Maintenance M7/M7L/M7LT Modular Satellite Modem
3-82 M7/M7L/M7LT - Rev. 0.05
There are two acquisition methods used by the modem. The normal mode for fastest possible
acquisition (especially at low data rates) is the “Fast” mode which utilizes an onboard digital signal
processor (DSP) to mathematically determine the location of the carrier and lock as fast as possible.
This mode initially goes for the largest carrier power within the acquisition range. A new acquisition
attempt will always repeat the same process and go to the same carrier. The “fast” acquisition mode
is optimized for the fastest possible acquisition speed, and is set as the default acquisition mode for
the modem.
A second mode called “Search” also uses the DSP but performs a piece-wise sweep of the
programmable acquisition range to locate the carrier and lock to it. If the modem cannot lock to the
first carrier it detects it will attempt to find another carrier in the next step of frequency. The sweep
always starts at the low end of the acquisition range and moves upward, wrapping around to the low
end when the top is reached.
NOTE: The Search mode is optimized for crowded spectrum applications where nearby high power
carriers may interfere with the standard “Fast” acquisition mode.
The acquisition mode is set by setting the <Demod: IF - Sweep Mode> option parameter to either
“Fast” (0), or “Search” (1). The “Fast” mode is the standard setting.
3.7.11.1. Initial Acquisition
For initial acquisition, a single setting allows programming the acquisition sweep range that the
modem will search to find an available carrier. This parameter can be set from 100 Hz 1.25 MHz,
where 30 kHz is common for standard demodulators. If all of the system offsets are known and
stable for a given installation, the initial acquisition range can be set to a low value which will slightly
reduce acquisition time, especially at low data rates. Conversely if system frequency offsets are not
known, or change over time, the initial acquisition range should be set very wide to allow locking to a
carrier well outside the typical range.
CAUTION!: If the acquisition range is set too small and the system offsets drift, then a carrier may
be locked out of acquisition or lost during operation. If the acquisition range is set too
wide and other compatible carriers are within the acquisition range, then the wrong
carrier may be acquired.
3.7.11.2. Carrier Re-acquisition
For <Demod: IF - Sweep Mode=Search>, the modem attempts to find a carrier in a reduced search
range for a specified period of time before reverting to the standard search range. The reduced
sweep range is equal to the symbol rate in Hertz. Once < Demodulator: IF - Sweep Mode=Search>
acquisition mode is set, the < Demodulator: IF - Sweep Time> parameter setting controls the
acquisition search time in the reduced range.
NOTE: The reduced sweep range is referenced the last demodulator lock offset.
3.7.12. Interface Type Configuration
The modem has the capability to implement several different interface types. The modem processor
automatically determines the presence and type of the installed interface(s) and options by querying
the interface card slots. The installed interface part number will be displayed in the < Unit: Status -
Slot 3 ID> front panel display. If there is more than one interface type installed in the modem, the <
Unit: Status - Slot 4 ID> will display the part number of the second installed interface. Selection of
the active interface is done in the < Modulator: Data - Send Interface> and < Demodulator: Data -
Rcv Interface> parameter settings.
The following interface types are currently available as options within the M7 modem.
Multi-protocol Synchronous Serial Data interface (S7)
Advanced IP interface with internal bridge/router (I7)
Express Ethernet interface (E7)
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