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{How to find the null point]
When you insért the regular screwdriver into the slit for the grating adjustment and change the grating angle, the
amplitude of the tracking error signal at TPi, Pin 2 changes. Within the range for the grating, there are five or six
locations where the amplitude of the wave reaches a minimum. Of these five or six locations, there is only one at which
the envelope of the waveform is smooth. This location is where the three laser beams divided by the grating are all right
above the same track. (See Figure 3.)
This point is called the null point. When adjusting the grating, this nul! point is found and used as the reference position.
Turning the grating counterclockwise
Turning the grating clockwise
eo
ie
°
e
%
%
0
ov
0
o
8
o
o
Wavetorm of TP1, Pin 2
WN lH Win Wi WA Hh i, IH Wh
Grating adjustment position"
Null point
Figure 3
[
!
Wd
1 | |
AN
A
WM
Maximum amplitude
Waveform other than the
Null point waveform
ons
null point
42
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