Mocomtech CDM-570 - Manual (Page 111)

Chapter 3. FUNCTIONAL
DESCRIPTION
The CDM-570/570L has two fundamentally different types of interface - IF and data.
• The data interface is a bi-directional path, which connects with the customer’s
equipment (assumed to be the DTE) and the modem (assumed to be the DCE).
• The IF interface provides a bi-directional link with the satellite via the uplink and
downlink equipment.
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 1950 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.
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