[Scott Dorsey]

In article ,
wrote:
Hello Everyone,

I am looking for a Greybeard of sorts. I have recently been thrown
into the audio realm, particularly testing with semiconductor PA's,
and I am curious to know where the 600 ohm impedance originated from.

If you have open-wire transmission lines with two 18 ga. wires about
five inches apart on the telephone pole, you have a line with a 600
ohm characteristic impedance. This was the standard telephone circuit
well into the 1920s, and as a result the phone company adopted 600 ohm
lines and termination for almost everything.

A sidelight: 20 ga twisted pair with thick cotton insulation tends to
be around 150 ohms characteristic, so the phone company also used that
as a standard, starting in the teens. For many years, CBS Radio used
150 ohms as their transmission line standards, so their equipment would
not interoperate with the rest of the industry without adding more
matching transformers. A lot of gear still had 150 ohm taps well into
the seventies.

For example, most testing I have done is with 4 ohm to 8 ohms with
PA's and 16 ohms or 32 ohms with headphones for portable audio
(computing, MP3, cell phone) and there is generally no need for
impedance matching.

Right, in the modern world almost everything has a high-Z input and a
low-Z output, and you don't care about the cable characteristic impedance
unless you are running cables for tens of miles (as the telcos do).

I have managed to piece together some basic information from multiple
Google searches that 600 ohms originated from the POTS and was adopted
by the pro audio crowd decades ago, but I would like some more
'historical' information of when, why, and how.
What prompted this question is that another group uses an HP 8903B
which has either a 50 ohm or 600 ohm impedance to test audio analog
CMOS switches and 600 ohms is selected for THD+N measurements.
The philosophy of the impedance difference intrigued me and thus has
lead me on a search to understand where the 600 ohms standard came
from and why some equipment only has this option.
Any tips, notes, or thoughts will be greatly appreciated.

You want goofy, look up where the 50 and 75 ohm transmission line
standards came from...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[mcdonaldREMOVE TO ACTUALLY REACH [email protected]]

Scott Dorsey wrote:


You want goofy, look up where the 50 and 75 ohm transmission line
standards came from...


That's not goofy. The impedance of free space is (about) 75 ohms, as
is, not accidentally, the impedance of a matched dipole antenna.

The minimum loss of a coaxial transmission line with air insulation
occurs at 75 ohms (for the same reason!) while the minimum
loss for a coax line with plain polyethylene insulation is at
50 ohms approximately. Foam insulation line is intermediate.

It is a pain in the butt that TV (cable and receiving antennas) uses 75 ohm
lines while almost all other RF electronics equipment is 50 ohm.

Doug McDonald

[Don Pearce[_3_]]

On Mon, 25 May 2009 11:40:48 -0500, "mcdonaldREMOVE TO ACTUALLY REACH
wrote:

Scott Dorsey wrote:


You want goofy, look up where the 50 and 75 ohm transmission line
standards came from...


That's not goofy. The impedance of free space is (about) 75 ohms, as
is, not accidentally, the impedance of a matched dipole antenna.


The impedance of free space is 377 ohms (120 pi)

The minimum loss of a coaxial transmission line with air insulation
occurs at 75 ohms (for the same reason!) while the minimum
loss for a coax line with plain polyethylene insulation is at
50 ohms approximately. Foam insulation line is intermediate.

Minimum loss (at which copper loss and dielectric loss cross) comes at
about 67 ohms. There are cables at that impedance, but I've never seen
one.

It is a pain in the butt that TV (cable and receiving antennas) uses 75 ohm
lines while almost all other RF electronics equipment is 50 ohm.


Very true.

d

[mcdonaldREMOVE TO ACTUALLY REACH [email protected]]

Don Pearce wrote:
On Mon, 25 May 2009 11:40:48 -0500, "mcdonaldREMOVE TO ACTUALLY REACH
wrote:

Scott Dorsey wrote:

You want goofy, look up where the 50 and 75 ohm transmission line
standards came from...

That's not goofy. The impedance of free space is (about) 75 ohms, as
is, not accidentally, the impedance of a matched dipole antenna.


The impedance of free space is 377 ohms (120 pi)

You are of course right! I had a brain lapse! I was equating FOLDED
dipole to plain dipole. It's bad getting old!


Minimum loss (at which copper loss and dielectric loss cross) comes at
about 67 ohms. There are cables at that impedance, but I've never seen
one.




I've actually used them. I've also used the 125 ohm lines,
and even 250 ohm rigid ones (with teensy inner conductor held
in place with nylon fishing line.

Doug McDonald

Doug McDonald

[[email protected]]

Minimum loss (at which copper loss and dielectric loss cross) comes at
about 67 ohms. There are cables at that impedance, but I've never seen
one.


Copper loss dominates at frequencies below about 1 GHz for most
standard cables.

For a given outer diameter Foam cable has lower loss because the lower
dielectric constant allows the cable to have a larger center conductor
therefore reducing the copper loss. It is not because the dielectric
losses are lower. This is a common misconception.

Dielectric losses are not an issue for most cables below microwave
frequencies.

http://www.epanorama.net/documents/w...impedance.html

Mark

[isw]

In article ,
"mcdonaldREMOVE TO ACTUALLY REACH wrote:

Scott Dorsey wrote:


You want goofy, look up where the 50 and 75 ohm transmission line
standards came from...


That's not goofy. The impedance of free space is (about) 75 ohms, as
is, not accidentally, the impedance of a matched dipole antenna.

The minimum loss of a coaxial transmission line with air insulation
occurs at 75 ohms (for the same reason!) while the minimum
loss for a coax line with plain polyethylene insulation is at
50 ohms approximately. Foam insulation line is intermediate.

75 ohms answers the question "what impedance has the lowest attenuation
per unit length for a given outside diameter?". I believe that is true
*regardless* of the dielectric.

The fact that 75 ohms (and 300 ohms) are antenna impedances is
convenient, but not the main reason for the prevalence of 75 ohm cable
-- the preponderance of antennas are vertical quarter-wave devices, and
those run around 50 ohms.

50 ohms (sort of) answers the question "what impedance has the greatest
power handling capacity for a given outer diameter?". I believe that is
true *regardless* of the dielectric.

The precise answer is around 37 ohms, but the curve is very broad, and
50 (or 51.5 or 52) ohms is useful for (vertical) antennas, so that's the
impedance cable is built to. Incidentally, the lower DC resistance of 50
ohm cable made it the best choice for Ethernet (over 75 ohm's lower
attenuation) because it makes collision detection work better.
Propagation delay limits the length of an Ethernet segment anyhow, and
that doesn't vary greatly with impedance.

It is a pain in the butt that TV (cable and receiving antennas) uses 75 ohm
lines while almost all other RF electronics equipment is 50 ohm.

I suspect that the length of coax in use for cable TV RF plus baseband
video far, far exceeds all other uses of any other impedance of cable,
and in those uses, low transmission loss is more important that anything
else. Plus, of course, the major antenna type used for TV is the
(folded) dipole, which, at 300 ohms, has an impedance that is
"convenient" for use with 75 ohm coax.

Isaac

[[email protected]]

On May 26, 12:27*am, isw wrote:
The precise answer is around 37 ohms

I like that: kinda like saying it's exactly approximately.

[Serge Auckland[_2_]]

wrote in message
...
On May 26, 12:27 am, isw wrote:
The precise answer is around 37 ohms

I like that: kinda like saying it's exactly approximately.


Or very unique, or one of the only...sort of thing that makes me cringe when
I hear it on TV News.

S.
--
http://audiopages.googlepages.com

[Ron Capik[_2_]]

Serge Auckland wrote:

wrote in message
...
On May 26, 12:27 am, isw wrote:
The precise answer is around 37 ohms

I like that: kinda like saying it's exactly approximately.


Or very unique, or one of the only...sort of thing that makes me cringe
when I hear it on TV News.

S.
....and we all know the value can be up to 37 ohms or more. ;-)
[cringe away]



Later...

Ron Capik
--

[David Nebenzahl]

On 5/27/2009 8:22 AM Serge Auckland spake thus:

wrote in message
...

On May 26, 12:27 am, isw wrote:

The precise answer is around 37 ohms

I like that: kinda like saying it's exactly approximately.

Or very unique, or one of the only...sort of thing that makes me cringe when
I hear it on TV News.

"Very unique" irritates me too. Like saying someone is "sort of pregnant".

You either is or you isn't.


--
Found--the gene that causes belief in genetic determinism

[Mr.T]

"David Nebenzahl" wrote in message
.com...
On 5/27/2009 8:22 AM Serge Auckland spake thus:
"Very unique" irritates me too. Like saying someone is "sort of pregnant".

You either is or you isn't.

Nah, "little bit pregnant" is often used for someone in the first trimester,
when they aren't showing. Not correct English sure, but then MANY of our
common sayings aren't.
At least "very unique" is not a contradiction like "precisely.... about"

MrT.

[Don Pearce[_3_]]

On Thu, 28 May 2009 16:55:30 +1000, "Mr.T" MrT@home wrote:

At least "very unique" is not a contradiction like "precisely.... about"

I'd say the two are identical. In both cases you have an extreme
adjective which is hauled back somewhat by its adverb. Ok, in the
second example the adverb is extreme and pulled back by the adjective,
but the effect is the same.

d