106
Constant
input
capacitance
for the
various
attenuator
positions
is achieved
by trimmers
Cl
01,
Cl
04 and
Cl 09.
The high-ohmic
voltage divider
sections
are made
independent
of the input
frequency
(i.e,,
the capacitive
attenuation
for
a.c.
signals is
adjusted
to
the
resistive
attenuation for
d.c. signals)
by means
of trimmers
C107
and
Cl
12.
A diode
clipper
VI
01,
in the
gate circuit
of FET-transistor
VI
02
protects the
input
source
follower
of the
impedance
converter from
excessive
voltage
swings.
The high
frequency
path
of the
input
signal
consists of
capacitor
Cl 14 and
FET-transistor
V102
connected
in
a source-follower
configuration.
The
low
frequency
path
of the input signal
consists
of
error amplifier D101
,
which
samples
the input
and
output
signals
of the
impedance
converter
over
a
frequency
range
from d.c.
to
1
kHz.
The
error
amplifier
generates
a correction
signal on
pin 6
which
is fed
to the
impedance
converter
to
replace
the
missing
low
frequency
components
of the
high frequency
path.
The
gain
of the low
frequency
path
is set
by adjusting
the
resistor divider
ratio
from which
the
output is
sampled.
Preset R
132
(L.F.
corr)
is adjusted
so that the ratio of the network
R134/R132
is the same as the
ratio
of network R122/R123.
The
off-set
voltage of the error
amplifier is corrected
by preset R124.
After
low-ohmic
attenuator switching,
the output from
the impedance
converter provides
a correct impedance
match for
the coaxial
cable to the
intermediate
amplifier.
3.2.
1 .3.
Intermediate
amplifier
The intermediate
amplifier
comprises
two
main
stages.
The first
stage comprises the gain
adjustments, vernier
and continuous control, level shifting,
and sensitivity
for
the
2
mV,
5
mV
and
1 0
mV
ranges.
The second
stage comprises
a series-shunt feedback
amplifier circuit formed
by
D202
input transistors and
transistors V502,
V504 on the
delay-line driver circuit. Interposed in this
stage are the normal/invert, shift
and
electronic
switch
facilities.
Both stages have
overall gains
of
approximately
3.
To
improve
temperature
control and stability,
the
intermediate amplifier mainly
comprises integrated
circuits.
The signal
paths
for
channel
A and channel B
are
identical in
the
input
stages,
consequently,
only the
channel
A input circuit
is described.
The
Y
signal from the channel
A attenuator is applied
to a
coaxial input socket
on
the intermediate
amplifier,
via R204
to
pin
3 of integrated circuit D201
.
The asymmetrical input is converted to a symmetrical output in
a transistor balance amplifier.
Potentiometer
R212
provides
a
continuous balance control
to
correct for
line
shift.
Four
diode-connected transistors across the base circuits of the D201 cascode
transistors provide control of
attenuation by means of GAIN control R12 and CONT. control R8,
which
vary
the
dynamic
resistance of the
diodes.
Control
R12 gives 5 %
loss
of gain in the mid-position
and
10 %
loss of
gain at
minimum. Control
R8
gives a 3 to 1 attenuation, which is sufficient to give the desired
overlap between the input attenuator
steps.
When the currents through the diode bridge are equal, there
is no gain and the transistors are cut
off. When
current
flows in
one diode and
not
in the other, the
gain
is
maximum.
The
cascode transistors V204 and V203 that
follow integrated
circuit D201
provide additional gain for the
most sensitive ranges by the selection of load resistors. By
switching this
additional gain at
intermediate
amplifier level
a
reduction in noise is achieved.
The different loads of V203, V204 are selected by
switching
diodes under the control of the
front-panel
AMPL switch
oositions.
In the
10
mV-5 V
positions
the
-
5.2 V
supply from
AMPL/DIV
switch
S9 contact 14
is applied to the
junction
of R241
,stabistor
V205 and diode V208. The stabistor
V205
conducts and applies the
negative
potential
via
switching diode
V206
to
load
resistors R228 and
R236 of V204
and V203 respectively.
Diode
V208 also conducts and blocks
V21
1 ,
thus causing transistor V21 2
to
switch off and disconnect the
load
resistors
R231 arid R234.
In the 5
mV position,
the
-
5.2
V
supply
from
AMPL/DIV
switch S9
contact
20 is
applied to
the
junction
of
R242, and diodes
V209, V210. Diode V210 conducts and
applies the
negative potential via switching
diodes
V207
to
the load resistors
R229
in series with
R228, and R233 in
series with
R236
of V204
and V203
respectively. Diode
V209
also conducts and
blocks V21
1,
thus
causing
transistor V21
2 to
switch
off and
disconnect
the remaining load
resistors. To compensate
for
the reduction
of
bandwidth in the 5
mV
position
because
of the
higher value of the load resistor,
an
additional
capacitor, C202,
is
switched into the
emitter
circuit
via R209
and diode V201.
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