Re: Filters - Quality



GM6TRS wrote:

"Eeyore" wrote:
JohnW wrote:

Now comes the problems...

Any filter is designed to work best in a known environment.
In telecomms, we practically have a 600ohm balanced
transmission line environment, so this (balanced, 600ohm) is
used in the design to ensure the filters work as required.

Here's the next problem. The characteristic impedance of the
telephone cables to the home is actually about 100 ohms !

Did you mean 1000 ohms?

Absolutely not. The characterisitc impedance of twisted pair wiring (and indeed
co-ax too) is determined explicitly by it's *mechanical construction*, i.e.
dimensions and the dielectric properties of the insulation.


When I worked in a Telephone Repeater Station,
a '1200 Line 600 Office' transformer matched a customer line to
transmission equipment quite well - a through to term dip close to the
nominal 3.5 on a dB meter on the Office side confirmed this.

How long was the customer line ? Audio frequency signals don't encounter the
transmision line effects until the cable is very long, comparable with some
significant fraction of the wavelength of the highest frequencies for example (
4kHz - which has a theoretical wavelength of 75 km ). Voice traffic on the local
loop simply isn't matched to cable impedance because it doesn't need to be.

Graham

.

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