#### Generating a DFT using Limit Cursors

To generate a DFT with the limit cursor approach,
ensure that the time marker is turned off (e.g., press F4 until off is
displayed in the marker display field of the bottom annotation line. Refer
to Time Marker for
more details on enabling/disabling the time marker). With the time marker
disabled, the transform algorithm assumes that the time series is defined
by the left and right limit cursors. Now move the channel you wish to
transform into display window one using the Variable
Waveform Assignments function (e.g., type “1=5” if you wish to transform
channel 5). After this is accomplished, scroll the display of the selected
channel to center the area you wish to transform on the screen. The DFT
can now be generated directly from either the Y-T or the X-Y display modes
as follows:

In
the Transform menu click on DFT.

Choose
Transform DFT (ALT, T,
D).

If an FFT was not done prior to this DFT, the
resulting calculation will consist of the points on the waveform bounded
by the Y-T window chart edges as shown below:

With the limit
cursors at the extreme left and right chart edges of the Y-T window, you
get a DFT of exactly what you see in Y-T window 1. Of course, you can
adjust the limit cursors to get any length of waveform you desire.

If an FFT was done prior to generating the
DFT, the limit cursors will not be at the extreme left and right chart
edges but instead will be some power of 2 apart (i.e., 512 pixels apart).
Which does not give you a DFT of exactly what you see in window 1. The
following illustration shows this subtle but important point:

Since the transform
algorithm assumes that the time series is defined by the left and right
limit cursors, a DFT performed after an FFT will produce a power spectrum
over the range defined by the position of the limit cursors as shown above.
A very different power spectrum will be obtained from this range compared
to one obtained with the limit cursors at extreme chart edge.

It is possible to reposition the limit cursors
at the chart edges (or wherever desired) of window 1 and then perform
the DFT. See Split
Screen Operations.

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