Optical Pickup; The Laser And Lens; Focus Coil; Tracking Coil - Sony CDP-CX235 Training Manual

Optical Pickup

The Optical pick up assembly is the single most complicated device in the
CD player. The Optical assembly has several parts:
•
The laser and objective lens
•

Focus coil

•

Tracking coil

•
Detector assembly
Each part has a role in the playback of the disc information.

The Laser and Lens

The laser diode is powered by a regulated voltage (LD). Within the as-
sembly is a photodiode that monitors the laser light intensity. This infor-
mation (PD) is fed back to the circuitry to regulate the LD drive voltage.
The laser light is channeled through an objective lens towards the CD.
The convex objective lens funnels the laser beam into a focused point
near where the disc's shiny information layer should be.
Focus Coil
The objective lens has a coil of wire attached to it in two planes. A station-
ary piece of metal is adjacent to it for magnetic reference. As current
flows through the focus coil, the lens is moved either toward or away from
the disc depending upon the direction of the current. Therefore, applying
sufficient current to this coil will move the lens and its focus point to the
disc's information layer.
Tracking Coil
The tracking coil on the objective lens is perpendicular to the focus coil.
Current through this coil moves the lens and the laser beam to one side or
another. This movement keeps the laser beam on the spiral track as the
disc rotates. When the tracking coil has moved the lens to the limit, the
sled moves the optical assembly to follow the spiral track. This re-centers
the lens within limits and the tracking coil can continue to follow the track
once again.
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Detector Outputs

Laser light striking the information layer of the disc is modulated by the
data and returned to the detector area next to the laser diode. Detectors
A-F receive the light, amplify and output this signal to the external RF
amplifier to produce derived signals.
Optical Assembly Output Applications
Output
Detectors A-D RFO (RF Output)
Detectors A-D FE (focus error)
Detector E & F TE (tracking error)
Derived signal Purpose
Audio, Spindle
motor lock
Focus coil
correction signal
Tacking and sled
motor correction
signal