
Signal Reflection - ColinWright
https://www.solipsys.co.uk/new/SignalReflection.html?th08hn
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stn8188
This is part of the field of Signal Integrity within EE - a rabbit hole which
I decided to base my career on. For anyone desiring more detail (with some
minimal EE knowledge as a prerequisite), two of the most popular texts are:
High Speed Digital Design: A Handbook of Black Magic by Johnson/Graham and
Signal and Power Integrity - Simplified by Eric Bogatin.

[https://www.amazon.com/High-Speed-Digital-Design-
Handbook/dp...](https://www.amazon.com/High-Speed-Digital-Design-
Handbook/dp/0133957241/)

[https://www.amazon.com/Signal-Power-Integrity-Simplified-
Pre...](https://www.amazon.com/Signal-Power-Integrity-Simplified-
Prentice/dp/013451341X/)

~~~
jcims
Not the same but related, I got started down the road of (RF) DSP a while back
and just got to the wtf parts, particularly around handling errors, where it
really did kind of feel like magic.

I couldn’t hack the deep end of the math pool though and tapped out, but the
awe remains. ‘The Signal Path’ and ‘w1aex’ are two YouTube channels i still
subscribe to for my fix. Watching a waveform on a scope writhe around like a
silent theremin when you just move a cable or even wave your hand past it
really drives the point home.

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VectorLock
This YouTube video from "Physics Videos by Eugene Khutoryansky" really game me
the "ah-ha" moment understanding reflection and impedance matching
[https://www.youtube.com/watch?v=ozeYaikI11g](https://www.youtube.com/watch?v=ozeYaikI11g)

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peter_d_sherman
>"Way back in the 70s when I was building logic circuits, I was warned
repeatedly that I'd have to get a few things right, or risk "locking up" or
"burning out" the CMOS chips I was using. The bogeyman was "Signal
Reflection", but no one could tell me what it was, or why it was so bad."

[...]

>"I remember hooking up a television, again, back in the 70s, and we had to
change the cable connecting the aerial, because it was 75-Ohm, and we needed
to use 300-Ohm.

I was confused.

It was wire, and certainly if you connected a meter to measure the resistance,
you didn't get 75 Ohms, or 300 Ohms. You got effectively 0 Ohms. It was wire,
after all, and wire didn't have resistance.

No, but it has _impedance_.

When you have a current flowing down the wire, you have electrons in motion
(although not as much as you may think!). And if you have a _moving charge_ ,
you have a _magnetic field_.

So _when you start the current, it starts to create the [temporarily changing]
magnetic field_ , and a _changing magnetic field creates a voltage in the
opposite direction_.

So that opposes the current, giving a dynamic [instantaneous] resistance.

So _starting_ the current off down the wire causes a [instantaneous]
resistance, and we can then talk about how much resistance we experience. That
depends on a lot of things, but it serves to show that

 _even just current on a wire is a complex thing_."

Observation: Changing the current in an existing circuit might not result in
an instantaneous change in the current -- that's because an instantaneous
change in current results in a instantaneous change in the magnetic field
surrounding the current, and an instantaneous change in the magnetic field
surrounding a current may in turn act to influence the exact amount of current
which can be carried in an infinitesimally short time period...

Over a very short, but not instantaneous amount of time, these forces balance,
and the current does indeed flow, _but that does not happen in an
instantaneous / infintesimally small - unit of time_...

Fascinating!

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082349872349872
Other nice example of reflection at impedance boundaries are the rainbow
patterns of oil on puddles, or, well, rainbows themselves.

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nudgeee
Also check out the excellent video from Bell Labs: “Similarities of Wave
Behaviour (1959)” [0], which showcases termination, reflection, resonance and
impedance using the Shive Wave Machine.

[0] [https://youtu.be/DovunOxlY1k](https://youtu.be/DovunOxlY1k)

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xellisx
I learned about termination when I had a 10b2 network.

