the
circuits
for
four
types,
or
modes,
of
triggering.'
In
the
DC
position,
the
triggering
signal
is
de
coupled
as
far
as
the
trigger
shaper
stage.
In
the
AC
position,
blocking
capacitor C4
removes
the
de
component
of
the
signal.
In
the
AUTO,
position
of
SW5,
the
plate
of
the
A
section
of
the
trigger
shaper, V30,
drives
the
grid
of
the
B
section
just
as
it
also
drives its
own
grid
through
R45
,
a
resistance
of
several megohms.
This
plate-to-grid
coupling
allows
the
trigger
shaper
to
free-run
when
no
triggering
signal
is
present. The
addition
of
R45
causes
the
trigger
shaper
to
free
run
when
no
trigger
signal
is present. For example,
when
the
plate
of V30A
rises,
the
grid
of
V30B
also rises,
carrying
with
it
the
right-hand
end
of
R45.
The
left-hand
end
of
R45 is
connected
to the
A
grid
through
R22.
The
time
constant
of
the
rc
circuit
between
the B
grid
and
ac ground
through
C20,
R22
and
R45
is
of
such
length
that
it
takes about
.01
second
for
the
V30A
grid
to
rise
exponentially
from
its
starting point
below
cutoff
to
a
point
where
plate
current
can
flow.
When
V30A
plate
current
flows,
the
plate
drops,
forcing
the
V30B grid
down,
and
thus
the right-hand
end
of R45
is
forced
down.
The
left-hand
end of
R45
and
the
A
grid
immediately begin
to
drop
exponentially
toward
cutoff.
When
the
A
grid
reaches
cutoff again
it
has
completed
one
cycle
of
the
approximately
50-cycle
triangular
waveform.
The
range
of
the V30A
grid
voltage
between
A
cutoff and
B
cutoff
is
about
3
volts
for
the
circuit
used
in the
AUTO.
mode.
This
is
increased
from
about
.5
volt
for
the
AC
and DC
modes
by
the
addition
of
R45
to
the
circuit.
Since
the
V30A
grid
is
never
more
than
3
volts
from
cutoff,
a trigger
signal with
a
peak-to-peak
voltage
of
three
volts or
more
can
drive
the
grid
to
cutoff at any
time
and
produce
a trigger
output.
Smaller
signals
can
also trigger
the
shaper
but
only
if
they
occur
at
a
time
when
the
grid
is
within
their
peak
voltage
of
cutoff.
The
duty
cycle
of
operation
of
the
time-base
generator
is
somewhat
reduced
therefore with
smaller
trigger
signals.
This
circuit
configuration
is
useful because
with
it the
time-base
generator
can
be syn
chronized
with
repetitive signals
over
a
wide
range
of
frequencies
without
readjustment.
When
not
triggered
externally, the
generator
con
tinues at
a
50-cycle
rate,
and
in
the
absence
of any
vertical
signal,
generates a base line
that
shows
that
the
oscilloscope
is adjusted
so
as
to
display any
signal
that
might
be
connected
to
the vertical-deflection
system.
In
the
H
F
SYNC
position
of
SW5,
the
trigger
amplifier and
trigger
shaper
stages
are
bypassed
and
the
trigger
signal
is
applied
directly
to
the
swep
multivibrator.
In
this
mode
the
STABILITY
control is
set so the
sweep
is
superimposed
on
the
negative-going
trigger-holdoff
waveform
at
the
grid
of
V110A
and
will
cause
the
multivibrator
to
synchronize
at a
submultiple
of
the
triggering
signal
frequency.
This
circuit
is
suitable
for
signals
in
excess of
five
megacycles.
Schmitt
Multivibrator
The
dc-coupled
multivibrator,
shown
in
the
time-base
diagram,
turns
on
the
time-base
generator
upon
receipt
of
a
negative
trigger
from
the
trigger
shaper,
and
holds
off
sub
sequent
trigger
signals
until
after
the
sweep
is
completed.
The
multivibrator
consists
of
V110A
and
V120
with
both common-cathode
and
plate-to-grid
coupling.
Plate-to-grid
coupl
ing
is by
means
of
a
cathode
follower.
In
the
quiescent
state
VI10
A
is
conducting
and
its
plate
is
down.
Cathode-follower
V110B
holds
the
grid
of
V120B below
cutoff
through
voltage
divider
R115
,
R116.
Cathode-follower
V110B
isolates
the
plate
of
VI
10
A
from
the
various
loads,
and thereby
permits
a
faster
step.
When
the
negative
trigger
pulse
from
the
trigger-shaper
stage
reaches
the
grid of
V110A
it
is
coupled
to
V120B
and
V120B starts
to
conduct.
The
multivibrator
switches
quickly
to its
second
state
with
V120B conducting
and
V110A
cut
off.
The
biases
and
plate
loads
are
adjusted
so
that
when
VI10A
is conducting,
the
grid
of
V120B
is held
below
cutoff,
and
when
V120B
is
conducting
the
cathode
of
VI
10
A
is
held
above
cutoff.
There
are
thus
two
stable
states,
in
either
of
which
the
multivibrator will
remain
until
a
signal
of
the
proper
polarity
and
amplitude
to
the
grid
of
VI
10
A
switches
it
to
the other
state.
To
return
the
multivibrator
back to
the quiescent
state with
V110A
conducting,
a
AA
Circuit
Description
-
Type
515/515A
3-3
