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FM Discriminator Fundamentals
The Frequency Discriminator Method
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
(Figure
10-1).
Figure 10-1
Basic delay line/mixer frequency discriminator method.
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
Agilent E5500A/B Phase Noise Measurement System and common coaxial
cable.
Basic Theory
The delay line implementation of the frequency discriminator
(Figure
10-1)
converts short-term frequency fluctuations of a source into voltage
fluctuations that can be measured by a baseband analyzer. The coversion is a
two part process, first converting the frequency fluctuations into phase
fluctuations, and then converting the phase fluctuations to voltage
fluctuations.
∆f
→
∆φ
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.
10-2 Agilent Technologies E5500 Phase Noise Measurement System
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