Comtech EF Data DST - Manual (Page 59)

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.