224 N5511A Phase Noise Test System User’s GuideFM Discriminator Measurement ExamplesIntroductionIntroductionThese two measurement examples demonstrates the FM Discriminatormeasurement technique for measuring the phase noise of a signal source usingtwo different calibration methods.These measurement techniques work well for measuring free-runningoscillators that drift over a range that exceeds the tuning range limits of thephase-locked-loop measurement technique. The Discriminator measurementis also useful for measuring sources when a VCO reference source is notavailable to provide adequate drift tracking.The setup for a discriminator measurement uses a delay line to convertfrequency fluctuations to phase fluctuations and a phase shifter to setquadrature at the phase detector.Figure 10-1 FM Discriminator measurement setupIn the Discriminator measurement, the source is placed ahead of the powersplitter. One output of the splitter feeds a delay line with enough delay todecorrelate the source noise. The delay line generates a phase shiftproportional to the frequency. The phase shift is measured in the phasedetector by comparing the delay output with the other output from the splitter.The output of the phase detector is a voltage proportional to the frequencyfluctuations of the source.For more information about FM Discrimination basics, refer to Chapter 9, “FMDiscriminator Fundamentals”.
