Rohde & Schwarz R&S UPP, Руководство пользователя

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Instrument Functions
R&S
®
UPP
410 User Manual 1411.1055.32 ─ 10
5.26.2 Window Function
According to system theory, the FFT analysis requires a periodic signal which contin-
ues continuously beyond the observed time period. Normally, however, there is no con-
tinuous continuation (even with periodic signals). The FFT would interpret any disconti-
nuity at the boundaries of the signal section as a pulse whose (white) spectrum would
be superimposed on the wanted signal spectrum. In order to prevent this "leakage"
effect, the input signal is weighted using a window which attenuates both ends of the
observed signal section with respect to zero. As a result, the signal is continuous for
the FFT; there is, however, a reduction in selectivity, indicated by a relatively wide bell-
shaped curve. By selecting the window function, the user makes a compromise
between selectivity (frequency selectivity, width of bell-shaped curve at top), crosstalk
between adjacent lines (width of bell-shaped curve at bottom), slope of the bell-shaped
curve and "leakage" suppression in the far-off range.
Selecting the rectangular window deactivates windowing.
Window
Selection of the window function.
With the Post FFT of the distortion and level measurements, the window cannot be
selected by the user but instead is preset by the respective measurement function and
displayed here.
"Hann" This window combines selectivity with good leakage suppression in
the "far-off range " but has a relatively wide bell-shaped curve around
the signal lines.
Recommended application: Standard window.
"Rectangular" Window function deactivated (by using a rectangular window with a
constant weighting of 1).
If the signal fits in the section for the FFT exactly with an integer mul-
tiple of periods, there is no discontinuity at the section boundaries. A
window is then not required and the maximum frequency resolution is
possible. This feature may be of advantage when using a special
generator signal ("FFT noise") as it allows fast measurement of fre-
quency responses at specific frequencies.
Recommended application: Feasible only with special signals.
"Blackman
Harris" Only for Function FFT.
The slope of the bell curve is very steep up to approx. 80
dB; below
that, however, this window has considerable leakage.
FFT Analysis