Comtech EF Data CDM-550T Manual page 142

Eb/No Measurement
The idea is to accurately measure the value of (C+N)/N, (Carrier + Noise/Noise) and 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, and use video filtering and/or video averaging to
reduce the variance in the displayed sweep to a low enough level that the difference can
be measured to within 0.2dB. 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.
In the diagram above, the (C+N)/N measured is 4.6 dB. If Rate 1/2 QPSK is being used,
this would correspond to an Eb/No of approximately 2.6 dB.
B–2
CDM-550T Satellite Modem
Rev. 1.3