LRD-200B Low-Rate Demodulator
LRD-200B Low-Rate Demodulator
©
Copyright 2012, Orbital Systems, Ltd.
63
Revision B.03
6.2 DIGITAL DEMODULATOR
Figure 38 - Digital Demodulator
The digital demodulator digitizes the 70 MHz IF signal with 10-bit accuracy at a 50
MHz sample rate. The digitized signal is passed to a Digital Quadrature Tuner,
which converts the 70 MHz signal to a complex format baseband (zero frequency)
representation.
The baseband signal is passed to a decimation filter that matches the effective
sample rate to the rate required by the particular signal being demodulated. The
output of the decimation filter is passed to the Digital Costas Loop.
The Digital Costas Loop is used to demodulate the transmitted symbols. It provides
an AGC signal to the input selection and conversion module to ensure that the
analog-to-digital conversion is accomplished within an optimal dynamic range,
filling all 10 bits of the converter. It also provides feedback to the Digital
Quadrature Tuner to provide carrier tracking.
The Digital Costas Loop locks to the recovered carrier, provides timing recovery,
and demodulates the signal using an RRC (square Root of Raised Cosine) filter with
a fixed alpha of 0.4. The digital demodulator output is
I
(In phase) and
Q
(quadrature phase) soft decision symbol output.
The soft decision output provides additional information about the accuracy of the
symbol decision, which is useful if the signal requires Viterbi decoding.
Содержание LRD-200B
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