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1、 Screened balanced cables Coupling attenuation measurement, triaxial method P U B L I C L Y A V A I L A B L E S P E C I F I C A T I O N IEC/PAS 62338 Edition 1.0 2002-10 Reference number IEC/PAS 62338 I N T E R N A T I O N A L E L E C T R O T E C H N I C A L C O M M I S S I O N Copyright Internation
2、al Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- Copyright International Electrotechnical Commission Provided by
3、IHS under license with IECLicensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- Screened balanced cables Coupling attenuation measurement, triaxial method P U B L I C L Y A V A I L A B L
4、E S P E C I F I C A T I O N IEC/PAS 62338 Edition 1.0 2002-10 Reference number IEC/PAS 62338 I N T E R N A T I O N A L E L E C T R O T E C H N I C A L C O M M I S S I O N Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employees/1111111001, User
5、=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- 2 Copyright IEC, 2002 CONTENTS FOREWORD. 3 1 General. 4 2 Principle of the measuring method 4 3 Definitions and the theoretical background 5 3.1 Electrical symbols 5 3.2 Th
6、eoretical background. 6 3.2.1 Unbalance attenuation au. 6 3.2.2 Screening attenuation as of the screen 7 3.2.3 Coupling attenuation ac 8 4 Measurement . 9 4.1 Equipment . 9 4.2 Balun requirements . 10 4.3 Sample preparing 10 4.4 Procedure 11 4.6 Measurement Precautions 11 5 Expression of results 12
7、6 Requirement. 12 7 Typical measuring curves of coupling attenuation. 13 Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted w
8、ithout license from IHS -,-,- Copyright IEC, 2002 3 INTERNATIONAL ELECTROTECHNICAL COMMISSION _ SCREENED BALANCED CABLES COUPLING ATTENUATION MEASUREMENT, TRIAXIAL METHOD FOREWORD 1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising all na
9、tional electrotechnical committees (IEC National Committees). The object of the IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, the IEC publishes International Standards
10、. Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. The IEC colla
11、borates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an international consensus of opini
12、on on the relevant subjects since each technical committee has representation from all interested National Committees. 3) The documents produced have the form of recommendations for international use and are published in the form of standards, technical specifications, technical reports or guides an
13、d they are accepted by the National Committees in that sense. 4) In order to promote international unification, IEC National Committees undertake to apply IEC International Standards transparently to the maximum extent possible in their national and regional standards. Any divergence between the IEC
14、 Standard and the corresponding national or regional standard shall be clearly indicated in the latter. 5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with one of its standards. 6) Attention is drawn
15、 to the possibility that some of the elements of this PAS may be the subject of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights. A PAS is a technical specification not fulfilling the requirements for a standard, but made available to the public. IEC
16、/PAS 62338 has been prepared by IEC technical committee 46: Cables, wires, waveguides, r.f. connectors, r.f. and microwave passive components and accessories. The text of this PAS is based on the following document: This PAS was approved for publication by the P-members of the committee concerned as
17、 indicated in the following document: Draft PAS Report on voting 46/107/PAS 46/110/RVD Following publication of this PAS, the technical committee or subcommittee concerned will investigate the possibility of transforming the PAS into an International Standard. This PAS shall remain valid for no long
18、er than 3 years starting from 2002-10. The validity may be extended for a single 3-year period, following which it shall be revised to become another type of normative document, or shall be withdrawn.“ Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee
19、=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- 4 Copyright IEC, 2002 SCREENED BALANCED CABLES COUPLING ATTENUATION MEASUREMENT, TRIAXIAL METHOD 1 General This test method determines the co
20、upling attenuation aC of screened balanced cables. Due to the concentric outer tube, measurements are independent of irregularities on the circumference and outer electromagnetic field. A wide dynamic and frequency range can be applied to test even super screened cables with normal instrumentation f
21、rom low frequencies up to the limit of defined transversal waves in the outer circuit at approximately 4 GHz. For balanced cables the upper frequency is limited by the properties of the baluns. The procedure to measure the coupling attenuation aC is based on the procedure to measure the screening at
22、tenuation aS according to IEC 61196-1, Amendment 1. 2 Principle of the measuring method The test set up is a triaxial system consisting of the cable under test and a solid metallic tube. The matched cable under test which is fed by a generator forms the disturbing respectively the inner or primary c
23、ircuit. The disturbed respectively the outer or the second circuit is formed by the outer conductor (or the outer most layer in the case of multiscreen cables) of the cable under test and a solid metallic tube having the cable under test in its axis. The voltage peaks at the far end of the secondary
24、 circuit have to be measured. The near end of the secondary circuit is short-circuited. For this measurement a matched receiver is not necessary. The likely voltage peaks at the far end are not dependant on the input impedance of the receiver, provided that it is lower than the characteristic impeda
25、nce of the secondary circuit. However, it is an advantage to have a low mismatch, for example by selecting a range of tube diameters for several sizes of coaxial cables. Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employees/1111111001, User=
26、listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- Copyright IEC, 2002 5 3 Definitions and the theoretical background 3.1 Electrical symbols Z1 is the characteristic impedance of the primary circuit (cable under test) Z2 is
27、the characteristic impedance of the secondary circuit ZS is a normalised value of the characteristic impedance of the environment of the cable under test (150 secondary circuit impedance Z2 ) R is the input impedance of the receiver ZT is the transfer impedance of the cable under test in /m ZF is th
28、e capacitive coupling impedance of the cable under test in /m, ZZZjC FT = 12 (1) f is the frequency, in Hz CT is the through capacitance of the outer conductor per unit length F/m r1 is the relative dielectric permittivity of the cable under test r2 is the relative dielectric permittivity of the sec
29、ondary circuit r2,n is a normalised value of the relative dielectric permittivity of the environment of the cable l is the effective coupling length o is the vacuum wavelength co is the vacuum velocity as is the screening attenuation which is comparable to the results of the absorbing clamp method P
30、1 is the feeding power of the primary circuit P2 is the measured power received on the input impedance R of the receiver in the secondary circuit Pr is the radiated power in the environment of the cable, which is comparable to P2,n + P2,f of the absorbing clamp method PS is the radiated power in the
31、 normalised environment of the of the cable under test, ( Z S =150 and |v/v1| =10 % ) () () 1120 2120 32120 2 2 4 = =+ = rr rr r l l l / / / (2,3,4) Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employees/1111111001, User=listmgr, listmgr Not
32、for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- 6 Copyright IEC, 2002 U1 U2 open tube calibrated receiver or network analyzer balun in a high screened box Signal generator ferrite rings termination resistors and the remaining cablelength in a
33、 high screened box Figure 1 Principle test set-up 3.2 Theoretical background 3.2.1 Unbalance attenuation au Screened balanced pairs may be operated in the differential mode (balanced) or the common mode (unbalanced). In the differential mode one conductor carries the current +I and the other conduct
34、or carries the current -I; the screen is without current. In the common mode both conductors of the pair carry half of the current +I/2; and the screen is the return path with the current -I, comparable to a coaxial cable. Under ideal conditions respectively with ideal cables both modes are independ
35、ent of one another. Actually both modes influence each other. Differences in the diameter of the core insulation, unequal twisting and different distances of the pair. The unsymmetry is caused by the capacitive unbalance to earth e (cross-unsymmetry) and the difference of the inductance and resistan
36、ce between the two wires r (longitudinal - unsymmetry). eCC= 1020 (5) ()()rRj LRj L=+ 2211 (6) The coupling between the two lines is then expressed by: ( )( )() () T ZZ je xZZr xedx u n diffcom diffcom x l diffcom , = + + 1 4 1 0 (7) ( )( )() ()() T ZZ je xZZr xedx u f diffcom diffcom l x l diffcom
37、, = 1 4 1 0 (8) Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employees/1111111001, User=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- Copyright IEC, 2002 7 Where
38、 Zdiff is the characteristic impedance of the differential mode (balanced) and Zcom of the common mode (unbalanced). These are in principle the same coupling transfer functions compared to the coupling through the screen. The integral could only be solved if the distribution of the unsymmetry along
39、the cable length is known. For an unsymmetry being constant along the cable length, the transfer function results in the same way as for cable screens. () Tje ZZr ZZ l S u f n diffcom diffcom f n = 1 4 (9) If the cable is electrical long there is the same phenomenon as for the coupling through the s
40、creen. Depending on the velocity difference between the differential and the common mode circuit the envelope of the transfer function approaches a constant value which is frequency and length independent. However if the velocity difference is zero, then the transfer function at the far end increase
41、s by 20 dB per decade over the whole frequency range (Sf=1). In praxis we have small systematic couplings together with statistical couplings. Thus Tu,n increase by approx. 10 dB per decade and Tu,f by less then 20 dB per decade. 3.2.2 Screening attenuation as of the screen At coaxial cables, respec
42、tively in the common mode of screened balanced cables, the logarithmic ratio of the feeding power P1 and the periodic maximum values of the power Pr,max which may be radiated due to the peaks of voltage U2 in the outer circuit is termed screening attenuation aS = 1 maxr, sEnvlog P P 10 10a (10) At h
43、igh frequencies and when the cable under test is electrically long: P P c ZR ZZZZ 2 1 1 ,max + + + oTF r1r2 TF r1r2 (11) For exact calculation, if feedback from the secondary to the primary circuit is negligible, the ratio of the far end voltages U1 and U2 are given by ()() U U ZZ e ZZ e Z c ZRe TF
44、rr j TF rr j j 2 1 1212 1 0 2 11 1 211 12 3 + + + + / (12) i.e. formally | A + B | CD, where AC is the far end crosstalk, BC is the reflected near end crosstalk and D is the mismatch factor. Copyright International Electrotechnical Commission Provided by IHS under license with IECLicensee=IHS Employ
45、ees/1111111001, User=listmgr, listmgr Not for Resale, 03/06/2007 05:50:14 MSTNo reproduction or networking permitted without license from IHS -,-,- 8 Copyright IEC, 2002 Total oscillations of D are 3. Maximum values of AC and BC are given, if 1,2 = (2N + 1) and N is an integer 3.2.3 Coupling attenua
46、tion ac Balanced cables which are driven in the differential mode will radiate a part of the input power, due to irregularities in the cable symmetry. For unscreened balanced cables, this radiation is depicted by the unbalance attenuation aU. For screened balanced cables the disturbing power from th
47、e pair is additional attenuated by the outer screen. The unbalance causes a current in the screen which is then coupled by the transfer impedance and capacitive coupling impedance into the outer circuit. Consequently the effectiveness against electromagnetic disturbances of shielded balanced cables is the sum of the unbalance attenuation au of the pair and the screening attenuation as of the screen. Since both quantities usually are given in a logarithmic ratio, they may simply be added into the coupling attenuation
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