Brian D Heaton wrote:
IIRC, proper functioning of the 2-wire to 4-wire hybrid depends on
proper balance between the the individual wires of the pair. If you
upset the balance you're going to get all kind of problems.
I just finished modelling a standard 4-transformer hybrid coupled to a
balanced RC transmission line. Cross talk was zero when the hybrid was
balanced. Inserting a single resistor in series with tip or ring
imbalanced the hybrid and cross talk appeared. This could be completely
compensated with the proper RC on the opposite side of the hybrid, as
predicted. It made absolutely no difference to the cancellation if the
resistor was split. Since a balanced hybrid appears as a pure
resistance (complex terms are 0) to the transmission line, placing a
simple resistor in series with the hybrid (on either side) at the
termination point will just look like 2 resistors in series and will
properly terminate the line. There should be no effects at all from
doing this other than the loss of some energy in the termination
resistor, which can be made up for with a boost in Rx gain.
Along the same lines, overdriving the hybrid is a big source of echo.
That's because the cores saturate on transformer based hybrids. This is
not as likely to occur with active hybrids built with op-amps (which are
found in almost all modern line cards), although it is possible if the
gains are high enough. However the distortion from the clipping would
be far worse than the echo.
The correct way to set the RX/TX gain values would be to get the number
for the "milliwatt test signal" from a friendly telco tech. You can
dial that number through * and then adjust the RX gain value so that the
signal is loud, but not overdriving the ADC on the analog interface
card. A good starting point on the TX gain would be whatever the RX
gain ends up at. The proper way to set TX gain would be to use a
digital version of the miliwatt tone (1004Hz at 0dBm IIRC) outgoing and
adjust it for a proper level through to another line on the same telco
switch or across a T1 span that you can observe with a test set.
I don't recall the exact specs for what percentage of the full-scale
value milliwatt tones should be at on the digital systems. Anyone have
the reference?
THX/BDH
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