Advantech AMT-70, Installation And Operation Manual, Page: 140 / 160

AMT-70/AMT-73/AMT-75 Installation and Operation
CHAPTER 13 APPENDIX B S/N TO EB/NO
CONVERSION
Eb/No or more properly, Eb/N0, is defined as the ratio of Energy per Bit to Spectral Noise Density in a
1 Hz bandwidth. It is used in digital communications to define the signal strength required at the input
of the demodulator to provide a desired Bit Error Ratio (BER). Eb/No is independent of the system data
rate and symbol rate. Eb/No is difficult to measure with standard test equipment, such as Spectrum
Analyzers. The typical Spectrum Analyzer displays a ratio of Signal plus Noise/Noise, or (S + N)/N.
The charts below provides the (S + N)/N to Eb/No conversions for the most popular modulation and
code rates.
A simple but quick method of determining Eb/No is obtained by measuring the (S + N)/N value using
an accurate Spectrum Analyzer. The analyzer needs to be looking at the same signal that the
demodulator will see. A two-way power divider in the receive path with one output connected to the
Spectrum Analyzer and the other to the demod input is acceptable. The common input would be from
the satellite feed. Follow the steps below to obtain a reasonable measurement of (S + N)/N, which then
can be translated to Eb/No values by using the following charts.
•
Set the Resolution Bandwidth to less than 20% of the transmitted symbol rate.
•
Set the Video Resolution to reduce the noise variation. Video averaging is also acceptable, if
the analyzer has that feature.
•
Start at 10 dB per vertical division and scale down to 2 dB (if possible) to obtain the noise floor
/ carrier within the same display. This will allow the measurement to be more accurate.
•
Using the Spectrum Analyzer marker, place the marker over the center part of the wanted
carrier. If the analyzer includes a delta measurement capability, then use the delta marker
placed over a blank area of the noise (no carriers present), and read the difference between
the carrier level and the noise level. This value is the (S + N)/N.
•
If the spectrum analyzer does not have a delta marker function, then record the carrier signal
level, then record the noise level, and subtract the two. This value will constitute the (S + N)/N
reading.
•
Then using the following charts, select the Eb/No that is closest to the obtained (S + N)/N,
remembering that you must follow the proper modulation type and FEC code rate.
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