Crosstalk effects of shielded twisted pairs
This article deals with the modeling and simulation of shielded twisted pairs with CST CABLE STUDIO™. The quality of
braided shields is investigated with respect to perfect solid shields. Crosstalk effects are calculated for unshielded twisted
pairs, poorly shielded twisted pairs, and twisted pairs with high-quality shields. Explanations are given how to create realistic
simulation setups in order to be able to compare them with measurement results.
Cable shielding
Shielded cables are widely used in industrial applications in order to suppress unwanted crosstalk effects between
neighbouring wires. A typical shielded cable is the coaxial cable with an inner wire and a concentric outer screen.
Ideal shielding conditions can be achieved by realizing the screen as a solid conductor with a specific thickness. With
increasing frequency the penetration depth of the electric field decreases until it is less than one half of the shields thickness.
At higher frequencies the current tends to flow mainly on the conductors surfaces (skin effect), thus completely decoupling the
inner part of the screen from the outer part. For a solid screen we can state: the higher the frequency the better the screening.
Due to cost and engineering reasons the use of solid shields is rather limited. More popular are braided shields since they are
easier to manufacture, are lighter in weight, and more flexible than solid shields. There are, however, different quality levels.
Braided shields have tiny apertures that are permeable to the electric field. As a consequence, the shielding effectiveness
decreases with increasing frequency. Hence the goal is to find the optimum between cost saving and shielding effectiveness.
A measure for the shielding effectiveness of shielded cables is the transfer impedance. It describes the frequency dependent
transmission of electromagnetic signals from one side of the shield to the other side of the shield. The denser the braid is or
the more braid sheets are used, the better the shielding.
Figure 1: Transfer impedance of a tubular braid with different number of sheets
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 1 of 7
Figure 1 illustrates the transfer impedance of various tubular braids - a single braid (top curve), a double braid (middle curve)
and a triple braid (bottom curve). As a comparison, the transfer impedance of a solid shield would strongly monotonically
decrease as the frequency increases.
The shielding effectiveness of the triple braid shows the best quality since its transfer impedance in the frequency range up to
about 50 kHz is quite similar in behaviour to that of a solid shield. At 50 kHz the transfer impedance reaches a local minimum
and rises again with increasing frequency. This means that 50 kHz is the threshold frequency above which the shielding
diverges from perfect behavior and falls off in quality at higher frequencies. The double and single tubular braids are even less
perfect: they have got higher transfer impedances even at low frequencies, and the local minimum is already reached earlier or
does not show up at all.
The question now is: how does this affect the crosstalk behavior? In order to answer this question it is reasonable to perform
three simulations of the same cable configuration but different shielding conditions.
Cable configuration and simulation tasks
The cable structure of interest can be seen in figure 2. It consists of four shielded twisted pairs in a common outer screen.
Twisted pairs are often used since they are easy to manufacture, are inexpensive, and exhibit reasonable crosstalk
characteristics. Since twisted pairs are quite susceptible to asymmetric load conditions and might cause certain crosstalk
effects they are shielded at times.
In the following four S-parameter simulations are carried out. The first simulation demonstrates the crosstalk effect in case of
perfect load conditions. The second simulation deals with the non-shielded twisted pairs and, in the subsequent two
simulations, different shields are investigated.
Figure 2: Cross-sectional view of shielded twisted pair cable
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 2 of 7
Perfect load conditions
The aim of this investigation is to demonstrate what happens if one simulates a perfect cable configuration with
homogeneously twisted pairs and fully symmetric loads. Figure 3 depicts the simulation configuration.
Figure 3: Simulation setup for a perfectly symmetric cable configuration
The screens of the twisted pairs are grounded to zero potential at both sides. Ports have been defined at the input and output
of the four twisted pairs. Figure 4 shows the simulation results.
Figure 4: Simulation result of perfectly symmetric cable configuration. Besides the reflection and transmission to
the line’s end there is no crosstalk effect visible. The numeric value for crosstalk is less than -200 dB
As expected, there is a certain reflection at the input and quite good transmission to the end of the line, but there is no
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 3 of 7
perceptible crosstalk into neighboring lines visible. Actually, the numerical value is less than -200 dB and is only limited to this
level in order to obtain a reasonable result view.
It is important to note that this result is not caused by an insufficiency of the simulation method but because no crosstalk exists:
the twisted pairs are arranged symmetrically, are twisted with identical number of twists per meter, and are loaded in exactly
the same way.
Real measurements, on the other hand, always show perturbations and crosstalk effects. When comparing their results with
ideal simulation results, there is often an uncertainty because of the discrepancy. It has to be understood, however, that a
measurement setup is never as perfect as a simulation setup. It is necessray to reproduce all potential perturbations of the
measurement in the simulation order to find the same results. Since such perturbations are often not known this is rather
difficult.
In order to overcome this problem tendencies need to be investigated rather than trying to reproduce reality. In the given
context it is reasonable to check the crosstalk effect of an unbalanced (asymmetric) termination and to investigate how
different shielding conditions contribute to its improvement.
Unshielded twisted pairs
It is interesting to understand what might be the crosstalk effect in an unshielded twisted pair cable configuration with
asymmetric termination. Figure 5 shows the cables cross-section, figure 6 the schematic, and figure 7 the simulation results.
The termination is realized by using a 50 Ohm resistor. In order to reproduce production tolerance of about 10% the resistance
values have been increased to 55 Ohm and decreased to 45 Ohm, respectively.
Figure 5: Twisted pair configuration without shields
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 4 of 7
Figure 6: Simulation setup with asymmetric load conditions
Figure 7: Crosstalk effect in case of unshielded twisted pairs with asymmetric load conditions. The transmission
factor into the neighboring lines lies between -60 dB and about -15 dB in frequency range up to 1 GHz
Shielded twisted pairs low shielding effectiveness
When returning to the case of shielded twisted pairs the different qualities of shielding can be investigated. A direct measure
for the shielding effectiveness and therefore for the crosstalk is the transfer impedance as discussed at the beginning of this
article. The higher the transfer impedance the greater the crosstalk effect since surface currents will induce higher voltages
inside the screen. As a consequence, the shielding effectiveness worsens.
Figure 8 shows the simulation result of the cable configuration with specific transfer impedance. The chosen values for the
simulation are Rt = 0.09 Ohm, Lt = 1e-9 H, and Ct = 1e-14 F.
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 5 of 7
Figure 8: Simulation result in case of low shielding effectiveness (high transfer impedance). The transmission
factor into the neighboring lines lies between -200 dB and about -90 dB in frequency range up to 1
GHz
Shielded twisted pairs high shielding effectiveness
When changing the transfer impedance to a lower value less voltage can be induced, thus yielding higher shielding
effectiveness. Figure 9 shows the simulation result of the cable configuration with Rt = 0.001 Ohm, Lt = 1e-10 H, and Ct = 1e15 F. As expected, the crosstalk effect has been drastically reduced.
Figure 9: Simulation result in case of high shielding effectiveness (low transfer impedance). The transmission
factor into the neighboring lines now lies between less than -200 dB and about -120 dB in frequency
range up to 1 GHz – a noticeable improvement
Summary
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 6 of 7
Investigations of (multiple) shielded cables are possible with CST CABLE STUDIO™. A measure of the shielding effectiveness
is the transfer impedance value that can be determined by means of the three parameter transfer resistance (Rt), transfer
inductance (Lt), and transfer capacitance (Ct). Its should be noted, however, that a direct comparison between simulation and
measurement is not possible unless one considers exactly the same conditions in both simulation and measurement setup.
Since parasitic effects such as production tolerances are generally not entirely known, it is recommended to investigate
tendencies during simulation rather than trying to reproduce measurement results. This article describes how such
investigations can be done, and it illustrates how simulation helps to understand basic correlations of various parameters in
order to find the optimum solution.
Crosstalk effects of shielded twisted pairs
© 2016 CST AG - http://www.cst.com
Page 7 of 7