Comtech EF Data RCS10 - Manual (Page 225)

RCS10 Redundant Communication System Principles of Operation
TM058 - Rev. 2.3 Page 5-9
Receiver, Convolutional Encoder/Viterbi Decoder, Sequential Decoding Option, Differential
Encoder/Decoder, and a V.35 Scrambler/Descrambler.
5.8.1 Modulator
Processed baseband data ready for transmission enters the modulator and undergoes, if the
functions have been enabled, V.35 scrambling and differential encoding. The data then undergoes
convolutional encoding and is fed to the dual variable interpolating FIR filter. The FIR filter shapes
the data waveform to a predefined spectral mask and vectorizes the data for mapping into a PSK
constellation. The data is then converted to an analog waveform and is vector-modulated onto an RF
carrier produced from the transmit IF synthesizer circuitry. The final output is then fed to the IF
loopback circuitry where under microprocessor control the transmit signal may be routed to the
demodulator. Due to its nearly complete digital implementation, the modulator is capable of
performing virtually any modulation format, and can produce almost any desired spectral mask. The
modulator also houses the SCT oscillator and the reference oscillator. The reference oscillator
provides the frequency standard for both the modulator and demodulator. An external reference may
also be selected. In this case, the reference oscillator is locked to the external reference.
5.8.2 Demodulator
The demodulator performs a complete digital implementation of a variable rate phase lock satellite
receiver utilizing state-of-the-art digital signal processing (DSP) techniques. The demodulator is
capable of receiving nearly any modulation format. Signals enter the demodulator, are converted to
baseband, split into ‘I’ In phase and ‘Q’ Quadrature channels and digitized. The digitized I and Q
channels are then applied to a decimating FIR matched filter. After filtering, the signal is
demodulated using a Costas loop for recovery of the carrier and a clock recovery loop for recovery of
bit timing. The demodulated data is then fed to a 1650 Viterbi decoder, or sequential decoder if the
option is installed. After decoding, the data is differentially decoded and descrambled.