PATENT ABSTRACT
A test and measurement system including a test and measurement instrument, a probe connected to the test and measurement instrument, a device under test connected to the probe, at least one memory configured to store parameters for characterizing the probe, a user interface and a processor. The user interface is configured to receive a nominal source impedance of the device under test. The processor is configured to receive the parameters for characterizing the probe from the memory and the nominal source impedance of the device under test from the user interface and to calculate an equalization filter using the parameters for characterizing the probe and nominal source impedance from the user interface.

PATENT DESCRIPTION
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to signal acquisition systems and, more particularly, to a system, apparatus and method for reducing measurement errors due to, for example, probe tip loading of a device under test. 
       BACKGROUND 
       [0002]    Typical probes used for signal acquisition and analysis devices such as oscilloscopes and the like have an impedance associated with them which varies with frequency. As the bandwidth of test and measurement instruments and probe system become wider the effects of probe tip loading of non-flat through responses becomes more significant than in past systems. 
         [0003]    U.S. Pat. No. 6,725,170 entitled “Smart probe apparatus and method for automatic self-adjustment of an oscilloscope&#39;s bandwidth” to Barton Hickman, owned by Tektronix, Inc. and incorporated herein by reference, discloses storing S-parameters of a probe so that equalization filters can be computed when a probe is connected to different input channels of different types of test and measurement instruments. These equalization filters, however, are designed for device under test (DUT) source impedance of 50 ohms. What is needed is an equalization filter that can be calculated using the nominal source impedance of the DUT. Using the prior methods, if the source impedance of the DUT is not 50 ohms, then the acquired waveform received via a probe loading such a circuit may not accurately represent the voltage of the circuit prior to the introduction of the probe. 
       SUMMARY 
       [0004]    Certain embodiments of the disclosed technology include a test and measurement system including a test and measurement instrument, a probe connected to the test and measurement instrument, a device under test connected to the probe, at least one memory configured to store parameters for characterizing the probe, a user interface and a processor. The user interface is configured to receive a nominal source impedance of the device under test. The processor is configured to receive the parameters for characterizing the probe from the memory and the nominal source impedance of the device under test from the user interface and to calculate an equalization filter using the parameters for characterizing the probe and nominal source impedance. The equalization filter is adapted to compensate for loading of the device under test caused by a measurement of the device under test. 
         [0005]    Certain other embodiments of the disclosed technology include a method for calculating an equalization filter for use in a test and measurement system. The method includes receiving at a processor parameters for characterizing a probe of a test and measurement system, receiving at the processor via a user interface a nominal source impedance of a device under test, and computing an equalization filter adapted to compensate for loading of a device under test caused by measurement of the device under test based on the parameters for characterizing the probe and the nominal source impedance of the device under test. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates an ideal input waveform and the output waveform using a DUT with a 50 ohm source impedance loading the tip of the probe. 
           [0007]      FIG. 2  illustrates the ideal input waveform of the  FIG. 1  and output waveforms with varying DUT source impedance values. 
           [0008]      FIG. 3  illustrates a block diagram of a test and measurement system of the disclosed technology. 
           [0009]      FIG. 4  illustrates a user interface of the disclosed technology. 
           [0010]      FIG. 5  illustrates another user interface of the disclosed technology. 
           [0011]      FIG. 6  illustrates a variety of equalization filters calculated using the disclosed technology. 
           [0012]      FIG. 7  illustrates an ideal input waveform and various output waveforms using different equalization filters for various DUT source impedance. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In the drawings, which are not necessarily to scale, like or corresponding elements of the disclosed systems and methods are denoted by the same reference numerals. 
         [0014]    Traditional accessories for test and measurement instruments are designed so that an accessory&#39;s optimal frequency response occurs when the tip of the accessory is connected to a circuit under test in a DUT with a source impedance of 50 ohms. Accessories tend to load the circuit under test in the DUT, which then distorts the waveform read from the circuit in the DUT. Conventionally, an accessory will incorporate hardware equalization to correct an output wave-shape to look more like it did before the accessory loaded the circuit. An equalization filter is calculated and used to process the acquired samples from the DUT such that signal degradation or artifacts imparted to the waveform read from the circuit under test and in the DUT are compensated for within the system, effectively de-embedding the loading of the DUT by the probe tip.  FIG. 1  shows an output  100  response of a test and measurement instrument using the conventional approach given an ideal input pulse  102 . As can be seen in  FIG. 1 , the output  100  shows some distortion. 
         [0015]    A DUT source impedance, however, tends to fall in the range of 25 ohms to 100 ohms. The DUT source impedance tends to vary over that range even if the DUT is specified to have a source impedance of 50 ohms.  FIG. 2  shows in the conventional method how the outputs vary from the ideal input  102  shown in  FIG. 1  when the source impedance of the DUT is not 50 ohms. As can be seen in  FIG. 2 , the errors in the output waveforms can be quite large. 
         [0016]    The errors seen in  FIG. 2  can be reduced by allowing a user to specify the nominal source impedance of the DUT to calculate an equalization filter. 
         [0017]    As seen in  FIG. 3 , the system includes a test and measurement instrument  300  and a probe  302  connected to a DUT  304 . The test and measurement instrument  300 , may be for example, an oscilloscope. The test and measurement instrument may also be any other test and measurement instrument, such as a spectrum analyzer, logic analyzer, etc. 
         [0018]    The probe  302  includes a memory  306  for storing the S-parameters of the probe. Alternatively, the T-parameters or some other form of parameters to characterize the probe may be stored in the memory  306 . The parameters are measured at the time of manufacturing the probe  302  and then stored in memory  306 . Alternatively, the parameters may be stored in a memory  308  of the test and measurement instrument  300 , or on an external storage device (not shown), the internet (not shown), etc. The parameters merely must be supplied to the processor  310  to calculate the equalization filter as will be discussed in more detail below. 
         [0019]    The test and measurement instrument  300  also includes a memory  308 , as discussed above. Memory  308  stores the S-parameters of the test and measurement instrument  300  that are measured at the time of manufacturing. Alternatively, T-parameters or other forms of parameters to characterize the scope may be used and stored in memory  308 . Along with memory  308 , the test and measurement instrument  300  includes a display  312  and a processor  310 . 
         [0020]    During operation, the test and measurement instrument  300  is connected to the DUT  304  through the probe  302 . The display  312  contains a user menu or user interface  400  as shown in  FIG. 4 . The user menu  400  allows the user to specify the nominal source impedance of the DUT  402 . The user can specify either the real impedance or the complex impedance of the DUT at the menu  402 . The nominal source impedance is then used as part of the calculation for the equalization filter to obtain an ideal target response for the system. 
         [0021]    The user menu  400  also contains an option for the user to turn the probe equalization filter on or off  404 . A user may turn off the equalization filter if the results for a particular DUT are better without the filter. The user menu  400  also allows a user to select whether to use nominal equalization view  406 . The nominal equalization view shows the waveform as if the probe did not load the DUT circuit. The user can also select the option of using a probe load filter  408  in the user menu. The probe load filter shows the voltage at the probe tip with the probe loading the DUT circuit. 
         [0022]    The user menu  400  may also include a menu  500  to allow a user to load the S-parameters for the DUT test point, as shown in  FIG. 5 . The equalization filter is then computed using both the nominal impedance of the DUT and the S-parameters for the DUT test point. 
         [0023]    When the user has entered all the desired information into the user menu  400  on display  312 , the information is sent to processor  312  in the test and measurement instrument  300 . Further, the S-parameters of the probe stored in the probe memory  306  are also sent to the processor  310  in the test and measurement instrument  300 . The processor then uses the S-parameters of the probe, the nominal impedance of the DUT provided by the user to compute an equalization filter to provide a more accurate view of the signal from the DUT. To provide an even more accurate view, the processor may also use the S-parameters of the test and measurement instrument  300  stored in the test and measurement memory  310  and the S-parameters of the DUT if the S-parameters of the DUT are loaded into the test and measurement instrument  300  by the user via menu  500 . 
         [0024]      FIG. 6  illustrates the various equalization filters created by the processor  310  for each of the various nominal source impedance values. As can be seen in  FIG. 6 , the equalization filters are different for each of the nominal source impedance values, which helps create a more accurate view on the display to the user of the signal from the circuit under test in the DUT. 
         [0025]      FIG. 7  illustrates output waveforms with the equalization filter applied using nominal input impedance values inputted by the user. For example,  FIG. 6  shows an input waveform  700  and the output waveforms  702  for multiple DUT source impedance values. As can be seen in  FIG. 7 , for each of the DUT source impedance values, after applying an equalization filter calculated with the nominal DUT impedance input by the user, the output waveforms are nearly identical to the input waveform, unlike the output waveforms shown in  FIG. 2 . 
         [0026]    The disclosed technology allows a user to control the equalization filter applied to the tip of a probe. The user can specify a DUT source reference impedance at the probe tip and then an equalization filter is computed by the test and measurement instrument based on the measured S-parameters read from the probe. To create an even more refined equalization filter, the S-parameters of the test and measurement instrument and/or the test point of the DUT may be used. Although S-parameters are described above for calculating the equalization filter, as will be readily understood by one skilled in the art, other parameters may be used that characterize the probe, oscilloscope and/or the DUT test point, such as T-parameters. 
         [0027]    Although the embodiments illustrated and described above show the disclosed technology being used in an oscilloscope, it will be appreciated that embodiments of the present invention may also be used advantageously in any kind of test and measurement instrument that displays frequency domain signals, such as a swept spectrum analyzer, a signal analyzer, a vector signal analyzer, a real-time spectrum analyzer, and the like. 
         [0028]    In various embodiments, components of the invention may be implemented in hardware, software, or a combination of the two, and may comprise a general purpose microprocessor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the like. 
         [0029]    Having described and illustrated the principles of the disclosed technology in a preferred embodiment thereof, it should be apparent that the disclosed technology can be modified in arrangement and detail without departing from such principles. We claim all modifications and variations coming within the spirit and scope of the following claims.