QPSK transmission system having phaselocked tracking filter for spectrum shaping
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United States of America Patent
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-
Apr 12, 1988
Grant Date -
N/A
app pub date -
Oct 25, 1985
filing date -
Oct 25, 1985
priority date (Note) -
Expired
status (Latency Note)
Importance
Abstract
A system for transmitting digital data employs an angle modulated carrier signal having constant amplitude and a limited spectrum. The system comprises an offset quadrature phase shift key (QPSK) modulator with a 'tracking filter' phaselocked loop at the output of the QPSK modulator. The phaselocked loop has a voltage controlled oscillator which produces a constant amplitude carrier signal at the output of the loop, and also has a low pass loop filter which reduces the spectrum of the phase modulated carrier signal. Additional spectrum shaping is obtained by connecting L-R-C circuits to the low pass loop filter to eliminate predetermined frequency components from the output of the loop.
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