Re: The decibel

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

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.


Regards,

John Byrns

.

Relevant Pages

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