Comtech EF Data SDM-9000 - Manual (Page 142)

Theory of Operation SDM-9000 Satellite Modem
5–14 Rev. 4
The digital data is then sent to four separate circuits:
•
AGC/AOC
•
Carrier recovery (Costas) loop
•
Clock recovery loop
•
Soft decision mapping
The AGC/AOC provides a gain feedback signal to the RF module, and an offset
feedback signal to the offset amplifiers just prior to the Nyquist filters. This closed-loop
control ensures that the digital representation of the I and Q channels is optimized for the
Costas and clock loops, as well as the soft-decision mapping circuitry.
The digital Costas loop, in conjunction with a Direct Digital Synthesizer (DDS),
performs the carrier recovery function. The Costas loop consists of the following circuits
(all implemented digitally):
•
Costas phase detector
•
Loop filter
•
DDS
The DDS performs the function of a VCO in an analog implementation, but can be easily
programmed to the desired center frequency via the local M&C. The output of the DDS
is sent to the RF module and provides the reference to which the quadrature local
oscillator is locked. The local M&C sweeps the local oscillator (via DDS programming)
through the user specified sweep range. When the Viterbi decoder determines that the
modem is locked, the local M&C stops the sweep and begins the de-stress process. This
involves fine tuning the DDS based on the phase error in the Costas loop. The de-stress
process continues as long as the modem is locked. If the carrier is interrupted, the local
M&C resumes the sweep process.
The digital clock loop, in conjunction with the other half of the DDS, performs the clock
recovery function. The clock loop consists of the following circuits (all implemented
digitally):
•
Phase detector
•
Loop filter
•
DDS