Abstract:
A time domain reflectometer having a first impedance when in a first test mode and a second impedance when in a second test mode. The first impedance is substantially the same as the nominal characteristic impedance of a network link cable not connected to a network and the second impedance is substantially different from the impedance of a network link cable that is terminated into a network. A method for measuring the length of a terminated network cable includes the steps of determining that the network cable is terminated at a network, selecting a test mode suitable for testing the terminated network cable, and performing time domain reflectometry testing on the terminated network cable.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to time-domain reflectometry and more particularly to a time-domain reflectometry apparatus and method for testing a terminated network cable. 
   In a network environment, such as Ethernet systems, devices are connected to the network by cables, commonly referred to as link cables.  FIG. 1  illustrates a portion of a network where device  18 , such as a computer or printer, is connected at node  16  to one end of link cable  14 . The other end of link cable  14  is connected at node  12  to the network, which in  FIG. 1  is represented by cable  10 . 
   Technicians troubleshooting networks need to test, among other network elements, the various link cables in the system. To verify that link cables meet specifications, technicians use cable test instruments, which have the ability to test, among other parameters, cable length. In an Ethernet system, for example, the length of a link cable is limited by system standards to a maximum of 100 meters. Cable test instruments typically employ time domain reflectometry (TDR) to measure the length of link cables. In  FIG. 1 , cable tester  20  is connected to link cable  14  at node  16 . From this location, cable tester  20  may perform TDR testing on link cable  14 . 
     FIG. 2  illustrates a typical TDR circuit  22  such as may be utilized in tester  20  and found in the prior art. TDR circuit  22  is coupled to terminals  24 ,  26  of instrument  20 . A link cable  28 , which is to be tested, is connected to terminals  24 ,  26 . Cable  28  is illustrated as a twisted pair, that is, a pair of wires  28   a ,  28   b  terminated at one end to terminals  24 ,  26 . Link cable  28  has a characteristic impedance, Z 0 . 
   TDR circuit  22  includes voltage sources V 1  and V 2  coupled between ground and resistors R 1  and R 2 , respectively. The other end of R 1  is connected to terminal  24  and the other end of R 2  is connected to terminal  26 . As depicted in  FIG. 2 , V 1  supplies a positive voltage pulse through R 1  to cable  28  and V 2  supplies a negative pulse through R 2  to cable  28 . 
   The nominal value of Z 0  is typically 100 ohms. Values of R 1  and R 2  are typically selected to be 50 ohms. In this way the impedance of the TDR circuit  22  is matched to the characteristic impedance Z 0  of link cable  28 . 
   Also connected to terminals  24 ,  26 , is amplifier  30 , with its positive input connected to terminal  24  and negative input connected to terminal  26 . The output signal of amplifier  30  is illustrated by waveform  70  depicted in FIG.  3 . 
   During TDR testing of cable  28 , voltages V 1 , V 2  supply a pulse signal to link cable  28 . A fault anywhere along cable  28  results in a reflected waveform that will be detected by TDR circuit  22 . The reflected signal is applied to amplifier  30  whose output generates waveform  70  in FIG.  2 . 
   In many circumstances, it is desirable to be able to test link cables without disconnecting them from the network. This saves technicians time and also reduces the chance of error when reconnecting the link cable. Unfortunately, prior art cable testers utilizing TDR circuits of the type depicted in FIG.  2  and described above, do not provide clear and simple testing solutions for cables that are connected to a network. This shortcoming of prior art TDR test methods is due to the matching of impedances of the instrument and the cable under test. While this fixed impedance matching is preferable in most testing methodologies, it is actually detrimental when testing terminated network cables. It is well known that matching these impedances results in minimal power loss and, therefore, produces the strongest signals during test by minimizing return signals. However, when the cable under test is terminated into a network, the mismatched impedance of the connection results in a small or non-existent return signal, which in turn reduces the effectiveness of the TDR test. 
   This problem is further illustrated by waveform  70  depicted in  FIG. 3 , which represents the reflected signal for a link cable that is tested while connected into a network. The portion A of the waveform illustrates the reflected signal identifying of the end of the link cable. As is clear from this portion of  FIG. 3 , the TDR reflection indicating the end of the link cable, and hence the length of the cable, is difficult to detect. It would be very difficult for a cable test technician to extract information from the reflected signal of  FIG. 3  if it were to appear on a display in a hand held instrument of the sort typically used in the field. 
   Accordingly, there is a need to provide an apparatus and method for TDR testing of a link cable while it is connected to the network. The apparatus and method should be simple and easily embodied into handheld test instruments so that cable test technicians may readily interpret the results provided by the instrument. The present invention is directed to an apparatus and method designed to achieve these results. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a time domain reflectometer is provided having a first test mode and a second test mode in which the time domain reflectometer has a first impedance when in the first test mode and a second impedance when in the second test mode. 
   In accordance with further aspects of the present invention, the first impedance is substantially the same as the nominal characteristic impedance of a network link cable, and the second impedance is greater than the first impedance. 
   In accordance with yet further aspects of the present invention, the second impedance is greater than the resulting impedance of a network link cable that is terminated into a network. 
   In accordance with the present invention, a method for measuring the length of a terminated network link cable includes the steps of: determining that the network link cable is connected to a network, selecting a test mode suitable for testing the terminated network link cable, performing time domain reflectometry testing on the terminated network link cable. 
   In accordance with further aspects of the present invention, the method further includes the steps of: determining that the network link cable is disconnected from a network, selecting a test mode suitable for testing the disconnected network link cable, performing time domain reflectometry testing on the disconnected network link cable. 
   In accordance with sill further aspects of the present invention, the method further includes the step of selecting between the test mode suitable for testing the terminated network link cable and the test mode suitable for testing the disconnected network link cable. 
   As will be appreciated from the foregoing summary, the present invention provides a time domain reflectometer for testing a terminated network link cable and a method for accomplishing the same. 

   
     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  illustrates a network environment in which time domain reflectometry (TDR) is commonly used; 
       FIG. 2  is a schematic diagram of a typical TDR circuit found in the prior art; 
       FIG. 3  illustrates a reflected waveform associated with the TDR circuit of  FIG. 2 ; 
       FIG. 4  is a block diagram of a cable test instrument suitable for employing a TDR circuit according to the present invention; 
       FIGS. 5 ,  5 A and  5 B are schematic diagrams of a TDR circuit according to the present invention; and, 
       FIG. 6  illustrates a reflected waveform associated with the TDR circuit depicted in FIG.  5 B. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Turning now to  FIG. 4  there is illustrated a block diagram of a cable tester  32  suitable for performing TDR testing. Cable testers, and their general operation, are well known by persons in the area of network testing and analysis. Accordingly, cable tester  32  is not described in detail, but rather is discussed in general terms to better allow the understanding of the present invention. 
   Cable tester  32  includes an input/output terminal  34  for coupling to the link cable which is to be tested. Typically, the instrument is not directly connected to the link cable, but is instead coupled to the cable by a test cable that runs from terminal  34  to the link cable. Terminal  34  is coupled to test mode circuit  36  which configures the tester for a particular test. Conditioning circuit  38  is connected to test mode circuit  36  and processor  40 , and performs, among other tasks, conditioning, filtering and organizing of the signal and data passing between processor  40  and test mode circuit  36 . Display  48  and input circuit  46  are coupled to processor  40 . Display  48  may be, for example, an LCD, and provides information to the instrument user, while input circuit  46  provides a means for the user to input instructions to the instrument  32 . Input circuit  46  may be, for example, a keypad or touch screen, or other input device. 
   TDR circuit  42  is coupled to the test mode circuit  36  and processor  40  and is utilized when TDR testing is performed by tester  32 . The operation of tester  32  is briefly discussed below to aid in further understanding the present invention. 
   When a technician desires to receive data from the network, appropriate instructions are supplied via input circuit  46  to processor  40 , which cause test mode circuit  36  to be configured to receive data from the network through input terminal  34 . When the technician desires to perform a TDR test, instruction are again provided to processor  40  via input circuit  46 , this time causing test mode circuit  36  to be configured to couple TDR circuit  42  to terminal  34 . In the latter configuration, TDR signals are applied to the cable under test and the reflected signal is detected by TDR circuit  42 . The output of TDR circuit  42  is applied to processor  40  where appropriate operations are performed on the signal. An output associated with the reflected signal is typically presented on display  48  so that the technician may view and analyze results of the TDR test. 
   Turning to  FIG. 5 , there is illustrated a TDR circuit  50  according to a preferred embodiment of the present invention. Voltage supplies V 3 , V 4  are coupled to ground and to one end of resistors R 3 , R 4 , respectively. The other end of resistor R 3  is connected to terminal  52 . The other end of R 4  is connected to switch SW 1 . Switch SW 1  is coupled to terminal  54 . Link cable  56  comprises twisted pair wires  56   a ,  56   b , that are connected to terminals  52 ,  54  and has characteristic impedance Z′. Output amplifier  58  has its positive input connected to terminal  52  and its negative terminal connected to SW 1 . 
   If link cable  56  is not terminated into (i.e., connected to) a network, the technician may select a TDR test mode such that TDR circuit  50  is configured as depicted in FIG.  5 A. The technician may select this mode by providing appropriate instructions to tester  32 , as discussed above with reference to FIG.  4 . That is, instructions provided via input circuits  46  to processor  40  cause test mode circuit  36  to configure tester  32  so that TDR circuit  42  sends and receives TDR related signals to and from the link cable being tested. 
   The configuration of TDR circuit  42  shown in  FIG. 5A  is identical in operation to the circuit discussed above and depicted in FIG.  2 . In  FIG. 5A , SW 1  is depicted as comprising two (2) switches SW 1 A, SW 1 B. Switch SW 1 A is connected between R 4  and terminal  54 . Switch SW 1 B is coupled at one end to either R 4  or ground and at the other end to the negative input of amplifier  58 . In this test mode, switch SW 1 A is closed and couples R 4  to terminal  54 . Switch SW 1 B couples the negative input of amplifier  58  to R 4  and terminal  54 . Thus, TDR circuit  50  in  FIG. 5A  is configured to operate in the same manner as TDR circuit  22 , which was described above and is depicted in FIG.  2 . 
   If, however, the link cable being tested is terminated into a network, then the technician may provide appropriate instructions to processor  40  and cause TDR circuit  42  to be configured as shown in FIG.  5 B. In  FIG. 5B , switch SW 1 A is open and switch SW 1 B connects the negative input of amplifier  58  to ground. Characteristic impedance Z′a is approximately equal to the cable impedance Z′ ( FIG. 5 ) due to the low impedance of the network connection. In this test mode, R 4  is removed from the TDR test, which increases the impedance of TDR circuit  42  so that it is mismatched with the characteristic impedance Z′a of the link cable/network combination. 
   In accordance with one particular embodiment, R 3  and R 4  are each 50 ohms, and the characteristic impedance (Z′) of link cable  56  is nominally 100 ohms. Thus, in the test mode depicted in  FIG. 5B  the impedance of TDR circuit  42  is 50 ohms at terminal  52  and infinite at terminal  54  and the impedance Z′a of the link cable/network combination is less than 100 ohms. As a result, the circuit configuration of  FIG. 5   b  mismatches the impedances of TDR test circuit  42  and the cable/network load. The reflected TDR signal appearing at the output of amplifier  58  as a result of this impedance mismatch is illustrated by waveform  80  as depicted in FIG.  6 . 
   In accordance with the preferred embodiment of the present invention, and as noted above, TDR circuit  42  in  FIG. 5B  has a higher impedance than the TDR circuit  42  depicted in FIG.  5 A. As a result, the return pulse associated with  FIG. 5B  has a greater magnitude than the return pulse associated with FIG.  5 A. Accordingly, the results of a TDR test represented by waveform  80  may be more readily detected by a technician when depicted on the display of a cable test instrument. 
   From the foregoing description, it may be seen that a TDR circuit for testing terminated network cables formed in accordance with the present invention incorporates many novel features and offers significant advantages over TDR test circuits and methods currently available. While the presently preferred embodiment of the invention has been illustrated and described, it is to be understood that within the scope of the appended claims, various changes can be made without departing from the spirit of the invention. For example, it is possible to have sensing circuitry in the test instrument that automatically configures the TDR circuit when a lower cable or load impedance is detected. Further, a variety of switches and methods for switching between test modes may be implemented. Therefore, the illustrated and described embodiment is to be considered as exemplary only and the invention itself should be evaluated only as defined in the claims that follow.