Fluke 93 Service Manual page 58

3-24
CIRCUIT
DESCRIPTIONS
ADC
The
output signal
SNGOUT
(pin 18) of the
A-ASIC
Is
fed to the
e-bit
Analog
Digital Converter
TDA
8703. This
component
operates
on a 2S
MHz
clocK
signal.
The
signal
TRACKN
Is
delayed
to
compensate
for
the
internal signal
delay
in
^e
A-ASIC
(behind the
Track
&
Hold
section)
and
is
fed to
ADC
pin 17.
The
ADC
provides
for
the reference voltage
needed by
the
A-ASIC.
This reference voltage
is
derived
from
the
ADC. V/REF
is
made
of
the voltages on
pin
4 (VRB
« Reference Bottom
Voltage:
.5V)
and
pen 9
(VRT
= Reference Top
Vottage:
+3.5V)
of the
ADC.
During normal operating conditions
this
reference
voltage,
VREF,
is
+2.5V
{+/-
3-6%,
ref.
to
ground).
VREF
is
adjusted with potent!
omeler
R2346,
marked "OFFSET" andean
be
measured between TP331 and
ground.
The
ser>sitivity of the
ADC
is
adjusted with
B2347, marked "GAIN".
These
calibrations are described
In
chapter
5,
section
5.6.1:
"Hardware
SCOPE
Calibration
Adjustments
The
8-bit
output
of
the
ADC: ADCO.mADC?
Is
connected
to the
Digital
ASIC on
the
digital
Af PCB.
3.4.6
ANALOG CONTROL
CIRCUIT
• Introduction
See
figure
3.13.
The
various
sectlor^s
of
the
ScepeMeter,
situated
on
the
analog
A2 PCB,
are controlled
by
the
microprocessor
on
the
digital
A1 PCB.
This
is
done
by
means
of
the
CCLK
(serial
clock),
CDAT
(serial
data)
and
DTAE
(data-iatch)
lines.
This
bus system
creates several
control signals,
which
for
example
drive the relays
switches
in
the attenuator
sections.
-
Detailed
circuit
description
See
figure 3.13
and
circuit
diagram
A2a
(figure 10.5).
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OFFSeT-CO«P€NS*TION
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WAVEFOflM
SeiKTION
Figure
3.
IS Schematic diagram analog
control
circuitry
RELAYS
swrrCHES
(ATT
SECnONi
Each
shift
register transforms the serial signal
COAT
Into
8
parallel
control signals.
This
Is
done
by
means
of
the
serial
dock
signal
CCLK
and
the data-latch signals
DTAEa, DTAEb
and DTAEc.
The
control circuitry
comprises two
series
of
cascaded
shift
registers:
D2907-D2908-D2909
(24
signals)
and D2904- 02906(16
aignals).
The
signals, that
are
made
by
the
shift registers,
are used:
-
to control
the
buffers
(D2901
/
D2902
/
D2903), which
drive the relays
In
the attenuator section.
-
for offset
compensation
(A-RANGE
and
B-RANGE)
in
the attenuator sections.
•
for
L.F. -calibration
(A-OFFSET
and
B-OFFSET)
In
the attenuator sections.
-
to
select the
waveform
in
the signal generator section
(sine
wave/square wave/DC).
•
to drive
the buzzer
(beeper).