In article ,
glen herrmannsfeldt wrote:

JohnO wrote:

"philo" wrote in message
(snip)

VGA cable is shielded
and CAT5 is not, so you would get some horrible ghosting.

BTW, there may still be some ghosting even with VGA cable

STP CAT5e or CAT 6 would do the trick, and at these
lengths the cost difference isn't an issue.

There might be a minimum amount that they will sell, but the
real problem isn't shielding but impedance and
balanced/unbalanced line.

If you transition from a balanced line (UTP) to an unbalanced
line (coax), unless exactly impedance matched, it won't
work right. UTP cable depends on the voltage and currents
on the two wires being exactly opposite to cancel out and
not radiate the signal. Coax depends on the voltage on the
shield being zero. To couple between them you either need
a transformer (if there is no DC component), or active
circuitry such as differential amplifiers. VGA has a
DC component so you can't use transformers.

-- glen

You can use a transformer, but wired as a *balun* (coils in series with
the lines instead of across them); this provides the required impedance
transformation while still passing DC.


--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Rich Seifert wrote:
(snip)

You can use a transformer, but wired as a *balun* (coils in series with
the lines instead of across them); this provides the required impedance
transformation while still passing DC.

So that is how they do it.

TV baluns, from 300 ohm balanced to 75 ohm coax, don't do that.

http://en.wikipedia.org/wiki/Balun

Does that restrict which impedance transformation you can make?

-- glen

In article ,
glen herrmannsfeldt wrote:

Rich Seifert wrote:
(snip)

You can use a transformer, but wired as a *balun* (coils in series with
the lines instead of across them); this provides the required impedance
transformation while still passing DC.

So that is how they do it.

TV baluns, from 300 ohm balanced to 75 ohm coax, don't do that.

http://en.wikipedia.org/wiki/Balun

Does that restrict which impedance transformation you can make?


The impedance transformation is determined by the winding ratio,
regardless of whether the coils are in series or parallel with the
lines.

Putting the balun in series provides better low-frequency response (down
to DC), but of course this configuration does not provide any electrical
isolation. It functions very much like a common-mode choke, with an
impedance change.


--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX

Send replies to: usenet at richseifert dot com