NIST Technical Note 1337: Characterization of Clocks and Oscillators

526LESAGE AND AUDOINQuartz ClJstalFrequelll;fSourceFrequenc,MultiplierI nPhaseShi rterr-------,Frequenc,ultiplierHl-------JI noutputFig. II. Principle of measurement of phase noise with a high-quality balanced mixer used as a phase comparator.insured by phase locking the reference oscillator,number 2 of Figure 12, to the oscillator under test.Fluctuations of the output voltage u(t) at frequenciesflargerthan the frequency cutoff of the phaseloop are proportional to phase fluctuations ofoscillator number l. On the contrary, componentsof u(t) at frequencies smaller than the above frequencycutoff are representative of frequency fluctuationsof oscillator number I.The requirement of having a reference oscillatorof the same quality as the oscillator to be testedmay be inconvenient. It has recently been shownthat the phase noise of a single oscillator can bemeasured by using the mixer technique, but witha delay line [Lance et 01., 1977]. Figure 13 showsa schematic of the setup. The signal from thefrequency source is split into two channels. Thereference channel includes a phase shifter for thepurpose of adjustment. It feeds one of the mixerinputs. The other channel delays the; signal beforeit is applied to the second mixer input. It can beseen that the power spectra density of the mixeroutput is proportional to (2Trf't d) 2 S