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19 Nov 2012

MODEM Comtech cdm 570



Overview


The CDM-570L (Figure 1-1) is an L-Band Satellite Modem, intended for closed network
applications. The CDM-570 Satellite Modem is the 70/140 MHz IF version of the same modem.
Apart from the IF frequency band, the two modems are essentially identical.





• Variable data rates from 2.4 kbps to 9.98 Mbps, in BPSK, QPSK, Offset QPSK, 8-PSK,
8-QAM, and 16-QAM modes are offered.
• Viterbi, concatenated Reed-Solomon (RS), Trellis Coded Modulation (TCM), and Turbo
Product Coding (TPC, IESS-315 compliant) are provided as Forward Error Correction
(FEC) options.
• A full range of interface types is built in, including T1 and E1 G.703 types.
• The CDM-570 IF frequency range covers 50 to 90 and 100 to 180 MHz.
• The CDM-570L IF frequency range covers 950 to 1950 MHz. and supports external
Block Upconverters (BUCs) and Low-Noise Block Downconverters (LNBs). An optional
BUC power supply, up to 150 Watts @ 50°C and 180 Watts @ 30°C, may be installed
internally. 10 MHz reference signals are available to drive both BUC and LNB. LNB
power and FSK for ‘smart’ BUCs is standard.
• The CDM-570L is compact – 1RU high and 16 inches deep – and consumes only 22
Watts (typical, not including BUC power supply or IP module). The CDM-570 is 12
inches deep and consumes 18 Watts (typical, not including IP module).
• They have a front panel VFD display and keypad for local configuration and control,
although they can be fully remote-controlled.
• An optional integrated 10/100 BaseT Ethernet interface offers a wide range of networkbased
management options, such as SNMP, http (web server), and Telnet.


Functional Description

The CDM-570/570L has two fundamentally different types of interface – IF and data:
• The IF interface provides a bi-directional link with the satellite via the uplink and
downlink equipment.
• 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).
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. Following this, the data passes to the
Plesiochronous/Doppler buffer, which has a programmable size, or may be bypassed. From here,
the receive clock and data signals are routed to the terrestrial interface, and are passed to the
externally connected DTE equipment.
The CDM-570/570L signal processing functions are performed in a single, large
Field-Programmable Gate Array (FPGA), which permits rapid implementation of changes,
additions and enhancements in the field. These signal processing functions are controlled and
monitored by a 32-bit RISC microprocessor, which also controls all front panel, serial and
Ethernet interfaces.
Physically, the CDM-570/570L modem is comprised of a single printed circuit board assembly,
with two expansion slots for FEC codecs and other option cards.


Front Panel



Figure 1-2 shows the front panel of the modem. The front panel features (from left): a Type B
USB connector, used with a PC for reflashing the modem firmware; eight Light-Emitting-Diode
(LED) indicators; a keypad; and a Vacuum Fluorescent Display (VFD).
The VFD is an active display showing two lines of 24 characters each. It produces a blue light
with adjustable brightness. Compared to a Liquid Crystal Display (LCD), the VFD has greatly
superior viewing characteristics and does not suffer problems of viewing angle or contrast.
The keypad comprises six individual keyswitches, mounted directly behind a fully sealed
membrane overlay. They have a positive ‘click’ action, which provides tactile feedback. The user
enters data via the keypad, and messages are displayed on the VFD. The LEDs indicate, in a
summary fashion, the status of the unit.
The function and behavior of the LED indicators, VFD, and keypad is described in detail in
Chapter 5. FRONT PANEL OPERATION.


Rear Panel



Figure 1-3 shows the rear panel of the modem. External cables are attached to connectors on the
rear panel of the CDM-570/570L. Each connector is described in detail in Chapter 3. REAR
PANEL CONNECTOR PINOUTS. They comprise:



Note: The European EMC Directive (EN55022, EN50082-1) requires using properly shielded
cables for DATA I/O. These cables must be double-shielded from end-to-end, ensuring a
continuous ground shield.
In addition to the connectors listed above, there are eight LEDs on the rear panel:
• Six of the LEDs, all orange, indicate the interface type currently selected: V.35, RS232,
RS422/EIA530, T1, E1-U, or E1-B.
• For systems in a redundant configuration, a green LED labeled “Online” indicates the
Online/Offline status of the unit.
• When the unit is connected to a 1:N switch, a red LED labeled “1:N CAUTION!”
indicates that caution is required, as there may be DC voltages and other control signals
present on certain pins on the 25-pin Data Interface connector.
Also associated with redundancy mode on the rear panel is a slide switch that selects the 1:N mode.










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