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Basics on I/Q Data Acquisition and Processing
R&S
®
ZNL
21
User Manual 1178.5989.02 ─ 06
Table 4-2: Characteristics of typical FFT window functions
Window type
Frequency
resolution
Magnitude
resolution
Sidelobe sup-
pression
Measurement recommendation
Rectangular
Best
Worst
Worst
No function applied.
Separation of two tones with almost
equal amplitudes and a small fre-
quency distance
Blackman-Harris
(default)
Good
Good
Good
Harmonic detection and spurious
emission detection
Gauss (Alpha
= 0.4)
Good
Good
Good
Weak signals and short duration
Flattop
Worst
Best
Good
Accurate single tone measurements
5-Term
Good
Good
Best
Measurements with very high
dynamic range
4.2.2
Overlapping
The I/Q Analyzer calculates multiple FFTs per measurement by dividing one captured
record into several windows. Furthermore, the I/Q Analyzer allows consecutive win-
dows to overlap. Overlapping "reuses" samples that were already used to calculate the
preceding FFT result.
In advanced FFT mode with averaging, the overlapping factor can be set freely. The
higher the overlap factor, the more windows are used. This leads to more individual
results and improves detection of transient signal effects. However, it also extends the
duration of the calculation. The size of the window can be defined manually according
to the record length, the overlap factor, and the FFT length.
An FFT overlap of 67%, for example, means the second FFT calculation uses the last
67% of the data of the first FFT. It uses only 33% new data. The third FFT still covers
33% of the first FFT and 67% of the second FFT, and so on.
Basics on FFT