R&S FSMR, Operating Manual, Page: 217 / 955

R&S FSMR Analyzer - Bandwidths and Sweep Time
1166.3363.12 4.8-1 E-1
Setting the Bandwidths and Sweep Time –
BW
Key
The BW key calls a menu for setting the resolution bandwidth ( RBW ), video bandwidth ( VBW ) and
sweep time ( SWT ) for the frequency sweep. The parameters may be coupled dependent on the span
(stop minus start frequency) or freely set by the user. When working with a split screen display, the
settings always refer to the active window.
The R&S FSMR offers resolution bandwidths from 10 Hz to 20 MHz (R&S FSMR43: 10 Hz to 10 MHz) in
1, 2, 3, 5, 10 steps and additionally 50 MHz (not R&S FSMR
)
as maximum bandwidth.
Resolution bandwidths up to 100 kHz are realized using digital bandpasses with Gaussian
characteristics. As far as the attenuation characteristic is concerned they behave like analog filters but
have a much higher measurement speed than comparable analog filters. This is due to the fact that the
transient response can be compensated as a result of an accurately defined filter behaviour.
Bandwidths above 100 kHz are realized using decoupled 5-circuit LC filters.
As an alternative to the analog filters, FFT filters are available for the bandwidths between 1 Hz and
30 kHz. When working with bandwidths up to 30 kHz, the FFT algorithm offers considerably higher
measurement speeds with all the other settings remaining the same. The reason is that with analog
filters the sweep time required for a particular span is proportional to (Span/RBW
2
). When using the FFT
algorithm, however, the sweep time is proportional to (Span/RBW).
The video bandwidths are available in 1, 2, 3, 5, 10 steps between 1 Hz and 10 MHz
*)
. They can be set
in accordance with the resolution bandwidth.
The video filters serve for smoothing the displayed trace. Video bandwidths that are small compared to
the resolution bandwidth average out noise peaks and pulsed signals, so that only the signal average is
displayed. If pulsed signals are to be measured, it is recommended to use a video bandwidth that is
large compared to the resolution bandwidth (VBW
10 x RBW) for the amplitudes of pulses to be
measured correctly.
For analog and digital filters, the R&S FSMR has overload reserves of different magnitude
above the reference level. Due to the LO breakthrough the overload display OVLD
responds with digital filters with RBW < 100 kHz, as soon as the start frequency is
selected <6 * resolution bandwidth, for RBW = 100 kHz, as soon as the start frequency is
below 3 MHz.
*)
to 30 MHz (for resolution bandwidth > 10MHz, not R&S FSMR43)