Comtech EF Data CDM-700 - Manual (Page 35)

3–1
Chapter 3. FUNCTIONAL
DESCRIPTION
3.1 Description
The CDM-700 has two fundamentally different types of interfaces - IF and data.
•
The IF interface provides interface with the satellite via the uplink and downlink
equipment and will operate in full duplex or simplex modes.
•
The data interface connects with the customer’s equipment (assumed to be the Data
Terminal Equipment [DTE]) and the modem (assumed to be the Data Communications
Equipment [DCE]). The data interface is usually full duplex but the modem also operates
in simplex Tx only and Rx only applications. (Note – the current version supports
duplex and Rx only operation.)
Tx data is accepted by the terrestrial interface where line receivers convert the clock and data
signals for further processing. A small First-In-First Out (FIFO) follows the terrestrial interface to
facilitate various clocking selections. The clock and data are passed to the Framer card, where the
data from all enabled
1
ports on the data interface are combined together in a Multiplexer (Mux) and
aligned into a frame structure and sent to the Forward Error Correction (FEC) encoder.
In the FEC encoder, the data is differentially encoded, scrambled, and then encoded.
Following the encoder, the data is fed to the modulator where Tx digital filters perform spectral
shaping on the data signals. The resultant in-phase (I) and quadrature (Q) signals are then fed to
the remainder of the modulator for QPSK/8-PSK/16-QAM/64-QAM modulation. 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 digitally IF sampled, converted, demodulated and split into an I and a Q
component. An Automatic Gain Control (AGC) circuit maintains the desired signal level constant
over a broad range.
1 On all interfaces each port is enabled or disabled. An enabled port operates at the data rate programmed
for that port. A disabled port is not included or zero data rate.