Abstract:
A method of controlling the gain or sensitivity of a test and measurement system. The test and measurement system includes a host, a controller with an optical transmitter and an optical receiver, optical-to-electrical converter, an accessory head, and a device under test. The method includes determining whether a gain or sensitivity adjustment of the test and measurement system is required, determining the amount of gain or sensitivity adjustment, and adjusting the output power of a laser of the optical transmitter in response to the determination of the gain or sensitivity adjustment of the test and measurement system.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to test and measurement systems and signal acquisition accessories and more particularly to signal acquisition accessories using an optical sensor. 
     BACKGROUND 
     There is an increasing need in the electronics industry for test and measurement instruments, such as oscilloscopes, logic analyzers and the like, to measure electrical signals that are galvanically isolated and contain higher frequency content. One way to measure electrical signals that are galvanically isolated and have higher frequency content is to use an electro-optic probe. 
     Traditionally, the gain and sensitivity of an electro-optic probe is adjusted by controlling a variable gain amplifier of an optical-to-electrical converter. Controlling a variable gain of the optical-to-electrical converter results in higher sensitivity but a reduction in the overall bandwidth and frequency response of the test and measurement system. That is, gain and/or sensitivity for the test and measurement system is adjusted at the optical-to-electrical converter after a light beam has been emitted and a measurement has been taken at a device under test (DUT). Typically, the electro-optic probe also requires a user to develop an application specific integrated circuit (ASIC) to achieve the desired parameters of the electro-optic probe. In order to achieve the ability to have an adjustable gain, the dynamic range, noise, frequency and bandwidth response and overall system complexity must be compromised. 
     SUMMARY 
     Certain embodiments of the disclosed technology include a method of controlling the gain or sensitivity of a test and measurement system. The test and measurement system includes a host, a controller with an optical transmitter and an optical receiver, an accessory head, and a device under test. The method includes determining whether a gain or sensitivity adjustment of the test and measurement system is required, determining the amount of gain or sensitivity adjustment, and adjusting the output power of a laser of the optical transmitter in response to the determination of the gain or sensitivity adjustment of the test and measurement system. 
     Certain embodiments of the disclosed technology also include a test and measurement system including a device under test, a host, an accessory head with an optical sensor connected to the device under test, and a controller connected to the accessory head and the host. The controller includes an optical transmitter with a laser, an optical receiver, and an optical-to-electrical converter. The controller is configured to adjust the output of the laser of the optical transmitter to adjust the gain or sensitivity of the measurements from the device under test. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the test and measurement system according to the disclosed technology. 
         FIG. 2  illustrates a block diagram of the test and measurement system according to the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     The test and measurement system of the disclosed technology has the capability to sense a measurement of a signal under test from a DUT  106  using an optical sensor in an accessory head  104 . Referring to  FIG. 1 , there is illustrated a test and measurement system which includes a host  100 , accessory head  104 , and an optical transmission path  108  extending from the accessory head  104  to a controller  102 . The accessory head  104  is connected to a DUT  106 . The controller contains signal acquisition probing circuitry needed to provide the optical signal to the accessory head  104  and convert the returning optical signal to an electrical signal. The controller  102  then transmits the resulting measurement to host  100 . 
     The electrical signal representing the measured signal from the DUT  106  output from the controller  102  is coupled to acquisition circuitry within the host  100  that converts the electrical signal into digital data values and stores the data values in memory (not shown) or displays the data on a display  110 . 
       FIG. 2  is a block diagram of the test and measurement system. As stated previously, the test and measurement system includes a host  100 , a controller  102 , an accessory head  104 , and a DUT  106 . 
     The accessory head  104  includes an optical sensor  200 , such as an optical voltage sensor. The optical sensor may be, for example, a Mach-Zehnder optical sensor. The inputs from the DUT  106  are connected to the input signal electrodes  202  and  204  of the optical sensor  200 . The optical sensor  200  also includes bias electrodes  206  and  208  that are connected to a bias control (not shown). Waveguide  210  in the optical sensor  200  directs light emitted from optical transmitter  212  located within the controller  102 , to allow for a measurement to be taken from DUT  106 . As light travels through the waveguide  210 , an electric field from the electrodes causes a phase shift of the guided light, which allows for a voltage measurement to be taken from DUT  106 . 
     As mentioned above, controller  102  includes an optical transmitter  212 , such as a laser. Controller  102  also includes an optical receiver  214  and an optical-to-electrical converter  216 . In operation, the optical transmitter  212  in the controller  102  emits a light beam to the optical sensor  200  along waveguide  210 . The optical sensor  200  reads the output from the DUT  106  based on the received light beam from the optical transmitter  212 , and outputs the resulting light beam to the optical receiver  214 . The optical-to-electrical converter  216  converts the optical signal received at the optical receiver  214  to an amplitude modulated electrical signal that is representative of the signal being measured on the DUT  106 . 
     Conventionally, the gain of the optical-to-electrical converter  216  is adjusted to change the sensitivity of the test and measurement system as a whole. For example, conventionally, the optical-to-electrical converter  216  includes two different gain settings for high sensitivity and low sensitivity. However, as mentioned above, achieving large changes in the gain of the receiver comes with trade-offs in dynamic range, noise, frequency response and bandwidth and overall system complexity. 
     In the disclosed technology, rather than changing the gain settings at the optical-to-electrical converter  216 , a fixed optical-to-electrical converter  216  is used. To control the gain and sensitivity of the overall test and measurement system, the output power of the optical transmitter  212  is adjusted. The optical transmitter  212  is typically contained in a feedback control circuit (not shown) that monitors the output power from the transmitter and precisely controls the drive current to the transmitter in order to keep the transmitter output power at the desired level. This drive current can be adjusted over a very wide range with a corresponding change in the optical transmitter  212  output power. The wavelength and line width of the optical transmitter  212  can also be controlled independently of the drive current. Since the overall system gain is directly proportional to the transmitter output power, this change in the transmitter drive level translates into a corresponding change in the overall system gain. If the output power of the optical transmitter  212  is adjusted, rather than adjusting an amplifier of the optical receiver  214  as was done traditionally, gain and sensitivity changes of over  10  times can be realized without degradation in the bandwidth and frequency response. 
     The output of the optical transmitter  212  is controlled by controller  102 . A user can input the desired gain or sensitivity of the accessory head  104  into the host  100 . The host  100  then sends the desired gain or sensitivity to the controller  102  and the controller  102  adjusts the output power of the optical transmitter  212  accordingly. 
     Rather than a user setting the desired gain or sensitivity, the gain or sensitivity of the measurement system may also be a variable gain control set during a calibration or compensation routine of the accessory head  104 . A calibration signal is sent to the accessory head  104  and then an output from the accessory head  104  is returned to the optical receiver  214 . The output from the optical receiver  214  is then converted via the optical-to-electrical converter  216  and the resulting converted signal is sent to the host  100 . Depending on the output of the signal sent to the host  100 , the host  100  can determine if the laser in the optical transmitter  212  needs to be adjusted. 
     A more intense light beam from optical transmitter  212  provides a higher reading from the DUT  106  through the optical sensor  200  in the accessory head  104 . A less intense light beam may provide a lower reading from the DUT  106 . Accordingly, if a greater gain is needed in the test and measurement system, the light beam of the optical transmitter  212  is set to a higher output power. 
     Adjusting the output power of the laser at the optical transmitter  212  rather than the gain of the optical-to-electrical converter  216  allows for greater gain and sensitivity without the loss of dynamic-range or frequency and bandwidth capabilities. Further, the overall measurement system does not become more complex because a user would no longer be required to have an ASIC to achieve the desired parameters, or to have a complicated gain control circuit in the receiver. Further, adjusting the gain and sensitivity of the system at the optical transmitter  212  allows for reduced noise in the overall system. 
     Host  100  may be, for example, 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. 
     Although controller  102  has been shown in  FIG. 1  as an independent component powered by host  100 , the controller  102  may also be an independently powered controller or may be placed within the host  100  rather than as a separate component. That is, the host  100  may include the optical transmitter  212 , the optical receiver  214 , and the optical-to-electrical converter  216 . 
     The term “controller” and “processor” as used herein is intended to include microprocessors, microcomputers, ASICs, and dedicated hardware controllers. 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. 
     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.