This paper describes a typical direct-detection noise-measuring system, with particular attention given to the use of Schottky-barrier diodes as envelope detectors. The rise of threshold at low modulation frequencies encountered in these systems has been found to be mainly attributable to FM-to-AM conversion occurring in comparatively narrow-band detectors rather than to diode-flicker noise and thus is largely removable. A sensitivity improvement of 10-20 dB may result at low frequencies, yielding a noise-to-carrier ratio threshold range of about -150 to -155 dB/100 Hz at modulation frequencies from 100 Hz to 50 kHz for measurements of AM noise, and below 0.01 Hz/100 Hz for FM noise, of microwave oscillators at C band.
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Fikart et al. (1972) studied this question.
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