  We describe the use of digital phase noise test sets at frequencies well
beyond the sampling rate of their analog-to-digital converters. The technique
proposed involves the transfer of phase fluctuations from an arbitrary high
carrier frequency to within the operating frequency range of the digital
instrument. The validity of the proposed technique has been proven via
comparison with conventional methods. Digital noise measurements eliminate the
need for calibration and improve consistency of experimental results.
Mechanisms limiting the resolution of spectral measurements are also discussed.
