Chapter 3. Functional Description - Comtech EF Data CDM-550T Manual

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Chapter 3.
FUNCTIONAL DESCRIPTION
The CDM-550T has two fundamentally different types of interface - IF and data. The
data interface is a bi-directional path which connects with the customer's equipment
(assumed to be the DTE) and the modem (assumed to be the DCE). The IF interface
provides a bi-directional link with the satellite via the uplink and downlink equipment.
Transmit data is received by the terrestrial interface where line receivers convert the
clock and data signals to CMOS levels for further processing. A small FIFO follows the
terrestrial interface to facilitate the various clocking and framing options. If framing is
enabled, the transmit clock and data output from the FIFO pass through the framer, where
the EDMAC data is added to the main data. Otherwise, the clock and data are passed
directly to the Forward Error Correction encoder. In the FEC encoder, the data is
differentially encoded, scrambled, and then convolutionally encoded. Following the
encoder, the data is fed to the transmit digital filters, which perform spectral shaping on
the data signals. The resultant I and Q signals are then fed to the QPSK/BPSK
modulator. The carrier is generated by a frequency synthesizer, and the I and Q signals
directly modulate this carrier to produce an IF output signal.
The received IF signal is first translated to a fixed IF frequency, using a frequency
synthesizer. An AGC circuit maintains the composite level within the IF bandwidth
constant over a limited range. Following this, the signal is sampled by a high-speed
(flash) A/D converter. All processing beyond this conversion is purely digital. The signal
is translated down to near zero frequency by a complex mix, and then is processed by a
digital Costas Loop, which performs the functions of Nyquist filtering, carrier recovery,
and bit-timing recovery. The resultant demodulated signal is fed, in soft decision form, to
the FEC decoder (Viterbi, Sequential or Turbo, and Reed-Solomon, if installed). After
decoding, the recovered clock and data pass to the de-framer (if EDMAC is enabled)
where the overhead information is removed. Following this, the data passes to the
Plesiochronous/Doppler buffer, which has a programmable size, or may be bypassed.
From here, the receive clock and data signals are routed to the terrestrial interface, and
are passed to the externally connected DTE equipment.
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