Comtech EF Data CDM-550T - Manual (Page 28)

CDM-550T Satellite Modem Revision 3
Functional Description MN/CDM550T.IOM
3–2
Receive Data
1. Rx IF signal is first translated to a fixed IF frequency, using a frequency synthesizer.
2. An Automatic Gain Control (AGC) circuit maintains the composite level within the IF
bandwidth constant over a limited range.
3. Following this, the signal is sampled by a high-speed (flash) Analog-to-Digital (A/D)
converter.
4. All processing beyond this conversion is purely digital.
5. 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.
6. The resultant demodulated signal is fed, in soft decision form, to the FEC decoder
(Viterbi, Sequential or Turbo, and Reed-Solomon, if installed).
7. After decoding, the recovered clock and data pass to the de-framer (if EDMAC is
enabled) where the overhead information is removed.
8. Following this, the data passes to the Plesiochronous/Doppler buffer, which has a
programmable size, or may be bypassed.
9. From here, the Rx clock and data signals are routed to the terrestrial interface, and are
passed to the externally connected DTE equipment.