Spectrum analysis
R&S
®
RTP
403User Manual 1337.9952.02 ─ 12
The overlapping factor can be set freely. The higher the overlap factor, the more
frames are used. This leads to more individual results and improves detection of transi-
ent signal effects. However, it also extends the duration of the calculation. The size of
the frame depends on the number of input signal values (record length), the overlap
factor, and the FFT size (number of samples used for FFT calculation).
Frame
length
Overlap area
Record
length
Window functions
Each frame is multiplied with a specific window function after sampling in the time
domain. Windowing helps minimize the discontinuities at the end of the measured sig-
nal interval and thus reduces the effect of spectral leakage, increasing the frequency
resolution.
There are several window functions that can be used in FFT analysis. Each of the win-
dow functions has specific characteristics, including some advantages and some trade-
offs. Consider these characteristics carefully to find the optimum solution for the mea-
surement task.
For details, see "Window type" on page 413.
Combining FFT results
After the window function has been applied to the FFT results for each segment, the
results for all segments of the data acquisition must be combined to obtain the final
waveform. Various arithmetic functions are available for FFT segments, such as aver-
aging, enveloping, or minimum and maximum calculation.
Gating functions
You can restrict the time base of the input signal for which FFT analysis is to be per-
formed. There are various methods to do so:
●
Define absolute start and stop times for the time base extract
●
Define relative start and stop values that define a percentage of the original time
base
●
Couple the time base extract for FFT to an active zoom area.
FFT analysis
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