5120A/5115A Operations and Maintenance Manual 87
Appendix B: Theory of Operation
B.1 Introduction
Traditionally, phase noise measurements are made by analog techniques. A transducer is used to
convert the phase fluctuations to a voltage that is sampled and Fourier analyzed. The transducer is
almost always a double-balanced mixer that, along with the following low noise amplifiers, must
be calibrated at all Fourier frequencies of interest. The double balanced mixer operates as a phase
detector only when the signals at the local oscillator (LO) and signal ports are approximately in
phase quadrature. Thus analog phase noise measurements of two sources require that the unit
under test be phase locked to the reference. Using digital techniques, it is possible to eliminate
both of these restrictions making it much simpler to make high quality phase noise measurements.
Figure B-1 shows the analog approach to source phase noise measurements.
Figure B-1: Analog source phase noise measurements require both calibration and a phase-lock loop.
The digital method samples the RF waveforms rather than the output of the phase detector. The
phase detection is performed by digital signal processing, which results in several advantages: the
phase detector has 2Nπ range, eliminating the need for a phase-lock loop (PLL), and all digital
filters are sufficiently flat to eliminate the need to calibrate individual measurements. It is possible
to make the signal processing errors fall below the noise of the analog-to-digital converters
(ADCs), which then determine the noise floor of the direct-digital, phase-noise measurement
system. As seen below, this noise is rather high, but can be reduced to useful levels through the
application of cross-correlation.
1
1.F. L. Walls, S. R. Stein, James E. Gray, and David J. Glaze, “Design Considerations in State-of-the-Art Sig-
nal Processing and Phase Noise Measurement Systems,” Proceedings of the 30th Annual Frequency Control
Symposium, 1976, pp 269-274.
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