Appendix A. Selecting Lnbs For Use With L-Band Modems; Introduction; Comparing Lnbs - Comtech EF Data DST Operator's Manual

Appendix A. Selecting LNBs For
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:
Frequency Stability
Phase Noise
Sensitivity To Thermal Gradients
Minimum Data / Symbol Rates
Cost
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.
Use With L-Band Modems
Item EXT REF LNB
±0.02 ppm (from modem)
Best
Least
Lowest
Higher
A–1
INT REF LNB
±3.0 ppm (inside LNB)
OK
OK
Higher
Lower