Pulse Width Measurements - Fluke 8010A User Manual

The manner
in
which your meter
takes duty cycle
measure-
ments allows
it
to
be
very
tolerant of
aperiodic
(repetitive
but
not periodic) signals.
Duty cycle
measurements on
low
fre-
quency (<400 kHz) aperiodic
logic signals,
especially
serial
communication
signals,
is
a simple form
of
signature
analysis.
A
known
pattern
will
read the
same
duty cycle every time
(if
the
pattern
repeats
in
less
than
1
/3 second).
The
precision
and
resolution of the duty cycle
measure-
ments
are achieved by averaging
many
repetitions of the
input
signal.
In
rare cases,
this
averaging technique (which
is
similar to
pulse-width averaging
in
a
conventional counter)
may
cause a measurement problem
called "aliasing." Alias-
ing results
when
the
frequency
of
the input
signal
happens
to
be
exactly
synchronized
with the reference
crystal oscillator
of
the meter. This occurs
when
the
frequency
of the input
signal
can be
exactly divided
into
the frequency
of
the
oscillator
(1
31 ,072 Hz) or
one
of
the
oscillator's
harmonics.
When
they are nearly synchronized, the meter
is
"blind"
to
the correct duty
cycle,
and
the display
will
alternate
between
incorrect readings.
If
this
occurs,
and
the frequency reading
was
stable,
press (Wmax)
to
select the
MIN
MAX
Record
mode
and
scroll to
the
average
display.
The average
display
will
stabilize
on
the correct duty
cycle.
A common
duty cycle
measurement
is
the "dwell" angle
in
an
automobile. Dwell
is
the
number
of
degrees
of distributor rota-
tion that
the
points
remain closed
(or
current
is
flowing
in
the
APPLICATIONS
Pulse Width Measurements
coil).
Use
the following
to
convert a dwell angle
to
duty cycle
(in
percent):
%
Duty Cycle
=
DweN
(degrees)
*
No.
of
Cylinders
*
100
360 degrees
To
make
a
dwell
measure
ment, set the rotary
switch
to
V
select the
40V
range,
press
(
)twice(the%annunci
atoron
the
right
side
of
the
LCD
should
turn
on),
and
press the
(
"»»
)
(to
select the
negative trigger slope so
the
measurement
will
be
the
"off"
or points closed
time).
Then
connect
the
COM
input to
ground, and connect
the VO-*+
inputto the
low
(or
switched)
side of
the
coil.
Most
automobiles have
the points closed
for
a
duty cycle
between 50-70%.
Pulse
Width Measurements
For a
periodic
waveform
(that
is,
repetitive at
equal time
intervals),
a
duty cycle
measurement can be
easily
converted
to
pulse width.
First
measur
e the frequency
and
then
measure
the duty
cycle.
Toggle r~^iT)
to
select the
polarity of
the pulse
you want
to
measure. To
convert frequency
and
duty cycle
measurements
into
a pulse
width,
use
the
following:
PulseWidth
=
^ut^yc!enpo
Frequency
27