Patent Publication Number: US-2015084655-A1

Title: Switched load time-domain reflectometer de-embed probe

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/882,292 titled Switched Load Time-Domain Reflectometer de-embed probe filed on Sep. 25, 2013, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosed technology relates generally to signal acquisition systems, and more particularly, to a de-embed probe with switched loads and an internal signal generator for reducing measurement errors due to the probe tip loading of a device under test. 
     BACKGROUND 
     Traditionally a vector network analyzer or a time-domain reflectometer (TDR) system with a sampling oscilloscope has been required to obtain scattering parameter (S-parameter) measurements for characterizations of a device under test (DUT). Once the S-parameters of the fixture have been measured and the S-parameters of the device under test have been measured, then a full de-embed operation can be performed to obtain only characteristics of the device under test. 
     De-embed probes as described in U.S. Pat. No. 7,460,983 titled SIGNAL 
     ANALYSIS SYSTEM AND CALIBRATION METHOD, U.S. Pat. No. 7,414,411 titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD FOR MULTIPLE SIGNAL PROBES, U.S. Pat. No. 7,408,363 titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD FOR PROCESSING ACQUIRES SIGNAL SAMPLES WITH AN ARBITRARY LOAD, and U.S. Pat. No. 7,405,575 titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD FOR MEASURING THE IMPEDANCE OF A DEVICE UNDER TEST, each of which is incorporated herein by reference in its entirety, use switched loads inside the probes across the probe tips to take measurements. The S-parameters of the de-embed probe are measured at manufacturing time and stored in an S-parameter memory inside the probes. A user then connects a probe to the device under test and presses a calibration button. The scope takes two or three averaged acquisitions each with a different de-embed load switched across the probe tip. 
     After the acquisitions, the oscilloscope can compute the impedance of the device under test as a function of frequency and also provide a fully de-embedded view of the waveform at the device under test as if the probe and oscilloscope had never been connected. This can also be done by incorporating the above discussed method into a vector network analyzer using two de-embed prove fixtures with a signal source and a setup to operate as a vector network analyzer using two de-embed probes, as discussed in U.S. Provisional Application No. 61/882,283, titled TWO PORT VECTOR NETWORK ANALYZER USING DE-EMBED PROBES. 
     U.S. Provisional Application No. 61/882,298, titled “ALTERNATE METHOD OF PROVIDING DE-EMBED PROBE FUNCTIONALITY”, hereby incorporated by reference in its entirety, discloses a TDR de-embed probe. This probe contains no switched loads but contains an internal TDR generator that is always attached across the TDR de-embed probe tips. The S-parameters of this generator are measured at manufacturing and stored in the probe. When the TDR de-embed probe is connected to either an active or passive device under test, a triggering scheme is used to desynchronize the device under test waveform with the TDR pulser to average the device under test signal to zero so the result can be measured. From the measured result, the de-embedded waveform can be computed. 
     What is needed is a de-embed probe that can be used to measure both active and passive devices under test with or without a device under test signal source. 
     SUMMARY 
     Certain embodiments of the disclosed technology include a de-embed probe, including two inputs configured to connect to a device under test, a memory, a signal generator configured to output a signal, a plurality of load components, a plurality of switches, and a controller. Each load component is configured to provide a different load. A first switch of the plurality of switches is associated with the signal generator and the other switches of the plurality of switches are each associated with one load component. The controller is configured to control the plurality of switches to connect combinations of the loads from the plurality of load components and the signal from the signal generator across the two inputs. 
     Certain other embodiments of the disclosed technology include a de-embed probe including two inputs configured to connect to a device under test, a memory, a signal generator configured to output a signal, a load integrated circuit with a plurality of different loads, a plurality of switches, a first switch of the plurality of switches is associated with the signal generator and the other switches of the plurality of switches are each associated with a load of the load integrated circuit, and a controller configured to control the plurality of switches to connect combinations of loads from the load integrated circuit and the signal from the signal generator across the two inputs. 
     Certain other embodiments include a test and measurement system, including a device under test, a test and measurement instrument, and a de-embed probe of the disclosed technology as described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of a de-embed probe of the disclosed technology. 
         FIG. 2  illustrates a test and measurement system using the de-embed probe of  FIG. 1 . 
         FIG. 3  illustrates a block diagram of a de-embed probe according to another embodiment of 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 disclosed technology includes a de-embed probe that includes both a signal generator and switched loads, as shown in  FIG. 1 .  FIG. 1  depicts a de-embed probe  100  according to some embodiments of the disclosed technology. The de-embed probe  100  can be a standard probe with standard probe tips. The de-embed probe  100  can also be implemented as a plug-in module. Preferably, the de-embed probe  100  would be implemented as a probe compensation box with a subminiature version A (SMA) connector input. This configuration of the de-embed probe  100  would allow room for a signal generator  102  and other circuitry, as discussed in more detail below. 
     The de-embed probe  100  includes an amplifier  104  and also the typical circuitry generally found in de-embed probes and as discussed in the above discussed patent publications. The typical circuitry is not shown in  FIG. 1 . The de-embed probe  100  also includes a set of switches  106 . Some of the switches  106  may be analog switches within an integrated circuit. Further, some of the switches  106  may be microelectromechanical systems (MEMs). Other types of switches  106  may be incorporated such as relay contacts. The switches  106  are controlled by controller  108 , as will be discussed in more detail below. The de-embed probe  100  also includes a memory component  110 . The memory  110  stores the measured S-parameters of the probe in each of the possible switch  106  positions used during operation of the probe. These S-parameters are used to provide a de-embedded view of the waveform depending on the position of the switches for the probe acquisitions. For example, the memory would store the S-parameters for the probe if only the signal generator  102  is switched to be connected to the probe inputs  112  and  114 . And the memory  110  stores the S-parameters for the probe when the switches  106  are in all the other positions. The memory component  110  may also store typical functions that probes already incorporate. Further, memory component  110  may be made up of multiple memory components. 
     The de-embed probe  100  also includes a plurality of loads  116  that can be switched across the probe inputs  112  and  114 . The loads  116  may be provided by either a load integrated circuit or discrete load components. A minimum of three loads  116  have to be switched across the probe inputs. However, the first load is considered to be when no loads are connected across the probe inputs. It is desirable and preferable to have numerous other loads so that the best loading for the device under test can be chosen by a user in a menu of the test and measurement instrument as discussed below. 
     The de-embed probe  100  also includes a signal generator  102 , as mentioned above. If the de-embed probe  100  is a differential probe, then the signal generator  102  is also differential. However, if the de-embed probe is a single-ended de-embed probe (not shown), then the signal generator  102  is single-ended (not shown). That is, the de-embed probe may contain only a single input and a single output, rather than two inputs and one or more outputs. Preferably the signal generator  102  is a TDR pulse generator because a TDR pulse generator is easier to incorporate into the small size needed to fit into a probe. However, the signal generator  102  may be any type of signal generator, such a sine wave generator. 
     De-embed probe  100  also includes an output  118  from amplifier  104  that is sent to a test and measurement instrument as described in more detail below with respect to  FIG. 2 . The output  118  includes the waveforms from inputs  112  and  114  after they have traveled through the circuitry of probe  100  and the amplifier  104 . 
     De-embed probe  100  described above with respect to  FIG. 1  can be used in a test and measurement system as shown in  FIG. 2 . The de-embed probe  100  is connected to a test and measurement instrument  200  and a device under test  202 . 
     De-embed probe  100  can be used with any type of test and measurement instrument  200  that can accept an input from a probe. The test and measurement instrument  200  has the responsibility of controlling via a processor  204  the controller  108  to control the switches  106  during operation via path  120 . The processor  204  is also used to compute the math algorithms needed to perform the de-embed operations via a set of instructions stored in a memory and executed via the processor  204 . The S-parameters of the test and measurement instrument  100  are stored in a memory (not shown) in the test and measurement instrument  100  to be used as a part of the total de-embed process to provide more accurate results. 
     The test and measurement instrument  200  also includes a user interface  206 . A user is capable of controlling the de-embed probe  100  via the user interface  206 . That is, the user can control what loads and how many loads are connected across the probe inputs  112  and  114 . 
     The probe  100  may be attached to an extension cable to place the probe  100  closer to the device under test  202 . A user may insert into the user interface  206  the 
     S-parameters of any cable or fixture between the probe  100  and the device under test  200 . These are loaded or inputted to the user interface  206  to be included in the de-embed operation performed by the processor  204 . 
     The equations, math, and algorithms developed and defined in U.S. Pat. No. 7,460,983 titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD, U.S. Pat. No. 7,414,411 titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD FOR MULTIPLE SIGNAL PROBES, U.S. Pat. No. 7,408,363 Al titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD FOR PROCESSING ACQUIRES SIGNAL SAMPLES WITH AN ARBITRARY LOAD, U.S. Pat. No. 7,405,575 titled SIGNAL ANALYSIS SYSTEM AND CALIBRATION METHOD FOR MEASURING THE IMPEDANCE OF A DEVICE UNDER TEST, and U.S. Provisional Application No. 61/882,283, titled TWO PORT VECTOR NETWORK ANALYZER USING DE-EMBED PROBES each of which is incorporated herein by reference in its entirety, discussed above, may be used to derive algorithms to de-embed the acquired waveforms 
     Previously, the signal source was the device under test being measured in the test and measurement system and only passive loads were switched across the probe inputs. In the disclosed technology, if the device under test is passive then the signal generator  102  is switched across the inputs of the probe  112  and  114  to measure the impedance of the device under test  202 . The switched loads  116  are also used. The equations to perform the de-embed operation to obtain the characteristics of device under test  202  are the same as in the patent descriptions discussed above, except the signal generator  102  is located within the de-embed probe  100  rather than in the device under test  202 . 
     If the device under test  202  is active and includes a signal, the signal generator  102  is still switched across the inputs of the probes  112  and  114  along with the switched loads  116 . The signal from the active device under test  202  must be random with respect to the signal from the signal generator  102 . Then, a desynchronizing random delay trigger method may be used to insure that the device under test signal  202  averages to zero while the internal signal from the signal generator  102  does not. This provides an acceptable signal to noise ratio for the measurement. The random delay trigger would reside within the test and measurement instrument  200 . Compared with U.S. Provisional Application No. 61/882,298, titled “ALTERNATE METHOD OF PROVIDING DE-EMBED PROBE FUNCTIONALITY”, the plurality of loads can be switched in to be used with the signal generator  102 , and the de-embed results obtained from various loads can be then averaged to improve the accuracy. 
     The probe of the disclosed technology is not limited to a three-port probe  100  as shown in  FIG. 1 . The probe may also be a four-port probe  300  as shown in  FIG. 3 . Rather than the acquisition from inputs  112  and  114  traveling to a single amplifier  104  and a single output  118 , each input  112  and  114  includes an amplifier  302  and  304 , respectively, and an output  306  and  308 , respectively. Probe  300 , however, would still operate in the same manner as probe  100  discussed above with respect to the signal generator  102 , loads  116 , and switches  106 . 
     With a four-port probe  300  as shown in  FIG. 3 , both of the input waveforms from inputs  112  and  114  pass through the probe  300  to the test and measurement instrument. Both these waveforms may then be used in the measurement of S-parameter modeling processor to result in the desired de-embedding of the test equipment to provide a true waveform from the device under test. With the differential signal from the device under test fully represented by two waveforms, a user may be interested in any of four possible output waveforms: (1) a differential mode, which is the difference between the two waveforms on the two sides; (2) a common mode which is the sum of the two waveforms divided by 2; and (3) showing only one of the other waveforms. If the probe is only a three-port probe  100  as shown in  FIG. 1 , then the de-embedding operation only looks at the differential mode waveform. 
     The test and measurement instrument  200  may be an oscilloscope or spectrum analyzer. As mentioned above, the test and measurement instrument  200  includes a processor  204  and a memory (not shown) to store executable instructions. Such executable instructions may be computer readable code embodied on a computer readable medium, which when executed, causes the computer or processor to perform any of the above-described operations. As used here, a computer is any device that can execute code. Microprocessors, programmable logic devices, multiprocessor systems, digital signal processors, personal computers, or the like are all examples of such a computer. In some embodiments, the computer readable medium can be a tangible computer readable medium that is configured to store the computer readable code in a non-transitory manner. 
     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.