Comtech EF Data C5 - Manual (Page 218)

Theory of Operation C5/K1/K3 Integrated Satellite Terminal System
5-8 Rev. 0
The RF synthesizer is comprised of a Direct Digital Synthesis (DDS) chip; a voltage
controlled crystal oscillator (VCXO) loop, and an L-Band oscillator. It provides a signal
of 1498 to 1502 MHz in 200 Hz graduated steps to the RF Modulator chip. The
modulator chip divides the frequency of this input by two to generate I and Q carriers at
the desired IF output frequency. The modulator combines the I and Q carriers and the I
and Q baseband signals from the digital modulator to create a modulated carrier at 749 to
751 MHz in 100 Hz graduated steps.
For C-Band only - The output of the modulator is fed into a gain control circuit. The
attenuation level programmed by the M&C is a function of the desired C-Band output
power and the actual power measured at the ODU output.
Gain stages and a low pass filter to remove harmonics follow the attenuated signal. It is
then multiplexed onto the Modulator output cable to be transmitted to the ODU.
5.3.3 Theory of Modulation Types
The modulation types for the modem include BPSK and QPSK.
The PSK data transmission encoding method uses the phase modulation technique. This
method varies the phase angle of the carrier wave to represent a different bit value for the
receiver. The higher levels of modulation are required for an operating range that has a
limited bandwidth.
The order of modulation is represented by mPSK, where “m” relates to the number of
discrete phase angles. Refer to the following list for a brief description of the modulation
types.
! BPSK: 2 discrete phase angles represent the 2 possible states of a symbol.
! QPSK: 4 discrete phase angles represent the 4 possible states of a symbol.
Note: The code rate determines the number of symbols per bit.