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Focus Circuit - LG GSA-4040B Service Manual

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2. Focus Circuit

2-1. Block Diagram
Disc
Optical
Motor unit
Pick-up
HOP-8511T
FACT-
FACT+
FACT+
FACT-
2-2. Focus Servo
The aim of Focus Servo is to maintain the distance between object lens of P/U and disc surface, so that the
detected RF signal(A1, A2, A3, A4, B1, B2, B3, B4, EF1, EF2, EF3, EF4) can be maximized.
Focus Error Signal(FE) generates from focus error detection block in RF IC(HD153721) using Double knite
Edge method (DVD), spot size Detection method(CD).
Focus Gain and path can be changed at the RF IC(HD153721) according to the disc, and the resulting output
FE(HD153721 3pin) is input to DSP IC(HD153721 27pin).
The Focus Search operation is using FE, PE Signal, therefore check FE, PE signals when Focusing is failed.
The role of DSP IC(HD62604) is Focus Digital Controller. The operation path is as follows;
FE Signal is input to DSP IC(HD62604 27pin), and after A/D Conversion, Digital Equalizer Block and D/A
Conversion in HD62604, the output signal FOD(HD62604 39pin) is input to Drive IC(BD7905 53pin).
The drive output signal FCS+/FCS- generated according FCIN(BD7905 53pin), and drives focus actuator in the
P/U unit.
42
DVD : A1+A2+A3+A4
CD : EF1+EF2+EF3+EF4
(EF1~EF4)
A1~A4
B1~B4
(EF1~EF4)
DVD : (B1+B3)-(B2+B4)
CD : (EF1+EF3)-(EF2+EF4)
FOD
46
IC601
45
BD7905
LPF
IC101
HD153721U
LPF
Focusing
39
DAC
Compensator
IC701
HD62604
FCIN
53
VC
27
1.65V
CTL1
22
DRVCTL1
CTL2
23
DRVCTL2
HAVC
LS
G
4
to
0.8V
101
102
HAVC
LS
G
3
to
V14
ADC
C628
GND
PE
FEO
FE1
R701
C703
FE
GND
27
28
PE
R625
FOD

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