Comtech EF Data SDM-100A User Manual

Page 118

Theory of Operation

SDM-100A Satellite Modem

4–2

Rev.

Fault information from the modulator is sent to the host M&C, and includes:

• Synthesizers

out-of-lock

• RF output leveled

• Input data clock activity

• I channel digital filter activity

• Q channel digital filter activity

• AGC level faults

VCO

DDS

REF

OSC

MPC

IF OUTPUT

50 TO 180 MHz

-5 TO -30 dBm

MPC

IMPC

MPC

M&C

COMMAND

BUS

MICRO-

PROCESSOR

SCT

IF LOOPBACK

RX SAT CLK

EXT. CLOCK

TX_DATA

TX_CLOCK

SCRAMBLERS

AND

DIFFERENTIAL

ENCODER

CONVOLUTIONAL

ENCODERS

DIGITAL

NYQUIST

DAC

ALIAS

FILTER

DIGITAL

NYQUIST

DAC

ALIAS

FILTER

IF FILTER

VARIABLE

ATTENUATOR

SYNTH

Figure 4-1. Modulator Block Diagram

Data to be transmitted will come from the interface card, via the demodulator. The format
is RS-422, and includes a clock synchronous with the data. The data signal at this point is
clean and free of jitter.

The data signal then goes to the scrambler (which provides energy dispersal) and then to
the differential encoder. The differential encoder is a 2-bit encoder, which allows for
resolution of two of the four ambiguity states of the QPSK demodulator.