Digital Signal Processing Reference
In-Depth Information
To find the DVB-C/J83A,B,C carrier power, the spectrum analyzer is
set up as follows:
At the analyzer, a resolution bandwidth of 300 kHz and a video band-
width of 3 to 10 times the width of the resolution bandwidth (3 MHz) is se-
lected. To achieve some averaging, a slow sweep time (2000 ms) must be
set. These parameters are required because we are using the RMS detector
of the spectrum analyzer. The following settings are then used:
•
Center frequency:
center of the cable channel
•
Span:
10 MHz
•
Resolution BW:
300 MHz
•
Video BW:
3 MHz (due to RMS detector and log. dis-
play)
•
Detector:
RMS
•
Sweep:
slow (2000 ms)
•
Noise marker:
channel center (C' in dBm/Hz)
To measure power, the noise marker is used because of the noise-like
signal. The noise marker is set to band center for this. The prerequisite is a
flat channel which, however, can always be assumed at the modulator. If
the channel is not flat, other suitable but analyzer-dependent measuring
functions must be used for measuring the channel power.
The analyzer provides the C' value as noise power density at the posi-
tion of the noise marker in dBm/Hz, automatically taking into considera-
tion the filter bandwidth and the characteristics of the logarithmic ampli-
fier of the analyzer. To relate the signal power density C' to the Nyquist
bandwidth B
N
of the cable signal. the signal power C must be calculated 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 cable signal.
Example:
Measurement value of the noise marker:
-100.0 dBm/Hz
Correction value at 6.9 MS/s symbol rate:
+ 68.4 dB
Power in the channel:
- 31.6 dBm
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