  The close-in AM noise is often neglected, under the assumption that it is a
minor problem as compared to phase noise. With the progress of technology and
of experimental science, this assumption is no longer true. Yet, information in
the literature is scarce or absent. This report describes the measurement of
the AM noise of rf/microwave sources in terms of Salpha(f), i.e., the power
spectrum density of the fractional amplitude fluctuation alpha. The proposed
schemes make use of commercial power detectors based on Schottky and tunnel
diodes, in single-channel and correlation configuration. There follow the
analysis of the front-end amplifier at the detector output, the analysis of the
methods for the measurement of the power-detector noise, and a digression about
the calibration procedures. The measurement methods are extended to the
relative intensity noise (RIN) of optical beams, and to the AM noise of the
rf/microwave modulation in photonic systems. Some rf/microwave synthesizers and
oscillators have been measured, using correlation and moderate averaging. As an
example, the flicker noise of a low-noise quartz oscillator (Wenzel 501-04623E)
is Salpha = 1.15E-13/f, which is equivalent to an Allan deviation of
sigma_alpha = 4E-7. The measurement systems described exhibit the world-record
lowest background noise.
