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CDM-625 Advanced Satellite Modem
Introduction
For IF applications (50–180 MHz), the signal is first mixed up to the first IF frequency. The
second conversion is a complex mix, resulting in the signal once more being split into an in-
phase (I) and a quadrature (Q) component, producing an output at near-zero frequency.
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 CDM-625 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 CDM-625 modem 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 CDM-625 is fully backwards-compatible with the Comtech EF Data CDM-500, CDM-550, and
CDM-550T modems. As an Open Network Modem, the CDM-625 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 unit is also fully backwards-compatible with the CDM-600 and CDM-600L modems (with the
exception of a lower data rate range of 18 kbps). The modem includes software emulation of
CDM-600 and CDM-600L.
1–3
Revision 13
MN-CDM625
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