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Re: Can twisted wire replace shielded wire?

C

Cliff

Jan 1, 1970
0
the Speed of the transmission is signficantly less then the speed of light,
and can be calculated out by knowing the dielectric constant of the
dielectric

Oh? What about the speed of the field
conductor?
 
T

Terry Given

Jan 1, 1970
0
shu said:

if you re-do his test with a piece of what you consider to be "proper"
coax cable (eg RG174) you will see it behaves much the same as the RCA
cable. Actually I would expect to see less noise than the RCA, as the
braid on "proper" coax has pretty good coverage.

STP would give similar results to the RCA cable too.

If only he knew what he was actually measuring....


Cheers
Terry
 
F

Floyd L. Davidson

Jan 1, 1970
0
Phil Allison said:
"Floyd L. Davidson"

** A pair of parallel wires that are is not twisted is simply a long loop.

It is a transmission line. Just the same as a twisted pair.
Open wire, non-twisted, transmission lines are commonly used. TV
twin lead is of course one variation.
Any nearby alternating magnetic filed (ie like AC hum) will inject a small
voltage into that loop. If the wires are spread apart, the loop area
increases and so too does the injected voltage level.

Also, the phase of an injected voltage in a loop reverses if the loop is
rotated through 180 degrees - same as reversing the two connections at the
receiving amp end.

Now, if half a loop is rotated through 180 degrees and the other half is
not, then voltages induced in each half are in antiphase. If equal in
magnitude they will cancel each other.

So, by having many twists in a pair of signal carrying wires one maximises
the chance of cancellation in the general case - ie it *prevents* the
pair of wires acting like a loop receiver.

That is fascinating, but is not the way that twisted pair
transmission line works. A pair of parallel wires would have
*exactly* the same conditions. That is simply because the
distance between the centers of the conductors is very short
compared to a wavelength.
The two conductors in a balanced microphone cable are twisted quite tightly
for this reason.

Typically they are not twisted quite tightly, compared to
typical twisted pair used in the HF range.
** True - but the surprising thing about co-axial cable is that it does
NOT act like a loop.

Why is that surprising? Of course if you make the distance
between the center conductor and the outer conductor large
enough... just as with parallel wires it no longer acts like
a transmission line.
A co-axial cable suffers ( ideally) no voltage injection from a nearby
alternating magnetic field.

Since the outer conductor shields against external electric fields entering
the cable co-ax is largely immune from EM interference - making it ideal
for many wide band applications, 75 ohm video being one.

Ahem... you were doing fine until that last line. It is not
particularly ideal for wide band applications as opposed to
parallel conductor transmission line, which of course is even
better as far as wide band goes...
 
F

Fred Bloggs

Jan 1, 1970
0
** True - but the surprising thing about co-axial cable is that it does
NOT act like a loop.

A co-axial cable suffers ( ideally) no voltage injection from a nearby
alternating magnetic field.

The coaxial cable offers nearly no immunity from power line and other
low frequency magnetic fields, the shield is not thick enough.
 
P

Phil Allison

Jan 1, 1970
0
"Floyd L. Davidson
"Phil Allison"

It is a transmission line.


** It is also a loop.

The fact that is is a loop means it can have a voltage induced into it.

That is fascinating, but is not the way that twisted pair
transmission line works.


** It is exactly how a twisted pair line works to reject injection.

You are wrong.

A pair of parallel wires would have
*exactly* the same conditions.


** That is completely wrong.

That is simply because the
distance between the centers of the conductors is very short
compared to a wavelength.


** Absolute nonsense.

Magnetic field noise injection is a function of loop area and the field
intensity flowing through the loop at any point.

Typically they are not twisted quite tightly, compared to
typical twisted pair used in the HF range.


** Yawn.

Why is that surprising?


** Surprises most folk.

Of course if you make the distance
between the center conductor and the outer conductor large
enough... just as with parallel wires it no longer acts like
a transmission line.


** Yawn - cut it up with an axe and it stops doing that to.


Ahem... you were doing fine until that last line.


** You have yet to get even one thing correct - dickhead.


It is not
particularly ideal for wide band applications as opposed to
parallel conductor transmission line, which of course is even
better as far as wide band goes...


** Seeing as co-ax operates near ideally from DC up to GHz you are wrong
again.




......... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Fred Bloggs"
The coaxial cable offers nearly no immunity from power line and other low
frequency magnetic fields, the shield is not thick enough.


** That is both false and silly.





........ Phil
 
J

J. Clarke

Jan 1, 1970
0
shu said:
Read what i said again
Ideally you *want* the impedance to be as LOW as possible
i didn't say the Impedance WAS low relative to coax, nor did i imply
that...

A twisted pair typically has an impedance of 100-200 Ohms yes
the impedance on longer cables is there Simply because the cables are
Longer this isn't a GOOD thing.. you WANT it to be as LOW as possible

No, you want the impedence to be matched to the load, otherwise you get
reflections. You're confusing impedence with resistance I think.
are you channeling Cliff?






no no no no you're ignorant
here
http://www.westpenn-cdt.com/pdfs/T176o177.pdf
this is how Every single Coax cable iv'e seen is made
and RCA cable. does NOT have the dielectric
and the outer conductor is NOT the same as the outer shielding
the Outer shield is braided to spefically lessen outside interference,
it is entirly irrelevent that the outerconductor touches the shielding
the outer conductor however is NOT braided. it is designed to react to the
pulses between the inner conductor and the dielectric (read Capacitor) AS
well as aid in shielding.. YES it can do both.
because of this interaction it allows the Pulses to transmit further with
less impedance then a simple wire
the Speed of the transmission is signficantly less then the speed of
light, and can be calculated out by knowing the dielectric constant of the
dielectric
because of this if the impedances dont' match.. you can get Echos
(reflections) and actually notice them, on a VERY long cable.. like this
mile length cable.

That's not a "very long cable". TPC-1 is a "very long cable".
i'm doubtfull you would notice relfections because it's
just too damn long..

Tell that to AT&T.
it require TWO bounces to notice reflected images,...
in a real long cable. some of the signal will be reflected back, . and
then reflected again at the other end, but by that time it's too out of
phase, and too weak to produce much in the way of ghosting. instead it
looks like a simple signal loss. you can however have Very noticable
ghosting on shorter cables if the impedance is mismatched

Geez, you've sure got _that_ hashed up.
the Insulator in a SIMPLE rca type cable is no where near the quality of
the dielectric compounds used in coax cables, and hence has little if any
effect, and also the Shielded part of the RCA cable does NOT behave like
the outer conductor in a coax, . the shield is again braided and absorbs
signals if there is an outerconductor it's simply there to help shield, as
you want a Foil type shield and a braided shield, but again it doesnt'
beheave anyway
like a coax because of the lack of a decent dielectric
in short.. RCA cables are absolutely not like Coax cables.. you're just
wrong.

There is no such thing as an "RCA Cable" as distinct from a coaxial cable.
You can put RCA connectors on non-coaxial cables or on coaxial, depending
on the application.
coax cables are spefically designed for long range transmissions with
miniumal loss
by long.. i mean Longer then a simple insulated , or simple shielded wire
can transmit

Actually, coaxial cable is designed for the most part for RF applications.
For long range communication fiber is used today.
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Floyd L. Davidson said:
No that is not what eliminates noise and that is not what
"balance" means.

Both ends are _balanced_ _to_ _ground_, and each receiver has a
"differential input" circuit. Which is to say the input is the
voltage *between* the two wires, rather than the voltage between
ground and the wire. Almost all noise is between the wire and
ground and if the two wires are really well balanced there will
be no differential voltage due to noise, and hence no noise
voltage will be seen by the input. That is called "common mode
rejection". (Do a web search on it.)

The "balance" of the two wires is improved vastly by twisting
them. That assures that any electrical field which induces a
voltage into one of them will induce and equal voltage into the
other one. (If the wires are not twisted the one nearest to a
noise source will have a higher voltage.) The higher the
frequency, the more twist required to be effective.

Twisting wires helps reduce noise induced from ambient H fields. If you
have a high ambient E field, you can still get common mode noise coupled
to a bare twisted pair. Particularly on a high impedance circuit. This
becomes a problem at low signal levels, where a CMRR sufficient to
reject the noise may be impractical. Low signal level circuits are where
one finds high impedances as well. In this case, carefully designed
shielding can help.
 
J

Jamie

Jan 1, 1970
0
Phil said:
"Fred Bloggs"




** That is both false and silly.





....... Phil
now now boys, stop scratching each other! other wise
you will both have to come in!.
 
F

Floyd L. Davidson

Jan 1, 1970
0
shu said:
Read what i said again
Ideally you *want* the impedance to be as LOW as possible
i didn't say the Impedance WAS low relative to coax, nor did i imply that...

As I pointed out it could be much lower than it commonly is;
hence "as LOW as possible" is not true.
A twisted pair typically has an impedance of 100-200 Ohms yes
the impedance on longer cables is there Simply because the cables are Longer
this isn't a GOOD thing.. you WANT it to be as LOW as possible

And other twisted pair typically has an impedance of 900 or more
Ohms, *by* *design*. It isn't simply because the cable is long.
Indeed, a short length of the same cable has the same impedance
as a long length does. Rather the impedance is designed to be
higher because the cable runs will be longer and since the higher
impedance cable will have lower losses it is more suitable.

Read that paragraph again, and compare it to the URL you cite below.

I would suggest that you find a basic text on transmission line
theory. You are looking at perfectly good references, and simply
not understanding what they are saying. For example, as I said

*any* insulator is a dielectric... and *any* "shield" is in
fact the outer conductor of a coax cable.

There is nothing in your cited reference which suggests anything
different. At least not to anyone who understands the theory of
how coax cables actually work. I'm not going to try explaining
how the outer conductor (the shield) functions, because it is
quite complex. You should do some proper research and learn
about it though! On the other hand your misunderstanding of
what is a dielectric and what is an insulator is fairly easy to
clear up, because it is simply a matter of definitions.
this is how Every single Coax cable iv'e seen is made
and RCA cable. does NOT have the dielectric

Of course they have a dielectric.

dielectric:

1. A substance in which an electric field may be
maintained with zero or near-zero power dissipation,
i.e. , the electrical conductivity is zero or near
zero.

Note 1: A dielectric material is an electrical
insulator.

Note 2: In a dielectric, electrons are bound to atoms
and molecules, hence there are few free
electrons.

2. Pertaining to a substance that has a zero or near
zero electrical conductivity.

That is a direct quote from http://www.its.bldrdoc.gov/fs-1037/,
which is the Telecommunications: Glossary of Telecommunication
Terms, Federal Standard 1037C.

"Insulator" and "dielectric" are two terms for the same thing.
and the outer conductor is NOT the same as the outer shielding

It is indeed *precisely* the same physical entity.
the Outer shield is braided to spefically lessen outside interference,
it is entirly irrelevent that the outerconductor touches the shielding
the outer conductor however is NOT braided.

It may not be braided in the particular picture you looked at,
but you really should learn to do more than look at the
pictures. Reading the text you'll find that it describes,

There are basically three types of shielding techniques:
1. Foil Shielding,
2. Braid Shielding,
3. Combination Shielding

The picture of course is of coax constructed with technique 3,
combination shielding. Note that either the foil or the braid
could be removed. They are *both* referred to as "shielding".
That particular document does not use the terminology "outer
conductor", and *all* types of outer conductors are simply
referred to as a "shield".

As with the "insulator" vs. "dielectric", the shield *is* the
outer conductor.
it is designed to react to the
pulses between the inner conductor and the dielectric (read Capacitor) AS

If you are saying that dielectric is synonymous with
"capacitor", you are wrong. However, by definition a capacitor
*includes* an insulator (the dielectric material between the
plates).
well as aid in shielding.. YES it can do both.
because of this interaction it allows the Pulses to transmit further with
less impedance then a simple wire
the Speed of the transmission is signficantly less then the speed of light,
and can be calculated out by knowing the dielectric constant of the
dielectric
because of this if the impedances dont' match.. you can get Echos
(reflections) and actually notice them, on a VERY long cable.. like this

Are you saying the these reflections don't exist, or can't be
measured, on anything but "a VERY long cable"? What is your
definition of "VERY long"?
mile length cable. i'm doubtfull you would notice relfections because it's
just too damn long.. it require TWO bounces to notice reflected images,...

If you are saying that a "mile length" is a VERY long cable...
I'll point out that it depends on the frequency. And I don't
quite catch what you mean by requiring "TWO bounces".

Actually the impedance mismatch has an effect at each end of the
cable. At the load end there is less power transfered to the
load than would otherwise be expected. At the signal end, the
cable accepts less power than would otherwise be expected.

In the first case some power never does go down the cable, and
if it does exist it necessarily must be dissipated in something
other than the cable or the load at its distant end. In the
second case the power goes down the transmission line, is not
absorbed, and is reflected back to the source.

That usually means that either the signal generator 1) does not
produce as much power as it could, and/or 2) that power is
dissipated as heat in the device feeding the transmission line.
(Which is to say that the efficiency of the device is reduced.)
in a real long cable. some of the signal will be reflected back, . and then

That is *just* as true in a short cable. The only difference is
that some of the power will be absorbed by the long cable (cable
loss) and will not all end up being dissipated in the source
device.

The significance of the above is that if you do have a 1 mile
long cable, and feed a 100 MHz signal into it, it will make *no*
difference what the load at the end of the cable is! Almost no
power will reach the distant end anyway, and by the time it is
reflected back the loss will be so high that it is insignificant
(for example, if the cable loss is greater than 10 dB, giving 20
dB of loss for a round trip, the VSWR will be something like
less than 1.05 to 1, which of course is very good termination,
even if there is actually no load at all at the distant cable
end).

Of course with a short cable it *will* make a difference...
reflected again at the other end, but by that time it's too out of phase,

It may or may not be out of phase. That is a function of
frequency, distance, and velocity factor. At some points it
will be out of phase, and at others it will be in phase.
and too weak to produce much in the way of ghosting. instead it looks like a
simple signal loss. you can however have Very noticable ghosting on shorter
cables if the impedance is mismatched

Which is to say that all of the things you just said about LONG
cables have nothing to do with it being long or short.
the Insulator in a SIMPLE rca type cable is no where near the quality of the
dielectric compounds used in coax cables, and hence has little if any
effect,

The lower quality dielectric used in "SIMPLE rca type cable"
will have *more* effect. Of course some cables that use RCA
connectors are of equal quality to the best coax cables (because
they do in fact use the best coax cables).
and also the Shielded part of the RCA cable does NOT behave like the
outer conductor in a coax, .

Please find a good textbook on transmission line theory. That
is absolutely not true. A two conductor transmission line using
concentric conductors *is* a coaxial transmission line.
the shield is again braided and absorbs signals
if there is an outerconductor it's simply there to help shield, as you want
a Foil type shield and a braided shield, but again it doesnt' beheave anyway
like a coax because of the lack of a decent dielectric
in short.. RCA cables are absolutely not like Coax cables.. you're just
wrong.

"RCA cables" are virtually *all* made using coax.
coax cables are spefically designed for long range transmissions with
miniumal loss
by long.. i mean Longer then a simple insulated , or simple shielded wire
can transmit

Coax has higher losses per unit of distance than do parallel
wire transmission lines. Which is to say that coax cables are
*not* specifically "designed for long range transmission with
minimal loss". Generally the loss is far to high for anything
that would be called "long range transmission".
 
F

Floyd L. Davidson

Jan 1, 1970
0
Phil Allison said:
"Floyd L. Davidson

** You have yet to get even one thing correct - dickhead.


** Seeing as co-ax operates near ideally from DC up to GHz you are wrong
again.

Hilarious. Do you know what amplitude distortion is?
 
F

Floyd L. Davidson

Jan 1, 1970
0
Phil Allison said:
"Fred Bloggs"

** That is both false and silly.

Any good text on coax cable will explain if for you. In particular
the typical foil shielded cable is lacking. Copper braid works much
better, but is not effective. Typicall when that is significant the
cable is installed in a steel conduit.
 
J

Jasen Betts

Jan 1, 1970
0
["Followup-To:" header set to sci.electronics.basics.]
if your twisted pair video system worked really well, then it was because
both ends were balanced (quite possible) , or you dont' have a lot of EM
noise around,

Cat 5E (network cable) is cheap, special video baluns are available
off the shelf for putting composite video through it.

with 4 pairs, one can carry video, one audio, one audio back the other way,
and one power for the camera etc...

Tone, or DC, signalling on the audio return, can be used to
operate a latch or boom gate etc...


Bye.
Jasen
 
C

Cliff

Jan 1, 1970
0
In particular
the typical foil shielded cable is lacking. Copper braid works much
better, but is not effective.

The length of a conductor in the braided shield is quite long
between two points while it may be much shorter with
a foil shield.
Considering the speed of the electrical field in
the conductor ..... and the distances involved ...
but the center conductor is shorter than the conductors in
the braid ...
 
G

Gary

Jan 1, 1970
0
Cliff wrote:


The length of a conductor in the braided shield is quite long
between two points while it may be much shorter with
a foil shield.
Considering the speed of the electrical field in
the conductor ..... and the distances involved ...
but the center conductor is shorter than the conductors in
the braid ...


Good god a'mighty, which comic book did you read that piece of crap in.
You have just written the most compacted wad of horseshit I've ever
read with respect to transmission lines. Get a book, learn some theory
concerning (cables, open wire or co-axial etc.). Then, take a stab at
explaining things.

--
Regards,
Gary

I know not with what weapons World War III will be fought,
but World War IV will be fought with sticks and stones.

Albert Einstein
 
F

Floyd L. Davidson

Jan 1, 1970
0
Cliff said:
The length of a conductor in the braided shield is quite long
between two points while it may be much shorter with
a foil shield.
Considering the speed of the electrical field in
the conductor ..... and the distances involved ...
but the center conductor is shorter than the conductors in
the braid ...

That is not true.
 
J

Jasen Betts

Jan 1, 1970
0
["Followup-To:" header set to sci.electronics.basics.]
Twisting wires helps reduce noise induced from ambient H fields. If you
have a high ambient E field, you can still get common mode noise coupled
to a bare twisted pair. Particularly on a high impedance circuit. This
becomes a problem at low signal levels, where a CMRR sufficient to
reject the noise may be impractical. Low signal level circuits are where
one finds high impedances as well. In this case, carefully designed
shielding can help.

transformers typically have very a high CMRR. this may be whay they are used
in network cards.


taken from an old 10Mb/s card.

1nF
2KV
_______ || ______________\
_) || (_ /
from _) || (____||___ to RJ12
driver _) || (_ || | socket
chip_____) || (_________|____\
|| | /
___L__
/////

I think the capacitor is to reduce the amount of CM noise
transferred capacitatively through the transformer.

AIUI transformers are also often used for balanced microphone leads.
 
P

Phil Allison

Jan 1, 1970
0
"Floyd L. Davidson"
"Phil Allison"

Hilarious.



** You are nothing but a lying, posturing fool - Davidson.

The moose in Alaska can out think a bullshit artist of your ilk.




......... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Floyd L. Davidson"
Any good text on coax cable will explain if for you.


** The fact that a co-ax cable ( like RG58) is insensitive to low frequency
magnetic fields readily demonstrated.

At least 40dB less sensitive than a non twisted pair ( figure 8) cable.

As theory predicts.



......... Phil
 
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