Demodulation of Amplitude Modulated Noise: A Mathematical Evaluation of a Demodulator for Pathological Tremor EMG´s

A mathematical description is given of amplitude modulated noise, simulating EMG signals of pathological tremors, and its demodulation into a tremor signal by means of a fuli wave rectifier. In this description these signals are considered in the frequency domain in the form of power spectra. The mathematically derived formulas are compared with the results of an experimental setup using a demodulator designed for use in clinical practice. The purpose of the experiments was to determine the signal-to-noise ratio of the tremor signal with respect to the demodulation noise, the latter being an inevitable consequence of demodulating the amplitude modulated carrying noise. The theoretically derived data are in acceptable agreement with the experimental results obtained in this way. As a result of the theoretical approach to the amplitude modulated noise, the signal-to-noise ratio of the demodulated signal is proportional to the bandwidth of the carrying noise of the amplitude modulation, and, for small modulation depths, is proportional to the square of the modulation depth.