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Commit 01db95c9 authored by Erik Strand's avatar Erik Strand
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Answer 7.1

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...@@ -7,6 +7,23 @@ title: Problem Set 5 ...@@ -7,6 +7,23 @@ title: Problem Set 5
Cables designed to carry signals with minimum pickup of interference often consist of a twisted pair Cables designed to carry signals with minimum pickup of interference often consist of a twisted pair
of conductors surrounded by a grounded shield. Why the twist? Why the shield? of conductors surrounded by a grounded shield. Why the twist? Why the shield?
Twisted pairs are used for differential signaling, i.e. where we listen to the difference between
the voltages on the wires as opposed to either absolute value. Noise can be imparted to our
conductors by external electric or magnetic fields. In particular, we need to worry about changing
fields (since static electric fields won't affect our current, and static magnetic fields will
induce a [Hall effect](https://en.wikipedia.org/wiki/Hall_effect) but not overlay a different
signal.)
If they were weren't twisted, a close noise source would add more noise to the closer wire, thus
distorting the voltage difference. Twisting distributes noise on average evenly across the two
wires, helping preserve the differential signal.
A shield helps prevent noise sources from interacting with the conductors at all. As we've seen,
oscillating electric fields can only penetrate so far. So the shield effectively provides a low pass
filter for the sorts of noise that can reach the conducting pair, with the cutoff frequency
determined by the shield's thickness. Grounding it ensures that built up static charge has a place
to go, though not all shields are grounded.
## (7.2) ## (7.2)
Salt water has a conductivity ∼4 S/m. What is the skin depth at $$10^4$$ Hz? Salt water has a conductivity ∼4 S/m. What is the skin depth at $$10^4$$ Hz?
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