DVB-S Measuring Technology - Digital Video and Audio Broadcasting Technology - page 274

N=N' +10logB K = N' +10log( channel bandwidth/ Hz ) dB; [ dBm ]

The channel bandwidth of the signal corresponds to the symbol rate of
the DVB-S signal (DVB measurement guidelines).

Example:

Measured value of the noise marker: -120 dBm/Hz

Correction value at 27.5 MS/s symbol rate: + 74.4 dB

Noise power in the DVB-S channel: - 45.6 dBm

The resultant C/N is:

C/N [ dB ] = C [ dBm ] - N [ dBm ] ;

In the example: C/N[dB] = -25.6 [dBm] - (-45.6 dBm) = 20 dB;

In fact, to measure the C/N in the downlink, the noise is measured in the
gaps between the individual channels. The only other possibility of meas-
uring C/N would be if a constellation analyzer were available or via the de-
tour of measuring the bit error rate. The received power itself can be meas-
ured by means of a professional DVB-S receiver which supports this type
of measurement.

15.3.2 C/N, S/N and Eb/N 0

The carrier-to-noise ratio C/N is an important value in assessing the quality
of the satellite transmission link. From the C/N, direct conclusion can be
drawn with respect to the bit error rate to be expected. The C/N is the re-
sult of the power radiated by the satellite (< ~100W), the antenna gain at
the transmitting and receiving end (size of the receiving antenna) and the
loss in the space between. The alignment of the satellite receiving antenna
and the noise figure of the LNB also play a role. DVB-S receivers output
the C/N value as an aid for aligning the receiving antenna.

C/N[dB] = 10log(P Carrier /P Noise ) ;

In addition to the carrier-to-noise ratio, there is also the signal-to-noise
ratio:

S/N[dB] = 10log(P Signal / P Noise );