Digital Signal Processing Reference
In-Depth Information
sweep time set, the more samples can be accommodated in this window
and the smoother and better averaged will be the test result.
Because of the noise-like signal, we use the noise marker measuring
power. The noise marker is set to band center for this purpose. The prereq-
uisite is a flat channel but this can always be assumed to be the case in the
uplink. If the channel is not flat, other suitable measuring functions must
be used for measuring the channel power but these are dependent on the
spectrum analyzer.
The analyzer provides us with the value C' as the noise power density at
the position of the noise marker in dBm/Hz, and the filter bandwidth and
the characteristics of the logarithmic amplifier of the analyzer are auto-
matically taken into consideration. To relate the signal power density C' to
the Nyquist bandwidth B
N
of the DVB-S signal, it is necessary to calculate
the signal power C as follows:
C = C'+ 10log
B
N
= C'+ 10log (
symbol rate/Hz
)
dB;
[
dBm
]
The Nyquist bandwidth of the signal corresponds to the symbol rate of
the DVB-S signal.
Example:
Measured value of the noise marker:
-100 dBm/Hz
Correction value at 27.5 MS/s symbol rate:
+ 74.4 dB
Power in the DVB-S channel:
- 25.6 dBm
15.3.1 Approximate Determination of the Noise Power N
If it were possible to switch off the DVB-S signal without changing the
noise ratios in the channel, the noise marker at the center of the band
would now provide information on the noise ratios in the channel. How-
ever, this cannot be done in such a simple way. If not an exact measure-
ment value, then at least a “good idea”, is obtained if the noise marker is
used on the shoulder of the DVB-S signal for measuring in close proximity
of the signal. This is because it can be assumed that the noise fringe in the
wanted band continues similarly to its appearance on the shoulder.
The value N' of the noise power density is output by the spectrum ana-
lyzer. The noise power N in the channel with the bandwidth B
K
of the
DVB-S transmission channel is then calculated from the noise power den-
sity N' as follows:
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