Comtech EF Data CDM-600/600L - Manual (Page 31)

CDM-600/600L Open Network Satellite Modem Revision 3
Introduction MN/CDM600L.IOM
1–3
1.2 Functional Description
The CDM-600/600L 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).
Tx 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 Tx clock and data
output from the FIFO pass through the framer, where the overhead data (IDR, IBS, D&I, or
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
encoded. Following the encoder, the data is fed to the Tx digital filters, which perform spectral
shaping on the data signals. The resultant I and Q signals are then fed to the BPSK, QPSK, Offset
QPSK, 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.
The Rx IF signal is translated to baseband using the carrier recovery VCO. This is a complex
mix, resulting in the signal once more being split into an in-phase (I) and a quadrature (Q)
component. 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, comprising a Costas loop that performs 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, or Turbo 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 alternatively
bypasses the buffer. From here, the Rx clock and data signals are routed to the terrestrial
interface, and are passed to the externally connected DTE equipment.
Physically the CDM-600/600L modem is comprised of two main card assemblies.
• The first of these is the baseband framer card, which includes all of the interface circuits,
the framer/de-framer, plesiochronous/Doppler buffer, Reed Solomon outer codec, and the
main microcontroller.
• The second card is the modem itself, that performs all of signal processing functions of
modulation, demodulation, and Forward Error Correction.
These functions are illustrated in the block diagrams on the next page (Figure 1-2).