Comtech EF Data CDM-625, Инструкция по установке и эксплуатации

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CDM-625 Advanced Satellite Modem Revision 1
Eb/No Measurement MN-0000036 (Ref MN/CDM625.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 (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 1/2
QPSK is used, this corresponds to an E
b
/N
0
of approximately 2.6 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 = 1 for BPSK
• Bits/symbol = 2 for QPSK
• Bits/symbol = 3 for 8-PSK/8-QAM
• Bits/symbol = 4 for 16-QAM
• Code Rate for ‘uncoded’ = 1
• Pay close attention to the sign of the middle term
See Chapter 12. ESC++ for details of how the Eb/No performance degrades
when ESC++ is used, particularly at lower data rates, where the percentage
overhead is high.
IMPORTANT
B–2