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4) Tracking Error Amplifier
This produces the tracking error signal used in the
tracking servo loop.
VTEY = (25 x E) - (25 x Fx 2 x 10k / (T.BAL+10k) )
Normally, the sensitivity of E & F are the same and
T.BAL=10k
=> VTEY = 25 x (E-F)
If, however, the E and F sensitivities are different the
T.BAL volume can be used to cancel out the unbalance.
The offset adjustment TE OFFSET is to cancel any DC
offsets from the photo-detectors or op-amps to ensure
the reference bias for the servo
loop is at zero.
Maladjustment of either of these pre-sets will result in
poor tracking performance
and susceptibility to
skipping.
For a typical unit, the TEY level should be approx. 1.8
Vpp.
5) Tracking Zero Crossing Amplifier
TEC1 is basically an amplified, AC coupled, version of
the TEY waveform. It is used by the servo LSi IC1201,
UPD63700GF to located.the zero crossing points of the
TEY signal to:
1) Determine
how many tracks have been crossed
during track jumping or a carriage move operation.
2) Determine
in which
direction the lens is moving
when
attempting to close tracking. This is used in
the "tracking brake" circuit described later.
For signals in the range 500Hz - 5kHz :
VTEC1 = R1005/R1006 x (E-F) = 45.5 x (E-F)
Typically TEC1 is around 4.2Vpp, this means that the
TEC1 signal level may be greater than the saturation
limit of the op-amp and the signal will clip. However,
since the servo LSI only uses the zero-crossing points,
this is not critical.
1-20
IC 1001
UPC2571GS
Fig.12 : TRACKING ERROR AMPLIFIER
2
10k
Wk
REFO
|
IC 1001
_f
UPC2571GS
Fig.13 : TRACKING ZERO CROSSING AMPLIFIER
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