Rohde & Schwarz R&S UPP, Руководство пользователя

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Instrument Functions
R&S
®
UPP
408 User Manual 1411.1055.32 ─ 10
When using manual selection the FFT size should not be less than the recommended
value (which is produced when Post FFT is activated), otherwise there will be too few
FFT bins in the lower subbands.
With every increase in FFT size the measurement accuracy and stability increase, par-
ticularly in the relatively narrow lower subbands. At the same time, however, the mea-
surement time doubles.
Unnecessarily high FFT sizes should be avoided in order to reduce measurement
times.
Remote command:
SENSe[1]:FUNCtion:FFT:SIZE
on page 763
Window
Selection of the window function.
For details, see Chapter 5.26.2, "Window Function" , on page 410.
5.26 FFT Analysis
The "FFT" analysis ( F ast F ourier T ransformation) enables the input signal to be dis-
played in the spectral range (frequency range). For this purpose, the (digitized) input
signal is recorded over a selectable period and transformed into the frequency range
by a fast "DFT" (
D iscrete F ourier T ransformation). The result is a "list" of equidistant fre-
quency lines (bins) which indicate the magnitude or the phase of the respective fre-
quency components. The graphical display of the magnitude or phase bins is referred
to as a spectrum or spectral display.
According to system theory, the "FFT" analysis assumes that the signal is periodic, i.e.
it continues continuously beyond the observed time period. Since this ideal state only
exists in a few exceptional cases, the input signal can if required be weighted using a
window which attenuates both ends of the observed signal section with respect to zero.
The recording time for the "FFT", i.e. the length of the measurement interval (integra-
tion time), is determined using the
FFT size and, like the frequency resolution, also
depends on the sampling rate and bandwidth used.
In the R&S UPP, 2 different types of "FFT" can be selected:
●
The "Post FFT" for distortion measurements is calculated with double precision
using up to 256 k points, thus ensuring a very high frequency resolution and
dynamic range. The size of the FFT and the used window cannot be selected, but
instead are preset according to the requirements of the respective measurement
function. The Post FFT is displayed in the "FFT Function" graphical window.
FFT Analysis