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Receiver
There are several different types of receivers. In principle, they
work the same as the speakers in your radio and TV. The speaker
consists of a small coil attached to a diaphragm. The coil is
mounted over a permanent magnet as shown in Figure 3-6. Coil
current in one direction causes the coil and diaphragm to be
repelled from the permanent magnet. Coil current in the other
direction causes the coil and diaphragm to be attracted to the
permanent magnet.
If a current of audio frequency is sent
through the coil, the diaphragm vibrates and generates sound
waves in step with the current. Thus, if the current from the
transmitter is sent through the coil, the sound produced will
duplicate the sound striking the transmitter.
Ringer
As shown in Figure 3-2, the ringer is connected across the tip
and ring inputs in series with a capacitor to block the 48VDC.
The ringer consists of a permanent magnet attached to an
armature as shown in Figure 3-7. When an alternating current
of 20Hz is passed through the coils, the armature is alternately
attracted to one coil and then the other. The hammer attached
to the armature thus strikes one bell and then the other to
produce the ringing sound.
Induction Coil / Balance Network
When transmitting and receiving is done over the same two
wires, the problem arises that current from the transmitter flows
through the receiver. The speaker then hears their own voice
from the receiver. This is called sidetone. Too much sidetone
may be objectionable to the speaker and cause them to speak
too softly. A small amount of sidetone is desirable to keep the
telephone from sounding dead.
The induction coil and balance network limit the sidetone. The impedance of the balance network shown in
Figure 3-2 approximately matches the impedance of the local loop. Thus, about half of the current from the
transmitter flows through L1 and the local loop and the other half flows through L2 and the balance network. The
currents in L1 and L2 induce voltages in L3 of opposite polarity which limits the voltage across the receiver to an
acceptable level. When receiving a signal from the local loop, the currents in L1 and L2 induce voltages in L3 of
the same polarity. These voltages combine to drive the receiver.
-8-
Figure 3-6
Figure 3-7
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