Comtech EF Data CDM-570 - Manual (Page 42)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Introduction MN/CDM570L.IOM
1–2
• The CDM-570L and CDMR-570L are compact – 1RU high and 16 inches deep or 1RU
high and 11 inches deep, respectively – and consume only 29 Watts (typical, not
including BUC power supply or IP Module).
• The CDM-570 is 1RU high and 13 inches deep, and consumes 29 Watts (typical, not
including IP Module).
• The CDM-570/570L features a front panel VFD display and six-button keypad for local
configuration and control, although both modems can be fully remote-controlled.
• An optional integrated 10/100 BaseT Ethernet interface offers a wide range of network-
based management options, such as SNMP, HTTP (non-secure web server), and Telnet.
1.2 Functional Description
The CDM-570/570L 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 EDMAC overhead data 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 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, 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.
In the CDM-570L, the Rx IF signal in the range 950 to 2000 MHz is translated to an
intermediate frequency at around 465 MHz, and from there further translated to baseband using
the carrier recovery VCO.
In the CDM-570, the conversion of signals in the range 50 to 180 MHz is directly to baseband.
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, 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, TCM, Reed-
Solomon, or Turbo if installed).
After decoding, the recovered clock and data pass to the de-framer (if EDMAC framing is
enabled) where the overhead information is removed. Following this, the data passes to the