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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