Special Circuit Descriptions - Samsung MM-18 Service Manual

mm18(10) 3/23/98 3:14 PM

5. Special Circuit Descriptions

5-1 RF Amp Block
5-1-1 RF Amplifier
The two currents from input pins PD1 (A+C) and PD2 (B+D) are converted into voltages through I/V Amp,
and they are added to RF summing Amp. The photo diode (A+B+C+D) signal which is I-V changed is
outputted by RFO (pin74). At this pin, the eye pattern can be checked.
5-1-2 Focus Error Amp
The output of the focus error amp is the difference between RF I-V AMP(1) output Va and RF I-V AMP(2)
output Vb,just is the Photo Diode ((A+C) - (B+D)) signal which is I-V changed.
The focus error bias voltage applied to the (+) of focus error amp can be changed by D/A converter as shown
in diagram, so that the offset of focus error amp can be adjust automatical.
Focus error bias can be adjusted from the range of +100mV ~ -100mV by connectiong the resistor with pin 63
(FEBIAS).
Samsung Electronics
Page 5-1
58K
65
PD1 -
+
IV AMP
VC
58K
-
PD2
66
+
IV AMP
VC
sev-stopb
SW1
FEBIAS 63
sev-stop
<5 Bit Counter>
X1 X2
VA 10K
-
+
RF SUMMINGAMP
VC
VB 10K
164K
32K
VB>
32K
VA>
160K
4K
3K
X4 X8 X16
-
FEBIAS
+
VC
RFO
74
RF-
73
-
59
FE1
+
-
fcmpo
+
fe-stopb
5-1