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display
on
the crt so the waveform
can
be examined
in
detail.
For this type
of display,
the start of each
sweep
must
bear
a definite fixed-time
relationship
to the events
in the input signal.
This can be accomplished
by using the
displayed signal or another related signal to start (trigger)
single
or
repetitive
sweeps.
The
following
is a detailed
description of the control and switch settings which provide
complete
control
over
the
means
of triggering
the
sweep.
It is assumed
that the oscilloscopes
system
is a Tektronix
Type 561A.
TRIGGER
MODE
FREE RUN.
Free-running operation produces continuously
repetitive sweeps even in the absence of a triggerifg signal.
These sweeps
provide a reference trace, as does the AUTO
position.
This method of operation is useful in applications
where a device under test requires a trigger or input signal.
The front-panel +GATE OUT signal may be used to operate
the
device
under
test.
The
resulting
signal
displayed
on
the crt will then be synchronized with the sweep.
AUTO.
This position is frequently used for ease of opera-
tion and
because
of the reference
trace
produced
in the
absence of a triggering signal.
The time base free runs with-
out the application
of a trigger.
If a triggering
signal
is
received, the free running is interrupted, but this first event
in the signal does not trigger a sweep.
If the first signal
event
is followed
by a second
event within
about
80 msec,
a triggered sweep
is initiated, and if not, the free running
resumes,
Since the dormant
period
is limited to about 80
msec, signal frequencies below about 20 cps cannot produce
a triggered sweep in the AUTO
mode.
For such signals, the
NORM
mode of operation is used.
With
the SOURCE
switch
set to INT and
the TRIGGER
MODE
switch set to AUTO,
the sweep will trigger on any
signal
that will
trigger
the
NORM
mode
(about
1 minor
division
of deflection)
so
long
as the triggering
signal
is
above about 20 cps.
When
operating
in the AUTO
mode,
it is necessary to adjust the TRIGGERING
LEVEL contro! to
lock the sweep with the triggering signal.
NORM.
In the NORM
or normal mode, the time base is
dormant in the absence of a triggering signal.
Each sweep
must be initiated by a triggering signal.
SINGLE SWEEP.
Single sweep is often used when photo-
graphing
non-reptitive waveforms and in other applications
where the vertical input signal continually varies in amplitude
shape, or time interval.
A continuous display of such signals
would
appear
as
a
jumbled
mixture
of
many
different
waveforms
and would
yeild little or no useful information.
The Type 3B4 permits a single sweep to be presented with
the elimination
of all subsequent
sweeps.
The
information
in the one sweep
is thus clearly recorded without the con-
fusion
resulting
from
multiple
nonrepetitive
traces.
When the TRIGGER MODE switch is set to SINGLE SWEEP,
the time base becomes
inoperative.
The time base can be
"reset" to the triggerable condition
by pressing the RESET
button.
If there
is sufficient
delay
before
triggering,
the
RESET lamp will light to show that the time base is ready to
be triggered.
When
the time base has been triggered and
one sweep completed, the time base again becomes inopera-
tive and
the lamp
extinguishes.
@
Operating
Instructions—Type
3B4
SOURCE
INT.
It is usually easiest to obtain the sweep triggering
signal internally (INT) from the vertical deflection system.
LINE.
If the displayed signal frequency is related to the
power-line
frequency,
the
line source
can
be
used.
This
source
is useful when
the displayed signal does not allow
internal
triggering.
EXT.
External triggering is often used when signal tracing
in amplifiers, phase-shift networks, and wave-shaping circuits.
The signal from a single point in the circuit can be connected
to the EXT TRIG
IN connector through a signal probe or a
cable.
With this signal triggering the sweep, it is possible to
observe the shaping, amplification, and time relationship of a
signal
at various points in the circuit without resetting the
triggering controls.
EXT.
+
10.
The only difference
between
external
(EXT)
and
external
divided-by-10
{EXT
+
10) is that the latter
attenuates the external triggering signal. Attenuation of high-
amplitude external triggering signals is desirable to broaden
the TRIGGERING
LEVEL contro! range.
DC.
De
coupling
allows
the trigger
circuits to receive
signals of all frequencies from de upward.
It is best to de
couple for very low-frequency signals (below about 20 cps).
AC.
Ac coupling
rejects the de component
of triggering
signals and
increasingly attenuates ac triggering signals as
the frequency decreases.
This position of the coupling switch
is not normally used for triggering signals below about 20
cps, but this is the most used position of the switch.
AC
LF REJ.
Ac
low-frequency
reject coupling
rejects the
dc component of triggering signals and increasingly attenu-
ates ac signals
as frequency
decreases
(30%
down
point
17 ke).
If line-frequency hum
is mixed with a desired high-
frequency
triggering
signal,
best
results
are
obtained
by
using this position
of the COUPLING
switch.
Ac
low-frequency
reject
coupling
should
also
be
used
when
triggering
internally
from
multi-trace
plug-in
units
operated
in the
alternate-trace
mode
(unless
the
"trigger
from a single channel only" feature of the plug-in is used).
For additional information, see the multi-trace vertical plug-
in unit instruction manual.
SLOPE
Sweeps can be triggered during either the rising or falling
portion
of the triggering
signal.
When
the display consists
of several
cycles of the input signal, either setting of the
SLOPE
switch
may
be
used.
However,
if it is desired
to
display less than one full cycle of the input signal, the SLOPE
switch
permits
the
sweep
to
start
on
either
the
rising
(+
slope) or falling (— slope).
TRIGGERING
LEVEL
The
TRIGGERING
LEVEL
control
determines
the
instanta-
neous voltage on the triggering signal at which the sweep is
triggered,
(This instantaneous voltage can also include a dc
2-3
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