Patent Publication Number: US-9417275-B2

Title: Cable measuring device and method

Description:
FIELD OF INVENTION 
     The invention relates to the field of cable measurement. More particularly, the instant invention relates to cable measuring device and method wherein accurate reflection measurements of (jacketed) twisted pair cables at frequencies above a predetermined MHz. 
     PRIOR ART 
     A computer network or data network is a telecommunications network that allows computers to exchange data. The most common type of computer network is the Local Area Network or LAN. LANs are made up computer interconnected in a relatively small geographical area. 
     Twisted pair cables are widely used for interconnecting computers in LANs. LAN users and applications are constantly requiring faster data transfer speeds. Technological improvements in computers, interconnecting electronics cable and connectors allow for faster data transfer speeds. 
     The classification of twisted pair cables for LAN applications is based on the data rate that they can carry. The term category is used to classify the LAN cables according to the data rate that they can carry. The main categories that are presently used are: Category 3, Category 5E, Category 6, Category 6A, Category 7 and Category 7A. The higher the category number the higher the data speed than can be carried over the LAN cables. Domestic and international standardization bodies are presently working on Category 8 cables. 
     There are standards organizations which develop specifications requirements for the electrical transmission parameters of these category cables. These specifications require LAN cables to be tested at different frequency ranges as follows:
     Category 3 1 MHz to 16 MHz   Category 5E 1 MHz to 100 MHz   Category 6 1 MHz to 250 MHz   Category 6A 1 MHz to 500 MHz   Category 7 3 MHz to 600 MHz   Category 7A 3 MHz to 1000 MHz   Category 8 1 MHz to 2000 MHz   

     Different specification limit values are given for each transmission parameter at different test frequencies. For a cable to be compliant with a given category it has to pass all the test parameter specification limits at the different frequencies. 
     Testing the transmission parameters of LAN cables with automatic test systems provide:
         fixturing for cable connection   software to control the test instruments used for testing   acquire test data for comparison against specification limits   provide test reports   data storage of the data acquired.       

     It is well known that the effect of the length of jacket removed on the reflection measurements test results may be minimized by modifying the connecting fixtures. As the length of jacket is removed (shortens), the frequency at which the tailing up/down effect is seen increases. It is necessary to remove some of the cable jacket to perform the test of the cable and thus there will always remain a tailing up/down effect due to the length of jacket removed. This may be very confusing to the cable manufacturers since they may reject good cable or pass bad cable due to failures on the reflection test results. The reproducibility of the reflection test results is also affected since the end of cable prepared by different individuals provides different reflection test results. 
     It is therefore desirable to achieve repeatability and reproducibility (R&amp;R) of the test results. The connection of the cable under test (CUT) to the test equipment is theoretically not to be a factor that alters the cable performance. However, to connect the CUT to the test system, the cable ends need to be prepared. This cable preparation for connection requires removing a length of cable jacket to provide access to the individual pairs. See  FIG. 1 . 
     LAN cables transmission parameters are divided in two major groups: through measurements (from end to end) and reflection measurements (same end). 
     Through parameters, S 21  and S 12  include: 
     
         
         
           
             insertion loss 
             propagation delay 
             crosstalk
 
Reflection parameters, S 11  and S 22  include:
 
             input impedance 
             return loss. 
           
         
       
    
     The amount of cable jacket removed affects the results of the reflection measurements. Specifically, the connection hardware and cable ends result in a short section of the tested cable with distinctly different impedance at each end of the cable. This impedance mismatch causes variations in the measurement that are not present in the cable itself. 
     If the short length of mismatch is a bit higher than the cable impedance, then the impedance trace tends to tail up with increasing frequency. If lower, then the impedance trace tends to tail down. As the test frequency increases the length of jacket removed effect becomes more notorious. The effect of jacket removal manifests as a tailing (up or down) of the input impedance test results. See  FIG. 2 . 
     The input impedance results shown on  FIG. 2  input impedance results are evenly distributed around 100 Ohms in the frequency range from 1 MHz to 250 MHz. After 250 MHz this even distribution starts showing more data below 100 Ohms than above 100 Ohms. The input impedance is showing a tail-down effect. Because of the combined effects of the cable, cable preparation, and test connection fixtures. This tail-down effect is due to the cable itself or to the length of jacket removed for cable connection to the test system. Whether the impedance trace tends to tail up or down, the reflection coefficient is increased compared to that of the cable itself, causing an error in measuring inherent cable performance. 
       FIG. 3  shows the return loss test data of the same cable using the same connection. No evaluation of the return loss results to the length of jacket removed can be made yet. 
     Prior Art attempted to remove the above described variations using various techniques described below. 
     A. Fixture Design 
     A commonly used technique is to use highly engineered fixtures to minimize impedance discontinuities in the signal path. However, as test frequencies increase, this becomes more difficult. Constraints such as connector size, managing crosstalk performance in multi-pair fixtures and other physical design constraints limit the ability to maintain impedance control in the fixtures. Also the physical limitations of connecting to the pair under test also places limits on the ability to control impedance discontinuities. 
     B. Single Pair Tests 
     A single pair is connected to integrated circuit type sockets. This technique minimizes the amount of jacket removed providing accurate results at higher frequencies than other method. However with this technique is not possible to obtain a reasonable reproducibility. Lack of good reproducibility makes the test system unsuitable for gauge R&amp;R evaluation. 
     C. Using Connector Plugs to Terminate Pairs 
     A minimum length of jacket removal is required for installing the connectors. These additional connectors also increase the reflection coefficient between the test fixture and the cable under test. Because each time a cable is terminated with a connector, there are variations in the way the conductors are arranged. Measurement reproducibility is affected by the inherent mechanical limitations. 
     D. Gating Using the Network Analyzer (Time-Domain Reflectometer—TDR) 
     Gating within the network analyzer is a technique known to improve measurements due to impedance discontinuities. This technique ‘eliminates’ portions of the signal in the time domain. The resulting measured signal can be converted to frequency domain by using Inverse fast Fourier transform (IFFT) technique. However, a problem with this technique can be ‘masking’ which is a term used to describe the inaccuracies that can occur when reflected power is ‘removed’ from the measurement. This can alter the calculated transmitted power, causing errors in parameters such as insertion loss or crosstalk. 
     SUMMARY OF THE INVENTION 
     It is an object to improve cable measurement. 
     It is another object to improve devices for measuring cables. 
     It is a further object to improve repeatability and reproducibility in measuring characteristics of cable. 
     A solution is needed to guarantee that LAN cable reflection tests are not dependent on the length of jacket removed and the individual preparing the cable for connection to the test system. 
     Accordingly, one aspect of the instant invention is directed to a cable measuring device for measuring a cable of a predetermined LAN cable category wherein the cable has at least one twisted conductive pair and jacket covering the same. The device includes a computer based device having hardware and software, a first cable connecting device for connecting a near end of a cable, a second cable connecting device for connecting a far end of the cable, a first balun transformer device operably connected to the first cable connecting device, a second balun transformer device operably connected to the second cable connecting device, an automatic switching device operably interconnecting each of the balun transformer devices and the computer based device, and a vector network analyzer operably connected to the computer based device wherein the computer software is equipped to receive a signal indicative of a length of jacket removed from the cable, to perform a reflection test in a frequency range required by a predetermined LAN cable category associated with the cable using the vector network analyzer, to perform data conversion, to apply inverse fast Fourier transform to eliminate a length of a jacket removed from measurement of the cable, to apply fast Fourier transform to convert data back to the frequency domain and to provide and output signal indicative of a true LAN cable reflection. 
     A method of measuring reflection of a cable of a predetermined LAN cable category is provided wherein the cable has at least one twisted conductive pair and jacket covering the same. The method includes providing a computer based device having hardware and software, providing a first cable connecting device for connecting a near end of a cable and a second cable connecting device for connecting a far end of the cable, providing a first balun transformer device operably connected to the first cable connecting device and a second balun transformer device operably connected to the second cable connecting device, providing an automatic switching device operably interconnecting each of the balun transformer devices and the computer based device, providing a vector network analyzer operably connected to the computer based device, wherein the computer software is equipped to receive a signal indicative of a length of jacket removed from the cable, to perform a reflection test in a frequency range required by a predetermined LAN cable category associated with the cable using the vector network analyzer, to perform data conversion, to apply inverse fast Fourier transform to eliminate a length of a jacket removed from measurement of the cable, to apply fast Fourier transform to convert data back to the frequency domain and to provide and output signal indicative of a true LAN cable reflection and providing a cable having a portion of its cable jacket removed from a near end and a portion of cable jacket removed from a far end of the cable, connecting the near end of the cable to the first cable connecting device, connecting the far end of the cable to the second cable connecting device, initiating the computer based device to receive a signal indicative of a length of jacket removed from the cable, to perform reflection test in the frequency range required by the predetermined LAN cable category associated with the cable using the vector network analyzer, to perform data conversion, to apply inverse fast Fourier transform to eliminate a length of the jacket removed from measurement of the cable, to apply fast Fourier transform to convert data back to the frequency domain and and provide and output signal indicative of a true LAN cable reflection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a connecting fixture. 
         FIG. 2  shows an example of input impedance test results for a cable. 
         FIG. 3  shows the return loss test data of the same cable using the same connection. 
         FIG. 4  shows a pictorial description on how a LAN cable is prepared for connection to the device of the instant invention. 
         FIG. 5  shows a schematic block diagram of the instant invention. 
         FIG. 6  shows input impedance test results for the cable using the invention. 
         FIG. 7  shows return loss test data of the same cable using the same connection using the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, the cable measuring device of the instant invention is generally referred to by the number  10 . The cable measuring device  10  is for measuring a cable  12  of a predetermined LAN cable category wherein the cable  12  has one or more twisted conductive pair  14  and jacket  16  covering the same.  FIG. 4  shows a pictorial description on how the LAN cable is prepared for connection to the test system. 
     The cable measuring device  10  includes a computer based device  18  having hardware and software which is more fully explained hereinafter. A first cable connecting device  20  for connecting a near end  22  of cable  12 . A second cable connecting device  24  is for connecting a far end  26  of cable  12 . 
     A first balun transformer device  28  is operably connected to the first cable connecting device  20  and a second balun transformer device  30  is operably connected to the second cable connecting device  24 . An automatic switching device  32  operably interconnects each of the balun transformer devices  28  and  30  and the computer based device  18 . A vector network analyzer  34  operably connects to the computer based device  18 . 
     The computer software in the device  10  is to receive an input signal indicating a length of jacket  16  removed (Ln and Lf) from the cable  12 . The software performs a reflection test in a frequency range required by a predetermined LAN cable category associated with the cable  12  using the vector network analyzer  34 . The software performs data conversion, applies an inverse fast Fourier transform to eliminate a length (Ln and Lf) of a jacket  16  removed from measurement of the cable  12 , and applies fast Fourier transform to convert data back to the frequency domain thereby providing and output signal indicative of a true LAN cable reflection. 
       FIG. 4  shows the total length (usually 100 meters) of cable  12  that requires transmission parameters testing. A length of jacket  16  is removed from the near end (Ln) and the far end (Lf) of the cable  12  to expose the twisted pairs  14  for connection to the test device. This cable preparation is fine for end to end testing (through) but the length of jacket removed (Ln and Lf) affects the reflection measurements at frequencies above 250 MHz causing the tail up/down effect mentioned herein. 
     Accordingly,  FIG. 5  depicts a block diagram of instant invention&#39;s solution for LAN cable electrical parameters automatic measurements. This solution applies to measurements performed using a balance to unbalance (balun) transformer  28 ,  30 . The balun  28 ,  30  interfaces coaxial output of the vector network analyzer  34  to the balanced twisted pair  14 . 
     The instant invention employs computer based device  18  which has application software and which provides an automatic test system having an algorithm that mathematically removes the tail up/down effect of the reflection measurements. In addition to the software removing the tail up/down effects the instant invention removes dependence of a specific individual for cable preparation and provides excellent reproducibility of the reflection test results. 
     An exemplary technique employed by instant invention (other techniques are also contemplated) is based on a process that performs an Inverse Fast Fourier Transform (IFFT) on the data obtained from the vector network analyzer during reflection measurements. 
     The steps required to remove the tail up/down effects using IFFT are achieved the invention which employs a method of measuring reflection of a cable  12  of a predetermined LAN cable category is provided wherein the cable  14  has at least one twisted conductive pair  14  and jacket  16  covering the same. The method includes providing a computer based device  18  having hardware and software, providing a first cable connecting device  20  for connecting a near end  22  of a cable  12  and a second cable connecting device  24  for connecting a far end  26  of the cable  12 , providing a first balun transformer device  28  operably connected to the first cable connecting device  20  and a second balun transformer device  30  operably connected to the second cable connecting device  24 , providing an automatic switching device  32  operably interconnecting each of the balun transformer devices  28  and  30  and the computer based device  18 , providing a vector network analyzer  34  operably connected to the computer based device  18 . The computer software is to receive a signal, indicative of a length of jacket removed (Ln and Lf) from the cable  12  and to perform a reflection test in a frequency range required by a predetermined LAN cable category associated with the cable  12  using the vector network analyzer  34 . The software is to perform data conversion, to apply inverse fast Fourier transform to eliminate a length of a jacket removed (Ln and Lf) from measurement of the cable  12 , to apply fast Fourier transform to convert data back to the frequency domain and to provide and output signal indicative of a true LAN cable reflection. The method further includes providing cable  12  having a portion of its cable jacket removed (Ln) from a near end  22  and a portion of cable jacket removed (Lf) from a far end  26  of the cable  12 . The method further includes connecting the near end  22  of the cable  12  to the first cable connecting device  20 , connecting the far end  26  of the cable  12  to the second cable connecting device  24 . The final steps employed are initiating the computer based device  18  to receive a signal indicative of a length of jacket removed (Ln and Lf) from the cable  12 , to perform a reflection test in the frequency range required by the predetermined LAN cable category associated with the cable  12  using the vector network analyzer  34 , to perform data conversion, to apply inverse fast Fourier transform to eliminate a length of the jacket removed (Ln and Lf) from measurement of the cable  12 , to apply fast Fourier transform to convert data back to the frequency domain and provide and output signal indicative of a true LAN cable reflection. 
     It will be understood that the embodiment of the invention includes a computer based device having hardware and software, a first cable connecting device for connecting a near end of a cable, a second cable connecting device for connecting a far end of the cable, a first balun transformer device operably connected to the first cable connecting device, a second balun transformer device operably connected to the second cable connecting device, an automatic switching device operably interconnecting each of the balun transformer to the vector network analyzer. The vector network analyzer and the automatic switching device are connected to a computer that runs the application software that controls the vector network analyzer and the switching device in addition to performing all the required mathematical processes, i.e., inverse fast Fourier transform. The application software also generates the test reports of the cable under test. The actual length of the jacket removed from the cable as depicted in  FIG. 4  is understood by those skilled in the art to be manually entered in the software application graphical user interface. Once the cable information is entered in the graphical user interface the test starts. The application program commands the vector network analyzer to perform a reflection test in a frequency range required by a predetermined LAN cable category. With the frequency domain reflection data acquired by the vector network analyzer, the application program performs an inverse fast Fourier transform (IFFT) to convert the frequency domain data to the time domain data. In the time domain the length of jacket removed and the signal velocity of propagation are used to modify the time data (speed=velocity/time). The modified time domain data is converted back to the frequency domain using fast Fourier transform (FFT). This frequency domain data has the correction required to remove the impedance and return loss tailing up/down effects that are due to the length of jacket originally removed from the cable. This final result provides the cable under test true impedance and return loss results. The final result prevents the rejection of a good cable that due to the length of jacket removed may indicate to be faulty if this invention is not used. 
     It is also known that the frequency range for Category cables has continually increased, and is currently at 500 MHz for Category 6a, and the 1 GHz region for Category 7/7a. Upcoming cable specifications are now underway to extend this range to 2 GHz. 
     There are a wide range of cable types with balanced pair configurations that far exceed 2 GHz in operation. The instant invention employs a technique which is useful for any balanced pair transmission line, whether a cable or other device where termination effects cause errors in measurement accuracy. 
     It is important to maintain the accuracy of the calculations such that calculation errors do not significantly contribute to the test result. It has been found there are requirements and relationships for factors such as the maximum frequency of the test, the number of data points taken, FFT matrix size, and desired fixture/cable length to eliminate. 
     Corrected Results 
       FIGS. 6 and 7  show the input impedance and return loss test results after the tailing up/down effect is removed using the instant invention described above. These results are obtained from the data acquired for  FIGS. 2 and 3 . 
     A comparison between  FIG. 2  and  FIG. 6  shows that after the use of the instant invention described herein the tail down effect on the input impedance is removed. A similar result may be seen when  FIG. 3  and  FIG. 7  are compared. The return loss results improve after 250 MHz. The results on  FIGS. 6 and 7  are the true input impedance and return loss results of the cable under test. 
     By employing the instant invention, there is provided a device to remove this tail up effect on the impedance/return loss measurement which addresses the effects of jacket removed on the reflection test results of LAN cables, removes the dependence of a specific individual for cable preparation, and provides excellent reproducibility of the reflection test results. 
     Benefits achieved by the instant invention are as follows: 
     
         
         
           
             1) A measurement process that allows accurate data acquisition for proper application of IFFT techniques to remove effect of jacket removal. 
             2) A calculation method to remove reflection effects at the source end of a measured transmission line using FFT and IFFT. 
             3) Where the amount of transmission line length to remove from the measurement is about 0.1 to 1% of the entire measured transmission line. 
             4) Where the measurement errors induced by the FFT processing is about 1% or less. 
             5) Where impedance changes of at least 20% can be eliminated with no more than 2% error in the inherent cable measurement. 
             6) Where the number of points required for the IFFT size is automatically selected by the application program. 
             7) Where the signal processing techniques are suitable for frequency ranges of 250 MHz and above. 
             8) Automatic testing of twisted pair cables wherein the device controls the data acquisition process from a vector network analyzer to apply digital signal processing techniques to the implementation of IFFT for accurate reflection measurements. 
             9) Automatic testing of twisted pair cables that performs an IFFT function, and processes the data in the time domain and converts back to the frequency domain for data reporting. 
             10) Automatic testing of twisted pair cables that performs the required calculations for getting time domain information from acquired frequency data using digital signal processing techniques. 
           
         
       
    
     Although various embodiments of the invention have been shown and described, they are not meant to be limiting. Those of skill in the art may recognize various modifications to these embodiments, which modifications are meant to be covered by the spirit and scope of the appended claims.