Re: The decibel

In article <t2voc3lln7rbps6r089rfj8inpouhvod7i@xxxxxxx>,
The Phantom <phantom@xxxxxxx> wrote:

On Wed, 22 Aug 2007 15:58:52 GMT, John Byrns <byrnsj@xxxxxxxxxxxxx> wrote:

In article <2d7nc3pbeps39kopr6j8jiq7je93mptmpd@xxxxxxx>,
Chris Hornbeck <chrishornbeckremovethis@xxxxxxx> wrote:

On Wed, 22 Aug 2007 03:15:13 +0100, Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:

Do you think it is higher or lower than 600 ohms?

I was under the impression it could be lower but I see the standards
still
refer
to 600 ohms.

The characteristic impedance of the twisted pair cable is actually around
100
ohms and DSL circuits operate at this impedance. So it's 600 ohms for
audio
and
100 ohms for data.

The big numbers are for open-wire line with huge (by modern standards)
spacing. Modern plastic-insulated twisted pairs tend to fall, as
you say, around 100 ohms.

While I believe the characteristic impedance of open wire lines is
higher than that of cables, as you say, that is far from the entire
story. The characteristic impedance of cable is greatly dependent on
frequency as a result of not having the correct ratio of inductance to
capacitance.

Once you're above the corner frequency, this is no longer true, until the
frequency is very high. See:

http://www.prc68.com/I/Zo.shtml

Please note the second paragraph in the above referenced web page which
states, "It turns out the the "100 Ohm" UTP cable does have a
characteristic impedance near 600 Ohms at 1 kHz."

Note also that the "corner frequency" for UTP is about 51 kHz, while
this discussion concerns the characteristic impedance at audio
frequencies which are well below 51 kHz. This is the reason I used the
word "ultrasonic", rather than "RF", to refer to frequencies above the
audio band. As a point of interest, the "standard" cable that started
this discussion has a "corner frequency" of about 13.7 kHz if I pushed
the buttons on my calculator correctly.

It's the open wire lines that have the 600 ohm impedance. That is what
they were using in the early days of telephone, and we inherited that 600
ohm legacy for telephone equipment for many years. The example of an open
wire line given on the web page referenced is of a pair of 6 gauge (!!)
wires, 1 foot apart.

The result is that the characteristic impedance of cable
varies with frequency, being lower at ultrasonic frequencies than at
lower audio frequencies.

If the above referenced web page is to be believed, then it appears that
600 Ohms is also a good approximation for the characteristic impedance
of "100 Ohm UTP" in the audio band.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
.

Relevant Pages

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