Tektronix 545A Instruction Manual page 15

Operating
Instructions
—
Type
545A
HORIZONTAL SWEEP OF
CRT
BEAM
Fig.
2-5.
In
triggered-sweep
operation the horizontal
sweep
of
the
crt
beam
is
started
by
the trigger
waveform.
or
by an
externally derived waveform.
This
selection
is
made
by
the
setting of the
TRIGGER SLOPE
control.
Each
type
of triggering has certain
advantages
for
some
applica-
tions.
Triggering
from
the displayed
waveform
is
the
method
most
commonly
used.
The
displayed
waveform
is
selected
when
the
TRIGGER SLOPE
control
is
in
either the
+
INT. or
—
INT.
position. Internal triggering
is
convenient
since
no
external triggering connections
are required.
Satisfactory
results
are obtained
in
most
applications.
When
you are observing a waveform
which
is
related
to
the
line
frequency, you
may
wish
to
trigger the
sweep
from
a line-frequency
waveform.
This
can be done by
placing
the
TRIGGER SLOPE
switch
in
either the
+LINE
or the
—
LINE
position. This
type
of triggering
is
particularly use-
ful
in
observing waveforms
which bear a
definite relation-
ship
to
the
line
frequency. The shape and
the amplitude
of
the triggering
waveform
remains
constant
to
provide
ex-
tremely
stable triggering.
To
trigger
the
sweep
from
some
external
waveform,
connect
the triggering
waveform
to the
appropriate TRIG-
GER
INPUT
connector
and
place the
TRIGGER
SLOPE
switch
in
either the
+EXT.
or
—
EXT. position. External
triggering provides
definite
advantages
over other
methods
of triggering
in
certain cases.
With
external triggering, the
triggering signal usually remains
constant
in
amplitude
and
shape.
It is
thereby possible
to
observe
the
shaping
and
amplification of a signal
in
an
external
circuit
without
re-
setting the oscilloscope triggering controls
for
each observa-
tion. Also, time
and
phase
relationships
between
the
waveforms
at different points
in
the
circuit
can be
seen.
If,
for
example,
the external
triggering signal
is
derived
from
the
waveform
at the input to
a
circuit,
the
time
rela-
tionship
and
phase
of the
waveforms
at
each
point
in
the
circuit
are
compared
to
the
input signal
by
the display
pre-
sented on
the oscilloscope screen.
When
you are
using external triggering
and
a stable
triggering signal,
it
is
possible
to
observe and
accurately
measure
jitter
of the
displayed waveform.
This
is
not pos-
sible
when
the
sweep
is
triggered
internally.
Selecting the Triggering Slope
The
horizontal
sweep
can be triggered on
either
the
rising
j
+
slope)
or
falling
(
—
slope) portion of the triggering
waveform
as
determined
by the position of the
TRIGGER
SLOPE
switch.
When
the switch
is
in
one
of the
+
posi-
tions,
the
sweep
is
triggered
on
the
rising
portion of the
triggering
waveform;
when
the
TRIGGER SLOPE
switch
is
in
one
of the
—
positions, the
sweep
is
triggered
on
the
falling
portion of the
waveform
(see
Figure
2-7).
In
many
applications the triggering slope
is
not impor-
tant since triggering on
either slope
will
provide a display
which
is
suitable
to
the application.
However,
in
many
other
cases,
such
as pulse
measurements,
the triggering
slope
is
very important.
If,
while using a
fast
sweep, you
may
wish
to
observe
the
rise
of a pulse,
it
will
be
necessary
for
you
to
trigger the
sweep
on
the rising portion of the
waveform
by
placing the
TRIGGER SLOPE
switch
in
one
of
the
+
positions.
To observe
the
fall
of a pulse at a
fast
sweep
rate,
it
will
be necessary
to trigger the
sweep
on
the
falling
portion of the
waveform
by placing the
TRIGGER
SLOPE
switch
in
one
of the
—
positions.
In
either case,
selection
of the
wrong
triggering slope
will
make
it
im-
possible
for
you
to
observe
the
portion
of the
waveform
you wish
to
cheek.
Selecting the Triggering
Mode
After selecting the triggering
source
and
triggering slope,
it is
next necessary
to select
the triggering
mode
which
will
allow you
to
obtain
the desired display.
Time Base
A
has
five triggering
modes
while Time Base
B
has
three.
Not
available
for
Time Base
B are the
HF SYNC
and
AC
LF
REJECT
triggering
modes.
Each
of
the
triggering
modes
is
designed
to
provide
stable triggering
from a
certain
type
of
waveform.
For
most
applications,
however,
several of the triggering
modes
will
work
equally
well.
For applications of
this
type, the
trig-
gering
mode
used
is
purely a
matter
of choice.
The
primary
thing to consider
in
choosing
the triggering
mode
is
whether
or not
it
allows you
to
obtain
the display
you want.
To determine
the best
mode
of
operation
for
a
particular
application,
it is
usually best
to try
each
triggering
mode
in
the application.
The
AUTOMATIC
mode
should be
tried
first
since
this
triggering
mode
provides
stable triggering
in
most
applications without the necessity
of setting the
STABILITY
or
TRIGGERING
LEVEL
controls.
If
the
AUTO-
MATIC mode
does not provide the desired display,
it
will
then be necessary
for
you
to try
one
or
more
of the other
triggering
modes.
Automatic
Triggering
Mode
The
AUTOMATIC
mode
is
most
frequently used
because
of
its
ease of operation.
This
mode
is
useful
in
obtaining
stable triggering from
waveforms
with frequencies of from
approximately 60
cycles to 2
megacycles. The
principal
advantage
of this
type
of
operation
is
that
it is
not neces-
sary
to adjust either the
STABILITY
or
TRIGGERING
LEVEL
controls
to
obtain a
stable display. This permits
you
to
observe a
large
number
of
waveforms
with
different
shapes
®2
2-5