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Thread: Impedance Matching a Phone Line
face BY : Brian Hopkins email (remove spam text)

Telco subscriber loops (twisted pair cable) have a nominal characteristic
impedance (Z) of 900 ohms. That is the classic comprise number used when
designing subscriber side equipment, and why all the CO line simulators
(artificial lines) use that value.
This of course is not the DC loop resistance (R) which will depend on your
distance from the CO and the gauge of cable in your loop makeup.
So..if you design your line drive electronics to have an output impedance of
900 ohms you should have maximum power transfer, and minimize reflections
(echo) for... most.... average...typical ... loops your equipment may
connect to.
I can't find any of my tech college references, but these sites confirm what
I recall was drilled into me.

Any references you see to 600 ohms (Z) relate to 'open wire' , used for toll
circuits (open wire carrier) in the pre-microwave days which were the
pre-fibre optic days. The equipment side of the telco was designed to match
that 600 ohms.
Old standards...old history...

Brian Hopkins

{Original Message removed}
<002101bfc5c7$db568ef0$0965b8a1@ent.agt.ab.ca> 7bit

See also: www.piclist.com/techref/index.htm?key=impedance+matching
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