Why 75 Ohms not 50 Ohms?

[Joe McElvenney]

Hi,

Does anyone know why video connectors have impedance 75 Ohms not
50 or any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?

Usually 75-ohm cable has less capacitance per unit length than
50-ohm and so, at video frequencies where you can't have a match
over the complete range (which with short cables is not so
important anyway), roll-off at the high end will be less
pronounced.


Cheers - Joe

 

[Phil Allison]

"Joe McElvenney"

Usually 75-ohm cable has less capacitance per unit length than
50-ohm and so, at video frequencies where you can't have a match
over the complete range

** Err - why the heck not ???

(which with short cables is not so
important anyway), roll-off at the high end will be less
pronounced.

** There should be none unless the co-ax cable is very long ( eg 1000 feet
+ ) and di-electric losses come into play.




........... Phil

 

[Fred Bloggs]

Does anyone know why video connectors have impedance 75 Ohms not

50 or any other number Ohms?

Yes- lots of people know and it is well-documented.

Why is 75 Ohms better than 50 Ohms for video signals?

Given the basic constraint of coaxial cable geometry with center
conductor, its diameter, and distance to the shield and its type, with
intervening dielectric of some loss factor, it is possible to adjust all
these physical parameters in order to mathematically optimize cable
performance for the various applications. Some applications require high
withstanding voltage, some high power transfer, some signal integrity
across wide bandwidths. The cable geometries that result from these
optimizations turn out to have greatly different impedance levels. The
high power transfer capability solution lands at 30 ohms, the high
voltage at roughly 50, and the low loss, broadband, signal integrity at
75. Since most video applications are comparatively broadband small
signal, requiring good signal fidelity for end performance, the 75 ohm
cable solution is best for them, and that is why it was adopted.
According to experts within the cable industry, Robert Fano's volume 9
of the MIT Radiation Laboratory Series is the best scientific exposition
and documentary available on the topic- and the last word on it actually.

 

[Arny Krueger]

"Paul Hovnanian P.E."


** Sounds quite plausible.

The 75 ohm standard must date back to WW2.

It was pretty mature, even then.

http://lists.contesting.com/archives/html/Towertalk/1999-06/msg00184.html

 

[Arny Krueger]

It was pretty mature, even then.

http://lists.contesting.com/archives/html/Towertalk/1999-06/msg00184.html

http://www.astrosurf.org/lombry/qsl-ham-history7.htm

"On December 8, 1931 Lloyd Espenschied and H.A. Affel from
AT&T received their first patent No. 1,835,031 for their
"concentric conducting system", aka the coaxial cable. Their
invention was not intended for amateur transmissions but
rather for the first television signals that required a line
broadband enough to transmit a range of frequencies
compatible with television image. Espenschied and Affel 's
invention involved placing a central conductor inside a
hollow tube and holding it in place with washers spaced
equally along the length of the tube. The low-loss
dielectric was air."

 

[Phil Allison]

"Arny Krueger"

"Phil Allison"

** Nothing relevant a all.

http://www.astrosurf.org/lombr/qsl-ham-history7.htm

"On December 8, 1931 Lloyd Espenschied and H.A. Affel from AT&T received
their first patent No. 1,835,031 for their "concentric conducting system",
aka the coaxial cable. Their invention was not intended for amateur
transmissions but rather for the first television signals that required a
line broadband enough to transmit a range of frequencies compatible with
television image. Espenschied and Affel 's invention involved placing a
central conductor inside a hollow tube and holding it in place with
washers spaced equally along the length of the tube. The low-loss
dielectric was air."

** No mention of 75 ohms at all - RG58U is 50 ohms.

Yawn - usual pile of useless crap from Arny.





........... Phil

 

[John Fields]

Does anyone know why video connectors have impedance 75 Ohms not
50 or any other number Ohms?

Why is 75 Ohms better than 50 Ohms for video signals?

 

[Pooh Bear]

Better colour rendering 75 ohms is violet-green-brown, while 50 ohms is
only green-black-brown. Violet and green are complementary, so you can
get any colour along that axis by mixing. Brown is low-brightness yellow
(yellow would have given 750 kohms, which is obviously too high), thus
introducing the missing red element.

Or perhaps it's just a historical accident, based on early designs of
coaxial cables in different countries. In Germany, 60 ohms was used.

Does anyone really care ?

Apart from the dBm power level jihadists of course !

Graham

 

[Pooh Bear]

Coaxial cable impedance has nothing to do with radio antennas or
video. Its a matter of engineering economics.

50-ohm cable maximises power handling capability for a given amount of
expensive copper.

Makes sense for feeder cables.

75-ohm cable minimises attenuation for a given amount of expensive
copper.

Makes sense for general signal distribution.

Graham

 

[Tam/WB2TT]

"Tam/WB2TT"

** So what ?

My point is that video is being confused here with RF.




.......... Phil

Man said Video, not RF. These seem to be spec'd at around 1Volt. We also
used 75 Ohm cable for high speed data. Controlling parameter was the about
0.65V needed to turn on a transistor. The ~100 Ohm cable was too fat or too
fragile.

Also, you have to look at this in historical perspective. Of the cable
available in the 1940s, 75 Ohm cable had less loss.

Tam

 

[Phil Allison]

"Tam/WB2TT".

Man said Video, not RF.

** But God said " let there be light " .

These seem to be spec'd at around 1Volt. We also used 75 Ohm cable for
high speed data. Controlling parameter was the about 0.65V needed to turn
on a transistor. The ~100 Ohm cable was too fat or too fragile.

** The knee bone is connected to the thigh bone ......

Also, you have to look at this in historical perspective. Of the cable
available in the 1940s, 75 Ohm cable had less loss.

** Yawn - video does not have to go miles.



.............. Phil

 

[Reg Edwards]

If the source, line, and load are matched, for the same power out of
the source, a higher impedance system will develop a higher voltage
across the load.

John Fields
Professional Circuit Designer

========================================

If that's the best you can do as a professional circuit designer then
you are obtaining money under false pretences.

 

[Michael A. Terrell]

** Yawn - video does not have to go miles.

............. Phil

Gee, Phil, the US had 75 ohm coaxial cables to connect tv stations in
different cities all the way back to the early days there was no other
way to have a live TV feed through a wide region of the country. When
JFK was assassinated, ATT engineers quickly lashed all of the different
networks and segments together for the first live, nationwide feed that
was shown on every TV station that was part of a network. It was live
coverage for days in low resolution B&W because the signal had to go
through a lot more amplifiers than the system was designed for so both
the bandwidth and noise figures suffered. In fact, if it had happened
on one coast, it would have barely made it to the other, due to the
extra connections along the way. It did show that it was possible to
feed wide areas, though. The coaxial cables were still in place after
C-band network feeds were available, in case a transponder or a bird
died. Now, they use fiber optic backups, and to connect some TV
stations to local cable TV systems.

 

[RST Engineering \(jw\)]

I'm not seeing the fallacy in his statement, Reg.

BTW, 50 ohms is NOT the max power handler for a given size of coax. 32
ohms claims this distinction. 50 ohms may have been chosen for a lot of
reasons; the most plausible is that it is a good compromise between highest
power (32 ohms) and lowest loss (75 ohms).

Jim

 

[Paul Hovnanian P.E.]

"Richard Henry"
"Phil Allison"

** What colour are the sox you have on ?

Left or right?

 

[Richard Henry]

Left or right?

You made me look.

Luckily today both are the same color, a sort of linty brownish blue.

 

[John Fields]

========================================

If that's the best you can do as a professional circuit designer then
you are obtaining money under false pretences.

---
Well, let's just see:

Assume a voltage source feeding 1 watt into a 50 ohm coax line
through a 50 ohm resistor and the other end of the line terminated
by a 50 ohm load. Under those conditions each of the resistors will
be dissipating 0.5W and, since:

E²
P = ---
R

we can rearrange to solve for the voltage across the load like this:


E = sqrt(PR) = sqrt (0.5W * 50R) = 5V


Now, if we do the same thing with a 75 ohm setup we'll have:


E = sqrt(PR) = sqrt (0.5W * 75R) ~ 6.124V


Is 6.124V not 1.76 dB > 5V ?

 

[John Woodgate]

I read in sci.electronics.design that Paul Hovnanian P.E.

Who is General Failure

Officer in charge at New Orleans on the Monday.....

and why is he reading my hard drive?

Nothing better to do now.

 

[RST Engineering \(jw\)]

Ah, no. It is like saying that a 2:1 turns ratio transformer has 6 dB of
gain. It does not. For dB to make any sense whatsoever, you should be
working in the same impedance.

Jim

 

[Pooh Bear]

Ah, no. It is like saying that a 2:1 turns ratio transformer has 6 dB of
gain. It does not. For dB to make any sense whatsoever, you should be
working in the same impedance.

The dBu used in audio isn't 'impedance referenced', so it's entirely
legitimate to say a 1:2 transformer has 6dB of voltage gain.

Graham