Abstract:
Bias tees, according to certain embodiments of the present invention, include switches in the AC signal path, the DC signal path, or both, to improve the capability of the bias tees to be used for high impedance AC measurement, low current DC measurement, or both. Optical control of the switches, as well as control of the switches using a DC bias present within the AC signal input to the bias tee, is described. Including a set of diodes into the DC signal path, rather than a switch, provides enhanced capability of the bias tee to be used for high impedance AC measurements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/955,553 titled “Improved Bias Tee Designs with Extended Low Current Measurement and AC High Impedance Measurement Capability” filed Mar. 19, 2014, which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This disclosure relates to bias tees, and, more particularly to configurable bias tees that improve current processes for low current measurements and AC high impedance measurements. 
         [0004]    2. Description of Related Art 
         [0005]    A bias tee is typically a passive, three-port electrical network that may act as a diplexer. In one mode of operation, one port of the bias tee network is connected to a very low frequency or direct current (DC) source and another port is connected to a high frequency or alternating current (AC) source. The bias tee combines the DC source signal and the AC source signal so that the third port of the network is simultaneously coupled to both the DC and AC signals. Bias tees are well-known electrical devices which are useful in many applications where it is necessary to inject DC power into an AC signal. Typical applications include powering photodiodes, lasers, or remote antenna amplifiers. 
         [0006]    Bias tees are also typically bi-directional. Therefore, in another mode of operation, a combined AC and DC (“AC+DC”) signal is applied to the third port of the tee, and the bias tee network separates the AC and DC components of the signal so that the AC component of the signal can be measured at the AC port of the tee, and the DC component of the signal can be measured at the DC port of the tee. Examples of applications that use a bias tee in this mode include packaged device characterization and wafer probing. In these types of applications, connecting the combined AC+DC port of the tee to the output of the device under test allows a user to measure the DC characteristics of the device, and to measure the AC characteristics of the device, without having to re-configure the test setup between the DC and AC tests. In such applications, for certain types of devices, the bias tee carries very low DC current levels, as well as AC signals for high impedance measurements to the measurement instrument. Achieving good performance for both low current DC measurements and AC high impedance measurements presents special challenges to the designer of a bias tee. 
         [0007]    The simplest bias tee designs employ a capacitor, a resistor, and three coaxial connectors. The coaxial connectors serve as a DC signal port, an AC signal port, and a combined AC+DC signal port for the tee. The capacitor is connected between the AC signal port and the AC+DC signal port. The resistor is connected between the DC signal port and the AC+DC signal port. The overall DC performance of this bias tee design is limited because the resistive element limits the current that can travel through the DC path of the tee. 
         [0008]    Improved DC performance is achieved with a modified bias tee design in which the resistor in the DC path is replaced with an inductor. Although an ideal inductor would block the AC signal from passing back to the DC port, the AC performance of this design can be limited by the potential LC resonance effects. Also, since such a design uses coaxial connectors as the ports of the tee, its low current performance is limited due to the leakage current inherent in coaxial connectors. 
         [0009]    To improve low current performance, triaxial connectors, rather than coaxial connectors, are used for the DC port and the AC+DC port. The single capacitor in the designs described above is replaced with two capacitors in series. One of the capacitors is “guarded” by the DC signal, thereby minimizing the leakage current through this capacitor. However, because this capacitor usually has relatively large capacitance, it will tend to generate current noise, thereby still hampering the low current performance of the bias tee. 
         [0010]    Embodiments of the invention address these and other limitations of the prior art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    A configurable bias tee includes an AC signal port, a DC signal port, and an AC+DC signal port. The bias tee has triaxial connectors as the DC signal port and the AC+DC signal port. A first electrical network coupled between the DC signal port and the AC+DC signal port provides a DC signal path through the tee. A second electrical network is coupled between the AC signal port and the AC+DC signal port. The second electrical network includes a first capacitor, a switch, and a second capacitor in series. The second capacitor is “guarded” by the guards of the triaxial DC and AC+DC signal ports. The switch is configured to provide an AC signal path through the tee when closed, and to disconnect the AC path when opened. 
         [0012]    Methods of using a configurable bias tee with an AC signal path and a DC signal path include opening either the AC signal path or the DC signal path, and measuring a signal conveyed through the non-opened path at either, respectively, the DC signal port or the AC signal port. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of a conventional bias tee incorporating coaxial connectors, a capacitor, and a resistor. 
           [0014]      FIG. 2  is a schematic diagram of a conventional bias tee incorporating coaxial connectors, a capacitor, and an inductor. 
           [0015]      FIG. 3  is a schematic diagram of a conventional bias tee incorporating a coaxial connector, triaxial connectors, a capacitor, and an inductor. 
           [0016]      FIG. 4  is a schematic diagram of a bias tee network incorporating a switch into the AC signal path, according to some embodiments of the invention. 
           [0017]      FIG. 5  is a schematic diagram of a bias tee network incorporating switches into the AC signal path and the DC signal path, according to some embodiments of the invention. 
           [0018]      FIG. 6  is a schematic diagram of a bias tee network incorporating switches into the AC signal path and the DC signal path, with isolated control circuitry for the switches, according to some embodiments of the invention. 
           [0019]      FIG. 7  is a schematic diagram of a bias tee network incorporating switches into the AC signal path and the DC signal path, with control circuitry for the switches being driven by the AC signal, according to some embodiments of the invention. 
           [0020]      FIG. 8  is a schematic diagram of a bias tee network incorporating a switch into the AC signal path, and diodes into the DC signal path, according to some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  shows a simple conventional bias tee network  100  including a coaxial connector  105  serving as an AC signal port, a coaxial connector  110  serving as a DC signal port, a coaxial connector  115  serving as a combined AC+DC signal port, a capacitor  120  coupled between the AC signal port  105  and the AC+DC signal port  115 , and a resistor  125  coupled between the DC signal port  110  and the AC+DC signal port  115 . The capacitor couples an AC signal to the AC+DC port, but generally blocks a DC signal from passing back to the AC port, thereby providing an AC signal path  104  through the bias tee. The resistor couples a DC signal to the AC+DC port, thereby providing a DC signal path  103  through the bias tee. As explained above, this type of bias tee allows for generally good AC coupling, but its DC performance is limited by the resistor in the DC signal path. 
         [0022]      FIG. 2  shows a conventional bias tee network  200  that offers improved DC performance compared to the bias tee shown in  FIG. 1 . The bias tee  200  is similar to the bias tee depicted in  FIG. 1  except that the resistor in the DC signal path is replaced by an inductor  225 . This bias tee design has improved DC performance, because a full range of current can be delivered to an AC+DC signal port  215 , assuming the inductor  225  used is physically capable of handling the required current. But, the tradeoff for this improvement in DC performance is a reduction in AC performance due to LC resonance, depending on the selection of a capacitor  220  and the inductor  225 . Additionally, the bias tee  200  has limited low current performance due to the physical real-world non-ideal characteristics of the inductor  225 , as well as the leakage current inherent in a coaxial DC signal port  210  and the coaxial AC+DC signal port  215 . 
         [0023]    Another type of conventional bias tee design, a bias tee network  300  depicted in  FIG. 3 , improves the low current performance of the tee. The design of the bias tee network  300  is similar to the bias tee network depicted in  FIG. 2 , except that in bias tee network  300 , the single capacitor in an AC signal path  304  is replaced with two capacitors in series—a capacitor  320  and a capacitor  330 . Like the bias tee network depicted in  FIG. 2 , a coaxial connector  305  serves as the AC signal port. However, in the bias tee network  300 , the connector serving as the DC signal port is a triaxial connector  310 , and the connector serving as the AC+DC signal port is a triaxial connector  315 . In general, triaxial cables and connectors have an outer shield, a center conducting core known as the force, and an inner shield, known as the guard, between the force and the outer shield. The guard is kept at approximately the same electric potential as the force, thereby minimizing leakage current between the force and the guard. 
         [0024]    In the bias tee network  300  of  FIG. 3 , the capacitors  320 ,  330  are coupled in series between the center conductor of the AC signal port coaxial connector  305  and the force of the AC+DC signal port triaxial connector  315 , thereby providing the AC path  304  through the bias tee  300 . An inductor  325  is coupled between the force of the DC signal port triaxial connector  310  and the force of the AC+DC signal port triaxial connector  315 , providing a DC path  303  through the bias tee  300 . A guard  335  of the DC signal port is connected to a guard  340  of the AC+DC signal port  315 , and also to the node between the capacitors  320 ,  330 . In this configuration, low current performance of the bias tee  300  is better than that of the bias tee depicted in  FIG. 2 . Because the guard  335  voltage follows the DC signal voltage, the capacitor  320  has 0 V across it. Therefore, the leakage current through the capacitor  320  is minimized, which improves the overall low current measurement capability of the bias tee  300 . However, because the capacitor  320  typically has a relatively large capacitance, the low current measurement capability of the bias tee  300  is still limited due to current noise resulting from the presence of the large capacitor  320  between the guard  335  and the DC signal. 
         [0025]      FIG. 4  illustrates a configurable bias tee network  400  according to an embodiment of the invention. The bias tee network  400  includes a connector  405  serving as an AC signal port, a triaxial connector  410  serving as a DC signal port, and a triaxial connector  415  serving as an AC+DC signal port. The bias tee network  400  is bi-directional. The AC signal port connector  405  is typically a coaxial connector. The DC signal port and the AC+DC signal port triaxial connectors  410 ,  415  each have a force and a guard. The DC signal port guard  435  is connected to the AC+DC signal port guard  440 . 
         [0026]    The bias tee network  400  has a first electrical network  401  coupled between the force of the DC signal port  410  and the force of the AC+DC port  415  to provide a DC path  403  between these two ports of the bias tee  400 . In the bias tee network  400  of  FIG. 4 , the electrical network  401  includes an inductor  425 . The bias tee network  400  also has a second electrical network  402  coupled between the force of the AC+DC signal port  415  and the center conductor of the AC signal port  405 . This second electrical network  402  includes a capacitor  420 , a switch  445 , and a capacitor  430  coupled in series. The capacitor  430  is coupled between the center conductor of the AC signal port  405  and the guards  435 ,  440 . The capacitor  420  and the switch  445  are coupled in series between the force of the AC+DC signal port  415  and the guards  435 ,  440 . With the switch  445  closed, the electrical network  402  provides an AC path  404  through the bias tee  400  between the AC signal port  405  and the AC+DC signal port  415 . With the switch  445  open, the AC path  404  is decoupled, or otherwise disconnected. 
         [0027]    With the switch  445  closed, the capacitor  420  is said to be “guarded by” the DC signal in the DC path  401  through the bias tee  400 . That is, because the voltage at the guard  435  follows the voltage at the force of the DC signal port  410 , there is 0 V across the capacitor  420 , thereby minimizing the leakage current through the capacitor  420 . However, with the switch  445  closed, the performance of the bias tee network  400  when measuring DC low currents may still be negatively impacted by current noise. Current noise may be generated because the capacitor  420  typically has a relatively large capacitance. Preferably, the switch  445  is designed or selected such that, when open, the parasitic capacitance of the switch  445  is much lower than the capacitance of the capacitor  420 . Therefore, by opening the switch  445 , the AC path  404  is disconnected and current noise is reduced, thereby improving the performance of the bias tee network  400  when used for low current measurements. 
         [0028]      FIG. 5  illustrates a configurable bias tee network  500  according to another embodiment of the invention. Like the bias tee network  400  depicted in  FIG. 4 , the bias tee  500  of  FIG. 5  has an electrical network  502  coupled between the force of an AC+DC signal port triaxial connector  515  and the center conductor of an AC signal port  505 . The electrical network  502  includes a switch  545  that, when closed, provides an AC signal path  504  through the bias tee  500 , and that, when opened, disconnects the AC path  504  to effect a performance improvement for low current measurement applications by reducing current noise that may be generated by a capacitor  520 . 
         [0029]    The bias tee  500  also has an electrical network  501  coupled between the force of the AC+DC signal port triaxial connector  515  and the force of a DC signal port triaxial connector  510 . The electrical network  501  includes an inductor  525  and a switch  550  coupled in series such that when the switch  550  is closed, the electrical network  501  provides a DC signal path  503  through the bias tee  500 . Opening the switch  550  creates a high impedance in the DC path  503 , thereby improving the performance of the bias tee  500  for high impedance AC measurements. A resistor  555  is coupled in parallel with the switch  550  to enable the bias tee  500  to still have some reduced current DC bias capability even when the switch  550  is open. 
         [0030]    In operation, one method of using the bias tee  500  includes opening either the DC path  503 , or the AC path  504 , and then measuring a signal conveyed through the non-opened path. Measurements may be made at, respectively, the AC signal port  505 , or the DC signal port  510 , for example. Opening the DC path  503  may include opening the switch  550 . The switch  550  may be opened in response to a generated DC path switch control signal Likewise, opening the AC path  504  may include opening the switch  545 . The switch  545  may be opened in response to a generated AC path switch control signal. 
         [0031]    Both the switches  545 ,  550  are preferably designed or selected to be switches with very low leakage current. One design consideration is the specified impedance of the switch. For example, if the switch has a specified impedance of 1 GΩ from control to output, at 100 V, a current of 100 nA will flow to the output. Such a leakage current may be unacceptable for the bias tee  500  to be used for low current measurements. In practice, it may be difficult to include switches that enclose control and switch circuitry into one package, and that also have acceptably low enough leakage current. Therefore, the switches  545 ,  550  are preferably designed or selected to be switches that have the control and switch circuitry separated and isolated, such as, for example, switches that are optically controlled. 
         [0032]      FIG. 6  shows a configurable bias tee network  600  according to another embodiment of the invention. The bias tee  600  is similar to the bias tee  500  shown in  FIG. 5 , except that in the bias tee  600 , a switch  645  in an AC signal path  604  is activated by a photocell  670 , and a switch  650  in a DC signal path  603  is activated by a photocell  675 . The photocell  670  responds to an isolated control circuit  660  and the photocell  675  responds to an isolated control circuit  665 . The output of control circuits  660 ,  665  may include, for example, light from light-emitting diodes (LEDs). Preferably, the control circuits  660 ,  665  are insulated from the respective photocells  670 ,  675  by a very high impedance material, such as, for example, air. In some embodiments, a single control signal may be input to both control circuits  660 ,  665 . 
         [0033]    In operation, generating an output from the control circuits  660 ,  665  may be used to control the switches  645 ,  650 , respectively. Generating an output from control circuits  660 ,  665  may include generating a DC bias voltage. 
         [0034]      FIG. 7  illustrates a configurable bias tee network  700 , according to another embodiment of the invention, providing self-contained switch control. The structure of the bias tee  700  is similar to the bias tee  600  shown in  FIG. 6 , except that in the bias tee  700 , the inputs of control circuits  760 ,  765  are coupled to the input of an AC signal port  705 . Because an AC signal (not shown) input to the AC signal port  705  is AC-coupled to the bias tee  700 , and because the AC signal may have its own DC bias level, controlling this DC bias level may be used to selectively drive the control circuits  760 ,  765 . Although the control circuits  760 ,  765  are illustrated in  FIG. 7  as LEDs, the control circuits  760 ,  765  may be designed and arranged in a variety of different configurations to effect different switch control logic for the photocells  770 ,  775 , activating, respectively, the switches  745 ,  755 . Preferably, the current needed to drive the control circuits  760 ,  765  is relatively low, so that the impedance of the control circuits does not interfere with the measurements for which the bias tee  700  is used. Additionally, in the case where the control circuits  760 ,  765  are LEDs, these LEDs are preferably optically isolated from each other, so as to prevent crosstalk. 
         [0035]    Finally,  FIG. 8  illustrates a configurable bias tee network  800 , according to another embodiment of the invention. The bias tee  800  is similar to bias tee  500  depicted in  FIG. 5  in that the bias tee  800  of  FIG. 8  has an electrical network  802 —including a capacitor  820 , a switch  845  and a capacitor  830  coupled in series—coupled between the force of an AC+DC signal port triaxial connector  815  and the center conductor of an AC signal port  805 . The capacitor  830  is guarded by a guard  835  of a DC signal port triaxial connector  810  and a guard  840  of the AC+DC signal port triaxial connector  815 . The switch  845  is arranged to provide an AC signal path  804  through the bias tee  800  when closed, and to disconnect the AC signal path  804  when opened. 
         [0036]    The bias tee  800  also has an electrical network  801  coupled between the force of the DC signal port  810  and the force of the AC+DC signal port  815 , providing a DC signal path  803  through the bias tee  800 . The electrical network  801  includes a pair of diodes  860 ,  865  and a resistor  855  coupled in parallel. The diodes  860 ,  865  are coupled in opposite polarity to each other. This configuration gives the bias tee  800  improved AC high impedance measurement capability when DC signal current is low, but also allows the bias tee  800  to supply a high DC bias current with a reduction in AC high impedance measurement capability. Preferably, the DC drop though diodes  860  and  865  is calibrated out in the measurement system (not shown) in which the bias tee  800  is used, or remote sense capability is added to correct for the drop. Other embodiments of the invention add remote sense capability to the bias tees  400 ,  500 ,  600 , and  700 , described above. 
         [0037]    It will be appreciated from the forgoing discussion that the invention provides significant advances in bias tee performance. Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.