Comtech EF Data CDM-550T - Manual (Page 146)

CDM-550T Satellite Modem Revision 3
Eb/No Measurement MN/CDM550T.IOM
To make this measurement:
• 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 (C+N)/N, in dB.
• Use this value of (C+N)/N 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 (C+N)/N 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 (C+N)/N
(approximately less than 6 dB).
Example: In the diagram on the previous page, the (C+N)/N measured is 4.6 dB. If Rate 1/2
QPSK is used, this corresponds to an E
b
/N
0
of approximately 2.6 dB.
B–2