Fluke 93 Service Manual page 50

3-16
CIRCUIT
DESCRIPTIONS
In
the
Scope
Meter the
response
of
the
H.F
attenuator sections
is
adjusted
by
means
of
three variable
capacitors
C2209,
C2207
and C2114.
These
variable
capacitors are used
to
compensate
for parasitic
capacitors
of
the
printed
circuit
board.
The
1
to
1.48 divider
(1
to 1.48 section)
can be
adjusted
with
variable capacitor
C2209.
The
1
to
14.8 divider
(1
to
1
.46
and
1 to
10
sections)
can be
adjusted with variable capacitor
C2207.
The
1
to
148
divider
(1
to 1.48,
1
to
10
and
1
to
100
sections)
can be
adjusted with capacitor
C221
4.
NOTE:
These
capacitors do
not
have
to
be
readjusted
at
every
calibration,
(see chapters, section
5.6.
1)
The
capacitors are rough adjustments, used
to
compensate
for
hardware
differences.
The
attenuator
response
is fine
adjusted by
means
of the LF.
caiibration
section (see next
page).
Impedance
converter
The
output
of
the
H.F.
path
Is
connected
with
the
Impedance
converter,
formed by
transtsiors
V2207
and
V2209
(see
circuit
diagram A2a,
figure 10.5).
The
bias voltage of
V2207
Is
determined by R2216.
To
prevent
destruction of the
gate
of
V2207
by
high voltages or voltage peaks,
two clamps V2206 and
V2204
are provided.
Summation
of
the
H
R
and
the
L.R
signal parts
Is
obtained
in
transistor
V2207.
which
acts
as the
collector
impedance
of
V2208.
LF.
(Lew
frequency)
path
The
L.F.
part of the
inptrt
signal enters the
L.F.
path,
which
consists of
a
L.F.
attenuator section,
a
L.F.
cal
b
ration
section
and a
regulating
feedback
loop,
which
consists of
a summator,
Inverter,
another
summator, and an
emitter follower
(see
figure 3.7).
L.F.
attenuator
Fig
3.8
shows
the
L.F.
attenuator section
in detail:
OPPser vouTAoe
The
L.F.
attenuator consists
of
an
inverting amplifier.
N2201
,
which
atter>uates
the
L.F.
signal
by
a
factor,
depending on
the
settings of switches D2201.
These
switches are
controlled
by
signals
named
Srib...Sr4b.
A
'high" signal
switches on
the corresponding
latched relays.
Table 3.3 Attenuator drive signals Sr1b...Sr4b.
Attenuator
settings
Srib
Sr2b
Sr3b Attenuation
5
mV/dlv...100
rr>V/dfv
high low low
1
-48 times
200
mV/djv...1 V/div
high
high low 14.8 times
2V/div...10V/dlv low low
high
148
times
20
V/dlv...100 V/dIv low
high
high
1
480
times