Overview
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Twisted pair cables are insusceptible to external noise and emit less noise,
so they are used for such as speaker cables and power wires which require
strict noise reduction. When current frequency in the cable increases,
the current flows non-uniformly in the copper wire due to skin effect and
proximity effect, resulting in the increase of resistance and the reduction
of inductance. Losses are proportional to resistance, and the change in
inductance distorts the signal. Therefore, it is important to investigate
both resistance and inductance at different frequencies before designing
the cable. This note presents the use of magnetic field analysis to obtain
resistance and inductance in a cable at different frequencies. |
Current Density Distribution
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Figure 1 shows the current density distribution in the copper wire. At
10 kHz frequency, current distribution is slightly non-uniform due to proximity
effect, and at 100 kHz frequency, current distribution is extremely non-uniform
due to skin effect and proximity effect. This non-uniform current distribution
affects resistance and inductance.
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Resistance / Inductance
| Figure 2 shows the resistance in the copper wire at different frequencies.
Figure 3 shows the inductance in the copper wire at different frequencies.
Both resistance and inductance start to change at around 10 kHz due to
proximity effect, and as the frequency increases, they change sharply due
to skin effect. To reduce the proximity effect and the skin effect, the
cale needs to be designed to have a structure in which current passes through
a conductor uniformly within the usable frequency band. |
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