Rohde & Schwarz FSWP Series - Page 32

Basics on I/Q Data Acquisition and Processing
R&S
®
FSWP
32User Manual 1177.5856.02 ─ 07
4.5.4 Frequency Resolution of FFT Results - RBW
The resolution bandwidth defines the minimum frequency separation at which the
individual components of a spectrum can be distinguished. Small values result in high
precision, as the distance between two distinguishable frequencies is small. Higher val-
ues decrease the precision, but increase measurement speed.
The RBW is determined by the following equation:
LengthWindow
RateSample
BandwidthNormalizedRBW 
Equation 4-1: Definition of RBW
(Note: The normalized bandwidth is a fixed value that takes the noise bandwidth of the
window function into consideration.)
The maximum RBW is restricted by the Analysis Bandwidth , or by the following equa-
tion, whichever is higher:
3
RateSample*BandwidthNormalized
RBW
max

If a higher spectral resolution is required, the number of samples must be increased by
using a higher sample rate or longer record length.
The minimum achievable RBW depends on the sample rate and record length, accord-
ing to the following equation:
 
LengthcordRe,4096min
RateSampleBandwidth*Normalized
RBW
min

To simplify operation, some parameters are coupled and automatically calculated, such
as record length and RBW.
RBW mode
Depending on the selected RBW mode, the resolution bandwidth is either determined
automatically or can be defined manually.
Auto mode:
This is the default mode in the I/Q Analyzer. The RBW is determined automatically
depending on the Sample Rate and Window Length , where the window length corre-
sponds to the Record Length , or a maximum of 4096.
If the record length is larger than the window length, multiple windows are combined;
the FFT length is 4096.
A Flatop window function is used.
Manual mode:
The RBW is user-definable.
Basics on FFT