Abstract:
A system for calibrating cables attached to a time measurement device is disclosed. The system includes a calibration device that generates a plurality of synchronized measurement signals that are communicated to a time measurement device by a set of cables. The calibration device can also generate at least one arming signal that is also communicated to the time measurement device. By determining the relative arrival time of a common reference event of each of the measurement signals, the time measurement device can determine any skew that exists between each of the cables that input the measurement signals. The skew can then be used to compensate for varying cable skew in future time measurements. In one embodiment, the system of the present invention can also be used to determine the arming latency of the time measurement device by delaying the measurement signals in relation to the arming signals and observing the delay time which causes complete synchronization of the measurement signals.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Meeting timing requirements is a critical portion of the design and operation of systems in a wide variety of technological fields. Computer chipmakers continually reduce the minimum size of devices while simultaneously increasing speed and complexity to improve performance. Each reduction in size and increase in speed demands a higher level of precision in the timing of the electrical pulses input and output by the chip.  
           [0002]    A number of methods and devices have been developed for high-speed and high-precision timing testing. The devices, generally referred to as “Automated Testing Equipment,” come in a wide variety of forms.  
           [0003]    Automated Test Equipment includes devices, frequently referred to as “Time Measurement Units,” for measuring time intervals and signal events. An example of a time measurement unit is found in U.S. Pat. No. 6,091,671, issued to Kattan, for a time interval analyzer, which is incorporated herein by reference.  
           [0004]    A number of time measurement units are commercially available to measure time events and interpret data gathered from such measurements. These time measurement units are typically used in conjunction with some sort of measurement interface between the point of measurement (for example, an electronic circuit board connection) and the time measurement unit. The measurement interface, referred to as an Automated Testing Equipment (ATE) loadboard, is commonly connected to the time measurement unit via (typically coaxial) cables.  
           [0005]    The cables, while providing a signal path from the testing equipment to the time measurement unit, are not ideal components. As the frequency of a signal increases, physical factors such as a cable&#39;s length and frequency response become substantially more important.  
           [0006]    At extremely high frequencies, even the slightest deviations in the cables—deviations such as bends, kinks, or length variations—can have an adverse effect on the accuracy of time measurements. When the length of a signal pulse is many orders of magnitude smaller than the length of the cable it is sent through, a simple bend in the cable could delay the signal enough to render a time measurement useless. Since it is nearly impossible and not economical to eliminate all the causes of such problems, there must be compensation for the signal irregularities that the cables introduce.  
           [0007]    In the prior art there can be found several attempts to address the problem of cable-induced signal irregularities through calibration of test equipment. One such example can be found in U.S. Pat. No. 6,032,107 issued to Hitchcock, entitled “Calibrating Test Equipment.” The calibration methods and apparatus taught in the Hitchcock patent rely upon usage of a test article measured with calibrated test equipment. The test article is measured using the uncalibrated equipment, and the resulting measurements are compared to those taken by the calibrated equipment. One of the deficiencies of this method, however, is that it does not address the situation where there is no previously calibrated equipment.  
           [0008]    Additionally, the complex nature of many time measurement units can be the source of timing errors as well. Typically, a time measurement unit must be “armed,” or prepared, to take a measurement. The arming circuitry of the time measurement unit causes a delayed response to the arming signal. This arming latency can vary with temperature and other factors, and introduces another variable that must be taken into account in time measurement. Without knowledge of the arming latency, it can be very difficult to make accurate measurements because the exact time that the measurement is taken is indeterminate.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is generally directed to a system for calibrating a time measurement device to compensate for irregularities in interface connections to time measurement equipment. The irregularities can be introduced by various factors, particularly by variations in cables that carry signals to the time measurement device.  
           [0010]    For example, when calibrating a time measurement device to compensate for cables that carry signals to the device, the process of the present invention can include the steps of generating a plurality of synchronized signals. The measurement signals can be communicated to the time measurement device by the corresponding set of cables that undergo calibration upon a software command.  
           [0011]    The time measurement device is used to determine the relative arrival time to a common reference event of each of the measurement signals. Any skew that exists between each of the cables can then be determined by comparing the time differences between the relative arrival times of the common reference event to each of the transmitted synchronized signals.  
           [0012]    Once a specific cable skew has been determined, this data can be stored by the time measurement device for making adjustments in future time measurements.  
           [0013]    The synchronized measurements that can be generated and communicated to the time measurement device can be single ended signals or can be a pair of complementary differential signals. When the measurement signals are differential signals, the common reference event can be where the differential signals cross each other at one of the channels that is designated as a reference channel.  
           [0014]    In some applications, an additional compensation scheme may also be needed in order to calibrate the arming circuitry of a time measurement device. This compensation is needed to ensure that the test equipment is aware of any delay that exists between an arming circuit and a signal channel. This delay is often referred to as arming latency and needs to be known.  
           [0015]    If desired, the disclosed calibration system can also be used to determine the arming latency of a time measurement device including the cables that connect each arming signal from the loadboard to the time measurement device. As will be described in more detail below, the arming latency can be determined by generating multiple arming signal pulses and measurement pulses having an increasing delay between them that are communicated to the time measurement device. From the times of occurrence of the received measurement pulses and the times of occurrence of the arming signal pulses, the time measurement device can then determine the time interval between the time of receipt of the arming signal and a measured measurement pulse, thus determining the arming latency.  
           [0016]    The system of the present invention for calibrating irregularities in time measurement devices can include a time measurement device in communication with a calibration device. The calibration device can include at least one signal generator for generating a plurality of synchronized measurement signals. In addition, the calibration device may contain at least one signal that can be used as an arming signal. A transmission element can be provided for forming a signal path from the calibration device to the time measurement device.  
           [0017]    In one embodiment, the calibration device can include at least one output buffer-expander that is connected to the signal generator. The output buffer-expander can reproduce and convert the measurement and arming signal pattern input into a plurality of synchronized measurement and synchronized arming signal pattern outputs for communication to the time measurement device. The calibration device can also include adjustable delay circuitry interposed between the signal generator and the output buffer-expander which delays the transmission of one of the signal generator pattern outputs.  
           [0018]    The calibration device can further include a nonvolatile and programmable solid-state memory storage that is readable by the signal generator to store particular data needed to ensure complete synchronization of the output signals.  
           [0019]    Other features and aspects of the present invention will be discussed in greater detail below.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:  
         [0021]    [0021]FIG. 1 is a side view depicting the connection between a time measurement unit via cables to an automated testing equipment loadboard and a device under test.  
         [0022]    [0022]FIG. 2 is an exploded view of the signal input connections of the time measurement unit illustrated in FIG. 1.  
         [0023]    [0023]FIG. 3 is a block diagram illustration of a calibration device in accordance with one embodiment of this invention.  
         [0024]    FIGS.  4 ( a ) and  4 ( b ) are block diagram illustrations of two embodiments of a calibration device as connected to automated testing equipment in accordance with preferred embodiments of this invention.  
         [0025]    [0025]FIG. 5 provides respective timing diagrams  5 ( a ),  5 ( b ),  5 ( c ), and  5 ( d ) illustrating four measurement channels, measured reference events (a transition from high to low), and calculated skew intervals between the reference events.  
         [0026]    [0026]FIG. 6 displays timing diagrams  6 ( a )- 6 ( d ), respectively, where  6 ( a ) and  6 ( b ) illustrate the two components of a differential signal which has zero intra-channel skew.  6 ( c ) and  6 ( d ) are timing diagrams of the two components of a differential signal with a finite intra-channel skew detectable by this invention, with measured reference events and differential skew values illustrated.  
         [0027]    [0027]FIG. 7 displays timing diagrams  7 ( a ) and  7 ( b ) depicting a measurement and arming channel, respectively, along with measured edges in order to illustrate the principle of arming latency.  
         [0028]    [0028]FIG. 8 displays timing diagrams  8 ( a )- 8 ( d ), respectively, where  8 ( a ) is a timing diagram featuring a clocked arming channel. Timing diagrams  8 ( b ),  8 ( c ), and  8 ( d ) feature measurement signal pulses, all of which depict signal events (a transition from low to high) and known delays in order to illustrate a method of determining arming latency in one embodiment of this invention.  
         [0029]    Repeat use of reference characters throughout the present specification and appended drawings is intended to represent the same or analogous features or elements of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Reference will now be made in detail to presently preferred embodiments of the invention, examples of which are fully represented in the accompanying drawings. Such examples are provided by way of an explanation of the invention, not limitation thereof. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention, without departing from the spirit and scope thereof. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Still further variations in selection of materials and/or characteristics may be practiced, to satisfy particular desired user criteria. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the present features and their equivalents.  
         [0031]    [0031]FIG. 1 illustrates a conventional configuration for the testing of a device using a time measurement unit (TMU)  10  and automated testing equipment (ATE) loadboard  14 . As depicted in greater detail in FIG. 2, time measurement unit  10  features arming channels  20  and  24  and measurement channels  21 ,  22 ,  25 , and  26  for receiving signal inputs from a device under test (DUT). Four measurement channels and two arming channels are provided for illustration purposes; those skilled in the art will appreciate that the methods and apparatus disclosed herein could easily accommodate fewer or more measurement and arming channels.  
         [0032]    During testing of the device under test (DUT), the arming and measurement signals are provided by DUT  18  to time measurement unit  10  through automated testing equipment loadboard  14 . Automated testing equipment loadboard  14  serves as an interface, taking signals received from DUT  18  through a socket  16  and routing them to time measurement unit  10  via connecting cables  12 . DUT  18  can be any number of a wide variety of devices for which precise timing measurements may be required, including, but not limited to, electronic circuits and chips.  
         [0033]    For calibrating time measurement unit  10  in order to compensate for errors introduced by the connecting cables  12 , calibration board  40  as illustrated generally in FIG. 3 is connected to the Automated Testing Equipment loadboard  14 . The calibration board includes a signal source generator  46 , preferably generating fast and stable edges, measurement signal buffer-expander  50 , adjustable signal delay circuitry  48 , arming signal buffer-expander  52 , matched measurement signal traces  54 , matched arming traces  56 , matched routing cables  62  and  64  (shown in FIGS.  4 ( a ) and  4 ( b )), a memory storage device (in the embodiments illustrated, an EEPROM)  44 , and configuration switch bank  42 . While further description of the invention refers to EEPROM  44 , any suitable reprogrammable nonvolatile memory can be utilized in the aspects and embodiments of the present invention. In one embodiment, configuration switch bank may include switches and inverter circuitry. Thus, the routing of signal outputs can be selected, as well as the type of signal (such as single ended, differential, or a combination of the two types).  
         [0034]    When used to calibrate the time measurement unit  10 , calibration board  40  is interfaced with automated testing equipment loadboard  14  in place of a DUT, and routing cables  62  and  64  are connected from the measurement and arming signal outputs to the automated testing equipment loadboard signal connections  66  and  68 . Those skilled in the art will appreciate the, fact that the exact connection and routing of cables  62  and  64  to automated testing equipment loadboard  14  will vary depending upon the configuration of time measurement unit  10  and the particular calibration test that is desired, as discussed in detail further in this application.  
         [0035]    Upon execution of the calibration routine, signal source generator  46  accesses the EEPROM memory. This memory contains a specialized sequence of bit outputs that correspond to certain information relating to the specific calibration device  40 . Such information may include pulse frequency and interval information, nominal voltage levels, and test program identification/initialization data, as well as any data regarding residual skew on the synchronized signals generated by calibration device  40 .  
         [0036]    Because the calibration board  40  outputs digital signals, data containing information to prepare time measurement unit  10  to perform the calibration sequence can be included in the output sequence. The data bits are followed by the actual test bits required for the calibration tests to be performed.  
         [0037]    In one embodiment, the calibration sequence is repeated so that time measurement unit  10  can sample the entire sequence of bits and determine which parts contain calibration data. After reading and processing the calibration setup data, time measurement unit  10  can select the appropriate point in the bit sequence to begin calibration measurements. This selection process may be enabled by providing a special series of bits in the bit sequence recognizable by time measurement unit  10 .  
         [0038]    This calibration data provided by the calibration board preferably contains information relating to any slight on-board deviations in signal trace lengths. As discussed later in this application, precise synchronization of signals at the calibration board is required for many calibration procedures. The use of previously measured or calculated deficiencies in the calibration board can add another level of precision to the calibration of time measurement unit  10 .  
         [0039]    The signal output by signal generator  46  is routed to measurement signal output buffer-expander  50 , which converts the signal pattern from signal generator  46  into a plurality of identical differential measurement channel signals. These measurement channel signals are routed to the plurality of output connections  58  through matched signal traces  54 . The particular routing of the signals is dependent upon the configuration of switch bank  42  that is set by the user to correspond to the desired calibration procedure. For example, for single-ended skew calibration tests, only one of each complementary pair will be routed to the measurement channels. In additional embodiments, switch bank  42  may include inverter circuitry such that differential output components could be generated based upon a single-ended signal generated by the source generator, if desired.  
         [0040]    The measurement channel signals are then routed through matched measurement channel signal routing cables  62  to the appropriate connection of automated testing equipment loadboard  14  using connectors  66  and  168 . The signal traces  54  and routing cables  62  are precisely electrically matched in order to reproduce the signals as accurately as possible. In most applications, any deviation between the synchronized signals at any of the outputs should be no greater than about 20 picoseconds.  
         [0041]    The signal output of the generator  46  is also routed to arming buffer-expander circuitry  52 . The arming buffer-expander circuit  52  produces a plurality of single-ended arming signals. As in the case of the measurement channel signals, in order to ensure that the arming signals are as nearly identical as is possible, the calibration board provides precisely matched traces  56  and routing cables  64  for routing to the appropriate outputs for subsequent transmission to the time measurement unit.  
         [0042]    Single-Ended Cable Skew Calibration  
         [0043]    [0043]FIG. 4( a ) illustrates an embodiment of calibration board  40  that can be used to calibrate time measurement unit  10  to account for relative skew between cables. In this particular embodiment, four measurement signal outputs  58  are connected via matched routing cables  62  to dedicated connectors  66  on automated testing equipment loadboard  40 . The connectors  66  are connected to cables  12  from loadboard  14  to time measurement device  10 . Four associated arming signal outputs  60  are connected via matched routing cables  64  to arming outputs  68 . The arming signal outputs are similarly routed and connected to the appropriate arming inputs of time measurement device  10 .  
         [0044]    As part of a cable skew calibration algorithm or program, time measurement unit  10  is configured to recognize a reference event or events on all of the measurement channels, and to register the occurrence of such with a time stamp. A reference event preferably comprises a particular transition occurring in each of the multiple copies of the signal generated and provided to the time measurement unit  10 . In one embodiment, the cable skew program is executed by a control computer system operatively associated with time measurement unit  10 .  
         [0045]    For the particular example described hereinbelow, the reference event is the first falling edge detected relative to the beginning of the signal pulse pattern. Other embodiments of the present invention may utilize other reference events. Additionally, time measurements provided in this and other examples are in no way intended to limit the precision or accuracy of this invention; the principles of operation described are applicable to any range of time scales.  
         [0046]    Upon execution, the measurement signals simultaneously arrive at the appropriate outputs  66  that correspond and are connected to the measurement channels of time measurement unit  10 . The arming signals also arrive simultaneously at the particular outputs  68  connected to the arming channels of time measurement unit  10 .  
         [0047]    In one embodiment, the arrival of the measurement signals as a group at the particular outputs  66  can be delayed (relative to the group of arming signals) by adjustable signal delay circuitry  48 . This is in order to facilitate the determination of the arming latency.  
         [0048]    Upon arrival at time measurement unit  10 , the signals may have experienced cable-to-cable skew. For instance, as illustrated in FIG. 5, skew has occurred and the four measurement channel signals have not arrived at time measurement unit  10  simultaneously. Time measurement unit  10  produces an internal table of reference event time stamps taken at edges E A , E B , E C , and E D  (shown in FIG. 5) for the various channels such as the one illustrated below:  
                             TABLE 1                           Reference Event Time Stamps                Channel   Reference Event Time Stamp                       A   233.994 ns           B   236.281 ns           C   242.018 ns           D   236.435 ns                      
 
         [0049]    Time measurement unit  10  selects one channel as the “base” channel. The time stamps for the reference events on the various channels are compared to the base channel reference event time stamp to determine Δt values, which are also shown in FIG. 5. Δt represents the time difference between common reference events on different channels; for example, Δt AB  represents the time difference between when a reference event was measured on channel B as compared to base channel A. To determine Δt AB , time measurement unit  10  subtracts the base channel time stamp A from the comparison channel time stamp B.  
         Δt AB =236.281−233.994=2.287  (700)  
         [0050]    Equation 700 represents calculation of Δt AB  using a single reference event time stamp. Other embodiments of the present invention may utilize multiple reference events on the various measurement channels. Additionally alternative embodiments may include computation of the average skew from multiple skew measurements.  
         [0051]    The skews of the remaining channels relative to base channel A are determined similarly as illustrated below in Equations 701 and 702.  
         Δt AC =242.018−233.994=8.024  (701)  
         Δt AD =236.435−236.281=0.154  (702)  
         [0052]    The process is then repeated to determine Δt values with a different reference event such as a rising edge, which could have slightly different skew levels. At the completion of the process, a Δt calibration table such as the one illustrated below can be produced.  
                                                                             TABLE 2                           Cable Skew Calibration Table                Channel:                    A   B   C   D                            relative to base A− edge   0   2.287   8.024   0.154           relative to base A+ edge   0   2.297   8.028   2.453                      
 
         [0053]    This table contains cable skew values for each channel relative to the base for rising (+) and falling (−) edge transitions. It is stored in the time measurement unit and used to adjust future time measurements, thus substantially ridding the future time measurements of inaccuracies due to skew between cables.  
         [0054]    Adjusting Measurements for Cable Skew  
         [0055]    In the following example, which is provided for purposes of illustration, the system of the present invention is used for testing precision electronic equipment by checking for synchronous rising edges on two channels. Three output pulses are measured using time measurement unit channels A and C, with a desired variance of no larger than ±0.003 between the two outputs. The detection results are as follows:  
                                                           TABLE 3                           Example Two-Channel Unadjusted Measurements                Rising edge   Channel A   Channel C   Deviation                            #1   18.984   27.011   8.027           #2   20.635   28.656   8.021           #3   22.286   30.312   8.025                      
 
         [0056]    These results would indicate that the electronic equipment under test is defective. Recall, however, that significant skew exists between channels A and C, with Δt AC  calculated in Eq. 703 to be 8.024. Accordingly, time measurement unit  10  accesses the table of cable skew values and adjusts the measurements by subtracting Δt AC  from the channel C time stamps, resulting in the following corrected measurements.  
                                                           TABLE 4                           Example Two-Channel Measurements Adjusted for Skew                Rising edge   Channel A   Channel C   Deviation                            #1   18.984   18.987   −0.003           #2   20.635   20.632   0.003           #3   22.286   22.288   −0.002                      
 
         [0057]    The results, now corrected for cable skew, reflect measurements that give a more accurate indication of the performance of a device under test.  
         [0058]    Since the time measurements obtained are relative to one another, there are many possible variations on the method for adjusting for cable-to-cable skew. For example, in an alternative embodiment, the Δt CA  value could have been automatically subtracted from channel C to obtain the same result.  
         [0059]    Differential Skew  
         [0060]    Differential skew can be divided into two categories, namely inter-channel skew, which occurs between two differential channels, and intra-channel skew, between the two components of a differential pair. Inter-channel skew can be measured and compensated for in a very similar fashion to channel-to-channel skew for single-ended channels. In one embodiment, the only notable difference is that the reference event is when the two differential line voltages cross one another, rather than a particular edge. Intra-channel differential skew is a notable variety of cable-to-cable skew. Specifically, intra-channel differential skew is cable-to-cable skew between two lines of a differential pair.  
         [0061]    In differential signaling, two cables transmitting complementary waveforms are used. The difference between the two waveforms is taken to determine the data transmitted. A typical complementary signal pair is illustrated at FIG. 6( a ) and  6 ( b ). A logical “1” on differential line  6 ( a ) is expressed as V + (typically −0.8 Volts in ECL applications), while a logical “0” would be V −  (−1.8 Volts in ECL). Complementary line  6 ( b ) would reverse the voltage/logic level assignment (V −  for logical “1” and V +  for logical “0”). Differential signaling is useful in reducing the effect of common mode noise on data transmitted. It will be understood that the aspects and embodiments of the present invention are equally applicable with other differential (and single-ended) voltage schemes and are not limited to any specific range or assignment of logic/voltage levels.  
         [0062]    However, skew between the cables comprising a differential pair can cause potentially serious problems. The differential pair illustrated at  6 ( c ) and  6 ( d ) has experienced differential skew Δt DIFF . When the two differential signals are processed, portions of the data will be indeterminate or entirely faulty due to the fact that the transitions are no longer matched and complementary.  
         [0063]    Time measurement unit  10  can determine the differential skew when used with calibration board  40 . This calibration procedure will not compensate for intra-cable skew, but can be used to help determine if the skew is within the allowable specification of the test setup. Unlike the single-ended calibration sequence described earlier, all combinations of measurement channels are not compared to one another.  
         [0064]    Configuration switch bank  42  is set such that signals are routed to differential signal pairs on time measurement unit  10 . As part of the calibration sequence, a DC voltage (V o , which is the average between V +  and V − ) is output by calibration board  40  on one of the differential signal pair, for example, on channel A, while a known sequence of measurement pulses is output on the other channel A + .  
         [0065]    Time measurement device  10  is configured to recognize when the voltage value of the measurement pulses is equal to the voltage V o  output on the other member of the differential pair. For example, the voltage level between the two channels may be compared, and the time at which the two voltages register as being equal is stamped and stored in memory.  
         [0066]    Once a requisite number of pulses have been output and crossing times noted, the signal outputs are reversed: the voltage V o  is output on A +  and the same train of measurement pulses is output on channel A − .  
         [0067]    Due to the intra-cable skew, the reference event (crossing of V o ) times will be slightly different, even though identical pulses and V o  values were output to TMU  10 . From these two sets of measurements, the time stamps can be statistically analyzed to determine the severity of intra-cable skew for test result reporting.  
         [0068]    Adjusting for CSC Board Irregularities (Residual Skew)  
         [0069]    Recall that part of the calibration output sequence stored in EEPROM  44  can contain configuration data. In one embodiment, this data includes data relating to previously determined irregularities in the synchronization of signals provided by the calibration board. These irregularities, which may be referred to as “residual skew,” can be found by measuring synchronized outputs of the calibration board using an oscilloscope or a precisely calibrated time measurement device, or by calculations utilizing such factors as trace length and material as known to one skilled in the art. Examples of calibration board irregularities are illustrated in Table 5, which is representative of the data stored in EEPROM  44  on the calibration board.  
                                               TABLE 5                           Calculated CSC Board Synchronization Deviation       (t = 0 is the occurrence of the synchronized event)                channel   Event Time                            A   0   ns (reference)           B   0.021   ns           C   0.013   ns           D   −0.019   ns                      
 
         [0070]    The CSC synchronization irregularity data is transmitted directly to time measurement unit  10  using the same cable connections  12  between calibration device  40  and time measurement unit  10 .  
         [0071]    In the single-ended skew example described earlier in this application, the underlying assumption that the calibration board signals were absolutely synchronized was correct. However, in real-world applications, such an assumption may be inaccurate. Consider the results that would be obtained using a calibration board with the irregularities and delays depicted in Table 5. As illustrated below in Table 6, the resulting reference event time stamps would reflect the slight delays in synchronization at calibration board  40 .  
                             TABLE 6                           Reference Event Time Stamps                    Time Stamp               Corrected for       Channel   Event Time Stamp   irregularities               A   713.994 ns   713.994 ns       B   716.283 ns   716.262 ns       C   722.018 ns   722.005 ns       D   716.435 ns   716.454 ns                  
 
         [0072]    These new reference event time stamps include the cable-to-cable skew that was determined earlier in Table 2. However, the time stamps also include slight skew due to irregularities in calibration board  40 . Although great care can be taken to precisely match the signal paths (such as measurement traces  54  and  56 ) on the board, slight deviations are possible. Table 6 also shows the corrected time stamps after taking into account the information in Table 5. Using the same calculation methods as in the previous single-ended skew example would result in refined skew values such as Δt AB =2.268 ns, Δt AC =8.012 ns, Δt AD =2.450 ns, etc.  
         [0073]    The calibration board skew table values can be subtracted from the Cable Skew Calibration table values in matrix form. After subtraction of the relative skew values at the board, the skew detected at time measurement unit  10  is correct.  
         [0074]    Alternatively, the reference event time stamps may be directly adjusted at time measurement unit  10  for the deviations in synchronization in a manner similar to that for adjusting for cable skew. This method results in the “correct” reference event time stamps and eliminates the need for a calibration board skew table and the associated calculations.  
         [0075]    The manner in which the internal skew of calibration board  40  is transmitted to time measurement unit  10  can be either by manually keying in values such as those found in Table 5 into a file stored in time measurement unit  10 . The file is later used to correct skew measurements as illustrated above. Alternatively, the information can be transferred on one of the connecting cables  12  to time measurement unit  10  using a simple time modulation or other encoding/transmission methodology. Such an embodiment employing information transfer by the calibration device itself reduces labor associated with testing and could be performed in an automated setting.  
         [0076]    Arming Latency  
         [0077]    Calibration board  40  as illustrated in FIGS.  4 ( a ) and  4 ( b ) can also be used to determine the arming latency of time measurement unit  10 . FIG. 7 depicts a measurement channel signal  7 ( a ) featuring signal pulse edges at G 2 , G 4 , G 5 , and G 7  with an associated arming channel  7 ( b ) featuring arming pulse edges at G 1  and G 5 .  
         [0078]    In this embodiment, the system of the present invention is being used to detect and measure the rising edges at G 2  and G 7 . To that end, arming signals are provided with pulse edges at G 1  and G 5  on the associated arming channel. Generally, devices such as time measurement unit  10  can detect a reference event and produce a time stamp when the arm signal is logically “1” and then only after that which is the length of the time it takes the internal circuit to arm the timing measurement circuit.  
         [0079]    Due to arming latency Δt LAT , the rising edge at G 2  cannot be detected. The time interval Δt 12  from when ARM pulse at G 1  is received to the intended measurement point at G 2  is less than the arming latency Δt LAT . Thus, time measurement unit  10  does not have sufficient time to prepare its internal circuitry for measurement, and the pulse at G 2  is not detected. Instead, the subsequent rising edge at G 4  may be detected, or no measurement is made at all, resulting in an erroneous test.  
         [0080]    The situation is different regarding the measurement channel pulse at G 7  in relation to the arming pulse at G 5 . The time interval Δt 57  from when the ARM pulse is received at G 5  to the intended measurement point at G 7  is greater than the arming latency Δt LAT , thus giving time measurement unit  10  sufficient time to arm and detect the rising edge at G 7 .  
         [0081]    To provide for the measurement of the arming latency of time measurement unit  10 , calibration board  40  of the present invention provides a specialized arming signal and measurement channel signal output. A stable frequency oscillator within signal source generator  46  provides a constant frequency clock signal that is routed to the arming channels. Calibration board  40  also outputs a plurality of measurement channel pulses to time measurement unit  10  via the adjustable delay circuitry  48 .  
         [0082]    The adjustable delay circuitry  48  delays the pulses output to time measurement unit  10 , and once the delay has been increased enough to compensate for the arming latency of time measurement unit  10 , the same point or points of coincidence will be detected at time measurement unit  10 .  
         [0083]    At the point where the delay imposed on the output signal by delay circuitry  48  equals the arming latency, the time measurement unit will start detecting pulses. This point is used as an indication that the system has found its arming latency.  
         [0084]    In one embodiment, as illustrated in FIG. 8, pulses are output to the measurement channels with a delay that increases by a fixed amount with each clock pulse. The ARM pulse at time H 1  is received by time measurement unit  10 , followed shortly by a measurement pulse rising edge at H 2 . Initial delay Δt D1  is not sufficiently long to enable arming, and the rising edge at H 2  is not measured or time-stamped. Subsequent rising edges at H 4  and H 6  have been delayed from their respective clocked arming pulses at H 3  and H 5  by Δt D2  and Δt D3 , respectively. Rising edge H 4 , delayed from arming pulse H 3  by Δt D2 , is not measured, again because edge H 4  is received before sufficient time has elapsed from the reception of H 3  by time measurement unit  10 .  
         [0085]    The delay Δt D3  from arming pulse edge H 5  to measurement pulse edge H 6  is larger than Δt LAT , and therefore measurement pulse H 6  is detected and time-stamped. Additional pulses thereafter with greater delay times relative to associated arming pulses are also detected.  
         [0086]    After a series of incremented delays and measurements, time measurement unit  10  can prepare a table of the measurement pulse time stamps. The time stamps of the detected measurement pulses will increase at a regular interval corresponding to the clock rate and the rate of increase in the arming delay, except for points (such as those in signals  8 ( b ) and  8 ( c ) of FIG. 8) at which a pulse was provided before the arming latency time Δt LAT  had elapsed from the nearest prior arming signal.  
         [0087]    Since the interval and absolute time reference (from the clock provided on the ARM channel) are known, the number of “missed” pulses and their associated time stamps can be determined. The required information (the clock rate and delay increment times) can be included in the data transmitted from EEPROM  44  via cables  12  and used by the internal circuitry and/or processors of time measurement unit  10 . From this information, the arming latency can be determined, as described in the example below.  
         [0088]    Arming Latency Determination  
         [0089]    [0089]FIG. 4( b ) illustrates a calibration board featuring two differential measurement channels (four total outputs) and two ARM channels. Calibration board  40  is configured, for instance, to provide an arming pulse clock output of one MHz, and switch bank  42  is set to output the differential signals on pairs A and B and pairs C and D (as represented by signals  6 ( a ),  6 ( b ),  6 ( c ) and  6 ( d ), respectively, of FIG. 6).  
         [0090]    The skilled artisan will understand that the numbers included in the example described hereinafter are for exemplary purposes only; it is entirely within the spirit and scope of the present invention to use time values and scales different from those provided for delays, measurements, and the like.  
         [0091]    An arming pulse provided on the arming channel at a frequency of one MHz results in a pulse every microsecond. After an initial delay of (Δt D1 ) of 0.24 microseconds, pulses are output to the measurement channels of time measurement unit  10 , with the delay from associated clocked arming pulses increasing in increments of 0.01 μs every microsecond. The first measurement channel pulse is provided after the initial delay of 0.24 microseconds from the first arming pulse at t=0. The delay is then incremented by 0.01 μs, to 0.25 μs. The next measurement channel pulse is transmitted 0.25 ns after the next arming pulse, which occurs at t=1 μs; thus the measurement channel pulse is transmitted at 1.25 μs. Time measurement unit  10  detects some of the pulses and records a time stamp. The time stamps are illustrated in Table 8.  
                             TABLE 7                           Example Time Stamps                Measurement   Time stamp                       1   2.26 μs           2   3.27 μs           3   4.28 μs                      
 
         [0092]    The 1.01 μs difference between measurements is reflective of the incrementing delay of measurement pulses from the clocked ARM pulses. However, recall that measurement pulses were also transmitted at t=0.24 μs and t=1.25 μs. Because the rate of delay incrementation and clock rate of ARM pulses are known (from data contained in the EEPROM and transmitted to the time measurement unit), these additional time stamps can be correlated with the absolute time reference provided by calibration board  40 .  
                             TABLE 8                           Measurement Delays Determined by Time Measurement Unit 10                Delay from   Delay from nearest       Measurement   absolute time   ARM pulse               Not measured   0.24 μs   0.24 μs       Not measured   1.25 μs   0.25 μs       1   2.26 μs   0.26 μs       2   3.27 μs   0.27 μs       3   4.28 μs   0.28 μs                  
 
         [0093]    Recall that an ARM pulse was transmitted every 1 μs, starting at t=0 (ARM pulses were sent at t=0, 1, 2, 3, 4, . . . μs). The delay from the nearest ARM pulse can be found by subtraction. As illustrated in Table 8, time measurement unit  10  now has determined the delay of measurement pulses provided relative to the nearest ARM pulse.  
         [0094]    The measurement pulses that were delayed 0.25 μs from the nearest ARM pulse were not detected, while those that were delayed at least 0.26 μs from the nearest ARM pulse were detected. Thus, it can be concluded that the arming latency lies between 0.25 and 0.26 μs.  
         [0095]    Although a preferred embodiment of the invention has been described using specific terms and devices, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of the present invention, which is set forth in the following claims. In addition, it should be understood that aspects of various other embodiments may be interchanged both in whole or in part. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred version contained herein.