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Circuit des cription
A.
INPUT
CIRCUIT
(Fig. 23)
When the AC-DC
switch SK3 is in position 66AC"
it is possible to observe small AC signals super-
imposed on a maximal 400 V DC-voltage. At the
same time relay RE 101 is energised which switches
capacitor Cl02
into comparison circuit R1-R2
(Fig. 9). In this 'way any DC co'mponent at the input
circuit which may be due, among other things, to the
leakage of C26, is compensated by the action of the
chopper amplifier.
In the "AC"
position of SK3, blocking capacitor
C26 may be charged by a DC input voltage. On
switching over to position "DC"
the capacitor is
discharged to prevent the charge of
C26 from
destroying any delicate test object when switchin_e
over again from DC to AC.
When the step attenuator is put at the 4 smallest
deflection-coefficients (0,5 , l, 2, 5 mY /cm) the input
signal is not attenuated. Trimmers C I8-C2I-C24-C29
allow the input capacitance to be adjusted to precisely
15 pF. For the larger deflection coefficients, attenua-
tor networks are inserted between the input terminal
and the cathode follower 8101. With trimmers C33-
C36-C39-C43-C47-C5 lC54-C58, the input capaci-
tance can be adjusted to 15 pF. Trimmers C32-C34-
C38-C42-C46-C49-C53-C57 allow an adjustment
for optimum square-wave response.
The 1.1 1 Mohm resistor consists of the resistors
R103 (1 Mohm) and R102 (110 Kohm). This value
is choosen to obtain a I Mohm input resistance
because a grid-current compensating network with a
10 Mohm source resistor (R104) is connected at this
point, and it is parallel to the 1.1 1 Mohm resistance.
The grid-current compensating network consisting of
resistors R106-R107-R1 10-R1 15 and potentiometer
R 108 delivers a positive or negative current, de-
pending on the position of R108. This current is
required for cathode follower stage B 101, Which
supplies the input signal to the split-band amplifier
via overload-protecting diodes GRl04-GR106.
Push-button SK2 disconnects the input signal and
earthes the grid of the cathode-follower, to indicate
the zero-level of the display.
Diodes GR 101 and GR 102 protect the chopper
amplifier when the DC part of the split-band ampli-
fier is overdriven.
B. HF-AMPLIFIER
(Fig. 26'|
The AC components of the input signal are fed into
the HF amplifier via emitter-follower TS102. The
three nearly identical stages consist of an ordinary
emitter-amplifier and an emitter-follower.
13
Each stage provides an output which is in anti-phase
with respect to its input. As a whole the HF-amplifier
provides a 180 degrees phase-shift to match the
output of the DC-amplifier.
In the positions "2 - 1
.5 mY / cm" of SK4 the gain
of the H.F. amplifier is increased by decreasing the
current feedback respectively at the 3rd, 2nd and
lst stage. At these positions of SK4 the gain can
be matched to that of the DC amplifier by selecting
resistors as tabulated below.
Position
of SK4
Resístor
Coarse
Fine
5 mV/cm
2 mV/cm
1 mY/cm
.5 mV/cm
R l 38
R 1 7 3
R l 5 8
R 143
R 139
R l74
R l59
R l44
If the measurements permit this, the HF-amplifier
can be taken out of operation by throwing switsh
SK 1 which cuts off only the power supp,ly of the
second stage.
C. I)C-AMPLIFIER
(Fig. 26),
The DC and LF-components of the input signal enter
the DC coupled amplifier at the base of TSI 13,
which is part of difference amplifier TS ll2 - TS rl4.
The correction voltage delivered by the chopper-
amplifier enters at the other emitter-follower (TS11l).
The difference of the two signals is fed, via emitter-
follower TS 116, to two mo,re stages, which consist of
an amplifier and an emitter-follower. At the last
stage, zener-diode GR lo7 provides a low impedance
step-down of the DC level.
when the most sensitive deflection coefficients are
selected, SK4 reduces the emitter resistance of TS ll7
thus increasing stepwise the overall gain of the DC-
amplifier from 10 x to 25 x, 50 x and 100 x. An exact
match with the I i t
attenuator can be carried out
by adjusting potentiometer R208.
D. INTERSTAGE
NETWORK
(Fig. 26)
The outputs of the HF-amplifier and the DC ampli-
fier are combined in cross-over filter R216
Cr57
and fed to emitter-follower TS401 . The 1 / A attenua-
tor is connected to the emitter of TS40l . It consists
of precision resistors R406
R408
R409
R4l I
and R412. Its output is fed to the chopper amplifier
via the 1.1 I Mohm resistors R300
R301. The
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