Comtech EF Data CDM-625 - Manual (Page 39)

CDM-625 Advanced Satellite Modem Revision 15
Introduction MN-CDM625
1–3
An AGC circuit maintains the desired signal level constant over a broad range. Following this, the
I and Q signals are sampled by high-speed (flash) A/D converters. All processing beyond this
conversion is purely digital, performing the functions of 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, Sequential, TCM, Reed-Solomon, TPC, LDPC or
VersaFEC (if installed).
After decoding, the recovered clock and data pass to the de-framer (if IBS, IDR, D&I or EDMAC
framing 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.
1.3 Features
1.3.1 Physical Description
The unit is constructed as a 1RU-high rack-mounting chassis, which can be free-standing if
desired. Handles at the front ease placement into and removal from a rack.
The unit chassis assembly (CEFD P/N PL/12587-1 Standard AC Chassis or CEFD P/N PL/12587-2
Optional DC Chassis) is physically comprised of two main card assemblies:
• Baseband Framing Card (CEFD P/N PL/11963-1). This first card includes all of the interface
circuits, the framer/de-framer, plesiochronous/Doppler buffer, Reed Solomon outer codec,
HDLC framer, Ethernet switch, and the main microcontroller.
• Modem Card (CEFD P/N PL/12575-1). This second card is the modem itself. It performs all
signal processing functions of modulation, demodulation, and primary Forward Error
Correction.
1.3.2 Modem Compatibility
The unit is fully backwards-compatible with the Comtech EF Data CDM-500, CDM-550, and
CDM-550T modems. As an Open Network Modem, the unit is fully compatible with modems
from other manufacturers that are compliant with the IESS-308/309/310/314 specifications.
Note, however, that IESS-315 (VSAT Turbo) defines closed network operation, and this therefore
requires modems from the same manufacturer at both ends of the link.
The CDM-625 also serves as a ‘drop-in’ replacement product and is fully backwards compatible
with the CDM-600 and CDM-600L modems (with the exception of a lower data rate range of 18
kbps). An emulation mode is provided to accomplish this design intent, and is configurable via
either the unit front panel or by remote control.