2-30
Figure 2-29.—Simple crystal frequency synthesizer.
Q25. What is the primary function of an afc circuit?
Q26. What is frequency synthesis?
AUDIO REPRODUCTION DEVICES
The purpose of audio reproduction devices, such as loudspeakers and headphones, is to convert
electrical audio signals to sound power. Figure 2-30 shows you a diagram of a loudspeaker called the
PERMANENT MAGNET SPEAKER. This speaker consists of a permanent magnet mounted on soft iron
pole pieces, a voice coil that acts as an electromagnet, and a loudspeaker cone connected to the voice coil.
The audio signal has been previously amplified (in terms of both voltage and power) and is applied to the
voice coil. The voice coil is mounted on the center portion of the soft iron pole pieces in an air gap so that
it is mechanically free to move. It is also connected to the loudspeaker cone; as it moves, the cone will
also move. When audio currents flow through the voice coil, the coil moves back and forth proportionally
to the applied ac current. As the cone (diaphragm) is attached to the voice coil, it also moves in
accordance with the signal currents; in so doing, it periodically compresses and rarefies the air, which
produces sound waves.
Figure 2-30.—Permanent magnet speaker.
