Fast Estimation of Spectral Spreading in GSM OPLL Transmitters Based on Folding Effects Analysis in Quadrature Phase Modulator

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 4omega_m and 8omega_m. 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 4omega_m and 8omega_m 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.