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Model 1821A
Section
IV
Paragraphs
4-21
to
4-35
4-21.
As
the
main sweep was
being generated,
the
positive-going
ramp
was
also applied
to
the hold-off
emitter followers,
Q210
and Q211.
The
positive-
going
signal
at
the
emitter
of
Q210was
coupled through
CR218
and
CR219
and discharged
the
selected hold-off
capacitor (C514
throu^
C520).
When
the
ramp
ended,
GR218
turned
off
and
the
hold-off capacitor
began
to
charge
negatively.
As
the
capacitor charges,
CR217
turns
on and connects
the
hold-off capacitor
to
the
base
of
Q205.
When
the
charge on
the hold-off
capacitor
reaches
the
lower hysteresis
limit
of
the
Schmitt
trigger,
Q205
turns
off,
turning
on Q204.
Q204 now
supplies
current
to
CR201
"resetting"
it
to
a pretrig-
ger
condition.
The
time between
the
end
of
one
sweep and
the
turn-on
of
Q204
is
called hold-off.
A
new sweep
cannot be started
until this
time has
elapsed.
Hold-off
is
varied
slightly
as
the
Main
TRIGGER
LEVEL
control
is
adjusted
by varying
the voltage
at
R254.
This provides a
stable display
of
certain dis-
crete high
frequency
signals.
4-22.
SINGLE.
When SINGLE
is
selected with the
SWEEP MODE
switch,
the output
from
the hold-off
circuit
is
clamped
to
about
0 v
by
CR220.
S201
must
be
depressed
to
provide
the
negative signal
that
turns
off
Q205, switching
the
Schmitt
trigger
to
a pretrigger
state
and
resetting
CR201.
At
this
time,
Q203
is
turned
on,
shorting
R217
and
lighting
front-panel
in-
dicator
DS201
.
RESET
lamp DS201
therefore
indicates
that
CR201
is
"reset".
The
next trigger pulse
will
switch
CR201
to
a high-voltage
state
and
the
pre-
vious cycle
will
repeat.
4-23.
AUTO.
At
the
time
Q205
is
turned
off,
apositive
pulse
from
the collector
of
Q205
is
applied
to
the
base
of
Q109
(Figure
8-4).
If
the
SWEEP MODE
switch
is
set
to
either
NORM
or
SINGLE,
the
monostable mul-
tivibrator,
QllO/Qlll,
is
disabled,
causing
CR117
to
open.
Q112
is
then biased
into
conduction,
turning
off
Q109.
The
auto trigger
from Q205
is
blocked by
Q109
and has no
effect
on
the
circuit.
4-24.
If
however,
AUTO
is
selected
(no
trigger signal
applied)
QUO
turns
on,
turning
off
Qlll.
The
negative
voltage
from
the collector
of
Qlll
turns
off
Q112.
The
relatively positive
voltage
from
the collector
of
Q112
is
blocked by
CR114,and Q109
conducts
when
the
auto
trigger
from Q205
is
applied.
The
current
from Q109
triggers
CR201, and
the
main sweep
circuit
free-runs
—
CR201
being
fired
by
the
current
from Q109
each
time
it
is
reset by
the
current
from
Q204.
4-25.
When
a trigger signal
is
applied, the
negative
pulses
from
the collector
of
Q108
are coupled
to
the
base
of
Qlll,
switching
the
monostable
multivibrator
to
its
non-stable
state
(QUO
off
—
Qlll
on).
If
the
frequency
of
the
trigger signal
is
above approximately
40 Hz,
the
multivibrator does
not
return
to
its
stable
state.
The
relatively positive voltage
from
the col-
lector
of
Qlll
is
blocked by
CRl
17
and
Q112
is
biased
into
conduction.
The
negative signal
from
the collector
of
Q112
turns
off
Q109,blocking
the
auto trigger.
The
next pulse
from Q108
will fire the
gate circuit
and
start
a sweep.
If
the
frequency
of
the trigger signal
drops below 40 Hz,
the
sweep
circuit will
be
alternately
triggered and free-
run,
providing an unstable
display.
4-26.
DELAYED
SWEEP.
4-27.
A
block
diagram
of
the
delayed
sweep
circuit
is
shown
in
Figure
4-3.
The
delayed
sweep
circuit
is
similar
to
the
main sweep
circuit.
There
are,
how-
ever, three
main
differences:
(1)
the hold-off circuit
is
replaced by
the
sweep
comparator;
(2)
there
is
no
auto
circuit;
(3)
triggering
from
the
power-line
wave-
form
is
replaced by automatic
triggering.
4-28.
BLOCK
DIAGRAM
DESCRIPTION.
4-29.
The sweep comparator
generates a
positive
pulse
at
a point on
the
main
sweep, determined by
the
setting
of
the
CM
DELAY
control.
The
pulse
from
the
sweep comparator
switches
the
delayed Schmitt
trig-
ger
to
a pretrigger
state,
resetting the gate generator.
Setting the
delayed Trigger Source switch
to
AUTO
allows
this
same
pulse
to
generate a trigger
that fires
the gate
generator.
In
AUTO,
the
delayed
sweep
starts
immediately
at
the
end
of
the delay.
Selecting either
INT
or
EXT
with
the
delayed
Trigger
Source
switch,
allows
the
selected trigger signal
to
produce
the
pulse
that fires
the gate
generator.
Using
internal
or ex-
ternal trigger signals
cause
the
delayed
sweep
to
start
on
the
first
trigger signal after the delay.
4-30.
The
negative output
from
the gate
generator
(when
fired)
is
applied
to
the
sweep
display switch
and
to
the integrator.
The
integrator,
when
undamped
by
this
signal,
generates a positive-going
ramp
that
is
applied
to
the
sweep
display switch
and
to
the
Schmitt
trigger.
When
the
sweep
reaches a pres
elected
ampli-
tude, the
Schmitt trigger changes
state,
turning
off
the gate
generator, terminating
the
sweep.
4-31.
The sweep comparator
will
again generate a
positive
pulse
atsometime
duringthe
next
main sweep
and
the
previous cycle
will
repeat.
4-32.
CIRCUIT DETAILS.
4-33.
The
following
paragraphs
contain
more
detailed
information about
the
delayed
sweep
circuits.
Refer
to
the
schematic diagram
in
Figures 8-7 and
8-8.
4-34.
The
positive-going
main
sweep,
applied
to
the
base
of
Q411
(Figure 8-8) eventually
causes
the
tran-
sistor
to
conduct
hard enough
to
switch
tunnel diode
CR423
to
a high-voltage
state.
Since the
DELAY
(DIV)
control
determines
the gate
voltage
of
Q412
it
also
determines
the bias
on Q411.
As
the
DELAY
(DIV)
control
is
varied,
Q411
switches
the tunnel diode,
CR423,
to
a high-voltage
state at
various times dur-
ing
each sweep.
The
negative signal
generated by
CR423
is
inverted by
Q410
and coupled
to
the
base
of
Q403.
The
positive signal turns
on Q403,
switching
the
Schmitt trigger
to
a pretrigger
state,
resetting
CR401.
The
positive signal
from Q410
is
also
differ-
entiated
by C422 and
R459
and coupled
to the
base
of
Q409. SinceiQ409
is
normally
cut-off,
only
the positive-
going pulses are applied
to
the
front-panel
BNC
con-
nector,
J401.
4-35.
If
Auto
is
selected with
the
delayed Trigger
Source
switch (Figure
8-7)thepositivepulsefrom
Q410
02591-3
4-3
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