Noise Measurements R&S FSL
1300.2519.12 2.22 E-11
The following steps are performed:
• The linear power of all the trace pixels within the channel is calculated.
P
i
= 10
(L
i
/10)
where P
i
= power of the trace pixel i
L
i
= displayed level of trace point i
• The powers of all trace pixels within the channel are summed up and the sum is divided by the
number of trace pixels in the channel.
• The result is multiplied by the quotient of the selected channel bandwidth and the noise bandwidth
of the resolution filter (RBW).
Since the power calculation is performed by integrating the trace within the channel bandwidth, this
method is also called the IBW method (I
ntegration Bandwidth method).
Parameter settings
For selection of the sweep time, see next section. For details on the parameter settings refer to chapter
"Instrument Functions", section "Settings of the CP / ACP test parameters" or the Online Help.
Sweep time selection
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 proportional to the selected sweep time.
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 setting 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.
If the RMS detector is used, 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:
10
1000
100
10000
100000
0
0.5
1
1.5
2
2.5
3
Number of samples
max. error/dB
95 % Confidence
level
99 % Confidence
level
Fig. 2-17 Repeatability of channel power measurements as a function of the number of samples
used for power calculation
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