DATUM
SYSTEMS
PSM-2100L Satellite Modem
L-Band IF Addendum
1.0
Introduction
Small receive only satellite stations have used L-Band as an outdoor to indoor equipment IF link
for several years. The advent of full transmit and receive modems utilizing L-Band interface
frequencies promises to usher in a new era in small terminal design and construction. The
promise of this new equipment configuration is reduced complexity and cost.
The PSM-2100L modem brings all the advantages of Datum System’s direct modulation and
demodulation design, superior performance and high digital integration for low cost assembly to
the VSAT station. Because this new modem costs little more than a standard 70 MHz IF modem
and significantly reduces the complexity and cost of the up and down conversion equipment, it
promises to provide a new high in performance per dollar.
A significant aspect of small station design using an L-Band interface modem is that all of the
complexity and “smarts” are contained within the modem itself. The Block UpConverter or “BUC”
and the Low Noise Block DownConverter or “LNB” now each contain a single fixed local
oscillator, not required to tune for operation over the entire satellite band covering all
transponders. The PSM-2100L tunes over an extended range of 950 to 1650 MHz in 1 Hz
increments allowing it to access 700 MHz of RF spectrum.
Aside from the many advantages, using L-Band as an inter-facility link frequency results in the
need to carefully consider the components, frequencies and construction techniques used to
insure proper operation. Part of the purpose of this addendum is to spell out those areas where
special care must be used to achieve a reliable station operation.
For the purposes of the remainder of this document the names, acronyms and meanings used
which may be new for this type modem are:
•
“
Modem
” - Refers to the PSM-2100L modem capable of both transmit and receive
operation.
•
“
IF
”. The modems Intermediate Frequency used to connect to the Up and
DownConversion equipment.
•
“
BUC
” – Block Up Converter, Often with an integrated power amplifier for installation
directly to the feed at the antenna.
•
“
LNB
” – Low Noise Block Down Converter. Includes a low noise RF front end and single
down conversion stage to L-Band frequencies. In a VSAT, especially at low data rates,
this is a significantly better device than the typical free running LNB used for video
broadcast reception. A “data grade” LNB must have very low phase noise and a phase
locked LO for proper performance.
•
“
Bias T Mux
”. This is a device that multiplexes power, IF signals and often a reference
frequency onto a single cable going up to the BUC or LNB.
•
“
Terrestrial
” side. The Line or data side of the modem.
•
“
VSAT
” – Vary Small Aperture Station, referring to a station with a small antenna,
typically 1 to 4.5 meters in diameter.
•
“
LO
” – Local Oscillator frequency used for up or down conversion of RF frequenies.
