Influence of a Filter on the Phase and Amplitude Noise Spectra of an Oscillator: Consequence for Spectral Purity Measurements

If a signal goes through a linear filter, the transfer function of the filter modifies the spectra of the phase and of the amplitude fluctuations of the signal, and induces a cross spectrum. These phenomena are analyzed as a function of the filter characteristics and of the degree of correlation between the two initial noises. Theoretical and experimental results are presented and the implication for the measurement of the spectral purity of oscillators by carrier attenuation is given.