IEC PAS 62338:2002
(Main)Screened balanced cables - Coupling attenuation measurement, triaxial method
Screened balanced cables - Coupling attenuation measurement, triaxial method
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IEC/PAS 62338
Edition 1.0
2002-10
Screened balanced cables –
Coupling attenuation measurement,
triaxial method
PUBLICLY AVAILABLE SPECIFICATION
IN TE RNA TI ONA L
ELECTROTEC HNICAL
Reference number
COM M IS SION
IEC/PAS 62338
IEC/PAS 62338
Edition 1.0
2002-10
Screened balanced cables –
Coupling attenuation measurement,
triaxial method
PUBLICLY AVAILABLE SPECIFICATION
IN TE RNA TI ONA L
ELECTROTEC HNICAL
Reference number
COM M IS SION
IEC/PAS 62338
– 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 Theoretical background. 6
3.2.1 Unbalance attenuation a . 6
u
3.2.2 Screening attenuation a of the screen . 7
s
3.2.3 Coupling attenuation a . 8
c
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
6 Requirement. 12
7 Typical measuring curves of coupling attenuation. 13
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 national 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. 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 collaborates 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 opinion 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 and 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 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 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/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 This PAS was approved for
following document: publication by the P-members of the
committee concerned as 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 longer 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."
– 4 – Copyright IEC, 2002
SCREENED BALANCED CABLES –
COUPLING ATTENUATION MEASUREMENT,
TRIAXIAL METHOD
1 General
This test method determines the coupling attenuation a of screened balanced cables. Due
C
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 from 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 a is based on the procedure to measure
C
the screening attenuation a according to IEC 61196-1, Amendment 1.
S
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 circuit. 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 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 impedance 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 IEC, 2002 – 5 –
3 Definitions and the theoretical background
3.1 Electrical symbols
Z is the characteristic impedance of the primary circuit (cable under test)
Z is the characteristic impedance of the secondary circuit
Z is a normalised value of the characteristic impedance of
S
the environment of the cable under test
(150 Ω secondary circuit impedance Z )
R is the input impedance of the receiver
Z is the transfer impedance of the cable under test in [Ω/m]
T
Z is the capacitive coupling impedance of the cable under test
F
in [Ω/m], ZZ= ⋅Z ⋅jω⋅C (1)
FT12
f is the frequency, in Hz
C is the through capacitance of the outer conductor per
T
unit length [F/m]
ε is the relative dielectric permittivity of the cable under test
r1
ε is the relative dielectric permittivity of the secondary circuit
r2
ε is a normalised value of the relative dielectric permittivity of
r2,n
the environment of the cable
l is the effective coupling length
λ is the vacuum wavelength
o
c is the vacuum velocity
o
a is the screening attenuation which is comparable to the
s
results of the absorbing clamp method
P is the feeding power of the primary circuit
P is the measured power received on the input impedance
R of the receiver in the secondary circuit
P is the radiated power in the environment of the cable, which
r
is comparable to P + P of the absorbing clamp method
2,n 2,f
P is the radiated power in the normalised environment
S
of the of the cable under test, ( Z =150 Ω and |∆v/v | =10 % )
S 1
ϕπ=−2 ε ε l /λ
( )
11rr2 0
ϕπ=+2 ε ε l /λ (2,3,4)
()
21rr2 0
ϕϕ=−ϕ= 4πε l /λ
32 1 r2 0
– 6 – Copyright IEC, 2002
calibrated receiver termination resistors
or network analyzer and the remaining
U
cablelength in a high
screened box
U
Signal
generator
open tube ferrite rings
balun in a high
screened box
Figure 1 – Principle test set-up
3.2 Theoretical background
3.2.1 Unbalance attenuation a
u
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 conductor 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 independent 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 resistance between the two wires r (longitudinal - unsymmetry).
eC= −C (5)
10 20
rR=+jωωL−R+jL (6)
() ( )
22 1 1
The coupling between the two lines is then expressed by:
l
1 1 −+γγ ⋅x
()diff com
T=⋅ ⋅⋅jeω x⋅Z⋅Z+rx⋅e dx
() ()
()
(7)
un, diff com
∫
ZZ⋅
diff com
l
1 1
γγ−⋅()lx−
()
diff com
T =⋅ ⋅⋅jeω x⋅Z⋅Z−rx⋅e dx
() ()
() (8)
uf, diff com
∫
ZZ⋅
diff com
Copyright IEC, 2002 – 7 –
Where Z is the characteristic impedance of the differential mode (balanced) and Z of the
diff com
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 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.
1 l
n
n
Tj=⋅ωe⋅Z⋅Z±r⋅ ⋅⋅ S (9)
()
u diff com f
f
ZZ⋅
diff com
If the cable is electrical long there is the same phenomenon as for the coupling through the
screen. 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 increases by 20 dB per decade over the whole frequency range (S=1). In
f
praxis we have small systematic couplings together with statistical couplings. Thus T
u,n
increase by approx. 10 dB per decade and T by less then 20 dB per decade.
u,f
3.2.2 Screening attenuation a of the screen
s
At coaxial cables, respectively in the common mode of screened balanced cables, the
logarithmic ratio of the feeding power P and the periodic maximum values of the power P
1 r,max
which may be radiated due to the peaks of voltage U in the outer circuit is termed screening
attenuation a
S
P
r,max
a s=− 10⋅ log Env (10)
P
At high frequencies and when the cable under test is electrically long:
P
c ZZ− ZZ+
2,max
oTF TF
≈ ⋅ + (11)
P
ωεZR⋅ −εε +ε
1 r1 r2 r1 r2
For exact calculation, if feedback from the secondary to the primary circuit is negligible, the
ratio of the far end voltages U and U are given by
1 2
U ZZ− ZZ+ 1
−−jϕϕj
2 TF TF
≈ ⋅−11e + ⋅− e ⋅ ⋅
[] []
U ω⋅ Z
εε− εε+
1 1
rr12 rr1 2
(12)
c
− jϕ
21+−ZR/ ⋅1−e
()
()
i.e. formally | A + B | ⋅C⋅D, where AC is the far end crosstalk, BC is the reflected near end
crosstalk and D is the mismatch factor.
– 8 – Copyright IEC, 2002
Total oscillations of D are
< 2 dB, if 1 < Z / R < 1,25
. 3 dB, if Z / R = 1,4
but 10 dB and more, if Z / R >3.
Maximum values of AC and BC are given, if
ϕ = (2N + 1) ⋅ π and N is an integer
1,2
3.2.3 Coupling attenuation a
c
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 a . For screened balanced cables the
U
disturbing power from the pair is additional attenuated by the outer screen. The unbalanc
...
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