A Broadband and ADSL forum. BroadbanterBanter

Welcome to BroadbanterBanter.

You are currently viewing as a guest which gives you limited access to view most discussions and other FREE features. By joining our free community you will have access to post topics, communicate privately with other members (PM), respond to polls, upload your own photos and access many other special features. Registration is fast, simple and absolutely free so please, join our community today.

Go Back   Home » BroadbanterBanter forum » Newsgroup Discussions » uk.telecom.broadband (UK broadband)
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

uk.telecom.broadband (UK broadband) (uk.telecom.broadband) Discussion of broadband services, technology and equipment as provided in the UK. Discussions of specific services based on ADSL, cable modems or other broadband technology are also on-topic. Advertising is not allowed.

A short tutorial on characteristic impedance.



 
 
Thread Tools Display Modes
  #1  
Old June 21st 14, 07:37 PM posted to uk.telecom.broadband
The Natural Philosopher
external usenet poster
 
Posts: 2,728
Default A short tutorial on characteristic impedance.

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-toc'-ra-cy) - 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.

  #2  
Old June 21st 14, 08:08 PM posted to uk.telecom.broadband
Peter Able
external usenet poster
 
Posts: 114
Default A short tutorial on characteristic impedance.

On 21/06/2014 19:37, The Natural Philosopher wrote:
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.


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.

--

PA


  #3  
Old June 21st 14, 08:23 PM posted to uk.telecom.broadband
Graham.
external usenet poster
 
Posts: 216
Default A short tutorial on characteristic impedance.

On Sat, 21 Jun 2014 20:08:01 +0100, Peter Able [email protected] wrote:

On 21/06/2014 19:37, The Natural Philosopher wrote:
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.


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.





Date: Sat, 14 Jun 2014 18:22:50 +0100
From: Vir Campestris
Newsgroups: uk.telecom.broadband
Subject: Phone cable and networking

--

Graham.

%Profound_observation%
  #4  
Old June 22nd 14, 11:01 AM posted to uk.telecom.broadband
The Natural Philosopher
external usenet poster
 
Posts: 2,728
Default A short tutorial on characteristic impedance.

On 21/06/14 21:35, Dave wrote:
The Natural Philosopher wrote:

What is characteristic impedance?

When talking about faulty phone lines, characteristic impedance is what you
encounter from BT when you try to report a fault in their network that you
want fixed.

|



--
Ineptocracy

(in-ep-toc'-ra-cy) - 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.

  #5  
Old June 22nd 14, 05:23 PM posted to uk.telecom.broadband
grinch
external usenet poster
 
Posts: 68
Default A short tutorial on characteristic impedance.

On 22/06/14 11:01, The Natural Philosopher wrote:
On 21/06/14 21:35, Dave wrote:
The Natural Philosopher wrote:

What is characteristic impedance?

When talking about faulty phone lines, characteristic impedance is
what you
encounter from BT when you try to report a fault in their network that
you
want fixed.

|



Surely that's characteristic impudence
  #6  
Old June 22nd 14, 05:33 PM posted to uk.telecom.broadband
grinch
external usenet poster
 
Posts: 68
Default A short tutorial on characteristic impedance. OT

On 22/06/14 11:01, The Natural Philosopher wrote:
On 21/06/14 21:35, Dave wrote:
The Natural Philosopher wrote:

What is characteristic impedance?

When talking about faulty phone lines, characteristic impedance is
what you
encounter from BT when you try to report a fault in their network that
you
want fixed.

|



from wiki here is a list of facts and figures to keeps us all happy

http://en.wikipedia.org/wiki/List_of_device_bandwidths
  #7  
Old June 22nd 14, 08:44 PM posted to uk.telecom.broadband
Peter Able
external usenet poster
 
Posts: 114
Default A short tutorial on characteristic impedance.

On 21/06/2014 20:23, Graham. wrote:
On Sat, 21 Jun 2014 20:08:01 +0100, Peter Able [email protected] wrote:

On 21/06/2014 19:37, The Natural Philosopher wrote:
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.


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.





Date: Sat, 14 Jun 2014 18:22:50 +0100
From: Vir Campestris
Newsgroups: uk.telecom.broadband
Subject: Phone cable and networking


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.

--

PA
  #8  
Old June 22nd 14, 10:11 PM posted to uk.telecom.broadband
Mike Tomlinson
external usenet poster
 
Posts: 285
Default A short tutorial on characteristic impedance.

En el artículo , Dave
escribió:

characteristic impedance is what you
encounter from BT when you try to report a fault in their network that you
want fixed


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

--
(\_/)
(='.'=)
(")_(")
  #9  
Old June 23rd 14, 07:19 PM posted to uk.telecom.broadband
Jim
external usenet poster
 
Posts: 210
Default A short tutorial on characteristic impedance.

On Sun, 22 Jun 2014 20:44:22 +0100, Peter Able [email protected] wrote:

Thanks Graham. I tend to skip OPs with such a respect for grammar,
syntax and logic.


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

--
:: Jim,

Wessex
  #10  
Old June 29th 14, 11:52 AM posted to uk.telecom.broadband
Chris Kirby
external usenet poster
 
Posts: 5
Default A short tutorial on characteristic impedance.

brightside S9 wrote:
Mike Tomlinson wrote:

Dave escribió:

characteristic impedance is what you
encounter from BT when you try to report a fault in their network that you
want fixed


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


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


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

--
Chris

 




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
SPA-3102 FXS Port Impedance Bob Brewer uk.telecom.voip (UK VOIP) 3 July 11th 09 04:16 PM
VOIP Tutorial Fred Atkinson uk.telecom.voip (UK VOIP) 6 April 8th 06 10:46 PM
ADSL Line impedance (was DSL lead pinout) 317 uk.telecom.broadband (UK broadband) 4 June 18th 05 11:42 PM
Tutorial for Kerio 2.1.5 ? lubinok uk.telecom.broadband (UK broadband) 2 May 17th 04 10:21 PM
Tutorial for Kerio 2.1.5 ? Nick H uk.telecom.broadband (UK broadband) 9 May 17th 04 09:21 PM


All times are GMT +1. The time now is 01:16 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.Content Relevant URLs by vBSEO 2.4.0
Copyright ©2004-2019 BroadbanterBanter.
The comments are property of their posters.