Comtech EF Data C5 - Manual (Page 221)

C5/K1/K3 Integrated Satellite Terminal System Theory of Operation
Rev. 0 5-11
5.4.1 Demodulator Specifications
Refer to Chapter 2, Specifications for demodulator specifications.
5.4.2 Theory of Operation
The demodulator functions as an advanced, digital, coherent-phase-lock receiver and
decoder. Demodulator faults are also reported to the front panel. The demodulator
consists of the following basic subsections.
! Digital Costas Loop
! RF Section
! Automatic Gain Control
! Analog-to-Digital Converter (ADC)
! Soft Decision Mapping
! Programmable Vector Rotation
! Digital Nyquist Filters
! FEC Decoder
! Digital Clock Recovery Loop
! Decoder
The modulated IF signal enters the RF module for conversion to an IF frequency. The IF
is then sampled by an A to D converter and digitally demodulated. The I and Q data is
then sent to the digital Nyquist filters, resulting in a filtered, digital representation of the
received signal. The digital data is then sent to four separate circuits:
! Automatic Gain Control
! Carrier Recovery (Costas) Loop
! Clock Recovery Loop
! Soft Decision Mapping
The AGC provides a gain feedback signal to the RF section. 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.
When the active decoder determines that the modem is locked, the M&C stops the sweep
and begins the destress process. This involves fine tuning the DDS based on the phase
error in the Costas loop. The destress process continues as long as the modem is locked.
If the carrier is interrupted, the M&C resumes the sweep process.