Method and test unit for far end crosstalk measurements

[0002] This invention relates to the measurement of far end crosstalk in a multi-pair cable. In particular, the invention relates to a method and a test unit for providing phase coherent test signals at either end of a cable under test.

[0003] Crosstalk is the unwanted coupling of signals from one pair to another along a multi-pair cable. When crosstalk is measured at the same end of the cable where the crosstalk originates, the measurement technique is called near end crosstalk (NEXT). Twisted pair LAN technologies, such as 10BASE-T, 100BASE-T, and Token Ring are primarily vulnerable to cable crosstalk problems in a manner that is best tested by measuring the NEXT of the installed cable.

[0004] When crosstalk is measured at the end of the cable opposite from where the crosstalk originates, the measurement technique is called far end crosstalk (FEXT). FEXT is measured by applying a test signal to a wire pair at a far end of the cable and measuring the disturbance on the other wire pairs in the cable at the other or near end. It is relevant to specify the FEXT performance of cabling for network technologies, such as the new 1000BASE-T specification, that transmit simultaneously on multiple wire pairs in the same direction.

[0005] While it is easy to measure the FEXT performance of an installed multi-pair cable, it is difficult to specify certification limits for such measurements, since FEXT varies with the cable length. The equal level far end crosstalk (ELFEXT) measurement technique was developed as a practical alternative for field certification. Generally, ELFEXT equals FEXT minus attenuation caused by the cable. ELFEXT measurements compensate for the effect of varying cable length so that all installed cable can be certified to the same limit.

[0006] Residual crosstalk is any signal that is due to the test instrument itself. Residual crosstalk error must be taken into account in crosstalk measurement analysis.

[0007] With vector measurements, residual crosstalk errors can be mathematically cancelled. When an error term cancellation in a vector measurement is employed, the test instruments at either end of the cable need to generate test signals that are phase coherent with one another.

[0008] Accordingly there is a need for a method of generating phase coherent test signals at either end of the cable. There is also a need for a test instrument that can be used at either end of the cable and that is capable of producing a test signal that is coherent with another test signal generated at the other end of the cable.

[0009] The method of the invention is used in a system that conducts far end crosstalk measurements of a multi-pair cable that is under test. The system employs a near end test unit and a far end test unit, arranged at opposite ends of the cable. The near and far end test units each have a signal generator.

[0010] According to the method of the invention, a first test signal is generated with the far end signal generator and a second test signal is generated with the near end signal generator. The phases of the first and second test signals are synchronized to produce phase coherency of the two signals. The first test signal is transmitted from the far end. A FEXT measurement is performed using the first and second test signals and a crosstalk signal induced by the first test signal in one or more wire pairs of the cable.

[0011] In particular, synchronization is accomplished by generating a reference signal with the far end generator in phase synchronization with the first test signal. The reference signal is transmitted from the far end of the cable. At the near end, the near end signal generator is controlled, responsive to the received reference signal, to produce the second test signal in phase coherency with the first test signal.

[0012] A test unit in accordance with the invention includes a signal generator that is capable of operating as far end or a near end signal generator to perform the functions of the method described above.

[0013] Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the accompanying drawings, in which like reference characters denote like elements of structure.

[0014] FIG. 1 is a block diagram of a measurement system that uses test instruments according to the present invention.

[0015] FIG. 2 is a block diagram of the signal generator of the test instrument of the FIG. 1 measurement system.

[0016] FIG. 3 is a schematic circuit diagram of the transformers of the test instrument of the FIG. 1 measurement system.

[0017] FIGS. 4 and 5 are waveform diagrams of signals identified in FIGS. 2 and 3.

[0018] With reference to FIG. 1, a measurement system 10 includes a first test unit 11 and a second test unit 12 that are connected to opposite ends of a cable 13 that is under test. Cable 13 is a multi-pair cable that has four twisted wire pairs 14, 16, 18 and 20. Cable 13 is of a type that is used in a local area network (LAN) for coupling computer terminals and servers in a network. For the purpose of this description, it is assumed that cable 13 is installed in a LAN and is under test using various measurements, including FEXT and ELFEXT.

[0019] Test units 11 and 12 are substantially identical so that they can be used interchangeably at either end of cable 13. Accordingly, only test unit 11 will be described in detail.

[0020] Test unit 11 has a microprocessor or controller 30, a signal processor 32, a test switch matrix 34, a reference switch matrix 36, a transmit/receive (Tx/Rx) mode selector 38, a signal generator 50 and a transformer bank 70. Microprocessor 30 directs measurement procedures by communicating with test unit 12 over cable 13, transmitting or receiving signals carried by one or more of twisted wire pairs 14, 16, 18 or 20 and performing a measurement analysis of the signals.

[0021] For example, when a measurement procedure requires test unit 11 to transmit a test signal to test unit 12, microprocessor 30 issues a transmit enable signal to signal generator 50, Tx/Rx mode selector 38, reference switch matrix 36 and test switch matrix 34. Signal generator 50 generates a test signal which is applied via test switch matrix 34 and transformer bank 70 to one or more of wire pairs 14, 16, 18 and 20. The test signal is generally a swept frequency signal having a frequency in a range of frequencies, usually in the MHz portion of the spectrum. Crosstalk measurements are generally performed for each frequency in the range.

[0022] When a vector measurement method is employed, there is a need for a near end test unit, say unit 11, to replicate locally a test signal that is originally generated by the far end test unit 12 and transmitted via cable 13 to near end test unit 11. The replicated test signal must be substantially identical in frequency and phase coherent with the test signal originally generated by far end test unit 12. That is, the far end and near end test signals must have a constant phase relationship.

[0023] To provide phase coherent test signals at either end of cable 13, signal generator 50 generates a reference signal that has a frequency that is substantially lower than the test signal frequency range. For example, the reference signal frequency is generally in the kHz portion of the spectrum. The reference signal is applied via Tx/Rx mode selector 38, reference switch matrix 36 and transformer bank 70 to one or more of twisted wire pairs 14, 16, 18 and 20. The receiving test unit uses the reference signal to locally generate a replica of the test signal generated by the transmitting test unit as described below.

[0024] Referring to FIG. 2, signal generator 50 includes a crystal oscillator 52, a frequency synthesizer 54, a frequency divider 56, a differential receiver or amplifier 58 and a phase detector 60. Crystal oscillator 52 generally operates at a selected design frequency, for example, 15 MHz. The 15 MHz signal is used by frequency synthesizer 54 to generate a test signal and by frequency divider 56 to generate first and second reference signals at a frequency of about 200 kHz. The first and second reference signals are substantially identical, except that they are 180° out of phase.

[0025] Test unit 11 is operable in a transmit mode or a receive mode as directed by a transmit enable signal or a receive enable signal issued by microprocessor 30. When in the transmit mode, the transmit enable signal causes the test signal output of frequency synthesizer 54 to be applied to test switch matrix 34. Test switch matrix 34 applies the test signal, via transformer bank 70, to a selected one of the wire pairs 14, 16, 18 or 20 as directed by microprocessor 30, for transmission in a differential mode. The transmit enable signal causes TX/RX mode selector 34 and reference switch matrix 36 to apply the first and second reference signals via transformer bank 70 to two different selected wire pairs for transmission in a common mode. For example, if the test signal is applied to wire pair 14, the first reference signal is applied to wire pair 14 and the second reference signal is applied to a selected one of the other wire pairs 16, 18 or 20.

[0026] Referring to FIG. 3, transformer bank 70 has a plurality of transformers 74, 76, 78 and 80 that are connected to wire pairs 14, 16, 18 and 20, respectively. For example, transformer 74 has a primary winding 74P connected to test switch matrix 34 to receive the test signal. Transformer 74 has a secondary winding 74S that has its end leads connected to separate wires of wire pair 14. These connections result in the test signal being transmitted in a differential mode on wire pair 14, meaning that the test signal is split into two signals on the separate wires that are identical in amplitude and frequency, but are 180° out of phase.

[0027] Transformer secondary winding 74S has a center tap 74T that is connected to receive one of the reference signals. This connection results in the reference signal being transmitted on wire pair 14 in a common mode, meaning that the reference signal is split into two signals on the separate wires that are identical in amplitude, frequency and phase. The signals A and B on the two wires represent the composite test and reference signals. The waveforms of signals A and B are shown in FIG. 4. The higher frequency signals A and B are out of phase by 180°.

[0028] When in the receive mode, the receive enable signal causes the test signal generated by frequency synthesizer 54 to be applied to signal processor 32. The receive enable signal also enables test switch matrix 34 to transfer signals received from one or more wire pairs to signal processor 32 for signal shaping, filtering and digital conversion processes. Also in the receive mode, reference switch matrix 36 and Tx/Rx mode selector 38 are enabled to transfer the signals received on two of the wire pairs to signal generator 50.

[0029] When a FEXT measurement procedure is to be conducted, a far end test unit, say unit 12, generates a test signal and two phase displaced reference signals. The test signal and one of the reference signals are transmitted on a selected wire pair, such as wire pair 14 as signals A and B. This transmission induces crosstalk signals in the other wire pairs 16, 18 and 20. The other reference signal is transmitted simultaneously on one of the other wire pairs, say wire pair 18. The signals C and D represent the composite crosstalk and reference signals on wire pair 18. The waveforms of signals C and D are shown in FIG. 4 with the lower frequency reference signal being the carrier and the higher frequency crosstalk signal modulating the lower frequency carrier. The carriers of signals C and D are 180° out of phase with envelopes of signals A and B due to the 180° phase displacement of the two reference signals.

[0030] At near end test unit 11, transformer bank 70 acts to separate the carrier signals, A+B and C+D, from the modulating signals, A−B and C−D. The waveforms of these signals are shown in FIG. 5. The carrier signals are then applied to signal generator 50.

[0031] Referring to FIG. 2, carrier signals, A+B and C+D, are algebraically summed by differential receiver 58 to produce a difference signal (A+B)−(C+D). The difference signal is applied to phase detector 60 that detects any phase variance between the difference signal and the locally generated 200 kHz reference signal produced by frequency divider 56. The phase variance or error is used to lock oscillator 52 to the phase and frequency of the difference signal. Because of the frequency difference between the difference signal or reference signal and the oscillator, there will be a phase difference at the higher frequency of the received test signal A−B and the locally generated test signal at the output of frequency synthesizer 54. However, due to the phase locking action of oscillator 52, such phase difference will be maintained constant for the period when measurement samples are taken. That is the test signal locally generated by near end signal generator 50 will be phase coherent with the received test signal A−B. These two signals and the crosstalk signal C−D are used by signal processor 32 and microprocessor to perform a FEXT or ELFEXT measurement procedure.

[0032] According to the method of the invention, a first or far end test signal (sometimes referred to herein as a first test signal) and first and second reference signals are developed in phase synchronization with one another at the far end of the cable under test. The frequency of the far end test signal is in a relatively high frequency range and the frequency of the reference signals is substantially lower than such high frequency range. The reference signals are phase displaced from one another by 180°. The far end test signal is transmitted in a differential mode and the first reference signal is transmitted in a common mode on a first wire pair of the cable. The second reference signal is transmitted in a common mode on one of the other wire pairs in the cable.

[0033] At the receiving end of the cable, the received first and second reference signals are used to develop a second or near end test signal that is phase coherent with the received far end test signal. The far end and near end test signals and a cross talk signal received on a wire pair other than the one that carried the far end test signal are used to perform a far end crosstalk measurement.

[0034] The far end and near end signal generators each have an oscillator. The far end oscillator is use to generate the far end test signal and the near end oscillator is used to generate the near end test signal by adjusting the phase of the near end oscillator to the phase of the first and second received reference signals.

[0035] The present invention having been thus described with particular reference to the preferred forms thereof, it will be obvious that various changes and modifications may be made therein without departing from the spirit and scope of the present invention as defined in the appended claims.