Preamp output signals to
be
coupled to the Vertical
Output
Amplifier.
D201
through D204 control the Channel l
output
and
D206 through D209 control the Channel 2 output. These
diodes
are
in
turn controlled by the Switching Multivibrator
for dual-trace displays,
or
by the MODE switch for single-
trace displays.
CH
1.
In
the
CH
l position
of
the MODE switch,
-12
volts
is
applied
to the junction of D207-D208
in
the Chan-
nel
2 Diode
Gate
through
R227
(see simplified
diagram
in
.fig. 3-5).
This
forward biases D207-D208
and
reverse biases
D206-D209 since the input to the Delay-Line Driver
stage
is
at
about
-5.8
volts. D206-D209 block the Channel 2 signal
so it
cannot
pass to the Delay-Line Driver stage. At the
same
time,
in
the Channel l Diode
Gate,
D202-D203
are
con-
nected to ground through
R212.
D202-D203
are
held reverse
biased while D201-D204
are
forward biased. Therefore, the
Channel
l signal passes to the Delay-Line Driver stage.
CH
2.
In
the
CH
2 position
of
the MODE switch, the
above
conditions
are
reversed. D202-D203
are
connected to
-12
volts through
R217
and
D207-D208
are
connected to ground
through
R222.
The Channel l Diode
Gate
blocks the signal
and
the Channel 2 Diode
Gate
allows it to pass.
Switching
Multivibrator
Al
T.
In
this
mode
of
operation,
the
Switching
Multi-
vibrator
operates
as a
bistable
multivibrator.
2
In
the
ALT
position
of
the
MODE
switch,
-12
volts
is
applied
to
the
emitter
of
the
Alternate
Trace
Switching
Amplifier
stage,
0234
by
the
MODE
switch.
0234
is
forward
biased
to
supply
current
to
the
"on"
Switching-Multivibrator
tran-
sistor
through
R234
and
D218
or
D228.
For
example,
if
0225
is
conducting,
current
is
supplied
to
0225
through
D228.
The
current
flow
through
collector
resistors R221
and
R222
drops
the
D207-D208
cathode
level negative so
the
Channel 2 Diode
Gate
is
blocked
as
for
Channel 1
only
operation.
The
signal passes
through
the
Channel 1 Diode
Gate
to
the
Delay-Line Driver stage.
The
Alternate-trace
sync
pulse
is
applied
to
0234
at
the
end
of
each sweep. This negative-going
sync
pulse
moment-
arily
interrupts
the
current
through
0234
and
both
0215
and
0225
are
turned
off.
When
0234
turns
on
again
after
the
alternate-sync
pulse,
the
charge
on
C218
determines
whether
0215
or
0225
conducts.
For
example,
when
0225
was
conducting,
C218
was
charged
negatively
on
the
D218
side
to
the
emitter
level
of
0215,
and
positively
on
the
D228
side. This charge
is
stored
while
0234
is
off
and
holds
the
emitter
of
0215
more
negative
than
the
emitter
of
0225.
When
both
0215
and
0225
were
off,
the
voltage
at
their
bases
became
approximately
equal.
Now
when
0234
comes
back
on,
the
transistor
with
the
most
negative
emit-
ter
will
start
conducting
first
with
the
resulting negative
movement
at
its
collector
holding
the
other
transistor
off.
The
conditions
described
previously are reversed;
now
the
Channel 1 Diode
Gate
is
reversed biased
and
the
Channel 2
signal passes
through
the
Channel 2 Diode Gate.
The Reference Feedback stage,
0253,
provides common-
mode
voltage
feedback from the Delay-Line Driver
stage
to
allow the
diode
gates
to
be
switched with a minimum ampli-
tude
switching signal. The emitter level of
0253
is
con-
nected to the junction
of
the Switching Multivibrator collector
2
Jacob
Millman
and
Herbert
Taub,
"Pulse,
Digital
and
Switching
Waveforms"
McGraw-Hill, New York,
1965,
pp.
362-389.
REV. B, OCT. 1975
Circuit
Description-Type 453/R453
resistors,
R21
l
-R212
and
R221-R222
through D213
or
D223.
The collector level
of
the
"on"
Switching Multivibrator trans-
istor
is
negative
and
either D213
or
D223
is
forward biased.
This
clamps the
cathode
level of the forward-biased shunt
diodes
in
the
applicable
Diode
Gate
about
0.5 volts more
negative than the emitter level
of
0253.
The shunt diodes
are
clamped
near
their switching level
and
therefore they
can
be
switched very fast with a minimum amplitude switch-
ing signal. The level
at
the emitter
of
0253
follows the
average
voltage
level
at
the emitters of the Delay-Line
Driver stage. This maintains
about
the
same
voltage
dif-
ference across
the
Diode
Gate
shunt diodes so they
can
be
switched with a minimum amplitude switching signal regard-
less of the deflection signal
at
the
anodes
of the shunt diodes.
CHOP.
In
the CHOP position of the MODE switch, the
Switching Multivibrator free runs
as
an
astable
multivibrator
3
at
about
a 500-kHz rate. The emitters
of
0215
and
0225
are
connected to
-12
volts through
R218
and
R228.
At the
time of turn-on,
one
of
the transistors begins to conduct; for
example,
0225.
0225
conducts the Channel 2 current
and
prevents the Channel 2 signal from reaching the Delay-Line
Driver stage. Meanwhile, the Channel
l Diode
Gate
passes
the
Channel l signal to the Delay-Line Driver.
The frequency-determining components
in
the CHOP mode
are
C218-R218-R228. Switching action occurs
as
follows:
When
0225
is
on, C218 attempts to
charge
to
-12
volts
through
R218.
The emitter of
0215
slowly goes toward
-12
volts
as
C218 charges. The
base
of
0215
is
held
at
a nega-
tive point determined by
voltage
divider
R215-R224
between
-12
volts
and
the collector of
0225.
When
the emitter volt-
age
of
0215
reaches a level slightly more negative than
its
base,
0215
conducts. The collector level
of
0215
goes
negative
and
pulls the
base
of
0225
negative also, through
divider
R214-R225,
to cut
0225
off.
When
0215
turns on,
its emitter
is
pulled positive along with C218.
This
action
switches the Diode
Gate
stage
to connect the opposite half
to the Delay-Line Driver stage. Again C218 begins to
charge
towards
-12
volts but this time through
R228.
The emitter
of
0225
slowly goes negative
as
C218 charges, until
0225
turns on.
0215
shuts off
and
the cycle begins
again.
Diodes D218
and
D228 have no effect
in
the CHOP mode.
0253
operates
the
same
in
CHOP
as
in
ALT,
to allow the
Diode
Gates
to
be
switched with a minimum signal level.
The
Chopped
Blanking Amplifier stage, Q244, provides
an
output pulse to the Z Axis Amplifier which blanks out
the transition between the Channel
l trace
and
the Channel
2 trace.
When
the Switching Multivibrator changes states,
the current through
T241
momentarily changes. A negative
pulse
is
applied
to the
base
of
0244,
to turn
it
off. The width
of
the pulse
at
the
base
of
0244
is
determined by
R241
and
C241.
0244
clips the signal
applied
to
its
base,
and
the
positive-going output pulse, which
is
coincident with trace
switching,
is
applied
to the Z Axis Amplifier circuit through
R245.
ADD.
In
the
ADD
position of the MODE switch, the Diode
Gate
stage
allows both signals to pass to the Delay-Line
Driver stage. The Diode
Gates
are
both held on by
-12
volts
applied
to their cathodes through
R260
and
R270.
Since
both signals
are
applied
to the Delay-Line Driver stage, the
output signal
is
the
algebraic
sum of the signals on both
Channe·I
l
and
2.
3
lbid.,
pp.
438-451.
3-7
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