13.07.2015 Views

NIST Technical Note 1337: Characterization of Clocks and Oscillators

NIST Technical Note 1337: Characterization of Clocks and Oscillators

NIST Technical Note 1337: Characterization of Clocks and Oscillators

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The phase noise in the source can cancel out by 20 to40 dB depending on the similarity <strong>of</strong> resonantfrequencies Q's <strong>and</strong> the transmission properties <strong>of</strong>the two resonators. This approach was first used todemonstrate that the inherent frequency stability <strong>of</strong>precision quartz resonators exceeds the performance<strong>of</strong> most quartz crystal controlled oscillators [7].If only one resonance is used, the output includesthe phase fluctuations <strong>of</strong> both the source <strong>and</strong> theresonator. The calibration is accomplished bystepping the frequency <strong>of</strong> the source <strong>and</strong> measuringthe output voltage, i.e., bV = G(f)K1(f) bV1' Fromthis measurement the phase spectrum can be calculatedasVN (f)- (18)f K 1 (f)[ r(B:] .Fig. 8A shows one method <strong>of</strong> implementing thisapproach at X-b<strong>and</strong>. The cavity has a loaded qualityfactor <strong>of</strong> order 25,000. Fig. 8B shows the measuredfrequency discriminator curve. <strong>Note</strong> that K1(f) isconstant for f

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