R&S®FSWP Validity of Positive Phase Noise Values

With recent revisions to the R&S®FSWP data sheet (as of Version 07.00), the phase noise sensitivity table for the B61 option has been extended to include specifications for offset frequencies of 0.01 and 0.1 Hz. This change has prompted questions from several R&S®FSWP users regarding positive (greater than zero dBc/Hz) values in the 0.01 Hz column for carrier frequencies above 1 GHz.

The basis of these questions stems from an obsolete spectrum analyzer-based definition of single sideband phase noise L(f), where L(f) was defined as the spectral power density of the noise sidebands in a 1 Hz band at an offset frequency (f) away from the carrier divided by the total signal power. This old definition of L(f) results in confusion when measuring carrier noise with large angle phase fluctuations and has been corrected by the IEEE using more modern measurement methods.

This paper reviews the classical spectrum analyzer measurement of L(f) and its shortcomings and discusses how modern phase noise test sets measure phase noise and avoid the limitations imposed by direct spectrum measurements.

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