Comtech EF Data CDM-550T - Manual (Page 27)

Chapter 3. FUNCTIONAL
DESCRIPTION
The CDM-550T has two fundamentally different types of interface – Data and Intermediate
Frequency (IF):
• The data interface is a bi-directional path which connects with the customer’s equipment
(assumed to be the Digital Test Equipment [DTE]) and the modem (assumed to be the
Digital Circuit Equipment [DCE]).
• The IF interface provides a bi-directional link with the satellite via the uplink and
downlink equipment.
Transmit Data
1. Tx data is received by the terrestrial interface where line receivers convert the clock and
data signals to Complementary Metal-Oxide Semiconductor (CMOS) levels for
processing.
2. A small First In – First Out (FIFO) follows the terrestrial interface to facilitate the various
clocking and framing options.
3. If framing is enabled, the Tx clock and data output from the FIFO pass through the
framer, where the EDMAC data is added to the main data.
4. Otherwise, the clock and data are passed directly to the FEC encoder.
5. In the FEC encoder, the data is differentially encoded, scrambled, and then
convolutionally encoded.
6. Following the encoder, the data is fed to the Tx digital filters, which perform spectral
shaping on the data signals.
7. The resultant I and Q signals are then fed to the QPSK/BPSK modulator.
8. The carrier is generated by a frequency synthesizer, and the I and Q signals directly
modulate this carrier to produce an IF output signal.
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