C
Cliff
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?
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
shu said:
Phil Allison said:"Floyd L. Davidson"
** A pair of parallel wires that are is not twisted is simply a long loop.
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.
The two conductors in a balanced microphone cable are twisted quite tightly
for this reason.
** 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.
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.
** 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.
"Phil Allison"
It is a transmission line.
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.
Typically they are not twisted quite tightly, compared to
typical twisted pair used in the HF range.
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.
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...
The coaxial cable offers nearly no immunity from power line and other low
frequency magnetic fields, the shield is not thick enough.
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
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.
i'm doubtfull you would notice relfections because it's
just too damn long..
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
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.
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
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.
Phil said:"Fred Bloggs"
** That is both false and silly.
now now boys, stop scratching each other! other wisePhil said:"Fred Bloggs"
** That is both false and silly.
....... Phil
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
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. i'm doubtfull you would notice relfections because it's
just too damn long.. 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
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.
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
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.
Phil Allison said:"Fred Bloggs"
** That is both false and silly.
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,
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 ...
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 ...
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.
"Phil Allison"
Hilarious.
Any good text on coax cable will explain if for you.