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Basic Operation
DS O5000 Series Digital Storage Oscilloscope User Manual
22
2. Turn the VERTICAL POSITION knob to vertically move the YT waveform to the center (zero
division) so as to ensure the FFT will display a true DC value.
3. Turn the HORIZONTAL POSITION knob to position the part of the YT waveform to be
analyzed in the center eight divisions of the screen . The oscilloscope uses the 2048 center
points of the time-domain waveform to calculate the FFT spectrum.
4. Turn the VOLTS/DIV knob to ensure the entire waveform remains on the screen. If the entire
waveform is invisible, the oscilloscope may display wrong FFT results by adding
high-frequency components.
5. Turn the SEC/DIV knob to provide the resolution you need in the FFT spectrum.
6. If possible, set the oscilloscope to display multiple signal cycles.
If you turn the SEC/DIV knob to select a faster setting (fewer cycles), the FFT spectrum will display
a larger frequency range and reduce the possibility of FFT aliasing.
To
set the FFT display, follow the steps below.
1. Push the MATH MENU button;
2. Set the Operation option to FFT ;
3. Select the Math FFT Source channel.
In many situations, the oscilloscope can also generate a useful FFT spectrum despite the YT
waveform not being triggered. This is especially true if the signal is periodic or random (such as
noise).
Note:
Note:
Note:
Note: You
You
You
You should
should
should
should trigger
trigger
trigger
trigger and
and
and
and position
position
position
position transient
transient
transient
transient or
or
or
or burst
burst
burst
burst waveforms
waveforms
waveforms
waveforms as
as
as
as close
close
close
close as
as
as
as possible
possible
possible
possible to
to
to
to
the
the
the
the screen
screen
screen
screen center.
center.
center.
center.
Nyquist
Nyquist
Nyquist
Nyquist Frequency
Frequency
Frequency
Frequency
The highest frequency that any real-time digital oscilloscope can measure without errors is half of
the sample rate, which is called the Nyquist frequency. Frequency information beyond the Nyquist
frequency is undersampled which brings about the FFT aliasing. The math function can convert
the center 2048 points of the time-domain waveform to an FFT spectrum. The resulting FFT
spectrum contains 1024 points from DC (0Hz) to the Nyquist frequency. Usually, the screen
compresses the FFT spectrum horizontally to 250 points, but you can use the FFT Zoom function
to expand the FFT spectrum so that you can clearly view the frequency components at each of the
1024 data points in the FFT spectrum.
Note:
Note:
Note:
Note: The
The
The
The oscilloscope
oscilloscope
oscilloscope
oscilloscope ’
’
’
’ s
s
s
s vertical
vertical
vertical
vertical response
response
response
response is
is
is
is a
a
a
a little
little
little
little bit
bit
bit
bit larger
larger
larger
larger than
than
than
than its
its
its
its bandwidth
bandwidth
bandwidth
bandwidth (
(
(
( 60MHz
60MHz
60MHz
60MHz ,
,
,
,
100MHz
100MHz
100MHz
100MHz or
or
or
or 20
20
20
20 0MHz
0MHz
0MHz
0MHz ,
,
,
, depending
depending
depending
depending on
on
on
on the
the
the
the model;
model;
model;
model; or
or
or
or 20MHz
20MHz
20MHz
20MHz when
when
when
when the
the
the
the Bandwidth
Bandwidth
Bandwidth
Bandwidth Limit
Limit
Limit
Limit option
option
option
option is
is
is
is
set
set
set
set to
to
to
to Limited).
Limited).
Limited).
Limited). Therefore,
Therefore,
Therefore,
Therefore, the
the
the
the FFT
FFT
FFT
FFT spectrum
spectrum
spectrum
spectrum can
can
can
can display
display
display
display valid
valid
valid
valid frequency
frequency
frequency
frequency information
information
information
information above
above
above
above
the
the
the
the oscilloscope
oscilloscope
oscilloscope
oscilloscope bandwidth.
bandwidth.
bandwidth.
bandwidth. However,
However,
However,
However, the
the
the
the amplitude
amplitude
amplitude
amplitude information
information
information
information near
near
near
near or
or
or
or above
above
above
above the
the
the
the
bandwidth
bandwidth
bandwidth
bandwidth will
will
will
will not
not
not
not be
be
be
be accurate.
accurate.
accurate.
accurate.
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