Copper Lines and High Speed Application Note 52 Line quality is no coincidence Testing transmission lines for ADSL Test Solutions with the PSM-137 Selective Level Test Set
Contents Testing transmission lines for ADSL Verification of copper line parameters Page 3 Attenuation Page 3 Crosstalk Page 3 Longitudinal conversion loss Page 4 Impedance towards ground Page 4 Characteristic line impedance Page 4 Description of test procedures Page 4 Line attenuation test setup Page 4 Far-end crosstalk setup Page 5 Near-end crosstalk setup Page 5 Longitudinal conversion loss test setup Page 5 Impedance towards ground test setup Page 6 Characteristic impedance test setup Page 6 Test equipment Page 7 Imprint Author: Peter Ziemann, Dpt. VM Wandel & Goltermann GmbH Elektronische Meßtechnik Mühleweg 5 D-72800 Eningen u.a. Germany Subject to change without notice Order no.: E 9.97/D1/52 Printed in the Federal Republic of Germany 2
ADSL line qualification tests Asymmetric Digital Subscriber Line (ADSL), a modem technology, converts existing twisted-pair telephone lines into access paths for multimedia and high speed data communications. As the name implies, ADSL transmits an asymmetric data stream, with much more data going downstream to the subscriber and much less coming back. A modem must therefore be installed at both ends of the line. The maximum downstream rate depends on the length of the line between the central office (CO) and the subscriber. Typical values are: Up to 5.0 miles (8 km) 1.544 Mbit/s (T1) 4.5 miles (7 km) 2.048 Mbit/s (E1) 3.0 miles (5 km) 6.312 Mbit/s (DS2) 2.5 miles (4 km) 8.448 Mbit/s Measurements on existing copper lines are end-toend, with generator at one end and receiver at the other end of the twisted pair. Crosstalk Crosstalk is unwanted transfer of energy from one wire to an adjacent wire caused by electromagnetic coupling. Crosstalk attenuation is defined in terms of power level differences as follows: Near-end crosstalk (NEXT) ADSL uses discrete multi-tone coding (DMT), dividing the transmission channel into 255 sub-channels. Each subchannel is QAM modulated with a 4.3 khz bandwidth. This type of transmission is very similar to frequency division multiplex (FDM) systems. The lower frequency range (up to 20 khz) is used for POTS. The range from 20 khz to 1.1 MHz is used for duplex data transmission and is most important for line qualification tests. Far-end crosstalk (FEXT) Verification of copper line parameters Attenuation Attenuation is the main factor limiting cable length. It depends on the frequency of the transmitted signal. Normally, attenuation increases with increasing frequencies and causes unwanted reduction in the signal strength. As cables are never homogeneous, NEXT and FEXT should both be measured at both ends of the line. Longitudinal conversion loss Longitudinal conversion loss (LCL) or signal balance ratio is defined in ITU-T Rec. O.9 as the logarithmic ratio of the longitudinal voltage (V L ) to the transverse voltage resulting from it. 3
LCL is important in balanced systems when it comes to reducing the effects of common-mode voltages to ground, e.g. resulting from external interference fields. Impedance towards ground Transverse voltages (high impedance to ground) occur in balanced signal transmission. Damaged insulation, humidity effects and the capacitance between the twisted pair and ground result in a variable impedance / frequency response towards ground, resulting in higher attenuation distortion and lower common-mode rejection. The measurement frequencies of interest are below 100 khz, to reduce the influence of shield capacitance. Description of test procedures The test setups shown in this application note are for ADSL with a line impedance of 100 ohms. They are also suitable for other technologies, such as HDSL, ISDN, 2 Mbit/s PCM and leased lines with Z = 120, 135 or 150 ohms. In such cases, baluns are unnecessary and the balanced connectors of the instruments can be used. Matching resistors are always required to match the system impedance. The PSM-137-139 range of instruments is recommended (receiver + generator). Together with the LevelPRO control software, they form a powerful test system for fast measurements and documentation. The separate SPM-33A receiver and PS-33A generator may also be used for tests at up to 2 MHz. Normalization is required in order to achieve best accuracy. In the following, the term normalization means: - Set the reference memory to the displayed value (copy ABS to REF) - Display the normalized value ABS-REF (sets the display to 0 db) Line attenuation test setup Characteristic line impedance The transmission characteristic of a copper line mainly depends on the characteristic impedance Z 0. This parameter is determined by measuring the impedance of the line with one end open-circuit and then short-circuit. Z 0 is easily determined by finding the frequency points for which Z open is equal to Z short. There is no need to determine the phase angles with this method. The PSM-137 AUTOSTEP mode allows synchronized increments in the generator and receiver frequencies in up to 100 steps. The generator operates as master, with the receiver as a time-controlled slave. The receiver is triggered by pressing the generator start key. The far end instrument can be connected via modem (V.24) for PC-controlled measurements with LevelPRO. 4
Near-end crosstalk test setup Instrument display of attenuation vs. frequency NEXT is a single-end measurement. Users can select either AUTOSTEP or the faster SWEEP mode for determining the frequency response. The result display and the compensation for insertion loss are the same as described for line attenuation. LevelPRO display of attenuation vs. frequency If a generator level of 0 dbm is used, the receiver displays the attenuation with inverted sign. An easy way to compensate for the insertion loss of the baluns (< 0.5 db), is to increase the TX level. Longitudinal conversion loss test setup Far-end crosstalk test setup SWEEP mode provides a display of LCL vs. the desired frequency range. Before starting the measurement, the test setup must be normalized to compensate for the intrinsic attenuation of the SDZ-30 bridge. The setup for normalizing is similar to the test setup, with the line under test replaced by a reference element LCL = 0 (or a short-circuit between point a or b and ground). The test procedure is the same as for line attenuation. At low frequencies, crosstalk attenuation may be very high. Setting FSTART to f max and FSTOP to f min will ensure enough signal strength for the AUTOSTEP trigger. After normalization at one frequency point (in LEVEL mode), the test setup is ready for LCL measurements. The lower frequency limit for the SDZ-30 is 10 khz. The upper limit depends on the instrument used (PSM-139 up to 32 MHz). 5
LevelPRO provides convenient menu-driven normalization for measurements using external bridges. The figure below shows an example for the BMB-30 bridge. The instrument display is in db. LevelPRO shows the results in ohms. p /db Zx /ohm s p /db Zx /ohm s p /db Zx /ohm s p /db Zx /ohm s -21 102-31 345-41 1112-51 3538-22 116-32 388-42 1249-52 3971-23 131-33 437-43 1403-53 4457-24 148-34 491-44 1575-54 5002-25 168-35 552-45 1768-55 5613-26 190-36 621-46 1985-56 6300-27 214-37 698-47 2229-57 7069-28 241-38 784-48 2502-58 7933-29 272-39 881-49 2808-59 8903-30 306-40 990-50 3152-60 9990 IMB-30 conversion table Impedance towards ground test setup Characteristic impedance test setup SWEEP mode yields a display of impedance towards ground vs. the desired frequency range. Before starting the measurement, the test setup must be normalized to compensate for the intrinsic attenuation of the IMB-30 bridge. During normalization, the line under test should be connected and the button on the IMB-30 should be pressed. After normalization at one frequency point (in LEVEL mode), the test setup is ready for impedance measurements. The lower frequency limit for the IMB-30 is 50 Hz, the upper limit depends on the measured impedance (max. 1 MHz at 1kΩ, max. 100 khz at 10 kω). LevelPRO makes it very easy to determine the frequency points, where Z open and Z short are equal. The procedure starts with menu-driven normalization of the test setup. The results of the first measurement (e.g. with open end) are stored as TRACE B. The next step is to measure with the end short-circuit and to display the result of TRACE A&B. It is then a simple procedure to use the marker to find the points of intersection where the displayed impedance corresponds to the characteristic line impedance. The next figure shows the result graph for a twisted pair line with Z = 75 ohms. 6
Determination of characteristic line impedance using LevelPRO: The black trace (high impedance for low frequencies) shows the open-end values, the red trace shows the short-circuit values. Another interesting application using the same test setup is the estimation of the cable length. The length can be calculated if the λ/4 resonant frequency of the line has been measured and the NVP of the cable is known. Length = (NVP * c) / (4 * f res ) or Length /m = (NVP * 300) / (4 * f res /MHz) or Length /m = 300 / (4 * f res /MHz * SQR(ε r )) NVP = nominal velocity of propagation (0.5 to 0.9 c) ε r = dielectric constant of the cable insulation The resonant frequency is the first minimum in the impedance trace of the open-end pair or the first maximum of the short-circuit pair. The example above shows a first resonance at 0.428 MHz. The resulting cable length is 87 m (if ε r = 4 and NVP = 0.5). Test equipment PSM-137 Level test setup (8 MHz) BN 2203/15 with tracking generator (2 required) or PSM-138 Level test setup (18 MHz) BN 2203/16 with tracking generator (2 required) or PSM-139 Level test setup (32 MHz) BN 2203/17 with tracking generator (2 required) or SPM-33A Selective level meter (2 MHz) BN 2033/01 PS-33A Level generator (2 MHz) BN 2071/01 LevelPRO BN 2203/93.01 (control and evaluation software) SMZ-100/75 Balun 1 khz to 3 MHz BN 2078/14 (2 required for 100 Ω systems, e.g. ADSL) SDZ-30 Signal balance ratio bridge BN 2234/01 Reference element LCL=0 for SDZ-30 BN 2234/01.01 IMB-30 Impedance bridge BN 2234/20 BMB-30 Impedance bridge BN 2234/30 Reference element 100 Ω for BMB-30 BN 2234/30.01 (may also be used as a matching resistor) 7
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