and θ2. The values for θ1 and θ2 are in turn used to provide a combined differential measurement θ DIFF . The measurements of θ1 and θ2 are made using two single ended 50 Ohm S11 measurements between two test ports TEST PORT  1  and TEST PORT  2 . With RL=R 1 +R 2 , and RL=100 Ohms, a 100 Ohm differential S11 measurement is effectively obtained. Complex math is needed to determine the differential measurement value θ DIFF  using the equations shown to follow. 
   
     
       
         
           
               
           
           ⁢ 
           
             
               
                 
                     
                   ⁢ 
                   
                     
                       θ 
                       = 
                       
                         
                           R 
                           - 
                           50 
                         
                         
                           R 
                           + 
                           50 
                         
                       
                     
                     , 
                     
                       R 
                       = 
                       
                         50 
                         ⁢ 
                         
                           ( 
                           
                             
                               1 
                               + 
                               θ 
                             
                             
                               1 
                               - 
                               θ 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       R 
                       DIFF 
                     
                     = 
                     
                       R1 
                       + 
                       R2 
                     
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       R 
                       DIFF 
                     
                     = 
                     
                       
                         50 
                         ⁢ 
                         
                           ( 
                           
                             
                               1 
                               + 
                               θ1 
                             
                             
                               1 
                               - 
                               θ1 
                             
                           
                           ) 
                         
                       
                       + 
                       
                         50 
                         ⁢ 
                         
                           ( 
                           
                             
                               1 
                               + 
                               θ2 
                             
                             
                               1 
                               - 
                               θ2 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       θ 
                       DIFF 
                     
                     = 
                     
                       
                         
                           R 
                           DIFF 
                         
                         - 
                         100 
                       
                       
                         
                           R 
                           DIFF 
                         
                         + 
                         100 
                       
                     
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       θ 
                       DIFF 
                     
                     = 
                     
                       
                         
                           50 
                           ⁢ 
                           
                             ( 
                             
                               
                                 1 
                                 + 
                                 θ1 
                               
                               
                                 1 
                                 - 
                                 θ1 
                               
                             
                             ) 
                           
                         
                         + 
                         
                           50 
                           ⁢ 
                           
                             ( 
                             
                               
                                 1 
                                 + 
                                 θ2 
                               
                               
                                 1 
                                 - 
                                 θ2 
                               
                             
                             ) 
                           
                         
                         - 
                         100 
                       
                       
                         
                           50 
                           ⁢ 
                           
                             ( 
                             
                               
                                 1 
                                 + 
                                 θ1 
                               
                               
                                 1 
                                 - 
                                 θ1 
                               
                             
                             ) 
                           
                         
                         + 
                         
                           50 
                           ⁢ 
                           
                             ( 
                             
                               
                                 1 
                                 + 
                                 θ2 
                               
                               
                                 1 
                                 - 
                                 θ2 
                               
                             
                             ) 
                           
                         
                         + 
                         100 
                       
                     
                   
                 
               
             
           
         
       
     
   
   More recently a 50 Ohm to 100 Ohm balanced transformer was introduced to alleviate some of the problems encountered using the two measurement extraction approach. The balanced transformer approach, however, has limitations due to the need of a broad band 1:√2 turns ratio. The turns ration is provided by a balanced transformer, which is the heart of the converter. 
     FIG. 2  shows components of a differential bridge used such a balanced transformer  20 . The differential bridge uses the signal source  2  and resistor  4  as provided through a coupler  10 , as in  FIG. 1 . Unlike in  FIG. 1 , the balanced transformer  20  is used to connect the coupler  10  output to both test ports TEST PORT  1  and TEST PORT  2  in  FIG. 2  so that an additional coupler and switches as in  FIG. 1  are not needed. The output θ is provided from the coupler  10  through a resistor  6  connected to ground. The common mode current does not appear due to the “floating” secondary TEST PORT  2  replacing ground connections. Components carried over from  FIG. 1  to  FIG. 2  are similarly labeled, as will be components carried over in subsequent drawings. 
   The transformer  20  transforms from 50 Ohms to 100 Ohms with a turns ratio of 1:√2 turns ratio, or √(100/50)=1.4142. The transformer  20  is difficult to construct because of the non-integer or half-integer value of the turns ratio. A practical transformer typically requires a turns ratio ranging from 1 to 1.5. The RL to RP value would be 1.5 2  or 2.25. Using the specially constructed transformer to provide the turn ratio shown, a single test measurement of θ can be used to determine the resistance RL using the following formulas: 
   
     
       
         
           
             
               
                   
                 ⁢ 
                 
                   RP 
                   = 
                   
                     
                       RL 
                       
                         N 
                         2 
                       
                     
                     = 
                     
                       
                         RL 
                         
                           
                             2 
                           
                           2 
                         
                       
                       = 
                       
                         RL 
                         2 
                       
                     
                   
                 
               
             
           
           
             
               
                   
                 ⁢ 
                 
                   θ 
                   = 
                   
                     
                       
                         RL 
                         2 
                       
                       - 
                       50 
                     
                     
                       
                         RL 
                         2 
                       
                       + 
                       50 
                     
                   
                 
               
             
           
           
             
               
                   
                 ⁢ 
                 
                   θ 
                   = 
                   
                     
                       RL 
                       - 
                       100 
                     
                     
                       RL 
                       + 
                       100 
                     
                   
                 
               
             
           
         
       
     
   
   The 50 Ohm bridge would be balanced with 50 Ohms at its input and 50*2.25=112.5 Ohms between the differential test port outputs TEST PORT  1  and TEST PORT  2 . Because the RL typically cannot be 112.5, post processing can correct for this error after an open-short-load (OSL) calibration. 
     FIG. 3  shows components of a conventional basic measurement bridge. The bridge is balanced when the load resistor  8 , labeled RL, is 50 Ohms and the output OUT is at 0 Volts. The bridge of  FIG. 3  uses resistors  31 – 33  as opposed to one of couplers  10  or  12  of  FIG. 1  to provide reflected test signals from the test port to the output OUT for measurement. A transformer  30  (labeled T 1 ) connects the signal source  2  and resistor  4  to the bridge formed by resistors  31 – 33 . The T1 transformer  30  has a first winding connecting the resistor  4  to ground. A second winding of he T1 transformer  30  connects the TEST PORT and one end of resistor  31  to a common node  25 . The common node  25  is provided between resistors  32  and  33 . The common connection of resistors  31  and  32  form the output port OUT. The output port OUT is where test measurements are obtained, and is connected from the resistor  51  through a resistor  6  to ground. 
   Only a single ended measurement of the load resistor RL is available from TEST PORT  1 , so multiple measurements must be made to obtain a differential measurement, similar to that described with respect to  FIG. 1 . Further, with a ground path through resistor  6 , the bridge is subject to common mode impedance disturbances. 
     FIG. 4  shows modification of  FIG. 3  to provide a transmission line balun  30 A. A first winding of T1 balun  30 A connects from the input IN to the TEST PORT. A second winding of the T1 balun  30 A connects from the common node  25  to ground. The transmission line balun  30 A can be provided using a standard 50 Ohm transmission line with ferrite beads placed over it. The ferrite beads extend of the balun characteristics to low frequencies. This allows operation of the transformer T 1  from the low megahertz to the tens of gigahertz. 
   It would be desirable to provide a device to measure 100 Ohm components using a standard 50 Ohm system without requiring a two measurement process, and without requiring use of a balanced transformer. 
   SUMMARY 
   According to the present invention a measuring device is provided that uses a true 100 Ohm differential bridge that is not compromised by the translation from 50 to 100 Ohms through a non-ideal transformer. The measuring device although providing measurements of a 100 Ohm component also interfaces with a standard 50 Ohm system. The reflection bridge further allows for differential measurements between two test ports rather than single ended measurements, enabling elimination of common mode impedance disturbances. 
   The reflection bridge according to the present invention includes two baluns. A first traditional balun T 1  connects the input signal source to a resistor bridge. A second balun T 2  connects between the central node of the resistor bridge and the output OUT as well as to a second test port that eliminates a path to ground. With no ground path the bridge is immune to common mode impedance disturbances. 
   The system can be designed to test any load resistance. The system can be modified simply by changing resistors in the reflection bridge and changing the characteristic impedance of the transmission lines used for the baluns to the desired measurement impedance. 
   The first T1 and second T2 baluns can be connected as standard transformers or connected as transmission line baluns. Neither the T1 nor T2 balun is ideal, so non-standard baluns like ones that use a 1:√2 turn ratio are unnecessary. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the present invention are explained with the help of the attached drawings in which: 
       FIG. 1  shows components of a differential bridge that can be used to make a 100 Ohm differential S11 measurement using two single ended S11 measurements; 
       FIG. 2  shows components of a differential bridge used to make a 100 Ohm differential S11 measurement that uses a 50 Ohm to 100 Ohm transformer with a 1/√2 turns ratio; 
       FIG. 3  shows components of a basic measurement bridge; 
       FIG. 4  shows  FIG. 3  modified to provide the transformer T 1  using a transmission line balun; 
       FIG. 5  shows components of a differential measurement bridge according to embodiments of the present invention; 
       FIG. 6  shows modification of  FIG. 5  to provide transformers T 1  and T 2  using transmission line baluns; 
       FIG. 7  shows modification of  FIG. 5  to provide for measurement of a 100 Ohm resistance RL; 
       FIG. 8  shows modification of  FIG. 6  to provide for measurement of a 100 Ohm resistance RL using transmission line baluns for T 1  and T 2 ; 
       FIG. 9  shows modification of  FIG. 8  to show the inversion of the output of transformer T 2  to provide for common mode balance at the test ports; 
       FIG. 10  illustrates the hardware implementation for the circuit of  FIG. 9 ; 
       FIG. 11  shows a simplified equivalent circuit for the components of  FIG. 10 ; 
       FIG. 12  shows circuitry needed for a 50 to 100 Ohm attenuator; 
       FIG. 13  shows the attenuator of  FIG. 12  configured in a balanced form; 
       FIG. 14  shows the circuit of  FIG. 10  as modified to include balanced attenuators as shown in  FIG. 13 ; and 
       FIG. 15  shows a simplified equivalent circuit for the components of  FIG. 14 . 
   

   DETAILED DESCRIPTION 
     FIG. 5  shows a differential measurement bridge according to embodiments of the present invention. The bridge includes 50 Ohm resistors  31 – 33  similar to the resistive bridge of  FIG. 3 . A T1 transformer  30  couples a signal source  2  and resistor  4  to the resistors  31 – 33 . A resistor  6  connects a measurement output OUT to ground, similar to  FIG. 3 . 
     FIG. 5  according to the present invention, and in contrast with  FIG. 3 , further includes a second transformer labeled T 2 . The T2 transformer  40  has a first winding connecting from the output node OUT to ground. A second winding connects a node  38  between resistors  31  and  32  to a second test port TEST PORT  2 . The second test port TEST PORT  2  replaces the ground connection of  FIG. 3 . The second test port TEST PORT  2  is also connected to the resistor  33  and the 50 Ohm RL load resistor  8 . This allows RL to be measured differentially directly, rather than through single ended measurements. RL also now has no path to ground, making it immune to common mode impedance disturbances. 
     FIG. 6  shows a further embodiment of the present invention with transformers  30  and  40  of  FIG. 5  reconfigured as transmission line baluns  30 A and  40 A. As in  FIG. 4 , the transmission line balun  30 A has a first winding connecting the signal source  2  and 50 Ohm resistor  4  to TEST PORT  1 . A second winding connects the common node  25  to ground. According to the present invention, the second T2 transformer  40 A has a first winding connecting the output OUT to the node  38 . A second winding of transformer  40 A connects the second test port TEST PORT  2  to ground. 
     FIG. 7  shows modification to the bridge of  FIG. 5  to change the 50 Ohm resistors to 100 Ohm resistors. Resistors  4 ,  31 – 33  and the output resistor  6  are changed to 100 Ohms. The RL load resistor  8  is also changed to 100 Ohms. The bridge will be balanced when RL  8  is 100 Ohms, causing an output OUT of zero volts. 
     FIG. 8  shows modification to the bridge of  FIG. 6  to change to 100 Ohm resistors. The T1 and T2 transmission line baluns  30 A and  40 A are each formed using a piece of 100 Ohm coax transmission line with ferrite beads placed over it. 
     FIG. 9  shows modification to the bridge of  FIG. 8  to invert the T2 transmission line balun, now labeled  40 B. Inversion of the T2 balun  40 B is accomplished by connecting a first winding from ground to the node  38  between nodes  31  and  32 . A second winding is connected between the output OUT and the second test port TEST PORT  2 . The second winding connected to the bridge output OUT is formed by the center pin of balun  40 B, while the outer conductor of the balun  40 B forms the first winding. Connecting the balun  40 B in this manner provides common mode balance at the first and second test ports TEST PORT  1  and TEST PORT  2 . The inversion can be reversed using post processing techniques. 
     FIG. 10  illustrates how hardware is positioned to form the circuit of  FIG. 9 . The T1 and T2 baluns  30 A and  40 B are illustrated as a perspective drawing showing a section of 100 Ohm coax line with surrounding ferrite beads. The resistors  4 ,  6  and  31 – 33  and the RL resistor  8  show the relative position of these resistors in an actual circuit. 
     FIG. 11  shows a simplified equivalent circuit diagram for the components of  FIG. 10 . The equivalent circuit illustrates that the transformer  72  has a 1:1 turns ratio, and that dual TEST PORTS  1  and  2  connect to measure a 100 Ohm load RL. The test signal input IN and output OUT are also shown connected to 100 Ohm devices. 
   One additional modification to the bridge illustrated by  FIGS. 9–11  makes it more practical. The 100 Ohm transmission line used for baluns T 1  and T 2  are nonstandard. Additionally, the input IN and coupled output OUT impedances are also nonstandard, as these ports typically interface with 50 Ohm systems. Accordingly, some embodiments of the present invention use 50 Ohm input IN and coupled output OUT impedances. Transformation from 50 Ohm devices and 50 Ohm baluns is, thus, needed up to 100 Ohms for the bridge resistors  31 – 33 . The baluns for T 1  and T 2  can, thus, be provided with 50 Ohm transmission lines. This transformation can be accomplished by a minimum loss 50 to 100 Ohm matching pad. The penalty for using the matching pad is a power loss of 7.66 dB at the output Test Port. 
     FIGS. 12 and 13  illustrate components for providing a 50 Ohm to 100 Ohm transformation by using a minimal loss attenuator. In  FIG. 12 , the attenuator is provided by one 70.7 Ohm resistor  60  connected between an input IN and output OUT port. Another 70.7 Ohm resistor  61  connects from the input IN to ground. The input IN is at 50 Ohms, while the output OUT is transformed to 100 Ohms.  FIG. 13  shows a balanced attenuator that can more easily be used with the circuit of  FIG. 10 . The circuit of  FIG. 13  includes a first 35.35 Ohm resistor  64  connected from a first input IN 1  to a first output OUT 1 . A second 35.35 Ohm resistor  66  connects a second input IN 2  to a second output OUT 2 . A 70.7 Ohm resistor  66  connects the first input IN 1  and second input IN 2 . The inputs IN 1  and IN 2  are at 50 Ohms, while the outputs OUT 1  and OUT 2  are at 100 Ohms. 
     FIG. 14  shows placement of balanced attenuators  70  and  72  into the circuit of  FIG. 10 . Use of the balanced attenuators  70  and  72  allows input IN and output OUT impedances as well as balun impedances to be 50 Ohms, while the remaining impedances are 100 Ohms, as illustrated. The resistors  31 – 33 , as well as resistors of the attenuators  70  and  72  can be constructed with discrete resistors, thick film, thin film on substrate, or any technique that allows controlled impedances. 
     FIG. 15  is a simplified equivalent circuit diagram for the components of  FIG. 14 . The equivalent circuit illustrates that a transformer with attenuators now effectively provides a 1:√2 turns ratio. The dual test ports TEST PORT  1  and TEST PORT  2  connect to measure a 100 Ohm load RL. The test signal input IN and output OUT, however, are designed to connect to 50 Ohm devices. 
   Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many additional modifications will fall within the scope of the invention, as that scope is defined by the following claims.