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10
FM Discriminator Fundamentals
The Frequency Discriminator Method
The Frequency Discriminator Method
Figure 10-1 Basic Delay Line/mixer Frequency Discriminator Method
Basic Theory
10-2
E5500 Phase Noise Measurement System Version A.02.00
Unlike the phase detector method, the frequency discriminator
method does not require a second reference source phase locked to
the source under test. See
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.
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.
The frequency fluctuation to phase fluctuation transformation
(
) takes place in the delay line. The nominal frequency arrives
∆ f
→
∆φ
at the double-balanced mixer at a particular phase. As the frequency
changes slightly, the phase shift incurred in the fixed delay time will
Figure
10-1.
Document Part No. E5500-90024 Ed. 1.0
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