Eeyore said:
Twisted pair cable as used for telecoms has a nominal 100-110 ohm
inpedance. See the ADSL specs.
The twisted pair cable as used for telecoms has a nominal 100-120 ohm
impedance for high frequecies the ADSL system uses. The ADSL
system uses frequencies between 138 kHz and 1.1 MHz for downstream
data (and 25 kHz to 138 kHz ofr upstream). This 100-120 ohm
impedance holds pretty well for those frequencies above 100 kHz.
For lower frequencies the the impedance of the telephone cable
is not anymore that 100-120 ohm, but something else.
For voice frequencies used on on normal telephone (300-3400 Hz)
the impedance is normally considerable higher than 120 ohms.
Normal telephone subscriber lines in USA (0.4-0,6mm subscriber PE
insulated vaseline filled cable) are 770 ohm resistor (with 2uf series
capacitor) and 47nF parallel capacity.
2 uF
||
----+-----||--------+
| || |
| | |
--- | | 770 ohms
--- 47 nF | |
| |
----+---------------+
This diagram is referred to 800Hz, but impedance is rather complex,
and varies from high value at low frequency and drops to ca. 150 ohm
on 10kHz and 120-125 ohm above 100kHz.
Some telephone lines can have higher impedance (typically 1100 ohms in
lines with loading coils or telephone air cables).
In european specifications (for Finland etc..) I have seen this
that complex reference impedance Z = 270 + (750 //150 nF)
750 ohm
_____
270 ohm +--|_____|--+
_____ | |
--|_____|---+ +-----
| || |
+----||-----+
||
150 nF
Typical cable used in for subscriber lines has following
characteristics: 0.5 mm diameter wire, loop
resistance 182 ohm/km and pair capacitance 39 nf/km.
TRANSMISSION SYSTEMS FOR COMMUNICATIONS, revised 4th edition, Bell
Telephone Laboratories (1971) gives the followign information on
typical cable characteristics:
"The primary constants of twisted pair cables are subject to
manufacturing deviations, and change with the physical environment
such as temperature, moisture, and mechanical stress. The inductance,
L, is of the order 1 mH/mile for low frequencies and the capacitance,
C, has two standard values of 0.066 and 0.083 uF per mile although
lower capacitance cables are under development.
Of the primary constants, only C is relatively independent of
frequency; L decreases to about 70 percent of its initial value as
frequency increases from 50 kHZ to 1 MHz and is stable beyond; G is
very small for PIC (polyethylene insulated cables) and roughly
proportional to frequency for pulp insulation; and R, approximately
constant over the voiceband, is proportional to the square root of
frequency at higher frequencies where skin effect and proximity effect
dominate."
600 ohms is an irrelevant historical nonsense from the days when they used
telegraph wires for phone circuits.
600 Ohms is somewhat of a compromise between different real-life
impedances that could be seen. Normal telephone line connections are
theoreticallydesigned to be 600 ohm resistive impedance. This 600
ohm is kept as international reference for designing telephone line
equipment (typically the signal powers are measured to 600 ohm load).
In practice the telephone line does lot look like pure 600 ohm
resistance.
Telephone equipment which is designed to operate with 600 ohm loads
will operate with those real-life lines, but it's performance is worse
than in ideal situation. Typically the modems are designed for 600 ohm
reference impedance because they can handle the sidetone.
The return loss of the terminal equipment must be greater than 10 dB
when compared to 600 ohm reference. This measurement applies to
telephones, modems and other terminal equipments. NET4 technical specs
are European specs and they are used in many European countries (NET4
is actually a collection of different specs in use in different countries).
For best performance the telephones are designed to the exact line
impedance. Matching the hybrid circuit to the real line impedance
(instead of 600 ohm) will improve the feedback typically by
3-6dB. 20dB sidetone is easy to achieve, but 30dB is also not too
difficult provided you can measure the line impedance and take steps
to build a correct balancing network.