NIST Technical Note 1337: Characterization of Clocks and Oscillators

272A. L. LANCE, W. D. SEAL, AND F. LABAARa differential delay of 500 nsec between the two channels. The signal amplitudesare assumed to be almost equal, thus producing the familiar voltagestanding-wavepattern or interference display. Because this is a two-channelsystem, there is a null every 360°, as shown.2. System Sensitivity (Noise Floor) When Using theDifferential Delay-Line TechniqueHalford (1975) has shown that the sensitivity (noise floor) of the singleoscillatordifferential delay-line technique is reduced relative to the twooscillatortechniques. The sensitivity is modified by the factorFor WT d = 2ndTd ~ 1 a good approximation is(86)where 8 is the phase delay of the differential delay line evaluated at thefrequency.r Figure 17 shows the relative sensitivity (noise floor) of the twooscillatortechnique and the single-oscillator technique with differentdelay-line lengths. The f - 2 slope is noted at Fourier frequencies beyond-20-40-60~ -80co "-100:s0::00-'.~-...._~u.. -120UJVl0 -140z-160520 nsec}260 n!eC65nsec-180TWO OSCILLATORS-19010 10 3 10 4 10 5FOUR IER FREQUENCYSING LEOSCillATORFIG. 17 Relative sensitivity (noise floor) of single-oscillator and two-oscillator phasenoisemeasurement systems.TN-223