Abstract:
A circuit can include multiple data input ports and data output ports, pickoff tees coupled therebetween, and a resistive network coupled between the pickoff tees. A differential signal generator can be coupled with the resistive network and the pickoff tees. Resistances of the pickoff tees and resistive network can be selected such that impedances looking into the data input ports and data output ports are matched to a desired system impedance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 61/591,587, titled “INTEGRATED COMBINER WITH COMMON MODE CORRECTION” and filed on Jan. 27, 2012, the content of which is hereby fully incorporated by reference herein. 
     
    
     TECHNICAL FIELD  
       [0002]    Embodiments of the disclosed technology generally relate to test and measurement instruments, and more particularly to circuits for adding common mode and differential mode signals onto broadband differential data. Embodiments generally allow for broad and flat frequency response in common mode signals and low-loss, broadband, and flat frequency response in the data. 
       BACKGROUND  
       [0003]      FIG. 1  is functional diagram illustrating an example of a prior system  100  that includes three resistive combiners  108 ,  110 , and  112  and two directional couplers  114  and  116 . In the example, the system  100  includes a first signal generator  102  that provides a differential data signal to the directional couplers  114  and  116 . The system  100  also includes a second signal generator  104  that provides an input signal, such as a 2.1 GHz sine wave, to a balun  105 . Balun  105  creates a differential mode signal from the single-ended signal it receives from signal generator  104 , and provides one end of the differential mode signal to resistive combiner  108  and the other end to resistive combiner  110 . The system  100  also includes a common mode (CM) signal generator  106  that provides a CM signal to an amplifier  107  before being passed to resistive combiner  112 . 
         [0004]    Resistive combiner  112  is electrically coupled between the amplifier  107  and each of the other two resistive combiners  108  and  110 , which also receive signals from the balun  105 . In this manner the resistive combiners  108 ,  110 , and  112  effectively combine the input signal from the second signal generator  104  with the CM signal from the CM signal generator  106 . Directional couplers  114  and  116  effectively apply the differential data from the first signal generator  102  to the resulting signals from resistive combiners  108  and  110 , respectively. 
         [0005]    Prior implementations such as the one illustrated by  FIG. 1  are physically large and expensive. Such implementations also have a number of disadvantages. For example, loss from input data to output data is generally higher at high frequencies than at low frequencies. In other words, such loss is not flat aver frequency. Also, loss from other inputs to output data tends to roll off as frequency decreases and can be inadequate at certain frequencies, some of which can be important. 
         [0006]      FIG. 2  is a functional diagram illustrating an example of a prior apparatus  200  that includes five resistive combiners  202  and  206 - 212 , balun  204 , and no directional couplers. In the example, resistive combiner  202  may receive the common mode (CM) input and balun  204  may receive the differential mode (DM) input. Resistive combiners  206  and  208  are each electrically coupled with resistive combiner  202  and balun  204  and also with resistive combiners  210  and  212 , respectively. Resistive combiners  210  and  212  are typically pickoff tees. 
         [0007]    Prior systems such as would use the apparatus of  FIG. 2  generally use resistive combining, e.g., pickoff tees  210  and  212 , for a flatter loss in input data to output data aver frequency. Such systems have loss in the input data to output data path that is relatively low (2 dB). However, a consequence is that a pickoff tee that injects the differential mode and common mode signals onto the data generally has higher impedance at the injection point than the system impedance of 50 Ohms, thereby causing reflections that lead to a non-flat transfer function from the CM input and DM input to the data output. 
         [0008]    Another disadvantage of prior approaches is that, in order to avoid further impedance mismatches, the balun that the DM input signal goes into, e.g., balun  204  of  FIG. 2 , must have a 2:1 turn ratio, which limits its frequency response relative to what is achievable with transmission line baluns having a 1:1 ratio. 
         [0009]      FIG. 3  is a functional diagram illustrating an example of a prior system  300  that includes four resistive combiners  310 - 316  that effectively combine a DM signal from a DM source  302  and CM signal from two separate CM sources  306  and  308  with an input signal from a pattern generator  304 . As with the apparatus  200  of  FIG. 2 , this system  300  also uses resistive combining for flat loss in the input data to output data over frequency. The system  300  also has the disadvantage of high through loss, i.e., 6 dB, from the input data to the output data. It also cannot receive as input a single-ended differential mode input but instead requires two signals that are 180 degrees out of phase and each have 50 Ohm impedance. 
         [0010]      FIG. 4  is a graphing that illustrates an example  400  of a data signal resulting from a prior system or apparatus such as that illustrated by  FIG. 1 . One having ordinary skill in the art will appreciate that the attenuation over frequency in the example  400  is not flat, thus resulting in a data signal that is not clean. 
         [0011]    Accordingly, a need remains for improved systems that add CM and DM signals onto differential data. 
       SUMMARY  
       [0012]    A combining circuit in accordance with the disclosed technology may include a resistive combiner configured to receive a differential mode (DM) signal from a DM input and a common mode (CM) correction signal from a CM correction input, a balun electrically coupled between the resistive combiner and a second resistive combiner configured to receive a CM signal from a CM input. The balun and second resistive combiner may be electrically coupled between multiple input data ports and output data ports. Pickoff tees may be used to electrically couple the balun and the second resistive combiner with the input data ports and output data ports. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is functional diagram illustrating an example of a prior system that includes three resistive combiners, a balun, and two directional couplers. 
           [0014]      FIG. 2  is a functional diagram illustrating an example of a prior apparatus that includes five resistive combiners, a balun, and no directional couplers. 
           [0015]      FIG. 3  is a functional diagram illustrating an example of a prior system  300  that includes four resistive combiners and no directional couplers. 
           [0016]      FIG. 4  is a graphing that illustrates an example of a data signal resulting from a prior system or apparatus such as that illustrated by  FIG. 1 . 
           [0017]      FIG. 5  is a functional diagram illustrating an example of a system in accordance with certain embodiments of the disclosed technology. 
           [0018]      FIG. 6  is a circuit diagram illustrating a more detailed view of the system illustrated by  FIG. 5  in accordance with certain embodiments of the disclosed technology. 
           [0019]      FIG. 7  is a graphing that illustrates an example of a data signal resulting from an implementation in accordance with the disclosed technology, such as that illustrated by  FIGS. 5 and 6 . 
           [0020]      FIG. 8  is a functional diagram illustrating another example of a system in accordance with certain embodiments of the disclosed technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Embodiments of the disclosed technology generally include reduced size resistive networks, such as would be suitable to be implemented in any of a number of different circuits and systems. These and other features and embodiments of the present invention proceed with reference to each of the figures. 
         [0022]      FIG. 5  is a functional diagram illustrating an example of a system  500  in accordance with certain embodiments of the disclosed technology. In the example, differential data may be received from DataIn+ and DataIn− ports  516  and  517 , respectively, and delivered to DataOut+ and DataOut− ports  518  and  519 , respectively with a small amount of loss. 
         [0023]    A single-ended signal from a differential mode (DM) input  502  may be resistively combined with a signal from a common mode (CM) correction input  504  by a resistive combiner  506  and then split into differential mode signal by a balun  508 . The balun  508  may be a 1:1 ratio transmission line balun, for example. 
         [0024]    The resulting signal from the balun  508  may then be applied to the output data ports DataOut+  518  and DataOut−  519  by way of two pickoff tees  514  and  515 , respectively. The signal may be applied with equal amplitude but 180 degrees out of phase. 
         [0025]    A signal from a CM input  510  may be applied to the output data ports DataOut+  518  and DataOut−  519  by way of a resistive network  512  and the pickoff tees  514  and  515 , respectively. The signal may be applied with equal amplitude and in phase. 
         [0026]    The signal received from the CM correction input  504  may be applied to the output data ports DataOut+  518  and DataOut−  519  with equal amplitude but 180 degrees out of phase. In situations where this signal is applied at the same frequency as the signal applied by the CM input  510 , its amplitude and phase may be adjusted in order to cancel out any DM signal that is inadvertently caused by the signal applied by the CM input  510 . 
         [0027]    In certain embodiments, various components used to implement the system  500  may be integrated, on a single circuit board and, consequently, they may be kept in close proximity to each other. In such embodiments, some or all of the ports may advantageously maintain a certain impedance, e.g., 50 Ohms, over a broad frequency range. 
         [0028]      FIG. 6  is a circuit diagram illustrating a more detailed view of the system  500  illustrated by  FIG. 5  in accordance with certain embodiments of the disclosed technology. In the example, the DM input  502  is terminated by resistor  520  in series with resistor  530 , in parallel with the resistances to the DataIn+/− and DataOut+/− ports, i.e., resistors  522  and  524  to the DataIn+ port  516 , resistors  532  and  534  to the DataIn− port  517 , resistors  522  and  526  to the DataOut+ port  518 , and resistors  532  and  536  to the DataOut− port  519 . 
         [0029]    Because the CM input  510  is at a point around which the DM input  502  signal swings symmetrically, there is essentially no current flow from the DM input  502  to the CM input  510 . This is also the case for the CM correction input  504 . 
         [0030]    In the example, the CM input  510  is terminated by resistor  538  in parallel with resistor  540 , in parallel with the resistances to the DataIn+/− and DataOut+/− ports, i.e., resistors  522  and  524  to the DataIn+ port  516 , resistors  532  and  534  to the DataIn− port  517 , resistors  522  and  526  to the DataOut+ port  518 , and resistors  532  and  536  to the DataOut− port  519 . Current from the CM input  510  is limited from flowing out the DM input  502  and CM correction input  504  at low frequencies by capacitors  542  and  544  and, at high frequencies by the common mode rejection of balun  508 . 
         [0031]    In the example, resistors  520 - 526  and a 25 Ohm impedance into balun  508  effectively form a 50 Ohm impedance-matched T attenuator between the DataIn+ port  516  and, the DataOut+ port  518 . The attenuation of this T attenuator may be 1.5 dB, for example. Similarly, resistors  530 - 536  and the 25 Ohm impedance into balun  508  effectively form a 50 Ohm impedance-matched T attenuator between the DataIn− port  517  and the DataOut− port  519 . The attenuation of this T attenuator may also be 1.5 dB, for example. 
         [0032]    Combining the differential mode signals, e.g., from the DM input  502  and CM input  510 , with the CM correction input signal, e.g., from the CM correction input  504 , before applying them to the DataIn+/− and DataOut+/− ports in accordance with the disclosed technology advantageously results in minimal signal loss. This allows a designer to have higher loss when applying the combined signals to DataOut+/−, which in turn results in lower loss in the path from the DataIn+/− ports  516  and  517  to the DataOut+/− ports  518  and  519 . 
         [0033]    Resistor values of a circuit in accordance with the disclosed technology may be modified to change the tradeoff between the attenuation from the DataIn+/− ports  516  and  517  to the DataOut+/− ports and the transfer function from the other three inputs to the DataIn+/− ports  516  and  517  and DataOut+/− ports  518  and  519  while still maintaining an impedance of 50 Ohms at all of the ports  502 ,  504 ,  510 , and  516 - 519  over a broad frequency range. 
         [0034]      FIG. 7  is a graphing that illustrates an example  700  of a data signal resulting from an implementation in accordance with the disclosed technology, such as that illustrated by  FIGS. 5 and 6 . One having ordinary skill in the art will appreciate that the attenuation over frequency is flatter than would be for prior implementations that use a directional coupler, thus resulting in a cleaner data signal as can be readily seen in the illustrated example  700 , e.g., vis-à-vis the example  400  of  FIG. 4 . 
         [0035]      FIG. 8  is a functional diagram illustrating another example of a system  800  in accordance with certain embodiments of the disclosed technology. In the example, the system  800  includes DataIn+/− ports  816  and  817 , DataOut+/− ports  818  and  819 , and a CM input  810 . The system  800  also includes a DM input  802  that provides an input signal to a balun  803 , which converts the signal from single-ended to differential. In the example, a resistive network  812  is electrically coupled between the CM input  810  and two pickoff tees  814  and  815 . Implementation of the pickoff tees  814  and  815  may be substantially the same as that of the pickoff tees  51 . 4  and  515  of  FIGS. 5 and 6 , for example. 
         [0036]    In the example, the balun  803  is not on the same circuit board as the other components of the combiner circuitry; rather, two legs of the differential mode signal are provided on two separate 50 Ohm inputs  804  and  805 . Attenuators  806  and  807 , e.g., separate 6 dB attenuators, may be used on the legs of the signal to minimize the effects of any potentially undesirable reflections. In alternative embodiments, a differential amplifier may be used in place of the balun  803 . 
         [0037]    While a CM correction signal is not shown in the illustrated system  800 , the signal may be summed with the DM input  802  before being applied to the balun  803  as is done by the system  500  of  FIGS. 5 and 6 . 
         [0038]    Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments may be modified in arrangement and detail without departing from such principles, and may be combined any desired manner. And although the foregoing discussion has focused on particular embodiments, other configurations are contemplated. In particular, even though expressions such as “according to an embodiment of the invention” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. 
         [0039]    Consequently, in view of the wide variety of permutations to the embodiments described herein, this detailed description and accompanying material is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.