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CDM-625A Advanced Satellite Modem
Introduction
In the FEC encoder, the data is differentially encoded, scrambled, and then convolutionally or
block 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, 8PSK, 8-QAM, or 16-QAM modulator.
The carrier is generated by a frequency synthesizer, and the I and Q signals directly modulate
this carrier. For L-Band applications, the directly modulated signal comprises the main output.
For IF applications (50–180 MHz), the L-Band signal is mixed down and filtered to produce the
desired output. The Rx-IF signal at L-Band is processed by a dual IF superheterodyne receiver.
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.
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-625A 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-625A modem chassis assembly (CEFD P/N PL-0021325 Standard AC Chassis or CEFD
P/N PL-0021327 Optional DC Chassis) is physically comprised of two main card assemblies:
•
Baseband Framing Card (CEFD P/N PL-0021331 or PL-0021534). 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-0021330). This second card is the modem itself. It performs
all signal processing functions of modulation, demodulation, and primary Forward Error
Correction.
1–3
Revision 0
MN-CDM625A
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