Specifications; Theory Of Operation - Comtech EF Data SDM-650B Installation And Operation Manual

SDM-650B Satellite Modem

5.3.1 Specifications

Demodulation Type
Frequency Range
Frequency Select Method
Frequency Step Size
Channel Spacing
Input Level Desired Carrier
Input Overload
Input Impedance
Input Return Loss
Filtering
Image Rejection
Symbol Rate Range
I/O Connector

5.3.2 Theory of Operation

A Costas Loop on the decoder board is used for carrier recovery.
The demodulator board provides only the I and Q channel signals, and has a VCXO
input.
The incoming modulated carrier enters the demodulator in the AGC amplifier, where the
carrier is filtered and amplified. The AGC circuit, controlled from off-card, provides
variable gain to maintain the signal level into the IF converter at a constant level over the
entire input dynamic range.
The carrier signal is converted to the fixed IF by the frequency synthesizer. The
synthesizer is programmable externally, to allow for acquisitions of carriers in the 50 to
90 MHz, or 100 to 180 MHz range.
The IF is split into two channels in the quadrature demodulator. Using an in-phase and a
quadrature carrier from a VCXO controlled externally, the quadrature demodulator
produces the I and Q channel baseband signals.
The I and Q channel baseband signals are amplified and filtered in the Nyquist filters.
These filters are matched filters to those of the transmitter, resulting in optimal detection
of the transmitted data. In addition, they effectively remove adjacent channels which
could corrupt the detection process.
The signal is amplified to the final level in the baseband amplifier. At this point, the
signal looks like the classic "eye" pattern. The signal is then quantized in the soft
decision interface and output to the card I/O connector.
Rev. 6
QPSK (BPSK optional)
50 to 90 MHz, or 100 to 180 MHz
Synthesized
2.5 kHz
0.7 times the data rate divided by the encoding rate
-55 to -30 dBm
0 dBm max
75 Ω (50 Ω optional)
20 dB minimum
Nyquist, 8 pole, 1 of 4 selectable
40 dB minimum
9.1 to 3152 Ks/s
DIN, 96-pin
Theory of Operation
5–17