AT78C1503
11
Slow Servo Arithmetic Functions
Figure 11 shows the top level diagram for the slow servo.
The first block has inputs A, B, C, D,E, F, G and H. The
functionality is as follows:
First there is a Sample and Hold function on all the 4
inputs. The S/H is done from an external CMOS input pin
TBD. The minimum amount of time for the Hold state is 30
ns, and it should not exceed 200 ns. The minimum amount
of time for the Sample state depends on how much the sig-
nal changes between samples. For example if ak+1 - ak is
half of the dynamic range on FO1,2 TR1,2, then the sample
state should not be less than 20 ns. In reality the S/H signal
will be slew rate limited by the CMOS I/O input.
Second there is a Differential to Single ended conversion
and a VGA function on each input. The reference for the
D2S function is referenced from the supply and it varies
from VDD-1.5V to VDD-2V via a 3-bit D/A. This reference is
also sent as an output off chip, in case it needs to be used
to a reference in a preamp. The VGA range is between 0.3
to 4 (16 exponentially spaced steps) with a worst case
bandwidth of 15 MHz. The VGA outputs go also to ID
Detector, Wobble Detector, and Mirror Field Detector
blocks. There is also an option which resets the values of
A, B, C and D to their midrange point (ex. 0V differential).
This option is used for electronic offset correction on the
Focus Error and Tracking Error signal described later in
this section.
For the slow servo each of the four inputs contains a single
pole low-pass filter with a programmable cutoff frequency
between 150-500 kHz. The cutoff frequency is
programmable with a 3-bit SR control which does not track
the channel data rate.
Next block is a Voltage to Current converter followed by a
±30% gain adjust (4 bits + sign) and a ± 30% offset adjust
(4 bits + sign) on each individual channel. The input signals
A,B,C,D will get the gain and offset adjustment.
These gain and offset adjusted signals are now summed
together and the sum feeds a digitally controlled AGC loop
(also known as the normalization loop). Maintaining a con-
stant output voltage at the output of the AGC loop
normalizes the input to the focus and push-pull tracking
error signals, so the error signals are not dependent on the
strength of the light returned to the photo-diodes. This nor-
malization alleviates differences due to media reflectivity.
This loop consists of a VGA, counter (7 bits) and compara-
tors. The clock for the counter is selectable: It can either be
a divide by 3*X of the data rate (X ranges between 3 and
16) or a divide by 3*Y of the oscillator frequency (Y ranges
between 1 and 16). The reason behind the two different
clock domains is that when the normalization loop is used
for normal operation (read, erase, write) collecting data
along the tracks, will be desirable for the loop bandwidth to
follow the data rate. For the Tracking error during a seek
however, it might be desirable for the loop bandwidth to be
independent of data rate. The normalization loop has a fast
acquisition mode controlled by 2 bits TBD. Depending on
this setting the counter can count by either 1,2,4,8 every
time when the error signal exceeds a threshold which is
also programmable by 2 bits TBD. This feature helps the
normalization loop track fast slewing signals in the
A+B+C+D signal.
Focus error signal (FE) FE = (A+C)-(B+D). The FE signal is
then passed through a VGA which is slaved to the normal-
ization loop. The dynamic range on the FE is between 0.5
to 2.5V centered around 1.5V. There is also a +-0.5V offset
added to the FE signal using a 3-bit + sign D/A. In addition,
the gain on the Focus Error is adjustable between 1 to 5
using a 3-bit gain adjust (exponentially spaced). There is
high gain chip input TBD which flips between gain = 1 and
the gain set by the 3-bit gain adjust. For the gain of 1 set-
ting the FE is linear and covers 0.5 - 2.5 range. For a gain
higher than 1 the FE will saturate.
Push Pull Tracking error signal (PPTK) is PPTK = (A+B)-
(C+D). The PPTK signal is then passed through a VGA
which is slaved to the normalization loop. The dynamic
range on the PPTK is between 0.5 to 2.5V centered around
1.5V. There is also a ±0.5V offset added to the PPTK sig-
nal using a 3-bit + sign D/A. In addition, the gain on the
Tracking error is adjustable between 1 to 5 using a 3-bit
gain adjust (exponentially spaced). For the gain of 1 setting
the PPTK is linear and covers 0.5-2.5 range. For a gain
higher than 1, the PPTK will saturate.
The Differential Push Pull tracking error is
DPPTK=(A+B+E+F) - (C+D+G+H). The differential Push
Pull Tracking error signal is passed through a VGA slaved
to the normalization loop and conditioned similarly to the
PPTK error signal as described above.
The Track Zero Crossing (TZC) signal is needed for
counting tracks during a seek operation. Part of the TZC
function is an Average Detector function (see Figure 11)
which follows the average of either the PPTK or DFTE
(Differential Phase tracking error) signals. The architecture
of the Average Detect Function (ADF) is similar to the
normalization loop. The only difference is that rather than
keeping a constant output, this loop follows the input with
different bandwidths. The clock frequency for this ADF
block is a divide by 2*X of the crystal, where X is between 1
and 2048. The maximum clock frequency however should
not exceed 8 MHz. This bandwidth is controlled by 2 input
pins (BWUP, BWDWN). When each of these pins is
toggled by the servo chip the bandwidth of the ADF macro
goes UP or DOWN by a factor of 2. There are 11 steps for
the bandwidth.