To
better
understand
the
oscilloscope
controls,
you need
to know
a little
more
about
how
oscilloscopes
display
a
signal.
Analog
oscilloscopes
work
somewhat
differently
than
digital
oscilloscopes.
However,
several
of the
internal
systems
are
similar.
Analog
oscilloscopes
are
somewhat
simpler
in
concept
and are
described
first,
followed
by a
description
of
digital
oscilloscopes.
Analog
Oscilloscopes
When you
connect
an
oscilloscope
probe to
a
circuit,
the
voltage
signal
travels
through
the
probe to
the
vertical
system
of the
oscilloscope.
Following
Figure is a
simple
block
diagram
that
shows
how an
analog
oscilloscope
displays
a
measured
signal.
Analog
Oscilloscope
Block
Diagram
Depending
on how
you set
the
vertical
scale
(volts/div
control),
an
attenuator
reduces
the
signal
voltage
or an
amplifier
increases
the
signal
voltage.
Next,
the
signal
travels
directly
to the
vertical
deflection
plates
of the
cathode
ray tube
(CRT).
Voltage
applied
to these
deflection
plates
causes a
glowing
dot to
move.
(An
electron
beam
hitting
phosphor
inside
the CRT
creates
the
glowing
dot.) A
positive
voltage
causes
the dot
to move
up while
a
negative
voltage
causes
the dot
to move
down.
The
signal
also
travels
to the
trigger
system
to start
or
trigger
a
"horizontal
sweep."
Horizontal
sweep is
a term
referring
to the
action
of the
horizontal
system
causing
the
glowing
dot to
move
across
the
screen.
Triggering
the
horizontal
system
causes
the
horizontal
time
base to
move the
glowing
dot
across
the
screen
from
left to
right
within a
specific
time
interval.
Many
sweeps
in rapid
sequence
cause
the
movement
of the
glowing
dot to
blend
into a
solid
line. At
higher
speeds,
the dot
may
sweep
across
the
screen
up to
500,000
times
each
second.
Together,
the
horizontal
sweeping
action
and the
vertical
deflection
action
traces a
graph of
the
signal
on the
screen.
The
trigger
is
necessary
to
stabilize
a
repeating
signal.
It
ensures
that the
sweep
begins
at the
same
point of
a
repeating
signal,
resulting
in a
clear
picture
as shown
in
following
figure.
Triggering
Stabilizes
a
Repeating
Waveform
In
conclusion,
to use
an
analog
oscilloscope,
you need
to
adjust
three
basic
settings
to
accommodate
an
incoming
signal:
-
The
attenuation
or
amplification
of
the
signal.
Use
the
volts/div
control
to
adjust
the
amplitude
of
the
signal
before
it
is
applied
to
the
vertical
deflection
plates.
-
The
time
base.
Use
the
sec/div
control
to
set
the
amount
of
time
per
division
represented
horizontally
across
the
screen.
-
The
triggering
of
the
oscilloscope.
Use
the
trigger
level
to
stabilize
a
repeating
signal,
as
well
as
triggering
on a
single
event.
Also,
adjusting
the
focus
and
intensity
controls
enables
you to
create a
sharp,
visible
display.
Digital
Oscilloscopes
Some of
the
systems
that
make up
digital
oscilloscopes
are the
same as
those in
analog
oscilloscopes;
however,
digital
oscilloscopes
contain
additional
data
processing
systems.
With
the
added
systems,
the
digital
oscilloscope
collects
data for
the
entire
waveform
and then
displays
it.
When
you
attach a
digital
oscilloscope
probe to
a
circuit,
the
vertical
system
adjusts
the
amplitude
of the
signal,
just as
in the
analog
oscilloscope.
Next,
the
analog-to-digital
converter
(ADC) in
the
acquisition
system
samples
the
signal
at
discrete
points
in time
and
converts
the
signal's
voltage
at these
points
to
digital
values
called
sample
points.
The
horizontal
system's
sample
clock
determines
how
often
the ADC
takes a
sample.
The rate
at which
the
clock
"ticks"
is
called
the
sample
rate and
is
measured
in
samples
per
second.
The
sample
points
from the
ADC are
stored
in
memory
as
waveform
points.
More
than one
sample
point
may make
up one
waveform
point.
Together,
the
waveform
points
make up
one
waveform
record.
The
number
of
waveform
points
used to
make a
waveform
record
is
called
the
record
length.
The
trigger
system
determines
the
start
and stop
points
of the
record.
The
display
receives
these
record
points
after
being
stored
in
memory.
Depending
on the
capabilities
of your
oscilloscope,
additional
processing
of the
sample
points
may take
place,
enhancing
the
display.
Pretrigger
may be
available,
allowing
you to
see
events
before
the
trigger
point.
Digital
Oscilloscope
Block
Diagram
Fundamentally,
with a
digital
oscilloscope
as with
an
analog
oscilloscope,
you need
to
adjust
the
vertical,
horizontal,
and
trigger
settings
to take
a
measurement.
Sampling
Methods
The
sampling
method
tells
the
digital
oscilloscope
how to
collect
sample
points.
For
slowly
changing
signals,
a
digital
oscilloscope
easily
collects
more
than
enough
sample
points
to
construct
an
accurate
picture.
However,
for
faster
signals,
(how
fast
depends
on the
oscilloscope's
maximum
sample
rate)
the
oscilloscope
cannot
collect
enough
samples.
The
digital
oscilloscope
can do
two
things:
-
It
can
collect
a
few
sample
points
of
the
signal
in a
single
pass
(in
real-time
sampling
mode)
and
then
use
interpolation.
Interpolation
is a
processing
technique
to
estimate
what
the
waveform
looks
like
based
on a
few
points.
-
It
can
build
a
picture
of
the
waveform
over
time,
as
long
as
the
signal
repeats
itself
(equivalent-time
sampling
mode).
Real-Time
Sampling
with
Interpolation
Digital
oscilloscopes
use
real-time
sampling
as the
standard
sampling
method.
In
real-time
sampling,
the
oscilloscope
collects
as many
samples
as it
can as
the
signal
occurs.
See
following
figure
for
single-shot
or
transient
signals
you must
use real
time
sampling.
Real
Time
Sampling
Diagram
Digital
oscilloscopes
use
interpolation
to
display
signals
that are
so fast
that the
oscilloscope
can only
collect
a few
sample
points.
Interpolation
"connects
the
dots."
Linear
interpolation
simply
connects
sample
points
with
straight
lines.
Sine
interpolation
(or
sin x
over x
interpolation)
connects
sample
points
with
curves.
(See
Following
Figure)
Sin x
over x
interpolation
is a
mathematical
process
similar
to the "oversampling"
used in
compact
disc
players.
With
sine
interpolation,
points
are
calculated
to fill
in the
time
between
the real
samples.
Using
this
process,
a signal
that is
sampled
only a
few
times in
each
cycle
can be
accurately
displayed
or, in
the case
of the
compact
disc
player,
accurately
played
back.
Linear
and Sine
Interpolation
Diagram
Equivalent-Time
Sampling
Some
digital
oscilloscopes
can use
equivalent-time
sampling
to
capture
very
fast
repeating
signals.
Equivalent-time
sampling
constructs
a
picture
of a
repetitive
signal
by
capturing
a little
bit of
information
from
each
repetition.
(See
Following
Figure) You
see the
waveform
slowly
build up
like a
string
of
lights
going on
one-by-one.
With
sequential
sampling
the
points
appear
from
left to
right in
sequence;
with
random
sampling
the
points
appear
randomly
along
the
waveform.
Equivalent-time
Sampling
Diagram |
