What limits digital data throughput in a wire?

[Eric R Snow]

I've been thinking about this but haven't come up with a satisfactory
answer. If a voltage pulse is sent down a wire does it get
lengethened? E.G. if a one picosecond pulse is sent down a wire will
it be longer at the recieving end? Is this why digital information
can't be sent in unlimited quantities? Or is it because the pulses are
in essence half wave ac? So the current eventually travels on the
outside of the wire? Just curious.
Thanks,
Eric

 

[John Larkin]

On Thu, 15 Jan 2004 08:49:46 -0800, Eric R Snow <etpm@whidbey.com>
wrote:

I've been thinking about this but haven't come up with a satisfactory
answer. If a voltage pulse is sent down a wire does it get
lengethened? E.G. if a one picosecond pulse is sent down a wire will
it be longer at the recieving end?

Yes. Any real wire or coaxial cable has resistive and maybe dielectric
and non-TEM mode losses. This causes amplitude loss, dispersion, and
ultimately, for long runs, Johnson noise that buries the signal.

Dispersion results in different frequency components traveling at
different velocities, which spreads out fast pulses and makes adjacent
pulses start to overlap, which is called "intersymbol interferance".

A true 1 ps pulse can't be propagated even a foot on high-quality coax
without severe loss and distortion. HP and Tek could make 200 GHz
sampling scopes, but anything above the current 70 GHz or so is mostly
useless, as you can't get a signal that fast to the scope over a
reasonable length of cable.

Fiber optics is *much* better, especially for long runs.

John

 

[Roy McCammon]

Signal to noise ratio and bandwidth are the ultimate limits.


Eric R Snow wrote:

I've been thinking about this but haven't come up with a satisfactory
answer. If a voltage pulse is sent down a wire does it get
lengethened?

Yes, but that is a linear effect and can be equalized out.
Its called dispersion.

E.G. if a one picosecond pulse is sent down a wire will
it be longer at the recieving end? Is this why digital information
can't be sent in unlimited quantities? Or is it because the pulses are
in essence half wave ac?

In long haul communication it is typical to use a protocol
that has a net 0 dc component.


--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

 

[Watson A.Name - "Watt Sun]

In article <26pd0091avl775rk2mjj425b3r88u5es00@4ax.com>,
jjlarkin@highSNIPlandTHIStechPLEASEnology.com mentioned...

On Thu, 15 Jan 2004 08:49:46 -0800, Eric R Snow <etpm@whidbey.com
wrote:

I've been thinking about this but haven't come up with a satisfactory
answer. If a voltage pulse is sent down a wire does it get
lengethened? E.G. if a one picosecond pulse is sent down a wire will
it be longer at the recieving end?

Yes. Any real wire or coaxial cable has resistive and maybe dielectric
and non-TEM mode losses. This causes amplitude loss, dispersion, and
ultimately, for long runs, Johnson noise that buries the signal.

Dispersion results in different frequency components traveling at
different velocities, which spreads out fast pulses and makes adjacent
pulses start to overlap, which is called "intersymbol interferance".

Another factor is crosstalk, in the case of Cat5 cable it's both NEXT
and FEXT. Right now, most PCs are sold with Gigiabit Ethernet cards
which send 1 gigabit per second down a 100 meter or 328 foot cable.
They are trying to up that to ten GB, maybe someday. But like he says
below, the solution will probably be fiber to everyones' desktop.

A true 1 ps pulse can't be propagated even a foot on high-quality coax
without severe loss and distortion. HP and Tek could make 200 GHz
sampling scopes, but anything above the current 70 GHz or so is mostly
useless, as you can't get a signal that fast to the scope over a
reasonable length of cable.

Fiber optics is *much* better, especially for long runs.

Well, that's why they use single-mode fiber. ;-)

John

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