Adding Noise and Predistortions, and Impairing the Signal
R&S
®
SMBV100B
257User Manual 1178.4460.02 ─ 03
To ensure flat noise within the selected system bandwidth, the noise bandwidth has
to be larger than the system bandwidth. The minimum noise bandwidth is sometimes
part of the specifications and is calculated as follows:
"Noise Bandwidth" = "System Bandwidth" x "Minimum Noise/System Bandwidth Ratio"
Logically, the calculated noise bandwidth does not exceed the total available band-
width of the instrument, as specified in the data sheet.
By default, the noise power within the system bandwidth is not defined directly but cal-
culated depending on the selected system bandwidth and the desired SNR. The noise
power over the noise bandwidth is calculated respectively.
The noise signal is not generated arbitrarily for a particular bandwidth, but instead it is
generated in steps. Noise therefore also occurs outside the set system bandwidth,
which means that the total measurable noise power usually exceeds the displayed
value "Noise Power".
For a correct measurement of the noise power within the system bandwidth, we recom-
mend that you perform channel power measurement with a signal analyzer.
In the "Additive Noise" mode, the output signal is the signal with the noise distribution.
Hence, the power level at the RF output corresponds to the carrier+noise power (see
Figure 4-3).
The noise power of digitally modulated signal is characterized by the parameter E
b
/N
0
indicating the ratio of bit energy to noise power density. The correlation to the SNR is
as follows:
C/N or S/N = (E
b
/N
0
)*(f
bit
/B
sys
), where B
sys
is the system bandwidth,
Bit Rate f
bit
= "Symbol Rate" * Modulation Value
Adding Noise to the Signal
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