Rohde & Schwarz R&S FSP Series, Operating Manual

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R&S FSP Getting Started
Noise Measurements
2.20 Operating Manual 1164.4556.12 - 06
With the RMS detector , the whole IF envelope is used to calculate the power for
each trace pixel. The IF envelope is digitized using a sampling frequency which is at
least five times the resolution bandwidth which has been selected. Based on the
sample values, the power is calculated for each trace pixel using the following for-
mula:
s
i
= linear digitized video voltage at the output of the A/D converter
N = number of A/D converter values per pixel of the trace
P
RMS
= power represented by a trace pixel
When the power has been calculated, the power units are converted into decibels
and the value is displayed as a trace pixel.
The number of A/D converter values, N, used to calculate the power, is defined by
the sweep time. The time per trace pixel for power measurements is directly propor-
tional to the selected sweep time. The RMS detector uses far more samples for
power measurement than the sample detector, especially if the sweep time is
increased. The measurement uncertainty can be reduced considerably. In the
default setting, the R&S FSP therefore uses the RMS detector to measure the chan-
nel power.
For both detectors (sample and RMS), the video bandwidth (VBW) must at least be
three times the resolution bandwidth, so that the peak values of the video voltage
are not cut off by the video filter. At smaller video bandwidths, the video signal is
averaged and the power readout will be too small.
Sweep time selection
If the sample detector is used, it is best to select the smallest sweep time possible
for a given span and resolution bandwidth. The minimum time is obtained if the set-
ting is coupled. This means that the time per measurement is minimal. Extending
the measurement time does not have any advantages as the number of samples for
calculating the power is defined by the number of trace pixels in the channel.
When using the RMS detector, the repeatability of the measurement results can be
influenced by the selection of sweep times. Repeatability is increased at longer
sweep times.
Repeatability can be estimated from the following diagram:
P
RMS
1
N
----
s
i
2
i 1 =
N
∑
×
=