Comtech EF Data CDM-625A - Manual (Page 639)

G–1
Appendix G. E
b
/N
o
MEASUREMENT
Although the CDM-625A Advanced Satellite Modem calculates and displays the value of receive
E
b
/N
0
on the front panel of the unit, it is sometimes useful to measure the value using a
spectrum analyzer, if one is available.
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:
• 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).