User`s guide

252 Agilent E5505A User’s Guide
10 FM Discriminator Measurement Examples
Introduction
These two measurement examples demonstrates the FM Discriminator
measurement technique for measuring the phase noise of a signal source using
two different calibration methods.
These measurement techniques work well for measuring free-running
oscillators that drift over a range that exceeds the tuning range limits of the
phase-locked-loop measurement technique. The Discriminator measurement is
also useful for measuring sources when a VCO reference source is not available
to provide adequate drift tracking.
The setup for a discriminator measurement uses a delay line to convert
frequency fluctuations to phase fluctuations and a phase shifter to set
quadrature at the phase detector.
In the Discriminator measurement, the source is placed ahead of the power
splitter. One output of the splitter feeds a delay line with enough delay to
decorrelate the source noise. The delay line generates a phase shift
proportional to the frequency. The phase shift is measured in the phase
detector by comparing the delay output with the other output from the splitter.
The output of the phase detector is a voltage proportional to the frequency
fluctuations of the source.
For more information about FM Discrimination basics, refer to Chapter 9, “FM
Discriminator Fundamentals.
Figure 180 FM Discriminator measurement setup