Harman Kardon FL8400 - Page 16

FL8400
2-2. Focus error detecting operation
The reflected laser beam from a disc is polarized 90° with the beam-splitter and sent to the cylindrical
lens. The beam passed through this cylindrical lens is then sent to the four division photodiodes and
focuses into an image whose shape varies with the distance between the disc and the objective lens.
Such change in the beam shape causes the current flowing from the photodiodes to vary.
Shown in Fig. 3 is the principle of the focus error detection.
The currents from the photodiodes (A+C and B+D) are applied to pins 7 and 8 of IC104 and converted to
voltage by RF I-V amplifiers (1) and (2) included in 1C104.
Fig. 3
2-3. Focus servo control operation
The focus error signal, after being converted to voltage by the RF I-V amplifier, is transmitted to the
operation amplifier in the IC and output from pin 19.
When the disc to objective lens distance is in focus, the beam forms a true round. In this state, the
beams applied to four elements of four division photodiodes become equal and thus the output
provided then is O0(zero). When the disc to objective lens distance is too close (near focus), the beam is
reflected divergently to form an oval in crosswise direction. In this state, the outputs provided from
photodiodes A and C are higher than those from B and D, resulting in negative (-) output voltage. On
the other hand, when the distance is too far (far focus), the beam is reflectd convergently to form an
oval in longitudinal direction. Then the outputs from photodiodes B and D are higher, resulting in
positive (+) output.
The output voltage (focus error signal) from pin 19 of 1C104 passes through IC 103, in from pin 6 and
out from pin11,as shown in Fig. 4. It is amplified in C103 and fed to the focus coil which then drives the
objective lens of the pickup.
1C104
RF J-V AMP. (1} -
RF I--V AMP. (2)