Comtech EF Data SDM-309B - Manual (Page 144)

Theory of Operation SDM-309B Satellite Modem
5-5–20 MN/U-SDM309B Rev. #
5.4.2 Reacquisition
Normally the modem sweeps full range of plus/minus 30 KHz during acquisition. The
time it takes to complete one full sweep cycle depends upon the symbol rate of the
demod as follows:
1. If the demod symbol rate is less than or equal to 112 KBPS the sweep time is:
sweep time (in seconds) = 8400/SR (symbol rate in KBPS)
2. If the demod symbol rate is greater that 112 KBPS but less than 257 KBPS the
sweep time is: sweep time (in seconds) = 4200/SR (symbol rate in KBPS)
3. If the demod symbol rate is greater than 256 KBPS the sweep time is: sweep
time (in seconds) = 2100/SR (symbol rate in KBPS)
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 - 999 seconds) the modem will terminate the reacquisition mode and
begin sweeping the full range of plus/minus 30 KHz.
The reacquisition mode is intended to reduce the time for reacquisition at lower symbol
rates. The following guidelines should be applied when using the re-acquisition mode:
1. The time specified for re-acquisition must be multiples of 10% of the total
nominal sweep time. This will insure that the modem has time to sweep across
the lock point at least once prior to termination of the re-acquisition mode.
2. Add a little to the re-acquisition time to account for the anticipated outage.
EXAMPLE: Demod operating at 64 Kbps, an anticipated outage of 2 seconds. What
should be the minimum re-acquisition setting?
Sweep time = 8400/64 = 132 second (round fractions up). Ten percent of the total sweep
time is 13.2 seconds. Add 2 seconds to the minimum re-acquisition time for the
anticipated outage.
ANSWER = 15.2 seconds, round up for 16 seconds minimum.
The additional function for sweep reacquisition can be found in the Configuration menu
of the front panel. SWP_RACQ is the parameter used to set the minimum reacquisition
time as described above.