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Fourier Transform Operations > DFT |
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The time marker approach to specifying a DFT range allows precise placement of end-point markers. Unlike the previous method of power spectrum generation, the time marker approach does not require the onscreen display of the full range of waveform information to be transformed. The location of the time marker on a waveform represents one limit of the transform, and the location of the waveform cursor the other limit. Since the time marker need not be displayed on the screen to remain active, large transforms are possible at low compression factors. Transforms up to 8192 points are possible at a compression factor of 1 allowing precise placement of end-points without the indecision of marker placement at compression factors greater than 1. This approach, however, does support compression factors greater than 1 adding more flexibility to its use.
Time marker and cursor placement on the waveform to be transformed may occur from either the Y-T or DFT modes of operation. Since the object of using the time marker approach to end-point definition is precision, it is recommended that the time marker and cursor positions be defined from the Y-T mode, using display format 1 to allow full screen resolution during the placement process. Just as with the limit cursor approach, the time marker and cursor should be adjusted to enclose an integer or whole number of cycles on the waveform to be transformed. This is best accomplished by picking a spot on the waveform where the slope is steep and a single points value is well defined, like near the midpoint of a waveform. Also keep in mind that the DFT is generated over the range including those points occupied by the time marker and cursor. So position the time marker and cursor accordingly, taking care not to enclose one-too-many points when defining your cycle. The recommended procedure is as follows:
1. Exit the DFT mode and enter the Y-T display mode by:
In
the View menu click on Exit/Enter
Split.
Choose
View Exit/Enter Split (ALT, V, X).
2. Assign the waveform to be transformed to window 1 by:
In
the View menu click on Assign
Channel…, then enter the desired channel number.
Typing
1, =,
X, where X is the desired channel
number.
3. Enter your desired compression factor by:
In
the Edit menu click on Compression….
Choosing
Edit Compression… (ALT, E, C).
Note that regardless of the range of waveform information to be transformed or the magnitude of the compression factor, DFT software will always ensure that the number of points in the result does not exceed 8192 by multiplying the compression factor by the smallest integer necessary. More on the effects of waveform compression follow shortly. The selected compression factor, however, must be 1 to allow precise end-point placement.
4. Scroll the window to your first desired end-point (using the ← or → cursor control keys).
5. Place the time marker at the desired location by pressing the F4 key. Make certain that the waveform cursor is enabled to allow precise placement of the time marker.
6. Scroll the window until the waveform cursor is directly over the second desired end-point.
7. Activate the DFT mode and display the power spectrum over the selected range by:
In
the Transform menu click on DFT.
Choosing
Transform DFT (ALT, T,
D).
Waveform compression operates slightly differently when the time marker approach is used as compared to the other method. When the time marker is used, the compression factor defines the number of consecutive Y-T points averaged before transforming the waveform. For example, if 2244 points exist between the cursor and time marker then the result is a 2244 point transform if the compression factor equals 1. Leaving the markers' position unchanged and incrementing the compression factor to 2 would yield a 1122 point transform since one point representing the average of each two consecutive points is passed through the transform. Whenever a DFT is activated using the time marker method, the analysis reporting area will display the actual averaging factor as follows:
