1、1TU-T RECMNxG.117 7b = 4862573 Ob37435 527 m INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STAN DARD IZATION SECTOR OF TU G.117 (02/96) TRANSMISSION SYSTEMS AND MEDIA GENERAL CHARACTERISTICS OF INTERNATIONAL TELEPHONE CONNECTIONS AND INTERNATIONAL TELEPHONE CIRCUITS TRANSMISSION ASPE
2、CTS OF UNBALANCE ABOUT EARTH ITU-T Recommendation G.117 (Previously “CCITT Recommendation“) FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (IT). The ITU-T is responsible for studying technical, operating and tariff ques
3、tions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Reco
4、mmendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). ITU-T Recommendation G. 117 was revised by IT-T Study Group 12 (1993-1996) and was approved under the WTSC Resolution
5、No. 1 procedure on the 6th of February 1996. NOTE In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. All rights reserved. No part of this publication may be reproduced or utilized in a
6、ny form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the i. ITU-T RECMN*G=ll7 96 W Y862591 Ob37437 3Tl CONTENTS Objective Principles of the scheme of nomenclature Summary of the descriptive terms used . 3.1 One-port networks 3.2
7、Two-pon networks . 3.3 Signal generating devices . 3.4 Signal receiving devices . Definitions and measuring techniques based on idealized measuring arrangements . 4 4.1 One-port networks 4.2 Two-port networks . . 4.3 Signal generating devices . 4.4 Signal receiving devices . 5 Other measurement defi
8、nitions 5.1 Common-mode rejection ratio Annex A - Aspects of conversion from longitudinal to transversal signals at analogue ports in some practical cases . A.l Introduction A.2 A.3 The difference between LCL, and LCL, in a practical case -two examples A.4 Additional Insight into Longitudinal Conver
9、sion Loss . Calculation of the longitudinal conversion loss References . Page 1 1 2 2 2 2 2 3 4 7 10 11 12 13 14 14 14 16 16 18 Recommendation G.117 (02196) i ITU-T RECMN*G.LL7 b 4862.593 Ob37438 238 SUMMARY In many applications, the balance performance of equipment is crucial in order to ensure a s
10、atisfactory suppression of unwanted signais from the normal transmission path. This Recommendation gives guidance of suitable measurement methods for evaluating different effects of unbalance. 11 Recommendation G.117 (OY96) ITU-T RECMN*G.LL7 7b 4862591 0637439 174 Recommendation 6.117 TRANSMISSION A
11、SPECTS OF UNBALANCE ABOUT EARTH (Geneva, 1980; amended at Malaga-Torremolinos, 1984 and Melbourne, 1988; revised in 1994) 1 Objective This Recommendation gives a comprehensive set of prescriptive measurements of various balance parameters for one- port and two-port networks. These are intended for u
12、se either in the field or in the factory with relatively simple test apparatus (e.g. standard transmission oscillators, level measuring sets), and a special test bridge. Measuring arrangements for assessing the degree of unbalance are covered in Recommendation 0.9 i. which are consistent with this R
13、ecommendation. Th definitions and methods are so devised that the results obtained from separately-measured (or specified) items of equipment (e.g. feeding-bridges, cable pairs, audio inputs to channel translating equipment, etc.) can be meaningfully combined though not necessady by simple decibel a
14、ddition. This allows the performance of a tandem connection of such items to be predicted or at least, bounds determined for that performance. Performance in this sense means those features affected by unbalanced conditions, e.g. levei of impulsive noise, sensitivity to longitudinal exposure, crosst
15、alk ratios, etc. 2 Principles of the scheme of nomenclature Many different terms have been used throughout the literature concerning unbalance about earth, some conflicting, or in other respects inadequate. The descriptive titles of the quantities given in this Recommendation are based on the follow
16、ing principles which have been adopted: Mode conversion, e.g. a poor (unbalanced) termination will develop an unwanted transverse signal when excited by a longitudinal signal. The measure of this effect is here termed longitudinal conversion ratio, and when expressed in transmission units Longitudin
17、al Conversion Loss, or LCL. When a two-port is involved where for example an excitation at one port produces a signal at the other port, then the designation will include the word transfer, for example longitudinal conversion transfer ratio and the corresponding loss, LCTL. The impedance of the long
18、itudinal path presented by a test object is a key parameter. The term longitudinal impedance ratio and the corresponding decibel expression, longitudinal impedance loss, are used to characterize the particular measurement defined. Active devices which are sources of signals (e.g. an oscillator. the
19、output port of an amplifier) are additionally characterized by the amount of unwanted longitudinal signal that is present in the output. The key word output is now included, to give longitudinal output voltage, and the corresponding longitudinal output level. When such unwanted signals are expressed
20、 as a proportion of the wanted (transverse) signal the key phrase is output signal balance ratio, the decibel expression of which is output signal balance. Devices which continuously respond to signais (e.g. level-measuring sets, the input port of an amplifier) and which can in principle respond to
21、unwanted longitudinal signals by reason of internal mechanisms (i.e. even if their input impedances were perfectly balanced) are characterized by measures containing the words input interference. These measures are input longitudinal interference ratio and the corresponding decibel expression input
22、longitudinal interference loss. The long-established and well-defined common- mode rejection ratio is maintained. The term sensitivity coeficient is avoided, since this is widely used in the Directives 2 and the work of Study Group 5 with a rather specialized meaning. Recommendation G.117 (0296) 1 I
23、TU-T RECMN*G.117 96 - Y862571 Ob17YYO 996 f) When a two-port network is involved, the input and output signais may not be the same, for example, they may have different levels, frequencies (FDM modems) or structure (PCM multiplex equipments). These aspects should be taken into account when formulati
24、ng proposals for the item under test. In the case of receiving devices in which the operation is not a linear continuous function of the level of the input signai (e.g. a group-delay measuring set or a data modem) the key principle is the threshold level of the interference; this is the level at or
25、above which an unacceptable amount of degradation of performance or misoperation occurs. Thus longitudinal interference threshold voltage and the corresponding levels are obtained. g) 3 Summary of the descriptive terms used 3.1 One-port networks a) transverse reflexion factor (transverse return loss
26、: TRL); b) transverse conversion ratio (loss: TCL); c) longitudinal conversion ratio (loss: LCL); d) longitudinal impedance ratio (loss: LE); e) transverse output voltage (level: TOL); f) longitudinal output voltage (level: LOL). Voltages e) and f) are unwanted signals uncorrelated to the wanted sig
27、nals. 3.2 Two-port networks 3.2.1 Separate measurement For each port taken separately the one-port measures: a) b) c) d) e) f) transverse reflexion factors (transverse return losses: TRL); transverse conversion ratio (loss: TCL); longitudinal conversion ratios (losses: LCL); longitudinal impedance r
28、atios (losses: LIL); transverse output voltage (levels: TOL); longitudinal output voltage (levels: LOL). 3.2.2 Measurement combined In addition, the following transfer parameters are for each of the two directions of transmission: a) b) c) d) transverse transfer ratios (losses: TL); transverse conve
29、rsion transfer ratios (losses: TCTL); longitudinal transfer ratios (losses: LTL); longitudinal conversion transfer ratios (losses: LCTL). 3.3 Signal generating devices - Output signal balance ratio (losses: OSB). This is in addition to the six one-port measures listed in 3.1. 3.4 Signal receiving de
30、vices a) b) Input longitudinal interference ratio (loss: ILIL). Longitudinal interference threshold voltage (level). 2 Recommendation 6.117 (OY96) ITU-T RECMNUGmLL7 96 4Bb2591 Ob17441 822 I These are in addition to the six one-port measures listed in 3.1. If the wanted signal is longitudinal (e.g. a
31、s in a signalling system) and the interfering voltage transverse, replace the word longitudinal with transverse in the descriptive terms. 4 Definitions and measuring techniques based on idealized measuring arrangements The illustrated definitions in this clause assume ideal test bridges (with lossle
32、ss infinite-inductance centre-tapped coils), zero impedance voltage generators and infinite-impedance voltmeters. An important aspect of this set of mutually consistent measurements is that the test bridge provides simultaneously defined reference terminations of 2 ohms for the transverse paths, and
33、 214 ohms for the longitudinal paths. From this starting point, the performance of cascaded items, each measured in the prescribed fashion, can be calculated. This takes account of the fact that the cascaded items do not, in general, exhibit the reference impedances provided by the test conditions.
34、It simplifies the mathematical treatment if the reference impedance is non-reactive and this also accords with the important objective of being able to use readily-available transmission test-apparatus to obtain field and factory measurement results. The ideal test bridge configuration used in the f
35、ollowing pages is shown in Figure 1. Note, however, that this test bridge configuration. with transversal impedance Z and longitudinal impedance Z/4, in general does not represent conditions met in practice. Thus, some care is needed when translating measured unbalance parameters into useful unbalan
36、ce information for practical cases. This applies in particular for the commonly used parameter LCL, Longitudinal Conversion Loss. This is discussed in more detail in Annex A. The transverse and longitudinal sources ET and EL are activated as required by the particular measurement being made. In Figu
37、re 6, neither source is active, and the bridge then provides only passive terminations of Z and Z/4. NOTE - It would have been in keeping with traditional transmission theory for the parameters to be defined in terms of half the open-circuit e.m.f. However, to harmonize with Recommendation 0.9, this
38、 Recommendation defines some parameters in terms of V,. If the input impedance of the device under test is nominally equal to the driving device, then the two methods are equivalent. termination ET 4 Lcnbdinal termination I - FIGURE 1/G. 11 7 Recommendation G.117 (OY96) 3 ITU-T RECMN*G*LL7 96 486259
39、3 Ob37442 769 4.1 One-port networks 4.1.1 Transverse reflexion factor (return loss) (see Figure 2) Z-Z, reflecteivdtage 2VR Transwce rle)rjtm factor p = - = =- Z+ZT fWWdW,ltage ET NOTES 1 The value of R is (theoretically) irrelevant. The potential divider aaoss the zero-impedance generator is only n
40、eeded to deriw half the generator voltage, which is numerically equal to the forward voltage needed for the definition. 2 Conventional return-loss measuring bridges do not terminate the longjtudinal path with 214. This is unimportant when the return loss is some 20 dB or so less than the longitudina
41、l conversion loss of the test object. in this case the reflected power is substantially greater than the power diverted to the longitudinal path, and there is negligible error. 3 If Z, is known then clearly p = 1 - 3 is not needed. If VTis measured p can be calculated from the expression p = 1 - % ,
42、 which is however somewhat inconwnient for high values of return loss. ET FIGURE 2/G.117 4 Recommendation G.117 (OZ96) ITU-T RECNN*G.117 Yb = 4862571 0637443 bT5 4.1.2 Transverse conversion ratio (loss) (see Figure 3) T12CZ30-Wd03 v, “T Tramverse cornersiun mijo, k = - arKi 1 k Transverse Cmversion
43、Loss (TLC) = 20 iog, I - I = 20 log, I dB NOTES . 1 the longitudinal conversion ratio c. However, this relationship is na true for other network arrangements. 2 and will not be explicitly referred to again. In the case where the network is linear passive and bilateral, the transverse Conversion Loss
44、 (EL) is equal to haif The dotted component is needed for a two-terminal device which, when in use, only bridges the transmission circuit FIGURE 3/G.117 4.1.3 Longitudinal conversion ratio (loss) (see Figure 4) - Longitudinal conversion ratio, c = - “T and EL Longitudinal Conversion Loss (LCL) = 20
45、log, I ; 1 I = 20 log ,O 121 dB NOTES 1 This measure is variously referred to in other Recommendations as: Longitudinal balance. Degree of unbalance. Unbalance. Degree of longitudinal balance. Signal balance ratio. f) Impedance unbalanced to earth. 2 The longitudinal conversion ratio is applicable t
46、o any one-port, even to those which are sources of signals (e.g.: oscillateur output terminais). in such cases the transverse voltage V, must be measured selectively if it is required to measure this loss in respect of a signal generator in operation. FIGURE 4/G. 1 17 Recommendation G.117 (0296) 5 I
47、TU-T RECMN*G.LLi 9b 48b2591 Ob17444 531 9 4.1.4 Longitudinal impedance ratio (loss) (see Figure 5) - Longitudinal impedance ratio, q = 5 v, and Longitudinal Impedance Loss (UL) = 20 log, I q I = 20 log, 151 dB NOTES .l 2 connection to earth) the value of V, will be very small and the corresponding r
48、atio (and loss) will be very large. in such cases the coupling introduced between longitudinal and transverse paths will be very small and the effect is not important. This is an additional measure that is needed if the performance of a cascade of items is to be predicted. In the case of test-object
49、s which are virtually earth free (e.g.: double-insulated, portable test apparatus with no deliberate FIGURE 5iG.117 4.1.5 Transverse and longitudinal output voltages (levels) (see Figure 6) NOTES 1 longitudinal path may deliver unwanted transverse signals. Similarly the output of an amplifier may deliver an unwanted longitudinal “hum” signal, or a cable pair may deliver unwanted longitudinal signais arising from induction o: radiation. 2 These measures relate to unwanted signals uncorrelated to the wanted signal. For exemple, a.d.c. signaling system in the Other referenc
