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In a quadrature phase modulator composed of a quadrature mixer and a limiter, spectrum folding effects are analyzed based on the Fourier series of a sinusoidal-carrier pulsewidth-modulated signal. The analysis reveals that the sideband tones after the limiter are created by two different folding mechanisms-dense and sparse folding when the distortion tones are around the odd and even harmonics of the carrier, respectively. The results are then applied to a practical offset phase-locked loop (OPLL) GSM transmitter to identify the origins of critical sideband tones at offset frequencies of 4omegam and 8omegam. A simple static equation is presented to relate the magnitudes of the sideband tones to the spectral spreading of a phase modulated signal. By using the equation, quantitative correlations of the 4omegam and 8omegam sideband tones with the 400-kHz emission level in a Gaussian minimum shift-keying signal are shown. The estimation method is instrumental for performance verification of OPLL transmitters. The analysis results are verified through simulations and measurements.
Microwave Theory and Techniques, IEEE Transactions on (Volume:56 , Issue: 2 )
Date of Publication: Feb. 2008