Noise Measurements R&S FSL
1300.2519.12 2.18 E-11
5. The measurement result is stabilized by averaging the trace.
Press the TRACE key.
Press the Trace Mode key.
Press the Average softkey.
The R&S FSL performs sliding averaging over 10 traces from consecutive sweeps. The
measurement result becomes more stable.
Conversion to other reference bandwidths
The result of the noise measurement can be referred to other bandwidths by simple conversion. This is
done by adding 10 log (BW) to the measurement result, BW being the new reference bandwidth.
Example
A noise power of –150 dBm (1 Hz) is to be referred to a bandwidth of 1 kHz.
P
[1kHz]
= –150 + 10 * log (1000) = –150 +30 = –120 dBm (1 kHz)
Calculation method for noise power
If the noise marker is switched on, the R&S FSL automatically activates the sample detector. The video
bandwidth is set to 1/10 of the selected resolution bandwidth (RBW).
To calculate the noise, the R&S FSL takes an average over 17 adjacent pixels (the pixel on which the
marker is positioned and 8 pixels to the left, 8 pixels to the right of the marker). The measurement result
is stabilized by video filtering and averaging over 17 pixels.
Since both video filtering and averaging over 17 trace points is performed in the log display mode, the
result would be 2.51 dB too low (difference between logarithmic noise average and noise power). The
R&S FSL, therefore, corrects the noise figure by 2.51 dB.
To standardize the measurement result to a bandwidth of 1 Hz, the result is also corrected by –10 * log
(RBW
noise
), with RBW
noise
being the power bandwidth of the selected resolution filter (RBW).
Detector selection
The noise power density is measured in the default setting with the sample detector and using
averaging. Other detectors that can be used to perform a measurement giving true results are the
average detector or the RMS detector. If the average detector is used, the linear video voltage is
averaged and displayed as a pixel. If the RMS detector is used, the squared video voltage is averaged
and displayed as a pixel. The averaging time depends on the selected sweep time (=SWT/501). An
increase in the sweep time gives a longer averaging time per pixel and thus stabilizes the measurement
result. The R&S FSL automatically corrects the measurement result of the noise marker display
depending on the selected detector (+1.05 dB for the average detector, 0 d for the RMS detector). It is
assumed that the video bandwidth is set to at least three times the resolution bandwidth. While the
average or RMS detector is being switched on, the R&S FSL sets the video bandwidth to a suitable
value.
The Pos Peak, Neg Peak, Auto Peak and Quasi Peak detectors are not suitable for measuring noise
power density.
Determining the noise figure
The noise figure of amplifiers or of the R&S FSL alone can be obtained from the noise power display.
Based on the known thermal noise power of a 50 resistor at room temperature (–174 dBm (1Hz)) and
the measured noise power P
noise
the noise figure (NF) is obtained as follows:
NF = P
noise
+ 174 – g,
where g = gain of DUT in dB
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