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33
The
overloaded
amplifier
(307/308)
makes from
the
20
V-supply
voltage
signals
with
steep edges.
The
signal
for searching stage
305
is
supplied
by
stage
309.
It
is
controlled
by
means
of
a
current,
which
is
taken
from
rectifier
arrangement 407/408. This
rectifiers
are
supplied
with
a
symmetrica!
20
V
voltage
from
the
mains
part.
When
push-button
R
is
not
depressed, the operating points of the
overloaded
amplifier
and
the control stage
(309)
are shifted
in
such
a
way
that control
from
the mains
part
is
blocked,
transistors
301 and
305
are
opened
and 302
remains blocked.
3
.
1
.
2
.
2
.
Operational amplifier
306
is
switched
as
a
voltage-current converter.
The
bridge
rectifier
is
d.c.-free
incorporated
in
the
feedback
arm
in series
with
resistor
662
(Fig.
25).
The
voltage
drop
across
this resistor
only depends
on
the current
which
is
taken
from
the
amplifier
output through
the bridge
rectifier.
In
such
a
way
a linear relation
between
the
rectified
voltage
and
the control voltage
is
obtained even
for
small
amplitudes
of the
control
voltage,
near the zero
passage.
In
order to adapt the
sensitivity
of the zero indicator
to
the application
purpose
(Fig.
15), a circuit
with diode
405
and
resistor
617
is
parallel-connected
in
flow
direction to
measuring instrument 821.
In
case
of
an
increasing diagonal
current of the
rectifier bridge,
the current
through diode
405
increases also
and
consequently
the
scale-deviation
change
of the
measuring instrument
decreases.
In this
way
the
characteristic
of the zero
indicator
becomes
logarithmic.
This curve
is
not
appropriate
in
the searching
mode;
therefore,
in
that case
diode
405
is
short-circuited
by
switch
801/D
and
the
indication
becomes
linear.
For
searching
mode,
see 3.1.4.
3.1.3.
Supply
voltage generator
A
switchable
supply
part
is
used
to supply the
bridges.
3
.
1
.
3
1
.
The
d.c.
supply
voltage
is
produced
in
bridge
rectifier
415, which
is
taken
from
a
well-screened
winding
of
the
mains
transformer. This
winding
is
floating
and symmetrical
to earth.
The
filtering
serves
to
reduce
the
ripple
voltage to
a
minimum.
The
internal resistance
of the source
amounts
to
approximately
1 1
Ohm.
3
.
1
.
3
.
2
.
The
supply
oscillator
contains
a
Wien-fourpole
in
the
positive
feedback loop and
an
amplitude-dependent
resistance
(409-410 and
640, 641)
in
the
negative
feedback loop
.
Operational amplifier
313
is
protected
against
overload
by means
of
anti-parallel
connected
diodes
413/414
at
the input
and
by
clamp
diodes
411/412
at
the
output.
Output
transformer
751
transforms
and
separates
the
output
voltage.
As
a
result
of the very
high voltage
feedback (open
amplification
reduced from
approximately 10
4
to
3)
the
output impedance
is
extremely
low.
The
oscillator
frequency
of
970
±
19
Hz
is
electronically
switchable to
97
± 2
Hz.
In
mode
97 Hz
capacitors
521 and
525
are short-circuited
by
parallel-mounted
transistors
314
and
312.
These
capacitors
are
part of the
double,
series
connected,
capacitors of the
Wien-fourpole
viz
518,
521/524,
525.
If
only capacitors
518
and
524
are
active,
the
frequency
amounts
to
97
Hz.
The
control
of
the transistor
switch
is
shown
in
Fig.
16.
Switch combination
D
(801/F)
and
SEARCH
(801/D) guarantee
this
frequency changeover
for
C mode
and
push-button
D
(801/F)
depressed.
In
mode
SEARCH
the
frequency remains
970
Hz.
In case
of external supply, the
shunt impedance
of the
Wien-fourpole
is
short-circuited
by
the
switch operated
from
socket 856.
The
external
supply
voltage
is
supplied to the
inverting
input of the operational amplifier
via resistor
647.
3.1.4.
Arrangement
for
mode
SEARCH
Push-button
SEARCH
(801/D)
switches over the
bridge
arrangements
into voltage dividers consisting
of
two
components, conform
to
Figs. 8,
9 and
12.
At
the
same
time
the
characteristic of
the zero indicator
is
linearized
(801/D
5-6),
the blocking of the
internal
frequency changeover
for
L
measurement
is
completed
(801/D
15-16)
and
the blocking of the
sensitivity
increase
is
prepared
for the three
first
R
measuring
ranges
(801/D
12-13).
This
last
blocking
becomes
active
when
push-button
R
(801
/A
12-13)
is
depressed.
In
this
searching
circuit,
the
impedance
of the
sample
is
compared
with the measuring-range
resistance
and
the
measuring-range switch
is
operated
until
the
indicator points to the
wvfield
on
the
measuring
instrument.
There
are
limit cases,
for
which
the coarse range indicated
does
not apply.
The measurement
then
must
be
performed
in
the adjacent
range.
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