Mocomtech CDM-570L Manuel D’Utilisation

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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