Comtech EF Data CDM-570A Installation And Operation Manual page 43

CDM-570A/570AL Satellite Modem with Optional Packet Processor
Revision 5
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 BPSK,
QPSK/OQPSK, 8-PSK, or 16-QAM 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.
In both L-Band and 70/140 MHz operation, receive signals are first converted to a Ultra High
Frequency (UHF) for Surface Acoustic Wave (SAW) filtering, and are then down converted to a
lower IF frequency for direct sampling in a high speed A/D converter. An Automatic Gain Control
(AGC) circuit controls the signal level presented to the A/D converter. Digital signal processing
performs the final quadrature conversion to baseband followed by Nyquist filtering, carrier
recovery, and symbol timing recovery. The resultant demodulated signal is fed, in soft decision
form, to the selected FEC decoder (which can be Viterbi, TCM, Reed-Solomon, Turbo,
VersaFEC, or VersaFEC-2, if installed).
After decoding, the recovered clock and data pass to the de-framer (if EDMAC framing is
enabled) where the overhead information is removed. Following this, the data either passes or
bypasses the Plesiochronous/Doppler buffer, which has a programmable size. From here, the
receive clock and data signals are routed to the terrestrial interface, and are passed to the
externally connected DTE equipment.
The modem signal processing functions are performed in a single, large Field-Programmable
Gate Array (FPGA), which permits rapid implementation of changes, additions and
enhancements in the field. These signal processing functions are controlled and monitored by a
32-bit Reduced Instruction Set Computing (RISC) microprocessor, which also controls all front
panel, serial and Ethernet interfaces.
Introduction 1–3 MN-CDM570A