Technics SL-P202A - Page 39

The photodiodes used in the optical
pick-up are connected in reverse
direction. The photodiodes require a
reverse bias of at least 5V. Bias
current is set by R3501 and R3502.
Supply noise and interference is kept
away by R3501 and C2521. Coupling of
the h.f. signal is performed by C2504,
R3502 and C2521 also prevent h.f.
coming into the supply.
The h.f. signal is effectively
separated; from the I.f. signals by
deriving the h.f. signal from the total
substrate current of the four
photodiodes and only the I.f. error
signals from the individual anode
currents.(see figure below).
+VB
The h.f. effectively
signal is
separated; from the I.f. signals by
deriving the h.f. signal from the total
substrate current of the four
photodiodes and only the I.f. error
signals from the individual anode
currents. (see figure below).
The h.f. amplifier and a.g.c. circuit
have to meet the correct transfer
function.
The parameters of the transfer function
are set by R3505, R3508, R3507, C2509,
and C2508.
With internal girators the qeualizer
provides a 3rd order bessel filter for
making the correct modular transfer
function to compensate the optical
pick-up system. The total IC bias
current is set by the value of R3508
and decoupled by C2510. The girator
circuits are set by the value of R3505.
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The 3rd pole of the modular transfer
function is made by the RC circuit
containing R3507, C2511 and the
capacitance of the h.f. wire from the
CDM to the demodulator circuits.
3-1-3 The amplitued level detector
For the a.g.c. the amplitude of the
h.f. signal is measured in a level
detector by comparing a é full-wave
rectified and fitered version of the
h.f. signal with an internal reference
level. The level detector is not data
path but parallel to the data path as
in the CDM4 system. For fast response
to drop-out the time constant of the
high pass filter formed by R3507, C2511
and the detector input resistance has
to be 10 sec. In the h.f. amplitude
slicer the amplitude of the h.f. signal
is sliced in 3 levels.
62% and below is HFD
12% and below is DO
The HFD output is used to switch the
PLL on and off ensuring minimum locking
loss while having a drop-out or during
searching. This provides fast relocking
afterwards.
Capacitor C2508 in combination with an
internal switch provides a more
constant gain control during drop-outs.
The switch is operated by the HFD
Signal. This set-up restricts the h.f.
Signal and gives the loop a bandwidth
of 50Hz.
When the pick-up is searching for a
track the DODS (drop-out detection
suppression) is activated to keep the
h.f. signal level clamped on the normal
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