Tektronix SC 502, Instruction Manual

Page: 35 / 127

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Section 3-SC 502
THEORY OF OPERATION
DIAGRAM
<D
&
<8>
CIRCUIT DESCRIPTION
Vertical Amplifier
The
vertical amplifier
pr
o
v
i d e s attenuation and
amplification for the vertical signal before it is applied to
the vertical deflection plates of the crt.
The v
e
rt
i
ca
l
amplifier circuitry
i n c
l
u d e
s
the delay
line
and
trigger
view
circuit.
which allows the oscilloscope
to
display
i
t
s
triggering
signal.
NOTE
Where both channels 1 and 2 are similar, only
channel 1 will be described.
Input Coupling
Signals
applied
to
the front-panel input connectors
may
be
capacitively
coupled (AC).
directly
coupled (DC).
or i n t e
r
n
a l l y disconnected (GND). Input
coupling
is
selected by 5100 at the input for channel 1.
Assuming that a s i
g
n a l Is applied to the input.
w
h e n
5100 Is
set to
DC,
the
a
p p l
i
e d signal is passed directly
to
the attenuators. When
5100 is
set to
AC,
C100 is placed
in
the circuit to couple
signals of
about
10 Hz
(-3 dB
pOint)
and higher to the attenuator. This capacitor blocks any dc
component of the signal. When 5100 is set to GND. a
ground
reference
is provided
to
the
input of the
amplifier
without the need to remove the applied signal from the
input connector.
NOTE
When dc levels (above
10
V)
are
to be blocked
by 8C
coupling. the AC-GND-DC
switch should be set to
GND while input
connections are made or broken. or
when
voltage levels
are changed. This will allow the
coupling capacitor
to
charge without overdriving the
amplifier.
Input Attenuator
The input attenuator is a frequency-compensated
voltage divider and provides
100X
attenuation in positions
0.5 to
20 of
the
VOL TS/DIV
switch. At dc and
for
low
frequency signals. the divider is essentially resistive
(attenuation ratio determined by the resistance ratio).
REV 0 APR 1980
In addition to providing constant 100X attenuation at all
frequencies
within
the bandwidth capabilities
of
the
instrument, the input attenuator maintains a constant
input rc characteristic (1 MO paralleled by about
47 pF).
Preamplifier Stage
The
preamplifier consists
of
two identical operational
amplifiers. connected in a differential configuration. Fig.
3-1 shows a simplified block diagram
of
the preamplifer.
Input
Slgn.1
G8In SettIng
RHI It
or
Selected by
VOL TSlDIY Switch
Supply
Supply
Output
Sign.'
1878·17A
Fig. 3-1. Input pr
•
...,pllflerdet
.. led block clagr..., Ihowlng IIgn.1
curr.nt
p8lhl.
3-1