User`s guide

246 Agilent E5505A User’s Guide
9 FM Discriminator Fundamentals
The Frequency Discriminator Method
Unlike the phase detector method, the frequency discriminator method does
not require a second reference source phase locked to the source under test.
See Figure 178.
.
This makes the frequency discriminator method extremely useful for
measuring sources that are difficult to phase lock, including sources that are
microphonic or drift quickly. It can also be used to measure sources with
high-level, low-rate phase noise, or high close-in spurious sidebands,
conditions with can pose serious problems for the phase detector method. A
wide-band delay line frequency discriminator is easy to implement using the
E5505A Phase Noise Measurement System and common coaxial cable.
Basic theory
The delay line implementation of the frequency discriminator (Figure 178)
converts short-term frequency fluctuations of a source into voltage
fluctuations that can be measured by a baseband analyzer. The conversion is a
two part process, first converting the frequency fluctuations into phase
fluctuations, and then converting the phase fluctuations to voltage
fluctuations.
The frequency fluctuation to phase fluctuation transformation ( ) takes
place in the delay line. The nominal frequency arrives at the double-balanced
mixer at a particular phase. As the frequency changes slightly, the phase shift
incurred in the fixed delay time will change proportionally. The delay line
converts the frequency change at the line input to a phase change a the line
output when compared to the undelayed signal arriving at the mixer in the
second path.
Figure 178 Basic delay line/mixer frequency discriminator method
Δf Δφ→