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Ultra-High Performance Amplitude and Frequency Modulation and Demodulation
Low distortion and high signal to noise ratio modulation and demodulation for both the AM and FM cases is possible, but several issues need to be recognized, among them: Filter characteristics, modulator and demodulator static and dynamic linearity, and topologies for modulators and/or demodulators. Approaches will be outlined which have resulted in successful achievement of distortion levels under 0.1% for AM and under 0.02% for FM, with signal to noise ratios of 80 to 90 decibels for AM and 70 to 80 decibels for FM. Better signal to noise ratios may be obtained at the sacrifice of distortion level. Distortion in amplitude modulators and demodulators is shown to be increased by processing passband amplitude asymmetry, and in frequency modulators and demodulators, by nonlinear phase response, as well as by modulator and demodulator amplitude distortion. A DFT (discrete Fourier Transform) approach is shown which can analyze the low-frequency theoretical distortion, and an example is given using the FM quadrature detector.
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