Instantaneous Voltage - Tektronix 545A Instruction Manual

Applications
—
Type
545A
As an example
of the
method, assume
that using a
1
OX
probe
and
a
deflection factor of
1
volt
per centimeter, you
measure
a
vertical distance
between
peaks
of
4
centimeters.
In this
case
then,
4 centimeters
multiplied
by
1
volt
per
centimeter gives you an indicated voltage
of
4
volts
peak-
to-peak.
The
indicated voltage
multiplied
by
the probe's
attenuation factor of 10 then gives
you
the true peak-to-
peak amplitude
of
40
volts.
When
sinusoidal
waveforms
are measured,
the peak-to-
peak
voltage obtained can be converted
to
peak, rms, or
average
voltage through use of standard conversion
factors.
1.
To
establish the voltage
reference
line,
touch
the
probe
tip
to
an
oscilloscope ground
terminal
(or
if
the reference
line
is
to represent a voltage other than ground,
to
a source
of that
voltage)
and
adjust
the oscilloscope controls
to
obtain a free-running sweep.
Vertically position the trace
to
a convenient
point
on
the oscilloscope screen.
This point
will
depend
on
the polarity
and
amplitude of the input
sig-
nal,
but should always be
chosen
so that the trace
lies
along
one
of the
major
divisions of the
graticule.
The
graticule divisions corresponding
to the position of the trace
is
the
voltage
reference
line
and
all
voltage
measurements
must be
made
with respect
to
this line.
(Do not adjust the
vertical positioning control after the reference
line
has
been
established.)
Instantaneous Voltage
Measurements
The
method
used
to
measure
instantaneous voltages
is
virtually identical to the
method
described previously for the
measurement
of the
ac components
of a
waveform. However
for instantaneous
voltage measurements
the plug-in
unit
input
selector
switch must be placed
in
one
of the
DC
positions. Also since instantaneous voltages are
measured
with respect
to
some
potential (usually
ground) a
reference
line
must be
established
on the oscilloscope screen which
corresponds
to that potential.
If,
for
example, voltage
measurements
are
to
be
made
with respect
to
+100
volts,
the reference
line
would correspond
to
+100
volts.
In
the
following procedure the
method
is
given
for establishing
this
reference
line
as
ground
since
measurements
with
re-
spect to
ground
are by far the
most
common
type.
The
same
general
method
may
be used
to
measure
voltage with
respect to
any
other potential,
however,
so long as that
potential
is
used
to establish
the reference
line.
To obtain an instantaneous voltage
measurement
with
respect to
ground, perform the following steps
(see
Figure
3-2):
2.
Remove
the probe
tip
from ground and
connect
it
to
the
signal source. Adjust the
triggering controls for a
stable
display.
3.
Using
the graticule,
measure
the vertical distance
in
centimeters from the desired point on
the
waveform
to
the
voltage reference
line.
4.
Multiply the
setting
of the
VOLTS/CM
control
by
the
dis-
tance measured
to
obtain
the indicated volage.
5.
Multiply the indicated voltage by the attenuation factor
of the
probe you
are using
to
obtain
the actual
voltage
with respect
to
ground
(or
other reference
voltage).
As an example
of this
method,
assume
that
you are
using
a 10X probe
and
deflection factor of
.2
volts
per
centimeter.
After
setting
the voltage reference
line
at the
second from
the
bottom
division
of the
graticule,
you measure
a distance
of
3 centimeters
to
the point
you wish
to
check.
In
this
case then, 3 centimeters multiplied
by
.2
volts
per
centi-
meter
gives
you an
indicated
.6
volts.
Since the voltage
point
is
above
the
voltage reference
line
the
polarity
is
Reference
Line
(usually
ground)
Vertical Deflection
I
J
ns, ®
n,OM
o«*
From
Reference Line
|
-
Voltage
Probe Attenuat
Factor
Volts/cm Switch
Setting
Fig.
3-2.
Measuring
the instantaneous voltage with respect
to
ground
(or
some
other reference voltage)
.