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Theory of Operation
5.4 Reacquisition, Fast Acquisition, and Directed Sweep
The fast acquisition algorithm has been permanently installed in the current version of the
modem software.
The carrier should be acquired within 30 seconds, regardless of the symbol rate.
The upper limit on the symbol rate for fast acquisition is 128 kbit/s. There is no lower
limit. Fast acquisition will work in conjunction with the sweep reacquisition function.
Fast acquisition will not work concurrently with the directed sweep function. The sweep
range parameter must be set at 50000 Hz to enable fast acquisition.
If the sweep range is set at any frequency less than 50000 Hz, the modem is in the
directed sweep mode. Turning ON the directed sweep function automatically disables the
fast acquisition function.
5.4.1 Reacquisition
Normally the modem sweeps full range of ± 25 kHz during acquisition. The time it takes
to complete one full sweep cycle depends upon the symbol rate of the demodulator as
follows:
•
•
•
After initial acquisition (decoder locked for at least three seconds), if the carrier is lost
(decoder becomes unlocked), the modem enters the reacquisition mode.
During reacquisition, the sweep is limited to 10% of the nominal sweep range, around the
last known lock point. If reacquisition does not take place during the time set for
reacquisition (0 to 999 seconds), the modem will terminate the reacquisition mode and
begin sweeping the full range of ± 25 kHz.
5–18
If the demodulator symbol rate is less than or equal to 112 kbit/s, the sweep time
is:
Sweep Time (in Seconds) = 8400/SR (symbol rate in kbit/s)
If the demodulator symbol rate is greater that 112 kbit/s but less than 257 kbit/s,
the sweep time is:
Sweep Time (in Seconds) = 4200/SR (symbol rate in kbit/s)
If the demodulator symbol rate is greater than 256 kbit/s, the sweep time is:
Sweep Time (in Seconds) = 2100/SR (symbol rate in kbit/s)
SDM-650B Satellite Modem
Rev. 6
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