Microdia Newsletter No 43 optimizing shielded pairs
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Incidence of single wire geometry and electric properties on the performance of LAN type cables.
Transcript of Microdia Newsletter No 43 optimizing shielded pairs
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N EWSLETTER N O 43 February 2015
Optimizing shielded pairs.
Introduction.
Twin axial cables were introduced as an inexpensive, stable and noise resilient connection media in
early computers and networks. Traditional design include two twisted signal lines, a simple helical me-tallic foil screen around the conductors and sometimes a drain wire located below the shield.
Twin axial cables are already adapted for 10 Gbits/s. Ethernet connectivity options in data centers, en-
terprises wiring closets and service provider transport application.
Applications of twin axial cables are numerous:
Networking cat 6, 6a, 7, 7a, etc – UTP, STP.
Computer storage – SAS, S-ATA. USB,
Computers PCI (Peripheral Component Interconnect) bus.
S-ATA (Serial Advanced Technology Attachment) computer bus.
Consumers HDMI (High Definition Multimedia Interface) cable.
Consumer USB (Universal Serial Bus) cable.
And more…
Twisted Pair bulk wire.
Inexpensive.
Various performance cables, “Category”
cables.
Shielded or non-shielded.
Susceptible to cross talk.
Shielded Parallel pairs (twinax) bulk wire.
Higher performances than twisted pairs.
Individually shielded pairs.
Various dielectrics, PE,PTFE, etc.
Foil or bulk braid shield.
Networking, PCI, SATA, Ethernet etc,
OptEM cable designer screen capture: Pair advisor tool able to optimize atte-
nuation and charachteristic impedance
of the cable for selected cable specs or
category
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Newsletter No 43 Pa ge 2
Sensitivity analysis.
Sensitivity analysis is a technique to determine how “sensitive” a model is to changes in the values
of its parameter and in its structure. Parameter sensitivity is usually performed as a series of tests in which different parameters are sets to see how a change in the parameter causes a change in the
dynamic behaviour of the model.
Sensitivity analysis is nowadays frequently used by the designers of electronic circuits. Less com-
mon and known is its use for interconnect analysis.
In the cable propagation properties: time delay [Td], attenuation [a] and characteristic impedance
[Z], each of them is a function of multiple parameters. For example, attenuation in a pair depends
independently on dimensional and material parameters of each wire.
Parameters sensitivity analysis:
It is the first step in guiding a designer into optimization of twin axial cables.
In our case, the parameter sensitivity analysis will show the incidence of parameter variation on
the cable properties.
For the cable designer:
Cable geometrical dimensions and material constants are the variable or parameters.
L, R, C & G and their corresponding propagation parameters Time delay [Td], attenuation
[a] and characteristic impedance [Z] are the properties of the design.
In circuits analysis:
Inductance [L], Resistance [R], Capacitance [C] and Conductance [G] are the parameters,
Voltage, Current and their ratios are the properties.
Name Symbol Units Unit symbol
loop resistance R ohms per metre Ω/m
loop inductance L henries per metre H/m
insulator capacitance C farads per metre F/m
insulator conductance G siemens per metre S/m
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Newsletter No 43 Pa ge 3
Insulated conductors. The main components of a twinax cable are the two copper wires insulated in extrusion lines. During the manufacturing of the insulated conductors:
Core diameters, over insulation diameter, coaxial capacitance, insulation concentricity are
measured on line.
Outer skin and inner skin thicknesses, main layer expansion, foam layer insulation dielectric
constant are calculated from measured values.
Conductivity, elongation, final diameter of conductor can only be measured off line.
Online measured and calculated values are displayed on the line monitoring screen and stored for sub-
sequent analysis.
Pair twisting.
Lay length of the pairs, back-twist ratio, tension in the conductors are recorded on-line and
saved during the manufacturing process.
Critical parameters.
On can look at individual properties and assess the impact that different parameters have on their values.
By sorting the parameters and by their impact strength, it is possible to find critical parameters that con-
trol time delay [Td], attenuation [a] and characteristic impedance [Z].
Attenuation
The main factor impacting attenuation is the skin diameter or overall insulation diameter, then come cop-
per and insulation eccentricities.
Copper diameter and copper conductivity by contrast have a relatively low level impact.
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Newsletter No 43 Pa ge 4
Characteristic impedance.
Regarding the characteristic impedance, the critical parameters are core diameter and skin diameter or
overall insulation diameter. To a smaller extend, insulation dielectric constant affect the impedance too.
It is interesting to note that copper eccentricity affects strongly the attenuation but has rather small im-
pact on the characteristic impedance.
Time delay
Foam dielectric constant which depend on foam expansion and compound dielectric constant has a
strong incidence on the time delay [Td]. The higher the level of foam expansion, the lower the dielec-
tric constant. Copper diameter, skin diameter or diameter over insulation and copper diameter affect to a lesser ex-
tend this cable property.
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STP critical design.
Combining sensitivities of all three properties control time delay [Td], attenuation [a] and charac-
teristic impedance [Z], one can present critical design of the STP cable.
The most critical design parameters for STP appear to be the insulation overall diameter. Copper diameter, copper eccentricity, skin eccentricity or insulation eccentricity, foam dielec-
tric constant are impacting the three selected properties with a similar magnitude. Other parameters play insignificant roles.
Newsletter No 43 Pa ge 5
Critical design parameters of a simulated STP calculated from Time delay [Td], attenuation [a] and characteristic impedance [Z] sensi-
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Newsletter No 43 Pa ge 6
Conclusion.
Calculated sensitivities. Not only provide a clear indication of how design and dimensions affect cable propagation properties but also offer a very valuable estimate of tolerances for cable propagation proper-ties based on expected distributions and tolerances of design parameters.
Copper diameter. Increase of copper diameter always leads to lower resistance values by reducing Eddy current
Insulation diameter. Causes reduction of attenuation and increase characteristic impedance.
Wires eccentricity. Has almost no impact on characteristic impedance but significant impact on attenuation.
Foam dielectric constant. Is almost entirely responsible for the cable time delay.tolerance.
Insulation diameter and copper/skin eccentricity. Are responsible for attenuation tolerance.
Copper and Insulation diameter. Are responsible for cable characteristic impedance.
Article extracted from:
Technical paper:
Optimizing shielded pair manufacturing.
Published in Wire Journal International September 2014
With kind autorisation of Dr. Juliusz Poltz.
Authors:
Dr Juliusz Poltz cofounder of OptEM Engineering Inc. software company in Calgary, Alberta, Cana-
da, that specializes in EM analysis and circuits simulation.
M. Mats Josefson works in the transmission technology dept. of Ericsson AB, Hudiskvall, Sweeden.
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Newsletter No 43 Pa ge 7
Microdia’s SFS cross head is a 3 layers crosshead designed to make it possible the extrusion of a wide
range of cable construction and compounds.
Configurations.
- Single layer
- Single layer foam
- Dual layer main layer + outer skin or inner skin
- Triple layer
- Dual layer inner skin + foam or foam + outer skin
- Triple layer skin-foam-skin
microdia mflex 9000-10 SLS skin foam skin cross head with microflex fine tuning
microdia mflex 9000-10 SLS three
layers skin-foam-skin cross head
with microflex fine tuning.
Configuration:
- horizontal main extruder
- 45° vertical auxiliary extruders
- pneumatic bypass on all three
extruders.
This cross head specially designed to extrude “category” and skin- foam-skin and railways signal wires is
available with the renown microdia microflex fine tuning system.
microflex heads are fixed centre heads with the possibil-
ity of a fine manual tuning.
Fine tuning is made via 4 radial adjusting screws.
The die assembly will not slide during the adjustment but only tilt thanks to a flexible element, part of the
head’s front part, therefore adjustment is possible dur-
ing the run , even with full head pressure.
When all the 4 screws are released the cross head
works as a fixed centre cross head giving a fairly good
level of concentricity approaching 0.95.
Once the line has reached its stable operating condi-
tion, trimming can be performed to bring the concen-
tricity factor close to 1.
microflex fine tuning
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Newsletter No 43 Pa ge 8
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