R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Instrument Functions
4.5.11.2 Measurement of Noise Density
Analyzer Mode
NOISE MEAS The NOISE MEAS softkey switches the noise measurement for the active marker on
or off. The corresponding marker becomes the NORMAL marker.
During noise measurement, the noise power density is measured at the position of
the marker. In the time domain mode, all points of the trace are used to determine
the noise power density. When measurements are performed in the frequency
domain, two points to the right and left of the marker are used for the measurement
to obtain a stable result.
The noise power density is indicated in the marker field. With a logarithmic amplitude
units (dBm, dBmV, dBmµV, dBµA) the noise power density is output in dBm/Hz
i.e. as level in 1 Hz bandwidth with reference to 1 mW. With linear amplitude units (V,
A, Ω) the noise voltage density is evaluated in µV/√Hz, the noise current density in
µA/√Hz or the noise power density in µW/Hz.
The following settings have to be made to ensure that the power density measurement
yields correct values:
Detector: Sample or RMS
Video bandwidth: ≤ 0.1 × resolution bandwidth with sample detector
(corresponds to RBW / VBW NOISE)
≥ 3 × resolution bandwidth with RMS detector (corresponds to
RBW / VBW SINE)
In the default setting, the R&S FSQ uses the sample detector for the noise function.
With the sample detector, the trace can additionally be set to AVERAGE to stabilize
the measured values. With RMS detector used, trace averaging must 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 measurement results.
The R&S FSQ uses the following correction factors to evaluate 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 × lg (1 Hz/
BWNoise), where BWNoise is the noise or power bandwidth of the set resolution
filter (RBW).
• 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 additionally. Logarithmic averaging
is thus fully taken into account which yields a value that is 1.45 dB lower than that
of linear averaging.
• RMS detector
With the exception of bandwidth correction, no further corrections are required for
the RMS detector since it already indicates the power with every point of the trace.
To allow a more stable noise display the adjacent (symmetric to the measurement
frequency) points of the trace are averaged.
Operating Manual 1313.9681.12 - 01 4.63