Figure 3-1.
Hybrid coil directs traffic through repeaters.
hybrid circuits are adjusted correctly, voice energy arriving from the left
transmission line travels through the top amplifier and out the transmission
line to the right. Likewise, voice energy arriving from the right transmission
line travels through the lower amplifier and out the transmission line to the
left.
Incorrect adjustment of hybrid circuits, especially the balancing
networks, allows undesirable feedthrough from the upper amplifier to the lower,
resulting in circuit singing. A hybrid coil circuit is essential in a two- to
four-wire converter.
a. Schematic Diagram.
The schematic diagram of a hybrid coil circuit
illustrated in figure 3-2 shows at least five windings that are magnetically
coupled because they are wound on the same transformer core.
A balancing
network represented by a variable resistor is used to separate talk (send) and
listen (receive) paths.
b. Principles of Operation. One side of the hybrid circuit connects to a
two-wire line, and the other side connects to a four-wire line. The two-wire
line serves the switchboard, and the four-wire line serves the circuits between
the switchboards including apparatus such as an echo suppressor or repeater
amplifier.
Each subscriber can talk and listen over the two-wire line.
However, the local subscriber's voice travels to the distant station on one
pair of the four-wire line, and the distant subscriber's voice returns through
the second pair of the four-wire line.
(1) A hybrid circuit is essentially a balanced bridge.
To operate
properly, the balancing network must be electrically equal to the
impedance of the switchboard loop, and the impedance of the send
line must equal that of the receive line. An imbalance of either of
the two lines will cause feedthrough as shown by the dashed line.
Another cause for feedthrough is the wrong value of a balancing
resistor.
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