Abstract:
A computer-based method and computer-readable medium containing computer-executable instructions for assimilating data collected by a time domain reflectometer and displaying more than two waves representing reflections of a pulse on conductors is provided. The method includes a means for wave reversal, wave shifting, multi-wave display, segmented velocity of propagation adjustment, multi-cursor option multi-flagging options and calculating of the total length of wet cable. The combination of these functions provides a highly accurate means for identifying the location of splices, faults, corrosion, cable damage and other anomalies that are typically found on any length of conductor cable. The ability of this method to display a greater number of waves simultaneously adds additional benefit to a technician attempting to locate particular anomalies with multi-conductor cables.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/169,229, filed Dec. 6, 1999, the disclosure of which is hereby expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to time domain reflectometers and, more particularly, to a method of minimizing signal errors and anomalies. 
     BACKGROUND OF THE INVENTION 
     With the ever-increasing number of communication and transmission cables being utilized throughout the world, it is desirable that anomalies such as faults, partial discharges, cable damage, and splices on communication and power transmission cables be located without the necessity of physical tracing and inspection. A Time Domain Reflectometer (TDR) can be used to analyze a cable for anomalies or changes in cable impedance in order to locate such anomalies. A typical TDR transmits a pulse of electrical energy onto cables that include two conductors separated by a dielectric material. When the pulse encounters a change in the impedance of the cable, part of the pulse&#39;s energy is reflected back toward the TDR. The amplitude and polarity of this reflection is proportional to the change in impedance. Such reflections are usually displayed in graphical form on the screen of a typical TDR whereby a technician can interpret the results and locate specific cable anomalies. 
     In the past, a technician&#39;s ability to interpret a displayed waveform has been limited because of a TDR&#39;s inability to provide high quality information. Information correlating to the portion of cable located closest to the TDR is of higher quality than that portion of the cable remotely located from the TDR. This is because the reflection signal degrades as the length increases. As a result, a waveform decreases in accuracy as the distance between that portion of the cable being measured and the TDR increases. Currently, there are several available solutions to overcome waveform degradation. One such solution is to locate the TDR at both ends of the cable being analyzed. This is undesirable because the technician would have to manually compare the two waveforms and make a calculation to determine the location of objects of interest, such as the location and determination of anomalies. 
     Another solution is to connect a signal wire to each end of the cable and simultaneously measure the refection wave. The TDR would then be able to process the two signals to better pinpoint anomalies. This is undesirable because a great length of test leads are necessary to measure two ends of a long portion of cable simultaneously with a single TDR. Additional problems arise when a standard three-phase power cable is analyzed and only one phase at a time can be recorded. This results in potential human comparison errors when deciphers splice and fault locations. In multiple conductor cables, this problem is even more evident. 
     Another problem that has arisen with the use of a currently available TDR, is cable medium with changing segment impedance. Often times, a cable contains several segments of different conductive mediums spliced together to form one cohesive length of cable. The reason segmented cables exist is due to portions of the length having been replaced with different conductor materials because of damage to the cable or the need replace particular sections of the cable with a different medium. A change in the medium will affect the impedance because of small differences in the cable&#39;s, manufactured geometry or materials thereby affecting signal&#39;s velocity of propagation (VOP). This results in inaccurate information of anomalies further down the conductor. Other factors may affect the VOP as well, such as a change in the dielectric material that separates conductors within a cable. Water flooding in the interior of cables that use air as part of the dielectric separation of conductors has been a particular problem that affects the VOP of a signal from a TDR. 
     Thus, there exists a need for graphically representing information collected from a device propagating a signal along the length of a cable. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention provides a method, apparatus and computer-readable medium for improving the quality and accuracy of information collected by propagating a signal along a length of cable in order to pinpoint specific anomalies along the length of cable. This embodiment improves quality and accuracy by displaying multiple waves simultaneously and combining several steps of signal processing to raw data collected by a TDR. The signal processing steps include: signal data collection, wave reversal, wave shifting, multi-wave display, segmented velocity of propagation, multi cursor, and wet cable calculator. 
     Using the various embodiments of the present invention in conjunction with a TDR, a technician can record, modify, and display several waveforms corresponding to specific cables from either end of specific cables and process the information collected and recorded at a later time. Specifically, a technician can take a set of two recorded waveforms that are collected from the same cable and compare the waveforms to determine the location of anomalies. If the two waveforms are recorded from opposite ends of the cable, then wave reversal can be used to process the waveforms in order to produce a more accurate representation of the location of anomalies along the cable. 
     As a non-limiting example, if the two waveforms are recorded from two different points on a cable in the same direction of propagation, then wave shifting can be used to process the waveforms in order to produce a more accurate representation of the location of anomalies along the cable. 
     In order to more accurately decipher the location of anomalies along a set of conductors, multiple waveforms can be displayed simultaneously. A technician can easily pinpoint the location of particular anomalies, such as three phase faults or severed cables, by analyzing several waveforms simultaneously. 
     Additionally, in another embodiment of the present invention, the accuracy of locating anomalies can be improved if the technician is aware of segments of differing mediums along the length of cable. By identifying the particular medium of the segment on which the signal is propagating, the TDR can compensate for a change in VOP which would affect the accuracy of the anomaly&#39;s actual location. A typical TDR will measure the time interval between two cursors that can be manually or automatically positioned. Because of this limitation of two cursors, several segments had to be analyzed separately. However, the various embodiments of the present invention are capable of employing several cursors simultaneously to analyze the entire length of cable with several different mediums, and subsequently each with a differing VOP. 
     Finally, in still yet another embodiment of the present invention, the calculation of the total length of water affecting the impedance of a cable is now possible. A technician knowing this information is able to adjust the signal processing in order to take this condition into account prior to identifying anomalies and their respective locations along the cable. This embodiment also improves the accuracy of locating anomalies. 
     A method, apparatus, and computer-readable medium capable of performing actions generally consistent with the foregoing data acquisition and signal processing for determining the location of anomalies along a cable is presented in further detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of a general-purpose computer system for implementing one embodiment of the present invention; 
     FIG. 2 is a block diagram of a prior art Time Domain Reflectometer (TDR); 
     FIG. 3 is a flowchart of an overall program architecture for a method of displaying waves collected by a TDR; 
     FIG. 4 is a flowchart of a wave reversal subroutine in a method of displaying waves collected by a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 5 is a flowchart of a wave shifting subroutine in a method of displaying waves collected by a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 6 is a flowchart of a multi-wave display subroutine in a method of displaying waves collected by a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 7 is a flowchart of a segmented velocity of propagation subroutine in a method of displaying waves collected by a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 7A is a flowchart of a calculation for total length of water affecting the impedance of a cable formed in accordance with one embodiment of the present invention; 
     FIG. 8 is a flowchart of a multi-cursor/flagging subroutine in a method of displaying waves collected by a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 9 is an exemplary wave form displayed on a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 10 is an exemplary reversed wave form displayed on a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 11 is an exemplary combination of a wave form and its reversed trace displayed on a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 12 is an exemplary wave form showing corrosion displayed on a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 13 is an exemplary comparison multi-wave form displayed on a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 14 is an exemplary three phase wave form displayed on a TDR formed in accordance with one embodiment of the present invention; 
     FIG. 15 is an exemplary set of wave forms displayed on a TDR with segmented VOP compensation formed in accordance with one embodiment of the present invention; 
     FIG. 16 is an exemplary set of wave forms displayed on a TDR with segmented VOP compensation formed in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Time Domain Reflectometers (TDR) transmit a pulse of electrical energy onto cables that includes two conductors separated by a dielectric material. When the electrical pulse encounters change in the cable that causes the impedance to change, part of the pulse&#39;s energy is reflected back toward the TDR. By measuring the amplitude and polarity of the reflected wave, the proportionality of the impedance change can be determined. Additionally, by measuring the time of propagation, the location of the impedance change can also be determined. Typical anomalies that will cause an impedance change include a change in the cable medium, splices, faults, partial discharges, and damage to the cable. 
     In accordance with one embodiment of the present invention, the TDR Display method source programs execute on a computer, preferably a general-purpose computer configured with basic input/output functions for a handheld device. FIG.  1  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which current embodiments of the invention may be implemented. Although not required, the embodiments of the present invention are described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the various embodiments of the present invention may be practiced with other computer system configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The various embodiments of the invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     With reference to FIG. 1, an exemplary system for implementing the embodiments of the invention includes a general purpose computing device in the form of a conventional personal computer  120 . The personal computer  120  includes a processing unit  121 , a system memory  122 , and a system bus  123  that couples various system components including the system memory  122  to the processing unit  121 . The system bus  123  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory  122  includes read only memory (ROM)  124 , random access memory (RAM)  125 , and a basic input/output system (BIOS)  126 , containing the basic routines that help to transfer information between elements within the personal computer  120 . 
     The personal computer  120  further includes a hard disk drive  127  for reading from and writing to a hard disk (not shown), a magnetic disk drive  128  for reading from or writing to a removable magnetic disk  129 , and an optical disk drive  130  for reading from or writing to a removable optical disk  131 , such as a CD ROM or other optical media. The hard disk drive  127 , magnetic disk drive  128 , and optical disk drive  130  are connected to the system bus  123  by a hard disk drive interface  132 , a magnetic disk drive interface  133 , and an optical drive interface  134 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  120 . 
     Although the foregoing exemplary environment employs a hard disk, a removable magnetic disk  129  and a removable optical disk  131 , it should be appreciated by those skilled in the art that other types of computer-readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital versatile disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROM), and the like, may also be used in the exemplary operating environment. 
     A number of program modules may be stored on the hard disk, magnetic disk  129 , optical disk  131 , ROM  124  or RAM  125 , including an operating system  135 , one or more application programs  136 , and program data  138 . A technician may enter commands and information into the personal computer  120  through input devices such as a keyboard  140  and pointing device  142 . Other input devices (not shown) may include a microphone, joystick, keypad, touch screen, scanner, or the like. These and other input devices are often connected to the processing unit  121  through a serial port interface  146  that is coupled to the system bus  123 , but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). A monitor  147  or other type of display device is also connected to the system bus  123  via an interface, such as a video adapter  148 . One or more speakers  157  are also connected to the system bus  123  via an interface, such as an audio adapter  156 . In addition to the monitor and speakers, personal computers typically include other peripheral output devices (not shown), such as printers. 
     The foregoing computer environment may be housed in a handheld device that can be coupled to a pair of conductor cables. FIG. 2 depicts a well known and typical handheld TDR. The computing unit, as described previously is housed in a compartment  210 . Depicted within the compartment  210  is the processing unit  121 , the display  147 , a keypad or touch screen interface  140 , system Memory  122 , a pulse generator  211 , and a pulse sensor  212 . When the program is implemented, a pulse is generated at the pulse generator  211  and propagated down the cable  213 . The pulse sensor  212  is then able to detect any reflection which occurs due to a change in impedance on the cable  213 . As the wave reflections are detected, the program receives pulse information from the pulse sensor  212  and assimilates the information to be displayed in a graphical representation on the display  147 . The technician of the TDR is able to interpret information from the graphical representation of the anomalies detected on the cable  213 . 
     One embodiment of the current invention is a method of recording, processing and displaying the information collected by the TDR. Information previously collected and stored on a computer may also be processed and displayed. FIG. 3 depicts the overall program architecture of the program. When the program is implemented, a technician selects a wave to be added to the display in Step  310 . By selecting a wave to be displayed, the data corresponding to the wave is loaded into the program. Loaded wave files can be modified by one of a number of methods described below. Loading a wave is done by using a browsing subroutine which allows the technician to select files from memory or the current live trace. If multiple waves have been loaded, the last wave to be modified (or recently loaded) is the active wave. Only the active wave can be modified individually. To modify a different wave, the technician must select the different wave as active. 
     Once a particular stored wave is selected to be loaded, the technician is prompted to select whether or not to implement the method of wave reversal in Step  315 . If wave reversal is selected, then the wave reversal subroutine is implemented which is depicted in FIG.  4  and discussed later. If the technician selects no wave reversal, then the wave file is loaded to an initial display screen, Step  320 . The technician is then asked if the technician wishes to select another wave to be loaded. The technician may repeat Steps  310 - 320  if another wave is desired, but if not, the program proceeds to an active wave display screen Step  325 . 
     The technician then selects one of the loaded waves to be the active one, step  325 . The technician may modify individual wave attributes, Step  330  which will only affect the active wave or may modify global wave attributes, Step  335  which will affect all loaded waves. Individual wave attribute modification include, wave shifting, depicted in FIG. 5 or multi-cursor flagging, depicted in FIG.  8 . Global wave attribute modifications include panning zooming and segmented velocity of propagation, depicted in FIG.  7 . Additionally, the technician may enable a wet cable calculation on any cable or portion thereof. After, each modification is implemented, all loaded waves are displayed on the display  147  in Step  340 . This multi-wave display method is interspersed within the overall flow of FIG.  3  and is presented in greater detail in FIG.  8 . Each attribute modification method is discussed in greater detail below. 
     FIG. 4 is a flowchart of the subroutine for wave reversal. If a technician chooses wave reversal in Step  315 , then this subroutine is implemented. As stated above, the wave reversal method is implemented when a particular wave is being loaded. A separate browser window is opened on the display in Step  410  that will allow a technician to select a particular wave file in Step  415 . The technician may then choose to implement wave reversal in Step  420 . When wave reversal is chosen, a file utility will be opened that renders the normal data in a transposed fashion. 
     With wave reversal, two traces of a reflected wave of the same cable can be displayed on the display  147  with one of the traces reversed. The first wave is a recorded trace or a live trace and depicted as a wave propagating from end A to end B as shown in FIG.  9 . End A represents the location of the TDR and end B represents the other end of the conductor. A second wave, which is the reversed wave is a recorded trace or a live trace and depicted from end B to end A as shown in FIG.  10 . Additionally, end A and B can be transposed, where end B represents the location of the TDR and end A represents the other end of the conductor. While two waves are used in the foregoing example, it should be apparent that the same invention can be applied to more waves, such as six waves (representing reflection waves from both ends of a 3 phase cable system) or more (when representing multiple conductor cable such as used in telecommunications). 
     As a pulse travels along a cable, its amplitude is attenuated. Imperfections, such as splices and corrosion, often called anomalies, will reflect a portion of the signal wave back to the TDR. Consequently, reflections coming from farther along a cable are smaller than reflections coming from close in. In addition to this attenuation from the cable, objects the pulse encounters will consume part of the pulse energy also attenuating the pulse. If there are two splices on a cable, the wavelike reflection from the second will generally appear smaller than the first. The reflection from neutral corrosion is a small positive only reflection. It is often small enough to be difficult to recognize. 
     Referring back to FIG. 4, in Step  430 , both traces are displayed at the same time vertically adjacent and with the either the first or the second trace live, but not both. Both may also be from memory, however. The second trace will be displayed reversed left to right so that ends A and B of both traces correlate. This is shown in FIG.  11 . As is shown the echoes do not match up vertically and it easily deciphered as merely an echo, whereas other anomalies occur in the same location. Also shown in FIG. 9 are representative anomalies that a TDR will locate and display. Corrosion  910 , a splice  920 , and an echo  930  are shown on this particular trace. 
     Corrosion reflections and sometimes splice reflections can also be confused with echoes. These echoes come from the pulse and reflection bouncing back and forth between objects like splices. Wave reversal will make the difference between echoes and true reflections more obvious. As a non-limiting example, with only one wave displayed, a small reflection which may be an echo or an anomaly far from the TDR cable cannot be easily identified. However, when the same wave is reversed and viewed from the second end, an echo will not be in the same place. When the second wave is reversed and placed along the first, anomalies that are echoes become much more obvious. 
     When the view from both ends are lined up using wave reversal, the reflection of some objects will appear to not line up. This is because the left edge of the reflection is the point where the pulse first encounters the left edge of the object. When a trace is reversed, the right side of the reflection is at the right side of the object. Since the two traces are views of the same cable from opposite ends, the difference in the positions of the two reflections is the difference between the true position of the left and right ends of the object. In this way, the length of an object can be measured. This is useful because the length of a reflection is longer than the length of the object that created it. It is particularly useful in measuring the extent of corrosion on power cables. This corrosion  1210  is shown in FIG.  12 . 
     FIG. 5 is a flowchart of the subroutine for wave shifting. Wave shifting will move an active wave horizontally, as represented on the display, relative to other waves, such that cable end reflections or anomalies can be correlated. This is shown generally is FIG.  13 . Wave shifting is necessary to aid in utilizing the previous function (wave reversal). Without wave shifting, the second trace, which is a reversed view of the same cable, the time coordinate would not correlate to the first, thus making any comparison moot. With both, it is possible to see when a reflection changes its apparent position if viewed from the other end. This will make echoes  1310  more obvious as they will not correlate to any reflection on a companion trace. 
     Referring back to FIG. 5, in Step  510 , a technician selects a particular wave to be shifted. In Step  520 , the technician selects starting point for the wave shift. In Step  530 , the program computes the time coordinate for the start of the wave shift. After these technician inputs are entered, the program edits the wave with starting point and time coordinate parameters. After computation, the new wave is displayed once again in Step  540 . 
     FIG. 6 is a flowchart of the multi-wave display function of the present invention. In the present art, a single channel TDR typically can display two waves from memory or one from memory and the other live (frequently updated with current data from the cable that the TDR is currently connected to). In one embodiment of the present invention, more than two waves can be displayed at the same time using a single channel TDR. Since many power cables being inspected are part of a three phase system (one circuit consisting of three parallel cables), with certain embodiments of the present invention all three phases can be surveyed, recorded, and then displayed with a single channel TDR reducing complexity and cost. 
     Multi-wave display will allow more than two (usually three and sometimes six) traces to be displayed simultaneously in any combination of a single live trace while the rest are from stored files. This will facilitate understanding and recognition of cable problems in multi-phase cable systems. This concept is exemplified in FIG. 14, whereby three cables of a three-phase system are shown vertically correlated for easy comparison. When used with wave reversal, up to six waves may be displayed simultaneously. The traces can be displayed vertically adjacent to aid visualization of differences or could be merged using datapoint addition, averaging, or subtraction, to form an composite trace to aid visualization of anomalies common to all. 
     During the multi-display method embodiment of the present invention, individual waves are loaded into the display program in Step  610 . With each addition, individual wave attributes can be modified in Step  620  (wave reversal, wave shifting) in addition to technician selections of whether the wave is to be visible in Step  630  and what distance of vertical separation is to be set between displayed waves (vertical offset value) in Step  640 . These steps roughly correlate to the steps of wave reversal  315 , individual attribute modification  330 , and global wave attribute modification  335 . Once all waves have been loaded and modified accordingly, each visible wave is displayed on the display  147  in Step  650 . 
     FIG. 7 is a flowchart of the segmented velocity of propagation subroutine of the program. Segmented Velocity of Propagation (VOP) will allow the trace(s) to be subdivided into segments with independent VOP settings. A VOP setting is a determination of the rate at which a pulse will travel along a cable and is governed by the physical attributes of the conductor. These VOP numbers are well known in the art for all typical conductor materials. This VOP setting can compensate and correct for sections of the cable having different speeds of pulse propagation. These different speeds can come from different types of cables being spliced together, or from the effects of other post manufacture differences such as water or filling compounds in telecommunication cables. Without segmenting, slow sections of cable would appear longer or shorter than actual length and all intermediate distance measurements would be inaccurate because a single VOP setting would only be able to arrange the total cable&#39;s VOP. FIG. 15 illustrates how a particular length of cable can be misrepresented in this fashion. 
     If the VOP between splice  1 , referred to by the number  1510 , and splice  2 , referred to by the number  1520 , is slower than the rest of the cable, the reflections  1530  and  1540  will appear in the wrong location. In FIG. 16, the VOP of the three segments  1610 ,  1620 , and  1630  that make up the cable are set independently. This will adjust the horizontal scale of the display to compensate for the different speeds and consequently splice  1   1640  will correlate correctly to its reflection  1660  as will splice  2   1650  correlate correctly to its reflection  1670 . 
     To set a desired VOP for a particular segment, a technician opens a dialog box in Step  710 . The technician chooses a “from flag” location in Step  720 , a “to flag” location in Step  730  and a VOP value for the particular segment in Step  740 . After these attributes are selected, the technician closes the dialog box and the value in the set in Step  740  replaces the default VOP variable “D” in Step  750 . At this point, if the new value of “global interval” is not “D”, the program will determine the new VOP of the segment containing the pertinent data point in Step  760  and modify the X interval displayed between the pertinent data points rendered in distance in Step  770 . Once the new wave files have been modified and once the technician enters any new zoom and scroll options in Step  780 , all new waves are displayed in Step  790  on the display  140 . 
     In one particular embodiment, a segment may be analyzed to determine the length of wet cable that is present. When some telecommunication cables are installed, they contain air between the conductors of the pairs. Over time, this space can become filled with water, which degrades the quality of the cable. In conventional use of a TDR, the water can be seen as a negative reflection and placing cursors at both ends of the reflection can approximate the length of the wet section. However, water may not fill a long contiguous section that is easily identified. It can be separated into many wet spots from a few inches long to hundreds of feet. In accordance with one embodiment of the present invention, a TDR can be used to automatically calculate the total length of a cable that contains water using the following equation: 
     
       
           Lw=Vw×{[L −( Dt×Vd ))]/( VOPw−VOPd )} 
       
     
     Where: 
     VOPw=speed of pulse in wet cable 
     VOPd=Speed of pulse in dry cable 
     Dt=Time required for pulse to transit cable segment 
     L=True length of cable segment 
     Lw=Total portion of segment that is wet 
     VOP w  and VOP d  are properties that can be predicted or measured for a given cable type. As seen best by referring to FIG. 7A, these values are entered by the technician in Steps  792  and  793  respectively. When a technician uses the wet cable function, known data from a cable information chart is determined and the technician inputs theses values into the TDR previous to calculation. Alternatively, the TDR would have this data stored in a file from which the technician would choose a cable type. Dt is measured with the TDR by placing cursors at the reflections from the beginning and end of the cable, Step  794 . The operator would input the true length of the cable (L) Step  795  after measuring with a wheel. With this information the TDR can automatically calculate Step  796  and display Step  797  the total length of all portions of the cable that are wet. 
     FIG. 8 is a flowchart of the method for adding, removing or adjusting flags and/or cursors to an active wave. A traditional TDR measures the time interval between two cursors that can be manually or automatically positioned on the displayed trace. A cursor is an indication of a point on a trace which the technician seeks to identify for the purposes of gaining information about that particular location. The cursor can be manually positioned at any point along a trace using an input device such as a mouse. The TDR can calculate the length between two cursors. The ability to position more than two cursors on the trace would facilitate the segmented VOP and multi-trace functions above. Any number of cursors could be created and individually positioned on a specified trace. The time interval between selected cursors would then be multiplied by that segment&#39;s VOP to derive and display each segment&#39;s length. 
     One embodiment of this invention would take the form of a single active cursor and many flags. The active cursor can be maneuvered along the X coordinate axis and will represent points corresponding to its X coordinate for all loaded waves. A flag can be placed on a particular loaded wave. Each flag would be represented by a tick mark on one particular wave of a multi-wave display. If that wave is shifted relative to the other waves, the flag would remain associated with the X coordinate of that single wave. On the other hand, the active cursor would not shift with a single wave. It is only associated with the X coordinate of the global display and would shift positions as the global zoom and scroll are adjusted. 
     Flags can be added by a technician by selecting an active wave in Step  810 . The technician then positions the cursor where a flag is to be added, removed or modified in Step  820 . The technician can then add, remove or modify a flag in Step  830 , the culmination of which is an edit of the flag field for the active wave with a new X coordinate for each flag added, removed or modified in Step  840 . As flags are added, removed or modified, they are displayed as tick marks on their respective waves in Step  850 , on the display  147 . 
     The foregoing functions can be accomplished using computer executable instructions embodied on a computer-readable medium. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.