From 01db95c900d83aa6f2534b5820132d63bf04556d Mon Sep 17 00:00:00 2001 From: Erik Strand <erik.strand@cba.mit.edu> Date: Wed, 13 Mar 2019 22:06:38 -0400 Subject: [PATCH] Answer 7.1 --- _psets/5.md | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/_psets/5.md b/_psets/5.md index b63f047..a5aa5c0 100644 --- a/_psets/5.md +++ b/_psets/5.md @@ -7,6 +7,23 @@ title: Problem Set 5 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? +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) Salt water has a conductivity ∼4 S/m. What is the skin depth at $$10^4$$ Hz? -- GitLab