Fluke 8012A - Pulse Width Measurements

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
measurement, set
the rotary
switch
to V
,
select
the 40V
range,
press
(
)twice(the%annunciatoron
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