Comtech EF Data SDM-9000, Installation And Operation Manual

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Theory of Operation SDM-9000 Satellite Modem
5–8 Rev. 4
The puncture patterns for the 3/4 and 7/8 rate encoders are as follows:
Puncture Pattern
Code Rate C0 C1
3/4 101 110
7/8 1000101 1111010
Notes:
1. 1 = code bits to be transmitted
2. 0 = deleted code bits
When using 5/6 rate 8PSK operation, 3/4 rate puncturing is used for every five input bits
to the encoder. No puncturing is done for 2/3 rate 8PSK. The 16QAM operation is
similar to QPSK, except that twice as much information is sent per symbol time as in
QPSK.
At the outputs of the encoders, the data is grouped and split into the following number of
separate data streams:
Modulation Type No. of Data Streams
QPSK 2
8PSK 3
16QAM 4
This data is fed into a look-up table for a Digital-to-Analog (D/A) converter to perform
amplitude equalization of the rectangular pulses. The amplitude-equalized data is passed
to the analog Nyquist filters for spectral shaping, as well as delay equalization.
The pulse-shaped I and Q baseband data is applied to the RF modulator, which performs
translation to the desired IF frequency. The RF synthesizer is a hybrid DDS-PLL design,
which accommodates a 2.5 kHz step size with low phase noise, using a single loop. The
RF section has a stability of
±
10 PPM. An optional high stability TCXO can be installed
to provide
±
0.2 PPM stability. The IF strip provides gain and amplitude stabilization to
control the output from -20 to +5 dBm, in 0.1 dB steps. An AGC circuit maintains the
output level to
±
0.5 dB over frequency and temperature.