Simultaneous time and frequency processing: a digital revolution
Time-Frequency instrumentation - High frequencies and phase noise measurement

Phase noise has traditionally been measured using analog techniques. As we have just seen, a transducer is used to convert phase fluctuations into a voltage which is sampled and analyzed by a Fourier transform analyzer. The transducer is almost always a double balanced mixer followed by a low-noise amplifier, and must be calibrated at all Fourier frequencies of the measurement band.

For phase noise characterization of two oscillators, the double balanced mixer functions as a phase detector only when the local oscillator (LO) and input signals are in phase quadrature. Thus, analog phase noise measurements of two sources require the unit under test to be phase-locked to the reference (figure