Selecting lnbs for use with l-band modems, Introduction, Comparing lnbs – Comtech EF Data DST User Manual
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A–1
Appendix A. Selecting LNBs For
Use With L-Band Modems
This chapter provides guidelines for choosing LNBs for use the DST Terminals.
A.1 Introduction
An option added to the DST Digital Satellite Terminals is the selection of either INT or
EXT REF LNBs. The 10 MHz reference for an EXT REF LNB is delivered up the
coaxial cable, while the reference for an internally referenced unit is contained within the
unit.
A.1.1 Comparing
LNBs
For adequate system performance, an INT REF LNB is the type phase locked to crystal
reference. While these have a larger frequency uncertainty than an EXT REF LNB they
are usable in many cases. The tradeoffs for INT and EXT REF LNBs include:
Item
EXT REF LNB
INT REF LNB
Frequency Stability
±0.02 ppm (from modem)
±3.0 ppm (inside LNB)
Phase Noise
Best
OK
Sensitivity To Thermal Gradients
Least
OK
Minimum Data / Symbol Rates
Lowest
Higher
Cost Higher
Lower
Because the reference for an EXT REF LNB is indoors and exposed to a smaller
temperature range and more benign environment, it provides the highest available
frequency stability. The INT REF LNB experiences the full range of environmental
exposure, but these units are capable good levels of performance. To decide, a frequency
uncertainty budget is evaluated to take into account all of the contributors and decide
whether an INT REF LNB is suitable.