Comtech EF Data CDM-710 Installation And Operation Manual page 242

CDM-710 Broadcast Satellite Modem
Eb/No Measurement
The idea is to accurately measure the value of (Co+No)/No, (Carrier density + Noise
density/Noise density). This is accomplished by tuning the center frequency of the Spectrum
analyzer to the signal of interest, and measuring the difference between the peak spectral density
of the signal (the flat part of the spectrum shown) and the noise density. To make this
measurement:
Note that the accuracy of this method degrades significantly at low values of (Co+No)/No
(approximately less than 6 dB).
Example:
In the diagram on the previous page, the (Co+No)/No measured is 4.6 dB. If Rate 1/2 QPSK is
used, this corresponds to an Eb/No of approximately 2.8 dB (DVB-S2) or 3.1 dB (DVB-S).
The relationship used to derive the table values is as follows (only simple way for DVB-S2):
Eb/No = 10 log
and:
The equation above applies to DVB-S and DVB-DSNG, or use the traditional relationship:
Eb/No = 10 log
Use a vertical scale of 1 or 2 dB/division.
Set the Resolution Bandwidth of the Spectrum Analyzer to < 20 % of the symbol
rate.
Use video filtering and/or video averaging to reduce the variance in the displayed
trace to a low enough level that the difference can be measured to within 0.2dB.
Place a marker on the flat part of the signal of interest, then use the MARKER
DELTA function to put a second marker on the noise to the side of the carrier.
This value is (Co+No)/No, in dB.
Use this value of (Co+No)/No in the table on the following page to determine the
Eb/No. You will need to know the operating mode to read from the appropriate
column.
If the (Co+No)/No value measured does not correspond to an exact table entry,
interpolate using the two nearest values.
(Co+No/No )/10)
(10
10
Eb/No and (Co+No)/No are expressed in dB
Spectral Efficiency includes the modulation type, code rate, overhead and
framing and is shown in the tables
(Co+No/No )/10)
(10
10
Eb/No and (Co+No)/No are expressed in dB
FEC Code Rate (Composite) = 3/4*(188/204), 7/8*(188/204) etc.
Bits/symbol = 2 for QPSK, 3 for 8-PSK, 4 for 16-QAM
Pay close attention to the sign of the middle term
-1) - 10 log (Spectral Efficiency)
10
-1) - 10 log (FEC Code Rate) - 10 log
10
B–2
Revision 9
MN/CDM710.IOM
(bits/symbol)
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