Theory of
Operation-213
Service
Sweep
Generator
and
Trigger
Integrated
circuit
U335
is
a
combination
trigger and sweep
generator. The trigger
input
sources
are
either internal
or
external
as
selected
by
S315. External trigger signals applied
through
J315
are
de coupled
through
a 10: 1
attenuator
net-
work
consisting
of
R314, R315,
C314, and C315. Internal
trigger signals
are
derived
from
a sample
of
the Oscilloscope
Preamplifier
output
and are
ac
coupled
through
C316. The
triggering level, and the slope at
which
triggering occurs,
is
set
by
R316.
CR332
and
CR333
provide
input
protection
to
U335.
The sweep generator
portion
of
U335
is
a
Miller
integrator
that
generates a linear
sawtooth
output
waveform. Its slope
is
controlled
by
the setting
of
S300, and
is
calibrated
by
R312. The ratio
of
R330
and R332
sets
the sweep length
to
about
12 divisions. When the generator
is
triggered
from
an
internal source,
it
operates either in a
normal
or
automa-
tic
mode depending
on
the voltage at
pin
15
of
U335.
In
the normal mode
of
operation
a negative 5 volts
is
applied
to
pin
15, and when the
input
signal
is
not
sufficient
to
cause triggering there
is
no trace. In the
automatic
mode
the negative 5 volts
is
removed, and when the
input
signal
is
not
sufficient
to
cause
triggering the sweep free-runs
at
a
rate set
by
C334.
Pin 4
of
U335
provides
an
unblanking
gate signal
coincident
with
the
sawtooth
waveform.
This
gate
controls
the
Z-Axis
Amplifier
and provides
crt
unblank-
ing
during
the
horizontal
sweep. C327 and C328 establish
the sweep
holdoff
time.
OUTPUT
SECTION 0
Vertical Output Amplifier
The
Vertical
Output
Amplifier
section consists
of
an
output
amplifier
driven
by
one
of
two
differential
amplifiers, one
for
the
digital
multimeter
function
and the
other
for
the
os-
cilloscope
function.
The Y signal
from
the Character Generator drives the digital
multimeter
vertical
differential
amplifiers
0350
and
0355.
They
are
turned
on
when
S355 applies voltage
to
the
emit-
ters and
bases,
and
their
gain,
which
sets
the vertical size
and
position
of
the characters,
is
controlled
by
R352 and
R354.
The vertical signal
from
the Oscilloscope Preamplifier drives
the oscilloscope vertical
differential
amplifier
0346
and
0348.
This
amplifier
is
turned
on
by
0347
and
0374
which
operates when S355 applies voltage
to
them.
0347
is
a
current
source
for
the
amplifier
and its
base
voltage
is
set
by
0374,
a temperature compensated voltage supply.
The
Vertical
Output
Amplifier
directly
provides the
crt
vertical
deflection
voltages and consists
of
transistors
0340
and
0345
whose
output
is
inductively
peaked
by
L340
and
L344. Rectifiers
CR350
and
CR355
prevent saturation
of
the
output
amplifier.
3-10
Horizontal Output Amplifier
The
Horizontal
Output
Amplifier
section consists
of
a hori-
zontal
output
amplifier
driven
by
a
differential
amplifier
for
the
digital
multimeter
function
or
a
differential
amplifier
for
the oscilloscope
function.
The X signal
from
the Character Generator drives
the
digital
multimeter
horizontal
differential
amplifier
0380
and
0385.
Transistors
0380
and
0385
are
turned
on when S355
is
in
the
DMM
mode,
which
applies voltage
to
their
emitters
and
bases.
Amplifier
gain,
which
controls
the
horizontal
size
of
the characters,
is
set
by
R382 and R384, whose
ratio
also
sets
the
horizontal
position
of
the characters. A
portion
of
the vertical signal
from
the Character Generator Y
output
is
fed
to
0385
through
R358
to
cause
a slight
tilt
in the nu-
merical readout display giving
it
a more pleasing appearance.
The sweep generator
output
drives the oscilloscope horizon-
tal
differential
amplifier
consisting
of
0366,
0368,
0370,
and
U365A
through
U365E. Transistors
0366
and
0368
are
turned
on
by
0370
and
0374
which
operate
only
when
S355 applies voltage
to
them. The
horizontal
position
of
the signal
is
determined
by
a
current
ratio between
0366
and
0368
that
is
set by R378
which
controls
the
base
vol-
tage on
0368.
Amplifier
gain
is
set
by
the
ratio
of
R366
and
R368
to
R360
and R364. Magnifier gain
is
controlled
by integrated
circuit
U365
operating
as
a gain cell whose
current
gain
is
controlled
by
the voltage on the
bases
of
U365E
and
0370.
The magnifier gain
is
set
by
the
ratio
of
the
current
through
R372
to
the
current
through
R366
and
R368
minus
the
current
through
R365 and R369. The cur-
rent
ratio
of
U365A
through
U3650
follows
the signal
input;
therefore, the gain cell maintains a constant
magnifier
gain
setting.
The
horizontal
output
amplifier
provides the
crt
horizontal
deflection
voltages and consists
of
high voltage transistors
0340
and
0345.
Rectifier
CR360
keeps
0360
from
satura-
ting.
2-Axis Amplifier
The
Z-Axis
Amplifier
(or
unblanking
amplifier)
consists
of
VR395,
0390,
and
0394.
When
an
unblanking
gate
from
the Sweep Generator
or
Character Generator
is
present at
the
base
of
0390,
it
turns
off
and
0394
turns
on. When
0394
turns
on, a positive voltage set
by
V R395
causes
the
crt
to
unblank
(or
turn
on).
0394
is
a speedup device
which
supplies additional
current
to
unblank
the
crt
more
rapidly.
CRT
CIRCUIT
~
The
crt
is
a
Tektronix
designed standard
battery
operated
oscilloscope device
which
has
its phosphor
modified
to
en
-
hance the
DMM
readout
function.
@
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