Comtech EF Data CDM-500 - Manual (Page 38)

CDM-500 Closed Network Satellite Modem Front Panel Operation
Rev. 1.3 6–9
NOTE: If BPSK is selected, only Rate 1/2 is permitted. A warning screen will be
displayed if this sub-branch is entered when in BPSK mode.
6.5.16 (CONFIG, RX) ACQ (Acquisition sweep range)
EDIT RX ACQ SWEEP RANGE:
+/- 25 kHz (PRESS ENTER)
Edit the acquisition sweep range of the demodulator by selecting the digit to be edited,
using the LEFT/RIGHT arrow keys). Change the value of the digit using the UP/DOWN
arrow keys. Press Enter.
The range varies from +/- 1 kHz to +/- 30 kHz. 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.
6.5.17 (CONFIG, RX) DESCRAM (Descrambler)
SELECT RX DE-SCRAMBLING:
ON OFF (PRESS ENTER)
Select ON or OFF, using the LEFT/RIGHT 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.
6.5.18 (CONFIG, RX) CLK (Clocking)
SELECT RX CLOCKING MODE:
BUFFER ON OFF(PRESS ENT)
Select OFF or ON, using the LEFT/RIGHT 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
rom 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-500 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 menu below: