Ultra High Performance Amplitude and Frequency Modulation and Demodulation
Low-distortion and high-signal-to-noise-ratio modulation and demodulation for both the amplitude- and the frequency-modulation cases are possible, but several issues need to be recognized, such as filter characteristics; modulator and demodulator static and dynamic linearity; and topologies for modulators and/or demodulators. Approaches are outlined which have resulted in the successful achievement of distortion levels under 0.1% for AM and under 0.02% for FM, with signal-to-noise ratios of 80-90dB for AM and 70-80dB for FM. Better signal-to-noise ratios may be obtained at the sacrifice of the distortion level. The 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 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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