Figure 3-2.
Hybrid coil circuit, schematic diagram.
(2) Since at least half of the energy passing through the hybrid circuit
in either direction of transmission is dissipated in the balancing
network, there will be at least a 3-db loss in either the talk or
listen signal path.
If the loss is not acceptable, an amplifier
must be inserted in the line to raise the signal level an amount
equal to the loss.
(3) The normal loss through a hybrid coil circuit is 4 db, meaning that
a signal of 0 dbm from the switchboard will leave the send line at a
level of approximately -4 dbm. Likewise, a signal of 0 dbm arriving
at the input to the receive line will arrive at the switchboard with
a level of approximately -4 dbm.
c. Delay.
Because of the large lumped inductance in the windings of a
hybrid coil, envelope delay may be excessive during transmission of high-speed
data signals.
The usual practice in data transmission is to use four-wire
operation to eliminate possible delay caused by hybrid coils.
3-3. ECHO SUPPRESSOR
a. Cause for Echo. In repeaters or carrier terminal sets, an amplifier is
used to raise the voice level in each direction of transmission.
If the
feedthrough from the receive line to the send line is great enough, the circuit
may sing.
If the feedback is not sufficient to make the circuit sing, the
talker may be able to hear his own voice come back to him like an echo. In
short lines, the echo will have no effect because the time lapse is small, but
if the lines are long, the delay of the signal as it returns to the talker may
cause the echo to be noticeable. An echo that is loud and delayed may cause
the talker to experience serious difficulty conversing over the circuit.
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