4.
CIRCUIT
DESCRIPTION
In
studying
the
operation
of
each circuit
in
your
Model
VT -121, please refer
to
Figure 1 ,
the
"Block
Diagram",
and
the
Schematic
Diagram on
the
back cover.
• Outline
of
Operation
A signal voltage
to
be measured, which
is
input
from
the
INPUT
connector,
is
passed
through
the
First
Attenuator
and
is
converted
to
a low impedance
by
the
Impedance
Converter.
The
impedance-converted signal
is
further
attenuated
through
the
Second
and
Third
Attenuators
and
is
magnified by
the
Main Amplifier.
The
magnified
si
gfi
al is
~~d
to
:he
O
l!tC'
u
~
~
rnC'
Ii
f
ier
)
f1
0
the
Re
lat;
ve
Reference Control.
The
Output
Amplifier magnifies
the
signal 50-fold
and
feeds
to
the
OUTPUT
connector.
The
Relative Reference
Control
sets
the
signal level for relative reference.
The
Absolute-Mean Value
Detector
converts
the
referenced
singal
to
DC
current
in
proportion
to
the
absolute mean
value.
The
converted
signal activates
the
Meter.
The
Attenuator
Control encodes
the
signal led from
the
RANGE selector
to
generate an
attenuator
control
signal. This signal
controls
the
First,
Second,
and
Third
Attenuators.
The
Power
Supply
feeds
to
the
functiona
l
circuits±
5 V DC voltages stabilized
by
its
IC
regulator.
• Description
of
Functional Circuits
1.
First
Attenuator
A resistance divider
attenuator.
The
amount
of
atten-
uation
is
switched
in
two
stpes
by
relay
contacts:
0 dB
and
-60
dB.
2. Impedance Converter
A voltage follower circuit having a F ET differantial
input.
This converts
the
First
Attenuator
output
signal
to
a sufficiently low impedance
and
feeds
to
the
Second
Attenuator.
3.
Second
Attenuator
A resistance divider
attenuator.
The
amount
of
atten-
uation
is
switched in
two
steps
by
relay
contacts:
0 dB
and
-30
dB.
4.
Third
Attenuator
A
1·e~
~
~'i.anL>e
d1\
1d€1
atte1' uatu1. I
h€
amuu1'\ 0i
a\te''
uation
is
switched in
four
steps by a F ET swltch:
0
dB,
-10
dB,
-20
dB, and
-30
dB.
5. Main Amplifier
A wide-band, non-phase-inversi
on
amplifier having a
differential input. This has high
input
impedance,
low
output
impedance,
and
20-fold gain. The
output
signal level
is
20
mVrms
for
the
full-scale read
on
the
Meter.
6.
Output
Amplifier
A wide-band non-phase-inversion amplifier with a
differential
input.
This works stably even
for
capaci-
tive loads.
The
gain
is
50-fold
and
the
output
imped-
ance
600
n.
The
output
singal level
is
1 Vrms
for
the
full-scale read
on
the
Meter.
7. Absolute-Mean Value
Detector
An absolute-mean value
detector
comprised
of
a high
5
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