NIST Technical Note 1337: Characterization of Clocks and Oscillators

TIME AND FREQUENCY525V(v)10- 11vFig. 7. Spectral density of fractional frequency fluctuationsof the same quartz crystal oscillator as in Figure 6 but frequencycontrolled by a Cs beam tube resonance.transforms frequency to voltage fluctuations. Thismethod can be applied to optical frequency sourcestoo, as shown in Figures 9 and 10. Here the sourceis, for instance, a CW dye laser, and the frequencyselective device is a Fabry-Perot etalon. The secondlight pass allows one to compensate for the effectsof amplitude fluctuations and to adjust to a nullthe mean value of the output voltage. The slopeof this frequency discriminator equals I V MHz-.,typically, with a good Fabry-Perot etalon in thevisible.c._ullti.. P ro• I H "I!V V.9 -/5.9.8.'.6 /'.sV/kf~VIt,-YI.A""- ~ 40 60 80 1m 1'$.3..2.15L"' ""-R~'" Ii1-"-"'-with T = (T.). Solid linescorrespond to the normal distribution of the same width.Fig. 8. Distribution of counting time results for white frequencynoise (cesium beam frequency standards, T = 10 s)in Galtonian coordinates. Circles represent the cumulative probabilitycorresponding to IT. - TIFig. 9. Principle of frequency to voltage transfer in a frequencydiscriminator.4.2. Use of a phase detectorThis technique is well suited for the study offrequency sources in the radio frequency domain0.2 MHz < Vo < 500 MHz, in a range where verylow noise balanced-diode mixers which utilizeSchottky barrier diodes are available. This techniquehas mainly been promoted by the NationalBureau of Standards [Shoaf, 1971; Walls and Stein,1977].Figure II shows the principle of the determinationof the phase fluctuations in frequency multipliers,for instance. The two frequency multipliers aredriven by the same source. A phase shifter isadjusted in order to satisfy the quadrature condition.One then basl'(t) = D [ep, (t) - ep2(t)] (II) *where D is a constant and 'P, and 'P2 are the phasefluctuations introduced in the devices under test.It is assumed that the mixer is properly used toallow a balance of the phase and amplitude fluctuationsof the frequency source.This technique is often used to characterize phasefluctuations of two separate frequency sources ofthe same frequency. The quadrature condition isF. P. elllon dirrerenti.1Source~i : ~ A";;".PhotocellsFig. 10. Principle of frequency noise analysis of a dye laser.* See Appendix Note # 25IN-175