Functional Description - Comtech EF Data CDM-625 Installation And Operation Manual

CDM-625 Advanced Satellite Modem
Introduction
1.2

Functional Description

The CDM-625 has two fundamentally different types of interface - IF and data:
• The IF interface provides a bidirectional link with the satellite via the uplink and
downlink equipment.
• The data interface is a bidirectional path which connects with the customer's equipment
(assumed to be the DTE) and the modem (assumed to be the DCE).
Transmit data is received by the terrestrial interface where line receivers convert the clock and
data signals to CMOS levels for further processing. A small FIFO follows the terrestrial interface
to facilitate the various clocking and framing options. If framing is enabled, the transmit clock
and data output from the FIFO pass through the framer, where the overhead data (IDR, IBS, D&I
o r EDMAC) is added to the main data; otherwise, the clock and data are passed directly to the
Forward Error Correction encoder.
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
s pectral shaping on the data signals. The resultant I and Q signals are then fed to the BPSK,
QPSK/OQPSK, 8-PSK, 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 outpu
a pplications (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
s econd 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
s ymbol timing recovery. The resultant demodulated signal is fed, in soft decision form, to the
selected FEC decoder, which can be Viterbi, TCM, Reed-Solomon, TPC or LDPC (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, w
here, the receive clock and data signals are routed to the terrestrial interface, and are passed to the
externally connected DTE equipment.
Physically, the CDM-625 modem is comprised of two main card assemblies:
• The first card is the baseband framer card, which includ
framer/de-framer, plesiochronous/Doppler buffer, Reed Solomon outer codec, HDLC
framer, Ethernet switch, and the main microcontroller
The second card is the modem itself, which performs all signal processing functions of
•
modulation, demodulation, and primary Forward Error Correction.
transmit digital filters, which perform
hich has a programmable size, or may be bypassed. From
1–2
MN-0000036 (Ref MN/CDM625.IOM)
frequency. The
es all of the interface circuits, the
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t. For IF