Actually, the network termination impedance for two wire phone lines is
important, as it is an important parameter in the design of the two to
four wire hybrid. http://www1.btwebworld.com/sinet/351v4p5.pdf gives
specifications for the UK network, which are somewhat higher than 600 ohms.
Note these are the system values at audio frequencies; they are not the
characteristic impedance of the line. The line will typically be too
short for that to be significant.
--
David Woolley
Owner K2 06123
On 15/04/14 07:37, Jim Brown wrote:
On 4/14/2014 4:46 PM, Harry_Yingst wrote:
Phone lines are Typically 600 ohms
Actually, the Zo of phone lines at radio frequencies is about 80 ohms,
and varies widely with frequency in the audio spectrum. That
characteristic is determined by the diameter, spacing, and insulation of
the twisted pairs used to carry the signal. They have not been 600 ohms
for at least 75 years. More than 40 years ago, it was standard practice
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