IEEE

EMC Experiments
and
Demonstrations Guide
Prepared by the
Education and Student Activities
Committee of the

IEEE EMC Society

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Crosstalk in Cables
Clayton R. Paul
Dept. of Electrical Engineering, University of Kentucky

1. Objective

To understand the mechanism of crosstalk in which the electromagnetic fields of electrical

signals on one pair of wires in a cable bundle couple to and induce signals in another pair of
wires. To investigate the factors that influence the coupling and methods for reducing the
crosstalk.

2. Equipment

 Sinusoidal oscillator (1 kHz to 1 MHz) with 50 ohm source impedance and at least 20V
p-p open-circuit voltage.
 Function or pulse generator capable of producing 100 kHz pulse trains having rise/fall
times of 1 s or less and an open-circuit voltage of 5V p-p.
 Dual trace oscilloscope (at least 50 MHz bandwidth).
 Cables
o Standard appliance cord (10').
o RG-58 coaxial cable (5').
o Two 6' lengths of insulated hookup wire (20 to 24 gauge).
 Four 10 ohm carbon resistors (1/4 watt).

3. Procedure

3.1 Crosstalk in Unshielded Wires

1. Place two 5' lengths of appliance cord flat on a nonmetallic table and tape them together
so that the insulations are touching as shown in Figure l(a).
2. Solder the 10 ohm resistors to the ends of these cords as shown in Figure l(a).
3. Attach the oscillator to one cord, and one channel of the oscilloscope to measure V1.
a. Adjust the frequency of the oscillator to 1 kHz and the output level at V1 to 3Vp-
p.
b. Attach the other channel of the oscilloscope to the other cord (across the 10 ohm
resistor) to measure V2.
c. Repeat the measurements at frequencies of 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 7.0,
8.0 and 9.0 in each decade up to 1 MHz.
d. Increase the oscillator frequency until you get a V2 that is measureable.
4. Plot the interference voltage transfer ratio; V2/Vl at the above frequencies on 3 cycle log-
log graph paper.
5. Repeat the above experiment with the two appliance cords parallel but separated by 1/2
inch.
6. Replace the sinusoidal oscillator with the function generator set to produce a 100 kHz
square wave with a peak voltage of 0.5 V and a pulse rise/fall time of 1 microsecond.

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 7. Sketch V1 and V2 versus time and note the effect on V2 of changing the rise/fall time of
V1. Observe that the level of V 2is directly dependent on the rise/fall time of V 1; the
shorter the rise/fall time of V1 the larger the amplitude of V2.

Figure 1.

3.2 Crosstalk by Common Impedance Coupling

1. Repeat 3.1 for the configuration shown in Figure l(b). The two appliance cords are to
remain taped together as in 3.1, step 1. Simply resolder two 10 ohm resistors between both ends
of one wire of each pair of cords.

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3.3 Crosstalk in Shielded Cables
1. Replace the driven pair of wires with a 5 foot length of RG-58 coaxial (shielded) cable as
shown in Figure 1 (c).
2. Repeat 3.1 steps 3 - 7 for this configuration.
3. Replace the coaxial cable with the pair of insulated hookup wire and twist these two
wires together to give about one twist every three inches.
4. Repeat 3.1 steps 3 - 7 for this configuration.

4. Theory

4.1 Crosstalk in Unshielded Wires

Currents and voltages associated with signal transmission on a pair of parallel wires (the
generator wires) generate electric and magnetic fields in the vicinity of those wires. These
electromagnetic fields interact with any neighboring wires (the receptor wires) and induce
voltages and currents into these lines as shown in Figure 2. Portions of these induced signals
appear at the ends of the receptor wire circuit. This unintentional coupling of signals from one
circuit to another can cause the devices at the ends of the receptor wires to be interfered with and
thus their performance can be degraded. This is commonly referred to as crosstalk. This crosstalk
is due to two mechanisms. The current of the generator line produces a magnetic field that is
coupled to the receptor line by the mutual inductance, Lm, between the two circuits. This is
referred to as inductive coupling. Similarly, the voltage of the generator line produces an electric
field that is coupled to the receptor line by a mutual capacitance, Cm, between the two circuits.
This is referred to as capacitive coupling. Both mutual impedances are functions of the cross-
sectional dimensions of the lines such as wire radii, and wire separation. Separating the two lines
reduces the mutual impedances. Both Lm and Cm are direct functions of the line length; doubling
the line length doubles the crosstalk, V2. The crosstalk also varies directly as the frequency of the
signal on the generator line; the higher the frequency the higher the crosstalk. Thus for the
experiment shown in Figure 1(a), the magnitude of the output voltage should be [1,2,3]:
V2  fK ( Lm )LI1  fK ( Cm )LV1 (1)
where I1 and V1 are the current and voltage of the generator line, L is the line length, f is the
frequency of I1 and Vl, and K(Lm) and K(Cm) are the inductive and capacitive coupling
coefficients, respectively, for the particular cross-sectional configuration. Note that the crosstalk
should increase linearly with frequency. This should show up on your graphs as a line with a
slope of 20 dB/decade. Normally, one component of the coupling will dominate the other. For
small terminal resistances where currents and magnetic fields are large, the inductive coupling
will be larger than the capacitive coupling. For large terminal resistances where the voltages and
electric fields are large, the situation is reversed.

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 Figure 2.

2. Time-domain crosstalk is also due to this mutual inductance and mutual capacitance between
the two circuits. The time-domain voltage induced across the load of the receptor line, v2(t), is
[2,3]
1 di ( t ) 1 dv ( t )
v2 ( t )  K ( Lm ) 1 L K ( Cm ) 1 L (2)
2 dt 2 dt
Note that the induced voltage is proportional to the time-derivative (slope) of the current and
voltage waveforms in the generator circuit. Typical waveforms resemble trains of triangular-
shaped pulses as shown in Figure 3. So the induced crosstalk voltage resembles pulses occurring
during the rise/fall times of the driven line voltage. The heights of the pulses are proportional to
the slope of these transitions on the generator line.

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 Figure 3.

4.2 Crosstalk by Common Impedance Coupling

1. Whenever two currents share a common return path, the current of this desired signal passing
through this impedance of the return path develops a voltage across the common impedance that
appears directly in the receptor circuit [21. For the case shown in Figure 1(a), the two circuits do
not share a common return path. However, suppose we wished to (,save wire" and choose to
have both circuits share one of the wires as a common return. This configuration is shown in
Figure 1(b).
In the case of Figure 1(b), the common impedance consists of one of the wires of the appliance
cord. This configuration is modeled as shown in Figure 4(a). Each wire of the appliance cord
consists of 41 strands of #34 gauge wire. From reference [2] each strand has a resistance of
0.2609 ohms/foot. The total resistance of each wire consists of 41 of these strands in parallel or

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 Figure 4.

0.00636 ohms/foot. For a 5-foot length, the total resistance of this common path is 0.0318 ohms.
The voltage developed across this common impedance is:
Vc = Rc IG (3)
which is equal to 9.6 mV p-p in this case. This voltage is divided across the two 10 ohm resistors
of the receptor circuit as shown in Figure 4 by voltage division to give:

V2 = Vc / 2 = 4.8 mV (p-p) (4)

For the configuration shown in Figure 1(b), the magnitude of the output voltage is again given by
Equation (1) but the coupling coefficients are different than for Figure 1(a).
As the frequency of the signal is decreased, the combined electric and magnetic field coupling
will decrease at a rate of 20 dB/decade until it reaches this "floor" produced by common
impedance coupling. As the frequency is further reduced, the crosstalk will remain at this level
due to co=on impedance coupling even as the frequency is reduced to DC !. (See Figure 4(a).)
2. Time-domain crosstalk involving common-impedance coupling is similar to the previous case
(time-domain crosstalk not involving common-impedance coupling) with the addition of a
constant level between the rise and fall times of the pulse. Between the rise and fall times, the
input voltage appears virtually constant at V 0 (see Figure 3) so that
v2(t) = Vc / 2 = 0.8 mV (5)
where the voltage across the common impedance is

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 v1 ( t )
vc ( t )  Rc  1.6mV (6)
10
The total time-domain crosstalk voltage is the superposition of (2) and (5).

4.3 Crosstalk Reduction Techniques

1. Replacing the pair of wires of the generator line with a coaxial cable as shown in Figure 1(c)
can reduce the crosstalk. The shield tends to confine the electric field to its interior, thus reducing
the electric field coupling. Also the generator line currents are essentially located on the same
axis, but are equal and oppositely directed. Thus the effect of the magnetic field is canceled in
the vicinity of the receptor circuit [1, 2].
2. Because the effects of the electric and magnetic fields from adjacent twists tend to cancel in
the vicinity of the receptor circuit, twisting a pair of wires together will have the effect of
cancelling the fields caused by the voltage and currents on those wires [2]. Thus the crosstalk
induced in a nearby receptor circuit will be reduced.

5. References

1. Ott, H.W., Noise Reduction Techniques in Electronic Systems, John Wiley & Sons, 2nd
Edition, 1988.
2. C.R. Paul, Introduction to Electromagnetic Compatibility, John Wiley & Sons, NY, 1992.
3. Paul, C.R., "Printed Circuit Board Crosstalk", 1985 IEEE International Symposium on
Electromagnetic Compatibility, Wakefield, Mass., August, 1985.

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