DVB-S Measuring Technology - Digital Video and Audio Broadcasting Technology - page 272

15.3 Measurements on DVBS Signals using a Spectrum
Analyzer

A spectrum analyzer can be used very well for measuring the power in the
DVB-S channel, at least in the uplink. Of course, it would also be quite
simple to use a thermal power meter but a spectrum analyzer can also be
used for determining the carrier/noise ratio in the uplink, possibly even di-
rectly. A DVB-S signal has the appearance of noise and has a rather large
crest factor. Because of its strong similarity with white Gaussian noise, its
power is measured exactly as in the case of noise.
To determine the carrier power, the spectrum analyzer is set as follows:
At the analyzer, a resolution bandwidth of 2 MHz and a video band width
of 3 to 10 times the resolution bandwidth (10 MHz) are selected. To
achieve some averaging, a slow operating time must be set (2000 ms).
These parameters are required because of the RMS detector used in the
spectrum analyzer. The following settings are used:

• Center frequency at the center of the DVB-S channel,

• Span at 100 MHz,

• Resolution bandwidth at 2 MHz,

• Video bandwidth at 10 MHz (because of RMS detector and log.

representation),

• Detector RMS

• Slow operating time (2000 ms)

• Noise marker at channel center (results in C' in dBm/Hz)

This results in a spectrum as shown in Fig. 15.2. The RMS detector cal-
culates the power density of the signal in a window with a bandwidth of
1 Hz, the test window being continuously pushed over the frequency win-
dow to be measured (sweep range). In principle, first the RMS (root mean
square) value of the voltage is determined from all samples in the signal
window of 1 Hz bandwidth:

1

2
1

2
2

2
3

U RMS = u + u + u + ... ;

N

From this, the power in this signal window is calculated with reference
to an impedance of 50 and converted into dBm. This is then the signal
power density in a window of 1 Hz bandwidth. The slower the selected