Abstract:
A test and measurement system including a device under test, an accessory, a controller and a test and measurement instrument. The accessory is connected to the device under test and includes a signal input to receive an input signal from the device under test, a compensation unit configured to apply a compensation signal internal to the accessory, and a signal output to output an output signal read from the device under test. The controller is connected to the accessory and includes one or more receivers to receive the input signal and the output signal from the accessory, and a microcontroller or correction circuit configured to compare the input signal and the output signal and in response to the comparison provide a compensation signal to the compensation unit.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of electronic test and measurement instruments and accessories therefore. The disclosed technology specifically addresses the problem of dynamically compensating a probe accessory while it is being used to make a measurement for the electronic instruments. 
       BACKGROUND 
       [0002]    Traditionally, compensating an accessory, such as a probe, in a test and measurement system involves removing the accessory from a device under test (DUT) and connecting the accessory to a calibrating or compensation stimulus. Conventionally, the test and measurement system includes a host, a controller, and a device under test. An accessory is attached to the device under test and measures a signal from the device under test and sends it back to the host. However, to compensate the accessory, the accessory has to be removed from the device under test and attached to a calibration/compensation stimulus, typically located on the host. Connecting the probe/accessory to the calibration or compensation stimulus may involve the use of special adapters and fixtures to interface between probe/accessory and the stimulus signal. 
         [0003]    Using optical sensors to accurately measure down to DC has not had much success in the past. Optical sensors are susceptible to changes in the surrounding environment. Temperature, mechanical stress, and the signals applied to the optical sensor can cause the DC/LF offset component of the sensor to shift dramatically making it unusable or result in significant measurement errors. 
         [0004]    What is needed is the ability to dynamically calibrate or compensate an accessory in real time, while it is operating, to eliminate the need to manually connect the accessory to a calibration or compensation signal and also the need to remove the accessory from a DUT to calibrate or compensate the probe. This dynamic compensation should be transparent to the user. 
       SUMMARY 
       [0005]    Certain embodiments of the disclosed technology include a test and measurement system including a device under test, an accessory, a controller and a test and measurement instrument. The accessory is connected to the device under test and includes a signal input to receive an input signal from the device under test, a compensation unit configured to dynamically apply a compensation signal internal to the accessory, and a signal output to output an output signal read from the device under test. The controller is connected to the accessory and includes one or more receivers to receive the input signal and the output signal from the accessory, and a microcontroller configured to compare the input signal and the output signal and in response to the comparison dynamically provide a compensation signal from the compensation unit in real time while it is making a measurement. 
         [0006]    Other embodiments include a method of internally calibrating an accessory in a measurement system including a host, a controller and a device under test. The method includes receiving at the accessory an input signal from a device under test, outputting an output signal from the accessory based on the input signal, comparing the output signal and the input signal at the controller and determining a compensation value, and providing the compensation value to a compensation unit in the accessory or in the host test and measurement instrument to compensate the output signal from the device under test. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1 and 2  illustrate embodiments a measurement system with an accessory head that contains a compensation unit. 
           [0008]      FIG. 3  illustrates a measurement system with an accessory head that includes an optical sensor and a compensation unit. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    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. 
         [0010]    There are times when an accessory attached to a DUT cannot be easily removed in order to calibrate or compensate the accessory. For example, the accessory could be permanently installed in a test fixture, soldered to a DUT, installed at a hard-to-access or remote location, in an environmental chamber, or in a hazardous location, such as a location with high voltage. Accordingly, in situations such as these, it is important to be able to calibrate or compensate the accessory without removing the accessory from the DUT. 
         [0011]    Some embodiments of the disclosed technology enable the use of an optical voltage sensor, as discussed in more detail below, to measure an electrical signal from direct current (DC) to Gigahertz (GHz) by dynamically compensating for the DC/LF (Low Frequency) instabilities of the optical sensor over time as it is making a measurement. The output of the optical sensor is susceptible to changes in the environment and the signal and bias applied to the sensor. Adding correction circuitry, as discussed in more detail below, enables the development of a completely electrically isolated, DC coupled, high bandwidth, high sensitivity differential accessory head with high common mode rejection and voltage range. 
         [0012]    Embodiments of the disclosed technology includes a test and measurement system that includes a host  100 , such as a test and measurement instrument, a controller  102 , an accessory head  104 , and a DUT  106 . One example of such a system is shown in  FIG. 1 . The accessory head  104  includes a compensation unit  108 , which is described in more detail below. 
         [0013]    During a measurement operation, a signal from the DUT  106  is received at inputs  114  and  116  of the accessory head  104 . The measured signal is then sent through an amplifier  118  and to the host  100  through the main signal path  120  and path  124 . The compensation unit  108  also sends back the monitored portion of the measurement signal that will be used for compensation of the system. 
         [0014]    The compensation is done continuously. Since the compensation signal from the compensation unit  108  is known, the compensation signal from the compensation unit  108  can be applied at the host  100  or controller  102  to the output signal. 
         [0015]    To determine the amount of compensation in the test and measurement system shown in  FIGS. 1 and 2 , the input signal from the DUT  106  is sent not only to amplifier  118 , but also to the compensation unit  108 . Compensation unit  108  then forwards the input signal from the DUT  106  to the controller  102  for analysis. The output signal from the amplifier  118  is also sent to the controller  102  for analysis. Although not shown, the output signal and input signal may also be analyzed internally in the accessory head  104 . The controller  102  compares the input signal and the output signal and determines the DC/LF offset error from the comparison of these signals. A resulting compensation value is determined based on the DC/LF offset error to minimize the DC/LF offset error when supplied by the compensation unit. The compensation value is then sent from the controller  102  to the compensation unit  108  in the accessory head  104 . This cycle is preferably continually repeated to maintain a minimum offset drift error in the measurement of the signals from DUT  106 . 
         [0016]      FIG. 1  shows a differential accessory head to receive two inputs from the DUT  106 . However, the compensation unit  108  may also be used in an accessory head  104  with a single-ended input, as shown in  FIG. 2 . The system of  FIG. 2  would work identically to that shown in  FIG. 1  except only a single input is received. 
         [0017]    As shown in  FIG. 3 , the accessory head  104  may include an optical sensor if the accessory head  104  is an optical accessory head. If such is the case, the measurement system shown in  FIG. 3  would still include a host  100 , a controller  102 , a compensation unit  108  and a DUT  106 , as discussed above with respect to  FIGS. 1 and 2 . Rather than using an amplifier  118 , the measurement system of  FIG. 3  includes an optical sensor  400 . The optical sensor  400  may be, for example, a Mach-Zehnder optical sensor. However, other optical sensors may be used as well 
         [0018]    Inputs  114  and  116  of the accessory head  104  are connected to signal input electrodes  402  and  404 . The output from the signal input electrodes  402  and  404  are sent from the optical sensor  400  to the controller  102  through the main signal path  120 . Compensation unit  108 , on the other hand, is connected to the control electrodes  406  and  408  of the optical sensor  400  which are separated and electrically isolated from the signal input electrodes  402  and  404 . 
         [0019]    When an optical sensor  400  is used, the controller  102  includes a laser controller  410 , a laser  412 , an optical transceiver  414 , an optical-to-electrical converter  416 , an analog-to-digital converter  418  and a microcontroller  420 . 
         [0020]    The amount of compensation to be applied to the control electrodes  406  and  408  is determined similar to that discussed above with respect to  FIGS. 1 and 2 . The input signals  114  and  116  from the DUT  106  are sent not only to the input signal path electrodes  402  and  404 , but also sent to the compensation unit  108 . The input signals are then sent to the optical transmitter  414  and finally microprocessor  420  for analysis. Although not shown, the input signals are converted to digital signals via an analog-to-digital converter connected to the compensation unit  108 . The output from the optical sensor  400  after reading the signal from the DUT  106  is sent to the optical-to-electrical converter  416  in the controller  102  and then further processed through an analog-to-digital converter  418 . The output from the analog-to-digital converter  418  could be sent to both the host  100  for display on a display of the host or storage in a memory (not shown) and the microcontroller  420  in the controller  102  for analysis. 
         [0021]    Similar to that discussed above, the microcontroller  420  compares the input signal and the output signal to determine a DC/LF offset error. Then, a compensation value can be determined based on the comparison that will minimize the DC/LF offset error when applied to the control electrodes  406  and  408  in the optical sensor  400 . The compensation value determined by the microcontroller  420  is sent back to the compensation unit  108  thru optical transceiver  414 . The compensation unit  108  then applies the compensation value to the control electrodes  406  and  408 . The input signals and output signals are constantly monitored and sent to the microcontroller  420  in the controller  102 . This allows for a compensation value to be continually determined to maintain a minimum offset drift error. A correction circuit may also be used rather than a microcontroller  420 . 
         [0022]    The disclosed technology is capable of not only calibrating, for example, direct current voltage, but can also be used to compensate the gain or frequency of an alternating current voltage. 
         [0023]    The disclosed technology is also not limited to use on a voltage probe. The accessory device may be any type of transducer device or general accessory device requiring voltage, current, power, etc., for operation, such as a measurement probe, measurement probe adapter, active filter devices, probe calibration fixture, probe isolation accessory, or the like. 
         [0024]    The host  100  may be a test and measurement instrument, such as an oscilloscope, logic analyzer, spectrum analyzer or similar such devices having an accessory device interface for accepting an accessory device. 
         [0025]    The connection to the controller  102  of the accessory head  104  may be a wired, optical fiber or a wireless connection as known to one of ordinary skill in the art. If the DUT  106  and accessory head  104  are located at a remote location, it may be necessary to have a wireless connection. Any of the signal paths  120 ,  122  and  124  may be a wired or wireless connection as known to one of ordinary skill in the art. 
         [0026]    In some embodiments (not shown) a controller is not required. Rather, all of the components of the controller  102  shows with respect to  FIGS. 1-4  are located with the host  100 . 
         [0027]    The term “controller” and “processor” as used herein is intended to include microprocessors, microcomputers, ASICs, and dedicated hardware controllers and associated memories. One or more aspects of the invention may be embodied in computer-usable data and computer-executable instructions, such as in one or more program modules, executed by one or more computers (including monitoring modules), or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types when executed by a processor in a computer or other device. The computer executable instructions may be stored on a non-transitory computer readable medium such as a hard disk, optical disk, removable storage media, solid state memory, RAM, etc. As will be appreciated by one of skill in the art, the functionality of the program modules may be combined or distributed as desired in various embodiments. In addition, the functionality may be embodied in whole or in part in firmware or hardware equivalents such as integrated circuits, field programmable gate arrays (FPGA), and the like. Particular data structures may be used to more effectively implement one or more aspects of the invention, and such data structures are contemplated within the scope of computer executable instructions and computer-usable data described herein. 
         [0028]    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.