Common Analysis and Display Functions
R&S
®
FSW
489
User Manual 1173.9411.02 ─ 19
level in 1 Hz bandwidth with reference to 1 mW. With linear amplitude units (V, A, W),
the noise voltage density is analyzed in µV/
√
Hz, the noise current density in µA/
√
Hz or
the noise power density in µW/
√
Hz.
The result is indicated as the
function result in the Marker Table
.
Prerequisite settings
The following settings have to be made to obtain correct values:
●
Detector: Sample or RMS
●
Video bandwidth:
≤ 0.1 resolution bandwidth with sample detector
≥ 3 x resolution bandwidth with RMS detector
●
Trace averaging:
In the default setting, the R&S
FSW uses the sample detector for the noise func-
tion. With the sample detector, the trace can additionally be set to "Average" mode
to stabilize the measured values. When the RMS detector is used, trace averaging
should not be used since in this case it produces too low noise levels which cannot
be corrected. Instead, the sweep time can be increased to obtain stable measure-
ment results.
Correction factors
The R&S
FSW uses the following correction factors to analyze the noise density from
the marker level:
●
Since the noise power is indicated with reference to 1 Hz bandwidth, the bandwidth
correction value is deducted from the marker level. It is 10 x lg (1 Hz/BWNoise),
where BWNoise is the noise or power bandwidth of the set resolution filter (RBW).
●
RMS detector: With the exception of bandwidth correction, no further corrections
are required since this detector already indicates the power for each point of the
trace.
●
Sample detector: As a result of video filter averaging and trace averaging, 1.05 dB
is added to the marker level. This is the difference between the average value and
the RMS value of white noise. With a logarithmic level axis, 1.45 dB is added addi-
tionally. Logarithmic averaging is thus fully taken into account which yields a value
that is 1.45 dB lower than that of linear averaging.
●
To allow a more stable noise display the adjacent (symmetric to the measurement
frequency) points of the trace are averaged.
●
For span > 0, the measured values are averaged versus time (after a sweep).
The R&S
FSW noise figure can be calculated from the measured power density level.
It is calculated by deducting the set RF attenuation (RF Att) from the displayed noise
level and adding 174 to the result.
Marker Usage
