Spectrum analysis
R&S
®
RTO6
373
User Manual 1801.6687.02 ─ 05
(im), the magnitude is displayed. For the "Advanced" mode FFT (
φ
) function, the phase
is displayed.
For magnitude display, you can select the scale and range of magnitudes to be dis-
played. For linear scaling, the vertical value range of the input signal is used. For loga-
rithmic scaling, the logarithmic power of the frequency is displayed. In this case, the
input signal must be given in either Volt or Watt. The resulting value range is defined by
a maximum value and a range size. Logarithmic scaling can also be set in relation to a
given reference value.
For phase display, you can select the unit and suppress phases beneath a threshold
value which are most likely caused by noise. The value range [-
π
, +
π
] or [-180°,
+180°] is used. Phase shifts due to a limitation of the value range can be eliminated
using the "Unwrap" function.
Dependencies between FFT parameters
FFT analysis in the R&S
RTO6 is highly configurable. Several parameters, including
the resolution bandwidth, frequency span and center frequency, can be defined
according to your requirements. Note, however, that several parameters are correlated
and not all can be configured independently of the others.
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 minimum achievable RBW depends on the integration time which is equivalent to
the number of samples available for FFT calculation. If a higher spectral resolution is
required, the number of samples must be increased by using a higher sample rate or
longer record length. To simplify operation some parameters are coupled and automati-
cally calculated, such as record length and RBW.
The
frequency span
and
center frequency
define the start and stop frequency of the
spectral diagram. By default, a suitable frequency range according to the resolution
bandwidth is selected, in respect to performance and precision. Span and RBW set-
tings are coupled, so that the parameters can be adjusted automatically as necessary.
With a
Span/RBW ratio
of 100 and a screen resolution of 1000 pixels, each frequency
in the spectrum is displayed by 10 pixels. A span/RBW ratio of 1000 provides the high-
est resolution. For full flexibility, the span/RBW coupling can also be disabled. Note,
however, that a higher span/RBW ratio (i.e. low RBW values and large frequency
spans) result in large amounts of data and extend the duration of the calculation.
Advanced FFT functions
In "Advanced" math definition mode, other FFT results than the basic frequency magni-
tude can be displayed.
●
FFT (
φ
)
: phase display
●
FFT (im)
: imaginary part of FFT value (magnitude)
●
FFT (re)
: real part of FFT value (magnitude)
FFT analysis
