NIST Technical Note 1337: Characterization of Clocks and Oscillators

12 FREOUENCY AND TIME MEASUREMENT2295.00688 MHz8-----~OSCILLATORUNDERTESTL.---'SMHzREFERENCEFIG.12-26 Use ofa simple divider as a substitute for a commercial frequency synthesizer ina heterodyne measurement system. Better noise performance can result from the initial mixingstage.12.3.3.2 THE DUAL-MIXER TIME-DIFFERENCETECHNIQUEThere is no best answer to the question of how to make freq uency-stabilitymeasurements. However, by combining versatility with low-noise perfonnance,the dual-mixer time-difference technique (Cutler and Searle, 1966: Allanand Daams, 1975) shown in Fig. 12-27 comes close to the ideal. The originalmotivation for this method was to use a transfer oscillator and two mixers inparallel to permit short-term frequency-stability measurements betweenoscillators that have an inconveniently small frequency difference. Thetransfer oscillator is most easily realized with a frequency synthesizer lockedto one of the oscillators, designated oscillator I in Fig_ 12-27. By conventionthe frequency of the synthesizer is set low compared to the oscillator undertest, so we write the frequency of the synthesizer asV = VI (1 - IIR). (12-83)sThe constant R is equal to the heterodyne factor, which can be seen bycalculating the beat frequency between oscillator 1 and the synthesizer:V BI = VI - V. = vt/R. (12-84)OSCILLATOR 1fREQUENCYSYNTHESIZERZERO­CROSSINGDETECTORZERO­CROSSINGDETECTORpOSCILLATOR 2FIG. 12-27 A dual-mixer measurement syslem. The scalars measure the number of wholecydes of elapsed phase. while the time-interval counter measures the fractional cycle.