Comtech EF Data CDM-550T - Manual (Page 52)

CDM-550T Satellite Modem Revision 3
Front Panel Operation MN/CDM550T.IOM
The value entered here determines the amount of frequency uncertainty the demodulator will
sweep over in order to find and lock to an incoming carrier. When operating at low bit rates, large
values of sweep range (compared to the data rate) will cause excessively long acquisition times.
For example: selecting
± 30 kHz with a data rate of 2.4 kbps, BPSK, will result in an average
acquisition time of around 5 minutes.
6.2.3.2.6 (CONFIG: RX) DESCRAM (Descrambler)
Select either ON or OFF, using the [
←] [→] arrow keys, then press ENTER.
When Transparent mode is selected, a V.35 descrambler is used. In Framed mode, a frame-synchronised
descrambler is used. It is recommended that scrambling/descrambling be used at all times.
(CONFIG: RX) CLK (Clocking)
Select either OFF or ON, using the [
←] [→] arrow keys, then press ENTER.
When OFF is selected, the Plesiochronous/Doppler buffer is disabled. The
receive clock will then be derived from the satellite signal, and will therefore be
subject to clock offsets relative to the local transmit clock, and a small amount
(< 5%) clock jitter, due to the demodulation bit-timing recovery process at low
values of Eb/No.
When ON is selected, the Plesiochronous/Doppler buffer is enabled. The input to
the buffer will be the signal from the satellite, with any clock offsets and jitter.
The output from the buffer will be derived from the local TRANSMIT clock. In
this way, the receive data will be perfectly synchronous with this local clock. The
CDM-550T can be operated with independent transmit receive and transmit data
rates. Even in this configuration, where RX data rate <> TX data rate, the output
clock for the buffer will be phase locked to the transmit clock. When the buffer is
enabled, the size of the buffer may be selected using the next menu:
SELECT RX DE-SCRAMBLING:
ON OFF (PRESS ENTER)
SELECT RX CLOCKING MODE:
BUFFER ON OFF(PRESS ENT)
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