Comtech EF Data CDM-700 - Manual (Page 252)

CDM-700 High Speed Satellite Modem Revision 5
Eb/No Measurement MN/CDM700.IOM
• 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.
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 3/4
QPSK is used, this corresponds to an E
b
/N
0
of approximately 1.1 dB.
The exact relationship used to derive the table values is as follows:
E
b
/N
0
= 10 log
10
(10 (Co+No/No )/10) -1) - 10 log
10
(FEC Code Rate) - 10 log
10
(bits/symbol)
and:
• E
b
/N
0
and (Co+No)/No are expressed in dB
• Bits/symbol = 2 for QPSK
• Bits/symbol = 3 for 8-PSK
• Bits/symbol = 4 for 16-QAM
• Bits/symbol = 6 for 64-QAM
• Pay close attention to the sign of the middle term
F–2