Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – With auxiliary means to condition stimulus/response signals
Reexamination Certificate
1999-07-20
2001-03-06
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
With auxiliary means to condition stimulus/response signals
C324S628000, C324S620000, C702S107000, C702S117000
Reexamination Certificate
active
06198292
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the measurement of crosstalk in a multi-pair cable. In particular, the invention relates to a test unit with minimized output signal imbalance and common mode susceptibility and to a method for calibrating the test unit.
BACKGROUND OF THE ART
Crosstalk in a multi-pair cable is the unwanted coupling of signals from one wire pair to another. When crosstalk is measured at the same end of the cable where the crosstalk originates, the technique is called near end crosstalk (NEXT) measurement. Twisted pair LAN technologies, such as 10BASE-T, 100BASE-T, and Token Ring are primarily vulnerable to cable crosstalk problems that can be tested by measuring the NEXT of the installed cable.
When crosstalk is measured at the end of the cable opposite from where the crosstalk originates, the technique is called far end crosstalk (FEXT) measurement. 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.
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, ELFEST 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.
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.
A test signal is generally applied to a wire pair in a differential mode. That is, the signals on the wires of a wire pair are ideally equal in amplitude and opposite in phase (180° out of phase). These conditions reflect an ideal output signal balance (OSB). A differential receiver, receiving a signal with common mode and differential mode components of the wires, will ideally reject the common mode component and respond only to the differential component. This characteristic reflects an ideal common mode rejection (CMR).
However, these conditions are hard to achieve due to imperfect components used to make the test unit and to asymmetries in layout. For example, the output terminals of a test unit can have unbalanced parasitic capacitances. In the past, such imbalance has been minimized by careful selection of components and layout or placement within the test unit assembly. This selection and placement process has been costly and time consuming.
Accordingly there is a need for a test instrument that minimizes deviation of OSB and CMR from the perfect or ideal conditions of equal amplitude and opposite phase. There is also a need for a method to calibrate such an instrument.
SUMMARY OF THE INVENTION
A test unit for conducting crosstalk measurements of a cable under test according to the present invention has a plurality of pairs of output terminals. Each pair of output terminals has a first terminal and a second terminal that have first and second parasitic capacitances, respectively. A first variable capacitor is coupled to the first terminal to form a first effective capacitance value that includes the first parasitic capacitance. A second variable capacitor is coupled to the second terminal to form a second effective capacitance value that includes the second parasitic capacitance. A controller sets the first and second variable capacitors to a first predetermined value and a second predetermined value, respectively. The first predetermined value and the second predetermined value are selected such that the first effective capacitance value is substantially equal to the second effective capacitance value.
The variable capacitors are preferably varactors that exhibit a variable capacitance that is voltage controlled. The controller sets the first and second predetermined values by controlling a first bias voltage and a second bias voltage applied to the first and second varactors, respectively.
A method according to the present invention calibrates the output signal balance of the test unit by calibrating, the test unit when it is not connected to a cable. First the first bias voltage is set to a predetermined value. The second bias voltage is then sequentially set to n voltage values. A test signal frequency sweep is applied to the first and second output terminals for each of the n voltage values. An OSB response is measured during each test signal frequency sweep. A first voltage value is marked when the OSB response becomes lower than a predetermined limit. A second voltage value is marked when the OSB response subsequently becomes larger than the standard limit. A bias voltage value is then calculated using the first and second voltage values. The bias voltage is then downloaded to the test unit.
REFERENCES:
patent: 4970466 (1990-11-01), Bolles et al.
patent: 5530367 (1996-06-01), Bottman
patent: 5532603 (1996-07-01), Bottman
patent: 5539321 (1996-07-01), Sciacero et al.
patent: 5698985 (1997-12-01), Bottman
Cook Ron
Haley Michael J
Kirk James W
Mlinarsky Fanny I
Agilent Technologie,s Inc.
Metjahic Safet
Nguyen Vincent Q.
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