# A short tutorial on characteristic impedance.

Discussion in 'Broadband' started by The Natural Philosopher, Jun 21, 2014.

1. ### The Natural PhilosopherGuest

What is characteristic impedance?

Its the INSTANTANEOUS impedance a cable looks like to a generator.
Before the signal;has had a chance to travel to the other end ( and
possibly back).

Does it vary with frequency?

No. Not theoretically, and very little in practice.

How is it calculated?

By a deeply dull calculation comprising Henrys per meter and Farads per
meter. Since these do not vary with frequency, you can see the
characteristic impedance is in fact constant with frequency. The more
inductance per meter the higher the impedance, the more capacititance
per meter the lower the impedance.

Why is it important?

Only really in one way at all. If you match a cable to the correct
terminating resistance the same as its characteristic impedance the
cable wont reflect signals from its ends, and so it won't RESONATE at
any particular frequency. Apart from that it has *absolutely no meaning
or use at all*.

So what does '600 ohm audio impedance' mean?

Almost **** all. Way back when it was discovered that running cables for
audio at high terminating impedance was a recipe for 'snap, crackle and
pop' and 600 ohms was settled arbitrarily as the 'sort of impedance to
terminate an long audio cable at' (or a long telephone one).

Outside of massively long copper lines, the reflections back from the
far end are utterly irrelevant.

the speed of light = 3.33564095 nanosecond per metre and its not a lot
less in a cable. So to get a delay of - say - 50us which might make a
difference above 10Khz, requires a cable length of around 8,000 meters.
(16m there and back)

So if you WERE running a shielded twisted pair to carry hifi audio 8km
you WOULD need to match to the cables characteristic impedance to get
full audio.

The characteristic impedance of most common cables you will find range
from 50 ohm air spaced dielectric coax, through 75ohm antenna coax
through twisted pair in the 100-200 range, and 300 ohm 'antenna ribbon'
cable. No cable I know of has a 600 ohm impedance.

Please, chaps, don't 'nod knowledgeably' if you do NOT understand the
engineering and physics.

Telephone cables are around 200 ohms IIRC and should be terminated at
that fir RF (DSL) use. On the audio side of the splitter/filter it
doesn't matter a damn.

High resistant joints make a BIG difference to these 'perfect twisted
pairs' and the Openreach engineers carry reflectometers than can
identify high impedance joints.

Yes SOME RF will jump an open circuit, BUT there will be horrible nulls
in the frequency response and you will lose a lot of bandwidth. High
resistant joints have the same effect putting ripples in the frequency
response leading to loss of some DSL frequencies.

--
Ineptocracy

(in-ep-tocâ€™-ra-cy) â€“ a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

The Natural Philosopher, Jun 21, 2014

2. ### Peter AbleGuest

Typos apart, I can't argue with that - but I've missed what has driven
you to this cri de coeur ;-}}

BTW, 600 ohm twin is not uncommon as an RF transmission line.

Peter Able, Jun 21, 2014

3. ### Graham.Guest

Date: Sat, 14 Jun 2014 18:22:50 +0100
From: Vir Campestris <>
Subject: Phone cable and networking

Graham., Jun 21, 2014
4. ### The Natural PhilosopherGuest

|

--
Ineptocracy

(in-ep-tocâ€™-ra-cy) â€“ a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

The Natural Philosopher, Jun 22, 2014
5. ### grinchGuest

Surely that's characteristic impudence

grinch, Jun 22, 2014
6. ### grinchGuest

grinch, Jun 22, 2014
7. ### Peter AbleGuest

Thanks Graham. I tend to skip OPs with such a respect for grammar,
syntax and logic. Persisting through to the 18th June though, I got to
the nub - and now all is revealed. A cri-de-coeur fully justified.

Peter Able, Jun 22, 2014
8. ### Mike TomlinsonGuest

and inductance is when you offer tea and chocky biscuits to the engineer
as an inducement to do a decent job

Mike Tomlinson, Jun 22, 2014
9. ### brightside S9Guest

and resistance is what the Talk Talk dor to door cold calling salesman
mmets.

brightside S9, Jun 23, 2014
10. ### JimGuest

Not to mention those who don't know the difference
between meter and metre.

Jim, Jun 23, 2014
11. ### Chris KirbyGuest

and admittance is when you let him/her into the house.

Chris Kirby, Jun 29, 2014
12. ### Bob EagerGuest

and conductance is when you usher them out again.

Bob Eager, Jun 29, 2014
13. ### MikeGuest

Only after a lot of coercivity!

A LOT!

Mike, Jun 29, 2014
14. ### 198kHzGuest

and reactance is his 'thank you very much', and capacitance is the
quantity of victuals consumed.

198kHz, Jul 6, 2014