218 N5511A Phase Noise Test System User’s GuideFM Discriminator FundamentalsThe Frequency Discriminator MethodThe Frequency Discriminator MethodUnlike the phase detector method, the frequency discriminator method doesnot require a second reference source phase locked to the source under test.See Figure 9-1.Figure 9-1 Basic delay line/mixer frequency discriminator methodThis makes the frequency discriminator method extremely useful for measuringsources that are difficult tN5511Ao phase lock, including sources that aremicrophonic or drift quickly. It can also be used to measure sources withhigh-level, low-rate phase noise, or high close-in spurious sidebands,conditions with can pose serious problems for the phase detector method. Awide-band delay line frequency discriminator is easy to implement using theN5511A Phase Noise Test System and common coaxial cable.Basic theoryThe delay line implementation of the frequency discriminator (Figure 9-1)converts short-term frequency fluctuations of a source into voltagefluctuations that can be measured by a baseband analyzer. The conversion is atwo part process, first converting the frequency fluctuations into phasefluctuations, and then converting the phase fluctuations to voltagefluctuations.The frequency fluctuation to phase fluctuation transformation (Δƒ→ΔΦ) takesplace in the delay line. The nominal frequency arrives at the double-balancedmixer at a particular phase. As the frequency changes slightly, the phase shiftincurred in the fixed delay time will change proportionally. The delay lineconverts the frequency change at the line input to a phase change a the lineoutput when compared to the undelayed signal arriving at the mixer in thesecond path.The double-balanced mixer, acting as a phase detector, transforms theinstantaneous phase fluctuations into voltage fluctuations (ΔΦ→V). With thetwo input signals 90° out of phase (phase quadrature), the voltage out isproportional to the input phase fluctuations. The voltage fluctuations can thenbe measured by the baseband analyzer and converted to phase noise units.
