Abstract:
A bias tee for connecting a measurement device to a DUT, where the measurement device has a guard output, includes a DC port; a HF port; and a measurement port. The HF input port is guarded with the guard output during operation of the bias tee.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to the measurement of electrical signals and, in particular, to bias tees used therein. 
   The use of bias tees in measurements is well-known. Referring to  FIG. 1 , a prior art bias tee  10  is used to connect a DC instrument  12 . (e.g, a source measurement unit) and a high frequency (HF) instrument  14  to a device under test  16  (DUT). The HF instrument  14  may be, for example, a combination RF signal generator and measurement device. It should be noted that the bias tee  10  can also be used in the case of pulses as opposed to radio frequency signals per se, the frequency content of typical interest being similar. 
   The typical purpose of the bias tee  12  is to allow the simultaneous application of HF and low frequency (e.g., DC) signals to the DUT  16 . At the same time, DC signals are blocked from the instrument  14  and HF signals are blocked from the instrument  12 . For example, the DC signals may serve to bias the DUT while it responds to RF signals, hence the name, bias tee. 
   The example of  FIG. 1  includes both a source path  18  and a measure path  20 , so that a so-called Kelvin connection can be made to the DUT to improve measurement performance with respect to the low frequency signals. 
   Unfortunately, the bias tee  10  introduces its own measurement errors, for example, leakage and stray capacitances. This particularly creates challenges in making very accurate and/or very sensitive measurements (e.g., pico- and femto-ampere measurements). 
   SUMMARY OF THE INVENTION 
   A bias tee for connecting a measurement device to a DUT, where the measurement device has a guard output, includes a DC port; a HF port; and a measurement port. The HF input port is guarded with said guard output during operation of the bias tee. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a prior art bias tee. 
       FIG. 2  is a schematic diagram of an example of a bias tee according to an aspect of the invention. 
       FIG. 3  is a schematic diagram of another example of a bias tee according to another aspect of the invention. 
       FIG. 4  is a schematic diagram of still another example of a bias tee according to still another aspect of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the example of  FIG. 2 , a bias tee  100  includes a guard input  102 . The guard input  102  is connected to the HF port  104  of the bias tee  100  by an impedance network formed from the inductors  116 ,  118  and the capacitor  120 . The DC instrument  126  is connected to the DC port  121  of the bias tee  100 . The inductors  122 ,  124  connect the source output of the DC instrument  126  to the output port  128  of the bias tee  100 . The inductors  130 ,  132  connect the measure input of the instrument  126  to the output port  128 . 
   In operation, the output port  128  applies both DC and HF to the DUT  134 . The capacitor  136  allows HF to travel to the port  128  and the inductors  122 ,  124 ,  130 ,  132  block HF from the instrument  126 . 
   The capacitor  120  blocks DC from the guard input  102  from entering the instrument  126  and the inductors  116 ,  118  block HF from the guard input  102 . 
   The guard output  140  of the DC instrument  126  is typically driven at a voltage equal to the forcing voltage of the DC instrument  126  (which is the nature of guards), as a result in this case, there will be no DC potential across the capacitor  136  and, hence, no leakage across the capacitor  136 . 
   This guarding of the HF port  128  results in substantial improvements in the measurement sensitivity of the DC instrument  126 . For example, from nanoamperes to picoamperes. 
   It should be pointed out that because of the high frequency content of fast rise/fall time pulses, the above circuit is suitable for not only RF signals, but also for pulsed signals as well. In addition, in many cases, the inductors of the bias tee may be replaced with resistors where the resistive impedance is sufficient to limit the HF component through the instrument  126 . 
   Referring to the example of  FIG. 3 , another bias tee  200  includes a guard input  202 . The guard input  202  is connected to the HF port  204  of the bias tee  200  by an impedance network formed from the inductors  216 ,  218  and the capacitor  220 . The DC instrument  226  is connected to the DC port  221  of the bias tee  200 . The inductors  222 ,  224  connect the source output of the DC instrument  226  to the output port  228  of the bias tee  200 . The inductors  230 ,  232  connect the measure input of the instrument  226  to the output port  228 . In addition, the bias tee  200  is located within a conductive enclosure  242  (e.g., a metal box) where the enclosure  242  is connected to the guard input  202 . The HF port  204  includes a DC isolation capacitor  244  to isolate the guard potential from the HF ground. 
   Similar to the previous example, the output port  228  applies both DC and HF to the DUT  234 . The capacitor  236  allows HF to travel to the port  228  and the inductors  222 ,  224 ,  230 ,  232  block HF from the instrument  226 . 
   The capacitor  220  blocks DC from the guard input  202  from entering the instrument  226  and the inductors  216 ,  218  block HF from the guard input  202 . 
   The guard output  240  of the DC instrument  226  is typically driven at a voltage equal to the forcing voltage of the DC instrument  226  (which is the nature of guards), as a result in this case, there will be no DC potential across the capacitor  236  and, hence, no leakage across the capacitor  236 . In addition, the connection of the guard to the enclosure  242  minimizes any DC potential between the enclosure  242  and the forcing voltage of the DC instrument  226 . This further minimizes leakage within the bias tee  200 , as the effect of stray capacitances to the enclosure are minimized. 
   This guarding of the HF port  228  results in substantial improvements in the measurement sensitivity of the DC instrument  226 . For example, from nanoamperes to femtoamperes. 
   It should be pointed out that because of the high frequency content of fast rise/fall time pulses, the above circuit is suitable for not only RF signals, but also for pulsed signals as well. In addition, in many cases, the inductors of the bias tee may be replaced with resistors where the resistive impedance is sufficient to limit the HF component through the instrument  226 . 
   Referring to the example of  FIG. 4 , a bias tee  201  similar to the bias tee  200  further includes a guard switch  246  that switches the enclosure  242  between ground or guard potential and a measure switch  248  that allows the Kelvin connection to be made either within the bias tee  201  or at the DUT  234  itself. Having the enclosure  242  at ground is useful when RF measurements are the primary interest and having the Kelvin connection at the DUT may be important in some tests. 
   It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.